A Guide To Programming In C++ Ulla Kirch Prinz, Peter Prinz Jones And Bartlett Publishers

User Manual:

Open the PDF directly: View PDF PDF.
Page Count: 846 [warning: Documents this large are best viewed by clicking the View PDF Link!]

Ulla Kirch-Prinz
Peter Prinz
A Complete Guide to
Programming in C++
World Headquarters
Jones and Bartlett Publishers
40 Tall Pine Drive
Sudbury, MA 01776
978-443-5000
info@jbpub.com
www.jbpub.com
Jones and Bartlett Publishers
Canada
2406 Nikanna Road
Mississauga, ON L5C 2W6
CANADA
Jones and Bartlett Publishers
International
Barb House, Barb Mews
London W6 7PA
UK
Copyright © 2002 by Jones and Bartlett Publishers, Inc.
All rights reserved. No part of the material protected by this copyright notice may be reproduced or utilized in
any form, electronic or mechanical, including photocopying, recording, or any information storage or retrieval
system, without written permission from the copyright owner.
Cover Image: Stones on shore-line and yellow leaf, Bjorkliden, Sweden, by Peter Lilja
Library of Congress Cataloging-in-Publication Data
Prinz, Peter.
[C++ Lernen und professionell anwenden. English]
A complete guide to programming in C++ / Peter Prinz, Ulla Kirch-Prinz; translated by Ian Travis.
p. cm.
ISBN: 0-7637-1817-3
1. C++ (Computer program language) I. Kirch-Prinz, Ulla. II. Title.
QA76.73.C153 P73713 2001
005.13'3—dc21 2001029617
2090
Chief Executive Officer: Clayton Jones
Chief Operating Officer: Don W. Jones, Jr.
V.P., Managing Editor: Judith H. Hauck
V.P., Design and Production: Anne Spencer
V.P., Manufacturing and Inventory Control: Therese Bräuer
Editor-in-Chief: Michael Stranz
Development and Product Manager: Amy Rose
Marketing Manager: Nathan Schultz
Production Assistant: Tara McCormick
Cover Design: Night & Day Design
Composition: Northeast Compositors
Text Design: Mary McKeon
Printing and Binding: Courier Westford
Cover printing: John Pow Company, Inc.
This book was typeset in QuarkXpress 4.11 on a Macintosh G4. The font families used were Goudy, Gill Sans,
Courier, Rubino Serif, and Seven Sans. The first printing was printed on 50 lb. Finch Opaque.
Printed in the United States of America
05 04 03 02 01 10 9 8 7 6 5 4 3 2 1
Dedicated to our children, Vivi and Jeany
This page intentionally left blank
v
This book was written for readers interested in learning the C++ programming
language from scratch, and for both novice and advanced C++ programmers
wishing to enhance their knowledge of C++. It was our goal from the begin-
ning to design this text with the capabilities of serving dual markets, as a text-
book for students and as a holistic reference manual for professionals.
The C++ language definition is based on the American National Stan-
dards Institute ANSI Standard X3J16. This standard also complies with ISO
norm 14882, which was ratified by the International Standardization Organi-
zation in 1998. The C++ programming language is thus platform-independent
in the main with a majority of C++ compilers providing ANSI support. New
elements of the C++ language, such as exception handling and templates, are
supported by most of the major compilers. Visit the Jones and Bartlett web site
at www.jbpub.com for a listing of compilers available for this text.
The chapters in this book are organized to guide the reader from elemen-
tary language concepts to professional software development, with in-depth
coverage of all the C++ language elements en route. The order in which these
elements are discussed reflects our goal of helping the reader to create useful
programs at every step of the way.
preface
Each double-page spread in the book is organized to provide a description of the lan-
guage elements on the right-hand page while illustrating them by means of graphics and
sample programs on the left-hand page. This type of visual representation offered by each
spread will provide students and professionals with an unmatched guide throughout the
text. The sample programs were chosen to illustrate a typical application for each lan-
guage element. In addition, filter programs and case studies introduce the reader to a
wide range of application scenarios.
To gain command over a programming language, students need a lot of experience in
developing programs. Thus, each chapter includes exercises followed by sample solu-
tions,allowing the reader to test and enhance his or her performance and understanding
of C++.
The appendix provides further useful information, such as binary number representa-
tion, pre-processor directives, and operator precedence tables, making this book a well-
structured and intelligible reference guide for C++ programmers.
In order to test and expand your acquired knowledge, you can download sample pro-
grams and solutions to the exercises at:
http://completecpp.jbpub.com
Content Organization
Chapter 1 gives a thorough description of the fundamental characteristics of the object-
oriented C++ programming language. In addition, students are introduced to the steps
necessary for creating a fully functional C++ program. Many examples are provided to
help enforce these steps and to demonstrate the basic structure of a C++ program.
Chapter 2 provides a complete introduction to the basic types and objects used by
C++ programs. Integral types and constants, fundamental types, and Boolean constants
are just a few of the topics discussed.
Chapter 3 describes how to declare and call standard functions. This chapter also
teaches students to use standard classes, including standard header files. In addition, stu-
dents work with string variables for the first time in this chapter.
Chapter 4 explains the use of streams for input and output, with a focus on formatting
techniques. Formatting flags and manipulators are discussed, as are field width, fill char-
acters, and alignment.
Chapter 5 introduces operators needed for calculations and selections. Binary, unary,
relational, and logical operators are all examined in detail.
Chapter 6 describes the statements needed to control the flow of a program. These
include loops with while, do-while, and for; selections with if-else, switch, and the condi-
tional operator; and jumps with goto, continue, and break.
Chapter 7 provides a thorough introduction to the definition of symbolic constants
and macros, illustrating their significance and use. Furthermore, a comprehensive exami-
nation of standard macros for character handling is included.
Chapter 8 introduces implicit type conversions, which are performed in C++ when-
ever different arithmetic types occur in expressions. Additionally, the chapter explores
an operator for explicit type conversion.
vi ■
PREFACE
Chapter 9 takes an in-depth look at the standard class string, which is used to repre-
sent strings. In addition to defining strings, the chapter looks at the various methods of
string manipulation. These include inserting and erasing, searching and replacing, com-
paring, and concatenating strings.
Chapter 10 describes how to write functions of your own. The basic rules are covered,
as are passing arguments, the definition of inline functions, overloading functions and
default arguments, and the principle of recursion.
Chapter 11 gives a thorough explanation of storage classes for objects and functions.
Object lifetime and scope are discussed, along with global, static, and auto objects.
Namespaces and external and static functions are also included in the discussion.
Chapter 12 explains how to define references and pointers and how to use them as
parameters and/or return values of functions. In this context, passing by reference and
read-only access to arguments are introduced.
Chapter 13 provides a complete description of how classes are defined and how
instances of classes, or objects, are used. In addition, structs and unions are introduced as
examples of special classes.
Chapter 14 describes how constructors and destructors are defined to create and
destroy objects. Also discussed are how inline methods, access methods, and read-only
methods can be used. Furthermore, the chapter explains the pointer this, which is avail-
able for all methods, and what you need to pay attention to when passing objects as argu-
ments or returning objects.
Chapter 15 gives a complete explanation of member objects and how they are initial-
ized, and of data members that are created only once for all the objects in a class. In addi-
tion, this chapter describes constant members and enumerated types.
Chapter 16 takes an in-depth look at how to define and use arrays. Of particular inter-
est are one-dimensional and multidimensional arrays, C strings, and class arrays.
Chapter 17 describes the relationship between pointers and arrays. This includes
pointer arithmetic, pointer versions of functions, pointers as return values and read-only
pointers, and pointer arrays. Students learn that operations that use C strings illustrate
how to use pointers for efficient programming, and that string access via the command
line of an application program is used to illustrate pointer arrays.
Chapter 18 explains sequential file access using file streams. Students will develop an
understanding of how file streams provide simple and portable file handling techniques.
Chapter 19 provides a complete description of the various uses of overloaded opera-
tors. Arithmetic operators, comparisons, the subscript operator, and the shift operators
for input and output are overloaded to illustrate the appropriate techniques. In addition,
the concept of friend functions, which is introduced in this context, is particularly
important for overloading operators. Students learn how overloading operators allows
them to apply existing operators to objects of class type.
Chapter 20 discusses how implicit type conversion occurs in C++ when an expression
cannot be compiled directly but can be compiled after applying a conversion rule. The
programmer can stipulate how the compiler will perform implicit type conversion for
classes by defining conversion constructors and functions. Finally, the chapter discusses
ambiguity that occurs due to type conversion and how to avoid it.
PREFACE
■vii
Chapter 21 describes how a program can allocate and release memory dynamically in
line with current memory requirements. Dynamic memory allocation is an important fac-
tor in many C++ programs, and the following chapters contain several case studies to
help students review the subject.
Chapter 22 explains how to implement classes containing pointers to dynamically
allocated memory. These include your own copy constructor definition and overloading
the assignment operator. A class designed to represent arrays of any given length is used
as a sample application.
Chapter 23 provides a thorough description of how derived classes can be constructed
from existing classes by inheritance. In addition to defining derived classes, this chapter
discusses how members are redefined, how objects are constructed and destroyed, and
how access control to base classes can be realized.
Chapter 24 discusses implicit type conversion within class hierarchies, which occurs
in the context of assignments and function calls. Explicit type casting in class hierar-
chies is also described, paying particular attention to upcasting and downcasting.
Chapter 25 gives a complete explanation of how to develop and manage polymorphic
classes. In addition to defining virtual functions, dynamic downcasting in polymorphic
class hierarchies is introduced.
Chapter 26 describes how defining pure virtual methods can create abstract classes
and how you can use abstract classes at a polymorphic interface for derived classes. To
illustrate this, an inhomogeneous list, that is, a linked list whose elements can be of vari-
ous class types, is implemented.
Chapter 27 describes how new classes are created by multiple inheritance and
explains their uses. Besides introducing students to the creation and destruction of
objects in multiply-derived classes, virtual base classes are depicted to avoid ambiguity in
multiple inheritance.
Chapter 28 explains how a C++ program uses error-handling techniques to resolve
error conditions. In addition to throwing and catching exceptions, the chapter also
examines how exception specifications are declared and exception classes are defined. In
addition, the use of standard exception classes is discussed.
Chapter 29 examines random access to files based on file streams, and options for
querying file state. Exception handling for files is discussed as well. The chapter illus-
trates how to make objects in polymorphic classes persistent, that is, how to save them in
files. The applications introduced in this chapter include simple index files and hash
tables.
Chapter 30 provides a thorough explanation of the advanced uses of pointers. These
include pointers to pointers, functions with a variable number of arguments, and pointers
to functions. In addition, an application that defines a class used to represent dynamic
matrices is introduced.
Chapter 31 describes bitwise operators and how to use bit masks. The applications
included demonstrate calculations with parity bits, conversion of lowercase and capital
letters, and converting binary numbers. Finally, the definition of bit-fields is introduced.
Chapter 32 discusses how to define and use function and class templates. In addition,
special options, such as default arguments, specialization, and explicit instantiation, are
viii ■
PREFACE
discussed. Students learn that templates allow the construction of functions and classes
based on types that have not yet been stated. Thus, templates are a powerful tool for
automating program code generation.
Chapter 33 explains standard class templates used to represent containers for more
efficient management of object collections. These include sequences, such as lists and
double ended queues; container adapters, such as stacks, queues, and priority queues;
associative containers, such as sets and maps; and bitsets. In addition to discussing how
to manage containers, the chapter also looks at sample applications, such as bitmaps for
raster images, and routing techniques.
Additional Features
Chapter Goals A concise chapter introduction, which contains a description of the
chapter’s contents, is presented at the beginning of each chapter. These summaries also
provide students with an idea of the key points to look for throughout the chapter.
Chapter Exercises Each chapter contains exercises, including programming problems,
designed to test students’ knowledge and understanding of the main ideas. The exercises
also provide reinforcement for key chapter concepts. Solutions are included to allow
students to check their work immediately and correct any possible mistakes.
Case Studies Every chapter contains a number of case studies that were designed to
introduce the reader to a wide range of application scenarios.
Notes This feature provides students with helpful tips and information useful to learning
C++. Important concepts and rules are highlighted for additional emphasis and easy
access.
Hints These are informative suggestions for easier programming. Also included are
common mistakes and how to avoid making them.
Acknowledgements
Our thanks go out to everyone who helped produce this book, particularly to
Ian Travis, for his valuable contributions to the development of this book.
Alexa Doehring, who reviewed all samples and program listings, and gave many valuable
hints from the American perspective.
Michael Stranz and Amy Rose at Jones and Bartlett Publishers, who managed the pub-
lishing agreement and the production process so smoothly.
Our children, Vivi and Jeany, who left us in peace long enough to get things finished!
And now all that remains is to wish you, Dear Reader, lots of fun with C++!
Ulla Kirch-Prinz
Peter Prinz
PREFACE
■ix
This page intentionally left blank
xi
Chapter 1 Fundamentals 1
Development and Properties of C++ 2
Object-Oriented Programming 4
Developing a C++ Program 6
A Beginner’s C++ Program 8
Structure of Simple C++ Programs 10
Exercises 12
Solutions 14
Chapter 2 Fundamental Types, Constants, and Variables 15
Fundamental Types 16
Constants 22
Escape Sequences 26
Names 28
Variables 30
The Keywords const and volatile 32
Exercises 34
Solutions 36
contents
Chapter 3 Using Functions and Classes 39
Declaring Functions 40
Function Calls 42
Type void for Functions 44
Header Files 46
Standard Header Files 48
Using Standard Classes 50
Exercises 52
Solutions 54
Chapter 4 Input and Output with Streams 57
Streams 58
Formatting and Manipulators 60
Formatted Output of Integers 62
Formatted Output of Floating-Point Numbers 64
Output in Fields 66
Output of Characters, Strings, and Boolean Values 68
Formatted Input 70
Formatted Input of Numbers 72
Unformatted Input/Output 74
Exercises 76
Solutions 78
Chapter 5 Operators for Fundamental Types 81
Binary Arithmetic Operators 82
Unary Arithmetic Operators 84
Assignments 86
Relational Operators 88
Logical Operators 90
Exercises 92
Solutions 94
Chapter 6 Control Flow 95
The while Statement 96
The for Statement 98
The do-while Statement 102
Selections with if-else 104
Else-if Chains 106
Conditional Expressions 108
Selecting with switch 110
Jumps with break, continue, and goto 112
Exercises 114
Solutions 116
xii ■
CONTENTS
Chapter 7 Symbolic Constants and Macros 119
Macros 120
Macros with Parameters 122
Working with the #define Directive 124
Conditional Inclusion 126
Standard Macros for Character Manipulation 128
Redirecting Standard Input and Output 130
Exercises 132
Solutions 134
Chapter 8 Converting Arithmetic Types 139
Implicit Type Conversions 140
Performing Usual Arithmetic Type Conversions 142
Implicit Type Conversions in Assignments 144
More Type Conversions 146
Exercises 148
Solutions 150
Chapter 9 The Standard Class string 153
Defining and Assigning Strings 154
Concatenating Strings 156
Comparing Strings 158
Inserting and Erasing in Strings 160
Searching and Replacing in Strings 162
Accessing Characters in Strings 164
Exercises 166
Solutions 168
Chapter 10 Functions 171
Significance of Functions in C++ 172
Defining Functions 174
Return Value of Functions 176
Passing Arguments 178
Inline Functions 180
Default Arguments 182
Overloading Functions 184
Recursive Functions 186
Exercises 188
Solutions 191
Chapter 11 Storage Classes and Namespaces 197
Storage Classes of Objects 198
The Storage Class extern 200
CONTENTS
■xiii
The Storage Class static 202
The Specifiers auto and register 204
The Storage Classes of Functions 206
Namespaces 208
The Keyword using 210
Exercises 212
Solutions 216
Chapter 12 References and Pointers 221
Defining References 222
References as Parameters 224
References as Return Value 226
Expressions with Reference Type 228
Defining Pointers 230
The Indirection Operator 232
Pointers as Parameters 234
Exercises 236
Solutions 238
Chapter 13 Defining Classes 243
The Class Concept 244
Defining Classes 246
Defining Methods 248
Defining Objects 250
Using Objects 252
Pointers to Objects 254
Structs 256
Unions 258
Exercise 260
Solution 262
Chapter 14 Methods 265
Constructors 266
Constructor Calls 268
Destructors 270
Inline Methods 272
Access Methods 274
const Objects and Methods 276
Standard Methods 278
this Pointer 280
Passing Objects as Arguments 282
Returning Objects 284
Exercises 286
Solutions 290
xiv ■
CONTENTS
Chapter 15 Member Objects and Static Members 297
Member Objects 298
Member Initializers 300
Constant Member Objects 302
Static Data Members 304
Accessing Static Data Members 306
Enumeration 308
Exercises 310
Solutions 314
Chapter 16 Arrays 321
Defining Arrays 322
Initializing Arrays 324
Arrays 326
Class Arrays 328
Multidimensional Arrays 330
Member Arrays 332
Exercises 334
Solutions 338
Chapter 17 Arrays and Pointers 349
Arrays and Pointers (1) 350
Arrays and Pointers (2) 352
Pointer Arithmetic 354
Arrays as Arguments 356
Pointer Versions of Functions 358
Read-Only Pointers 360
Returning Pointers 362
Arrays of Pointers 364
Command Line Arguments 366
Exercises 368
Solutions 372
Chapter 18 Fundamentals of File Input and Output 379
Files 380
File Streams 382
Creating File Streams 384
Open Modes 386
Closing Files 388
Reading and Writing Blocks 390
Object Persistence 392
Exercises 394
Solutions 398
CONTENTS
■xv
Chapter 19 Overloading Operators 411
Generals 412
Operator Functions (1) 414
Operator Functions (2) 416
Using Overloaded Operators 418
Global Operator Functions 420
Friend Functions 422
Friend Classes 424
Overloading Subscript Operators 426
Overloading Shift-Operators for I/O 428
Exercises 430
Solutions 432
Chapter 20 Type Conversion for Classes 441
Conversion Constructors 442
Conversion Functions 444
Ambiguities of Type Conversions 446
Exercise 448
Solution 450
Chapter 21 Dynamic Memory Allocation 453
The Operator new 454
The Operator delete 456
Dynamic Storage Allocation for Classes 458
Dynamic Storage Allocation for Arrays 460
Application: Linked Lists 462
Representing a Linked List 464
Exercises 466
Solutions 468
Chapter 22 Dynamic Members 477
Members of Varying Length 478
Classes with a Dynamic Member 480
Creating and Destroying Objects 482
Implementing Methods 484
Copy Constructor 486
Assignment 488
Exercises 490
Solutions 492
Chapter 23 Inheritance 499
Concept of Inheritance 500
Derived Classes 502
xvi ■
CONTENTS
Members of Derived Classes 504
Member Access 506
Redefining Members 508
Constructing and Destroying Derived Classes 510
Objects of Derived Classes 512
Protected Members 514
Exercises 516
Solutions 520
Chapter 24 Type Conversion in Class Hierarchies 529
Converting to Base Classes 530
Type Conversions and Assignments 532
Converting References and Pointers 534
Explicit Type Conversions 536
Exercises 538
Solutions 540
Chapter 25 Polymorphism 543
Concept of Polymorphism 544
Virtual Methods 546
Destroying Dynamically Allocated Objects 548
Virtual Method Table 550
Dynamic Casts 552
Exercises 554
Solutions 558
Chapter 26 Abstract Classes 565
Pure Virtual Methods 566
Abstract and Concrete Classes 568
Pointers and References to Abstract Classes 570
Virtual Assignment 572
Application: Inhomogeneous Lists 574
Implementing an Inhomogeneous List 576
Exercises 578
Solutions 580
Chapter 27 Multiple Inheritance 587
Multiply-Derived Classes 588
Multiple Indirect Base Classes 590
Virtual Base Classes 592
Constructor Calls 594
Initializing Virtual Base Classes 596
Exercises 598
Solutions 602
CONTENTS
■xvii
Chapter 28 Exception Handling 607
Traditional Error Handling 608
Exception Handling 610
Exception Handlers 612
Throwing and Catching Exceptions 614
Nesting Exception Handling 616
Defining Your Own Error Classes 618
Standard Exception Classes 620
Exercises 622
Solutions 626
Chapter 29 More About Files 637
Opening a File for Random Access 638
Positioning for Random Access 640
File State 644
Exception Handling for Files 646
Persistence of Polymorphic Objects 648
Application: Index Files 652
Implementing an Index File 654
Exercises 656
Solutions 660
Chapter 30 More About Pointers 681
Pointer to Pointers 682
Variable Number of Arguments 684
Pointers to Functions 688
Complex Declarations 690
Defining Typenames 692
Application: Dynamic Matrices 694
Exercises 696
Solutions 698
Chapter 31 Manipulating Bits 705
Bitwise Operators 706
Bitwise Shift Operators 708
Bit Masks 710
Using Bit Masks 712
Bit-Fields 714
Exercises 716
Solutions 718
Chapter 32 Templates 721
Function and Class Templates 722
Defining Templates 724
xviii ■
CONTENTS
Template Instantiation 726
Template Parameters 728
Template Arguments 730
Specialization 732
Default Arguments of Templates 734
Explicit Instantiation 736
Exercises 738
Solutions 742
Chapter 33 Containers 749
Container Types 750
Sequences 752
Iterators 754
Declaring Sequences 756
Inserting in Sequences 758
Accessing Objects 760
Length and Capacity 762
Deleting in Sequences 764
List Operations 766
Associative Containers 768
Sets and Multisets 770
Maps and Multimaps 772
Bitsets 774
Exercise 778
Solution 780
Appendix 783
Binary Numbers 784
Preprocessor Directives 787
Pre-Defined Standard Macros 792
Binding C Functions 793
Operators Overview 795
Operator Precedence Table 797
ASCII Code Table 798
Screen Control Sequences 800
Literature 801
Index 803
CONTENTS
■xix
This page intentionally left blank
1
Fundamentals
This chapter describes the fundamental characteristics of the object-
oriented C++ programming language. In addition, you will be introduced
to the steps necessary for creating a fully functional C++ program.The
examples provided will help you retrace these steps and also
demonstrate the basic structure of a C++ program.
chapter
1
2■
CHAPTER 1 FUNDAMENTALS
■DEVELOPMENT AND PROPERTIES OF C++
Characteristics
C++ C
-universal
-efficient
-close to the machine
-portable
OOP
-data abstraction
-data hiding
-inheritance
-polymorphism
Extensions
-exception handling
-templates
DEVELOPMENT AND PROPERTIES OF C++
■3
䊐Historical Perspective
The C++ programming language was created by Bjarne Stroustrup and his team at Bell
Laboratories (AT&T, USA) to help implement simulation projects in an object-ori-
ented and efficient way. The earliest versions, which were originally referred to as “C
with classes,” date back to 1980. As the name C++ implies, C++ was derived from the C
programming language: ++ is the increment operator in C.
As early as 1989 an ANSI Committee (American National Standards Institute) was
founded to standardize the C++ programming language. The aim was to have as many
compiler vendors and software developers as possible agree on a unified description of
the language in order to avoid the confusion caused by a variety of dialects.
In 1998 the ISO (International Organization for Standardization) approved a stan-
dard for C++ (ISO/IEC 14882).
䊐Characteristics of C++
C++ is not a purely object-oriented language but a hybrid that contains the functionality
of the C programming language. This means that you have all the features that are avail-
able in C:
■universally usable modular programs
■efficient, close to the machine programming
■portable programs for various platforms.
The large quantities of existing C source code can also be used in C++ programs.
C++ supports the concepts of object-oriented programming (or OOP for short),
which are:
■data abstraction, that is, the creation of classes to describe objects
■data encapsulation for controlled access to object data
■inheritance by creating derived classes (including multiple derived classes)
■polymorphism (Greek for multiform), that is, the implementation of instructions
that can have varying effects during program execution.
Various language elements were added to C++, such as references, templates, and excep-
tion handling. Even though these elements of the language are not strictly object-ori-
ented programming features, they are important for efficient program implementation.
4■
CHAPTER 1 FUNDAMENTALS
function1
data1
data2
function2
function3
object1
Properties
Capacities
object2
Properties
Capacities
■OBJECT-ORIENTED PROGRAMMING
Traditional concept
Object-oriented concept
OBJECT-ORIENTED PROGRAMMING
■5
䊐Traditional Procedural Programming
In traditional, procedural programming, data and functions (subroutines, procedures) are
kept separate from the data they process. This has a significant effect on the way a pro-
gram handles data:
■the programmer must ensure that data are initialized with suitable values before
use and that suitable data are passed to a function when it is called
■if the data representation is changed, e.g. if a record is extended, the correspon-
ding functions must also be modified.
Both of these points can lead to errors and neither support low program maintenance
requirements.
䊐Objects
Object-oriented programming shifts the focus of attention to the objects, that is, to the
aspects on which the problem is centered. A program designed to maintain bank
accounts would work with data such as balances, credit limits, transfers, interest calcula-
tions, and so on. An object representing an account in a program will have properties
and capacities that are important for account management.
OOP objects combine data (properties) and functions (capacities). A class defines a
certain object type by defining both the properties and the capacities of the objects of
that type. Objects communicate by sending each other “messages,” which in turn acti-
vate another object’s capacities.
䊐Advantages of OOP
Object-oriented programming offers several major advantages to software development:
■reduced susceptibility to errors: an object controls access to its own data. More
specifically, an object can reject erroneous access attempts
■easy re-use: objects maintain themselves and can therefore be used as building
blocks for other programs
■low maintenance requirement: an object type can modify its own internal data
representation without requiring changes to the application.
6■
CHAPTER 1 FUNDAMENTALS
Editor
Compiler
Linker
Executable
file
Source file Header file
Standard
library
Other
libraries,
object files
Object file
■DEVELOPING A C++ PROGRAM
Translating a C++ program
If the source file contains just one syntax error, the compiler will report an error. Additional error
messages may be shown if the compiler attempts to continue despite having found an error. So when
you are troubleshooting a program, be sure to start with the first error shown.
DEVELOPING A C++ PROGRAM
■7
✓NOTE
The following three steps are required to create and translate a C++ program:
1. First, a text editor is used to save the C++ program in a text file. In other words,
the source code is saved to a source file. In larger projects the programmer will nor-
mally use modular programming. This means that the source code will be stored in
several source files that are edited and translated separately.
2. The source file is put through a compiler for translation. If everything works as
planned, an object file made up of machine code is created. The object file is also
referred to as a module.
3. Finally, the linker combines the object file with other modules to form an exe-
cutable file. These further modules contain functions from standard libraries or
parts of the program that have been compiled previously.
It is important to use the correct file extension for the source file’s name. Although
the file extension depends on the compiler you use, the most commonly found file exten-
sions are .cpp and .cc.
Prior to compilation, header files, which are also referred to as include files, can be
copied to the source file. Header files are text files containing information needed by var-
ious source files, for example, type definitions or declarations of variables and functions.
Header files can have the file extension .h, but they may not have any file extension.
The C++ standard library contains predefined and standardized functions that are
available for any compiler.
Modern compilers normally offer an integrated software development environment, which
combines the steps mentioned previously into a single task. A graphical user interface is
available for editing, compiling, linking, and running the application. Moreover, addi-
tional tools, such as a debugger, can be launched.
In addition to error messages, the compiler will also issue warnings. A warning does
not indicate a syntax error but merely draws your attention to a possible error in the pro-
gram’s logic, such as the use of a non-initialized variable.
8■
CHAPTER 1 FUNDAMENTALS
#include <iostream>
using namespace std;
int main()
{
cout << "Enjoy yourself with C++!" << endl;
return 0;
}
■A BEGINNER’S C++ PROGRAM
Sample program
Screen output
Enjoy yourself with C++!
Structure of function main()
Function name
What the program does
(satements)
Type of function
End of function
Beginning of
function
Function block
int main()
{
}
.
.
.
.
What the program does
(statements)
A BEGINNER’S C++ PROGRAM
■9
A C++ program is made up of objects with their accompanying member functions and
global functions, which do not belong to any single particular class. Each function fulfills
its own particular task and can also call other functions. You can create functions your-
self or use ready-made functions from the standard library. You will always need to write
the global function main() yourself since it has a special role to play; in fact it is the
main program.
The short programming example on the opposite page demonstrates two of the most
important elements of a C++ program. The program contains only the function main()
and displays a message.
The first line begins with the number symbol, #, which indicates that the line is
intended for the preprocessor. The preprocessor is just one step in the first translation
phase and no object code is created at this time. You can type
#include <filename>
to have the preprocessor copy the quoted file to this position in the source code. This
allows the program access to all the information contained in the header file. The header
file iostream comprises conventions for input and output streams. The word stream
indicates that the information involved will be treated as a flow of data.
Predefined names in C++ are to be found in the std (standard) namespace. The
using directive allows direct access to the names of the std namespace.
Program execution begins with the first instruction in function main(), and this is
why each C++ program must have a main function. The structure of the function is
shown on the opposite page. Apart from the fact that the name cannot be changed, this
function’s structure is not different from that of any other C++ function.
In our example the function main() contains two statements. The first statement
cout << "Enjoy yourself with C++!" << endl;
outputs the text string Enjoy yourself with C++! on the screen. The name cout
(console output) designates an object responsible for output.
The two less-than symbols, <<, indicate that characters are being “pushed” to the out-
put stream. Finally endl (end of line) causes a line feed. The statement
return 0;
terminates the function main() and also the program, returning a value of 0as an exit
code to the calling program. It is standard practice to use the exit code 0to indicate that
a program has terminated correctly.
Note that statements are followed by a semicolon. By the way, the shortest statement
comprises only a semicolon and does nothing.
10 ■
CHAPTER 1 FUNDAMENTALS
/******************************************************
A program with some functions and comments
******************************************************/
#include <iostream>
using namespace std;
void line(), message(); // Prototypes
int main()
{
cout << "Hello! The program starts in main()."
<< endl;
line();
message();
line();
cout << "At the end of main()." << endl;
return 0;
}
void line() // To draw a line.
{
cout << "--------------------------------" << endl;
}
void message() // To display a message.
{
cout << "In function message()." << endl;
}
■STRUCTURE OF SIMPLE C++ PROGRAMS
A C++ program with several functions
Screen output
Hello! The program starts in main().
-----------------------------------
In function message().
-----------------------------------
At the end of main().
STRUCTURE OF SIMPLE C++ PROGRAMS
■11
The example on the opposite page shows the structure of a C++ program containing
multiple functions. In C++, functions do not need to be defined in any fixed order. For
example, you could define the function message() first, followed by the function
line(), and finally the main() function.
However, it is more common to start with the main() function as this function con-
trols the program flow. In other words, main() calls functions that have yet to be
defined. This is made possible by supplying the compiler with a function prototype that
includes all the information the compiler needs.
This example also introduces comments. Strings enclosed in /* . . . */ or start-
ing with // are interpreted as comments.
EXAMPLES:
/* I can cover
several lines */
// I can cover just one line
In single-line comments the compiler ignores any characters following the // signs up
to the end of the line. Comments that cover several lines are useful when troubleshoot-
ing, as you can use them to mask complete sections of your program. Both comment
types can be used to comment out the other type.
As to the layout of source files, the compiler parses each source file sequentially,
breaking the contents down into tokens, such as function names and operators. Tokens
can be separated by any number of whitespace characters, that is, by spaces, tabs, or
new line characters. The order of the source code is important but it is not important
to adhere to a specific layout, such as organizing your code in rows and columns. For
example
void message
( ){ cout <<
"In function message()." <<
endl;}
might be difficult to read, but it is a correct definition of the function message().
Preprocessor directives are one exception to the layout rule since they always occupy a
single line. The number sign, #, at the beginning of a line can be preceded only by a
space or a tab character.
To improve the legibility of your C++ programs you should adopt a consistent style,
using indentation and blank lines to reflect the structure of your program. In addition,
make generous use of comments.
exercises
12 ■
CHAPTER 1 FUNDAMENTALS
#include <iostream>
using namespace std;
void pause(); // Prototype
int main()
{
cout << endl << "Dear reader, "
<< endl << "have a ";
pause();
cout << "!" << endl;
return 0;
}
void pause()
{
cout << "BREAK";
}
■EXERCISES
Program listing of exercise 3
EXERCISES
■13
Exercise 1
Write a C++ program that outputs the following text on screen:
Oh what
a happy day!
Oh yes,
what a happy day!
Use the manipulator endl where appropriate.
Exercise 2
The following program contains several errors:
*/ Now you should not forget your glasses //
#include <stream>
int main
{
cout << "If this text",
cout >> " appears on your display, ";
cout << " endl;"
cout << 'you can pat yourself on '
<< " the back!" << endl.
return 0;
)
Resolve the errors and run the program to test your changes.
Exercise 3
What does the C++ program on the opposite page output on screen?
solutions
14 ■
CHAPTER 1 FUNDAMENTALS
■SOLUTIONS
Exercise 1
// Let's go !
#include <iostream>
using namespace std;
int main()
{
cout << " Oh what " << endl;
cout << " a happy day! " << endl;
cout << " Oh yes, " << endl;
cout << " what a happy day! " << endl;
return 0;
}
Exercise 2
The corrected places are underlined.
/* Now you should not forget your glasses */
#include <iostream>
using namespace std;
int main()
{
cout << " If this text ";
cout << " appears on your display, ";
cout << endl;
cout << " you can pat yourself on "
<< " the back!" << endl;
return 0;
}
Exercise 3
The screen output begins on a new line:
Dear reader,
have a BREAK!
15
Fundamental Types,
Constants, and Variables
This chapter introduces you to the basic types and objects used by C++
programs.
chapter
2
16 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
* without type void, which will be introduced later.
■FUNDAMENTAL TYPES
Overview*
bool
char
wchar_t
short
int
long
float
double
long double
For integers
For floating-point
values
For boolean values
For characters
FUNDAMENTAL TYPES
■17
A program can use several data to solve a given problem, for example, characters, inte-
gers, or floating-point numbers. Since a computer uses different methods for processing
and saving data, the data type must be known. The type defines
1. the internal representation of the data, and
2. the amount of memory to allocate.
A number such as -1000 can be stored in either 2 or 4 bytes. When accessing the
part of memory in which the number is stored, it is important to read the correct number
of bytes. Moreover, the memory content, that is the bit sequence being read, must be
interpreted correctly as a signed integer.
The C++ compiler recognizes the fundamental types, also referred to as built-in types,
shown on the opposite page, on which all other types (vectors, pointers, classes, ...) are
based.
䊐The Type bool
The result of a comparison or a logical association using AND or OR is a boolean value,
which can be true or false. C++ uses the bool type to represent boolean values. An
expression of the type bool can either be true or false, where the internal value for
true will be represented as the numerical value 1 and false by a zero.
䊐The char and wchar_t Types
These types are used for saving character codes. A character code is an integer associated
with each character. The letter Ais represented by code 65, for example. The character
set defines which code represents a certain character. When displaying characters on
screen, the applicable character codes are transmitted and the “receiver,” that is the
screen, is responsible for correctly interpreting the codes.
The C++ language does not stipulate any particular characters set, although in gen-
eral a character set that contains the ASCII code (American Standard Code for Informa-
tion Interchange) is used. This 7-bit code contains definitions for 32 control characters
(codes 0 – 31) and 96 printable characters (codes 32 – 127).
The char (character) type is used to store character codes in one byte (8 bits). This
amount of storage is sufficient for extended character sets, for example, the ANSI char-
acter set that contains the ASCII codes and additional characters such as German
umlauts.
The wchar_t (wide character type) type comprises at least 2 bytes (16 bits) and is
thus capable of storing modern Unicode characters. Unicode is a 16-bit code also used in
Windows NT and containing codes for approximately 35,000 characters in 24 languages.
18 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
#include <iostream>
#include <climits> // Definition of INT_MIN, ...
using namespace std;
int main()
{
cout << "Range of types int and unsigned int"
<< endl << endl;
cout << "Type Minimum Maximum"
<< endl
<< "--------------------------------------------"
<< endl;
cout << "int " << INT_MIN << " "
<< INT_MAX << endl;
cout << "unsigned int " << " 0 "
<< UINT_MAX << endl;
return 0;
}
■FUNDAMENTAL TYPES (CONTINUED)
Integral types
Sample program
Type Size Range of Values (decimal)
char
unsigned char
signed char
short
unsigned short
long
unsigned long
int
unsigned int
1 byte
1 byte
1 byte
2 byte resp.
4 byte
2 byte resp.
4 byte
2 byte
2 byte
4 byte
4 byte
—128 to +127 or 0 to 255
0 to 255
—128 to +127
—32768 to +32767 resp.
—2147483648 to +2147483647
0 to 65535 resp.
0 to 4294967295
—2147483648 to +2147483647
0 to 4294967295
—32768 to +32767
0 to 65535
FUNDAMENTAL TYPES (CONTINUED)
■19
䊐Integral Types
The types short,int, and long are available for operations with integers. These types
are distinguished by their ranges of values. The table on the opposite page shows the
integer types, which are also referred to as integral types, with their typical storage
requirements and ranges of values.
The int (integer) type is tailor-made for computers and adapts to the length of a reg-
ister on the computer. For 16-bit computers, int is thus equivalent to short, whereas
for 32-bit computers int will be equivalent to long.
C++ treats character codes just like normal integers. This means you can perform cal-
culations with variables belonging to the char or wchar_t types in exactly the same
way as with int type variables. char is an integral type with a size of one byte. The
range of values is thus –128 to +127 or from 0 to 255, depending on whether the com-
piler interprets the char type as signed or unsigned. This can vary in C++.
The wchar_t type is a further integral type and is normally defined as unsigned
short.
䊐The signed and unsigned Modifiers
The short,int, and long types are normally interpreted as signed with the highest bit
representing the sign. However, integral types can be preceded by the keyword
unsigned. The amount of memory required remains unaltered but the range of values
changes due to the highest bit no longer being required as a sign. The keyword
unsigned can be used as an abbreviation for unsigned int.
The char type is also normally interpreted as signed. Since this is merely a conven-
tion and not mandatory, the signed keyword is available. Thus three types are avail-
able: char, signed char, and unsigned char.
The current value ranges are available in the climits header file. This file defines
constants such as CHAR_MIN,CHAR_MAX,INT_MIN, and INT_MAX, which represent
the smallest and greatest possible values. The program on the opposite page outputs the
value of these constants for the int and unsigned int types.
In ANSI C++ the size of integer types is not preset. However, the following order applies:
char <= short <= int <= long
Moreover, the short type comprises at least 2 bytes and the long type at least 4 bytes.
✓NOTE
20 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
IEEE format (IEEE = Institute of Electrical and Electronic Engineers) is normally used to represent
floating-point types. The table above makes use of this representation.
✓NOTE
■FUNDAMENTAL TYPES (CONTINUED)
Floating-point types
Arithmetic types
Arithmetic operators are defined for arithmetic types, i.e. you can perform calculations with variables of
this type.
✓NOTE
Type Size Range of
Values
Lowest Positive
Value
Accuracy
(decimal)
float
double
long double
4 bytes
8 bytes
10 bytes
–3.4E+38
–1.7E+308
–1.1E+4932
1.2E—38
2.3E—308
3.4E—4932
6 digits
15 digits
19 digits
bool
char, signed char, unsigned char, wchar_t
short, unsigned short
int, unsigned int
long, unsigned long
float
double
long double
Floating-point types
Integral types
FUNDAMENTAL TYPES (CONTINUED)
■21
䊐Floating-Point Types
Numbers with a fraction part are indicated by a decimal point in C++ and are referred to
as floating-point numbers. In contrast to integers, floating-point numbers must be stored
to a preset accuracy. The following three types are available for calculations involving
floating-point numbers:
float for simple accuracy
double for double accuracy
long double for high accuracy
The value range and accuracy of a type are derived from the amount of memory allocated
and the internal representation of the type.
Accuracy is expressed in decimal places. This means that “six decimal places” allows a
programmer to store two floating-point numbers that differ within the first six decimal
places as separate numbers. In reverse, there is no guarantee that the figures 12.3456 and
12.34561 will be distinguished when working to a accuracy of six decimal places. And
remember, it is not a question of the position of the decimal point, but merely of the
numerical sequence.
If it is important for your program to display floating-point numbers with an accuracy
supported by a particular machine, you should refer to the values defined in the cfloat
header file.
Readers interested in additional material on this subject should refer to the Appendix,
which contains a section on the representation of binary numbers on computers for both
integers and floating-point numbers.
䊐The sizeof Operator
The amount of memory needed to store an object of a certain type can be ascertained
using the sizeof operator:
sizeof(name)
yields the size of an object in bytes, and the parameter name indicates the object type or
the object itself. For example, sizeof(int)represents a value of 2 or 4 depending on
the machine. In contrast, sizeof(float) will always equal 4.
䊐Classification
The fundamental types in C++ are integer types,floating-point types, and the void type.
The types used for integers and floating-point numbers are collectively referred to as
arithmetic types, as arithmetic operators are defined for them.
The void type is used for expressions that do not represent a value. A function call
can thus take a void type.
22 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
In each line of the above table, the same value is presented in a different way.
✓NOTE
■CONSTANTS
Examples for integral constants
Sample program
// To display hexadecimal integer literals and
// decimal integer literals.
//
#include <iostream>
using namespace std;
int main()
{
// cout outputs integers as decimal integers:
cout << "Value of 0xFF = " << 0xFF << " decimal"
<< endl; // Output: 255 decimal
// The manipulator hex changes output to hexadecimal
// format (dec changes to decimal format):
cout << "Value of 27 = " << hex << 27 <<" hexadecimal"
<< endl; // Output: 1b hexadecimal
return 0;
}
Decimal Octal TypeHexadecimal
16
255
32767
32768U
100000
10L
27UL
2147483648
020
0377
077777
0100000U
0303240
012L
033UL
020000000000
int
int
int
unsigned int
int (32 bit-)
long (16 bit-
CPU)
long
unsigned long
unsigned long
0x10
OXff
0x7FFF
0x8000U
0x186A0
0xAL
0x1bUL
0x80000000
CONSTANTS
■23
The boolean keywords true and false, a number, a character, or a character sequence
(string) are all constants, which are also referred to as a literals. Constants can thus be
subdivided into
■boolean constants
■numerical constants
■character constants
■string constants.
Every constant represents a value and thus a type—as does every expression in C++. The
type is defined by the way the constant is written.
䊐Boolean Constants
A boolean expression can have two values that are identified by the keywords true and
false. Both constants are of the bool type. They can be used, for example, to set flags
representing just two states.
䊐Integral Constants
Integral numerical constants can be represented as simple decimal numbers, octals, or
hexadecimals:
■adecimal constant (base 10) begins with a decimal number other than zero, such
as 109 or 987650
■an octal constant (base 8) begins with a leading 0, for example 077 or 01234567
■ahexadecimal constant (base 16) begins with the character pair 0x or 0X, for
example 0x2A0 or 0X4b1C. Hexadecimal numbers can be capitalized or non-
capitalized.
Integral constants are normally of type int. If the value of the constant is too large
for the int type, a type capable of representing larger values will be applied. The ranking
for decimal constants is as follows:
int, long, unsigned long
You can designate the type of a constant by adding the letter Lor l(for long), or U
or u(for unsigned). For example,
12L and 12l correspond to the type long
12U and 12u correspond to the type unsigned int
12UL and 12ul correspond to the type unsigned long
24 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
'H' 'e' '1' '1' 'o' '!' '\0'
"Hello!"
String literal:
Stored byte sequence:
■CONSTANTS (CONTINUED)
Examples for floating-point constants
Examples for character constants
Internal representation of a string literal
5.19
0.519E1
0.0519e2
519.OE-2
12.
12.0
.12E+2
12e0
0.75
.75
7.5e-1
75E-2
0.00004
0.4e-4
.4E-4
4E-5
Constant Character Constant Value
(ASCII code decimal)
Capital A
Lowercase a
Blank
Dot
Digit 0
Terminating null character
65
97
32
46
48
0
'A'
'a'
' '
'.'
'0'
'\0'
CONSTANTS (CONTINUED)
■25
䊐Floating-Point Constants
Floating-point numbers are always represented as decimals, a decimal point being used to
distinguish the fraction part from the integer part. However, exponential notation is also
permissible.
EXAMPLES: 27.1 1.8E–2 // Type: double
Here, 1.8E–2 represents a value of 1.8*10–2.Ecan also be written with a small letter
e. A decimal point or E(e) must always be used to distinguish floating-point constants
from integer constants.
Floating-point constants are of type double by default. However, you can add For f
to designate the float type, or add Lor lfor the long double type.
䊐Character Constants
A character constant is a character enclosed in single quotes. Character constants take
the type char.
EXAMPLE: 'A' // Type: char
The numerical value is the character code representing the character. The constant 'A'
thus has a value of 65 in ASCII code.
䊐String Constants
You already know string constants, which were introduced for text output using the
cout stream. A string constant consists of a sequence of characters enclosed in double
quotes.
EXAMPLE: "Today is a beautiful day!"
A string constant is stored internally without the quotes but terminated with a null char-
acter, \0, represented by a byte with a numerical value of 0— that is, all the bits in this
byte are set to 0. Thus, a string occupies one byte more in memory than the number of
characters it contains. An empty string, "", therefore occupies a single byte.
The terminating null character \0 is not the same as the number zero and has a differ-
ent character code than zero. Thus, the string
EXAMPLE: "0"
comprises two bytes, the first byte containing the code for the character zero 0(ASCII
code 48) and the second byte the value 0.
The terminating null character \0 is an example of an escape sequence. Escape
sequences are described in the following section.
26 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
#include <iostream>
using namespace std;
int main()
{
cout << "\nThis is\t a string\n\t\t"
" with \"many\" escape sequences!\n";
return 0;
}
■ESCAPE SEQUENCES
Overview
Sample program
Program output:
This is a string
with "many" escape sequences!
Single character Meaning ASCII code
(decimal)
alert (BEL)
backspace (BS)
horizontal tab (HT)
line feed (LF)
vertical tab (VT)
form feed (FF)
carriage return (CR)
" (double quote)
' (single quote)
? (question mark)
\ (backslash)
string terminating character
numerical value of a character
7
8
9
10
11
12
13
34
39
63
92
0
ooo (octal!)
\a
\b
\t
\n
\v
\f
\r
\"
\'
\?
\\
\0
\ooo
(up to 3 octal digits)
(hexadecimal digits)
hh (hexadecimal!)
numerical value of a character
\xhh
ESCAPE SEQUENCES
■27
䊐Using Control and Special Characters
Nongraphic characters can be expressed by means of escape sequences, for example \t,
which represents a tab.
The effect of an escape sequence will depend on the device concerned. The sequence
\t, for example, depends on the setting for the tab width, which defaults to eight blanks
but can be any value.
An escape sequence always begins with a \(backslash) and represents a single charac-
ter. The table on the opposite page shows the standard escape sequences, their decimal
values, and effects.
You can use octal and hexadecimal escape sequences to create any character code.
Thus, the letter A(decimal 65) in ASCII code can also be expressed as \101 (three
octals) or \x41 (two hexadecimals). Traditionally, escape sequences are used only to
represent non-printable characters and special characters. The control sequences for
screen and printer drivers are, for example, initiated by the ESC character (decimal 27),
which can be represented as \33 or \x1b.
Escape sequences are used in character and string constants.
EXAMPLES: '\t' "\tHello\n\tMike!"
The characters ', ", and \have no special significance when preceded by a backslash, i.e.
they can be represented as \',\", and \\ respectively.
When using octal numbers for escape sequences in strings, be sure to use three digits,
for example, \033 and not \33. This helps to avoid any subsequent numbers being eval-
uated as part of the escape sequence. There is no maximum number of digits in a hexa-
decimal escape sequence. The sequence of hex numbers automatically terminates with
the first character that is not a valid hex number.
The sample program on the opposite page demonstrates the use of escape sequences in
strings. The fact that a string can occupy two lines is another new feature. String
constants separated only by white spaces will be concatenated to form a single string.
To continue a string in the next line you can also use a backslash \as the last
character in a line, and then press the Enter key to begin a new line, where you can
continue typing the string.
EXAMPLE: "I am a very, very \
long string"
Please note, however, that the leading spaces in the second line will be evaluated as part
of the string. It is thus generally preferable to use the first method, that is, to terminate
the string with " and reopen it with ".
28 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
■NAMES
Keywords in C++
Examples for names
asm
auto
bool
break
case
catch
char
class
const
const_cast
continue
default
delete
do
double
dynamic_cast
else
enum
explicit
extern
false
float
for
friend
goto
if
inline
int
long
mutable
namespace
new
operator
private
protected
public
register
reinterpret_cast
return
short
signed
sizeof
static
static_cast
struct
switch
template
this
throw
true
try
typedef
typeid
typename
union
unsigned
using
virtual
void
volatile
wchar_t
while
valid:
a US us VOID
_var SetTextColor
B12 top_of_window
a_very_long_name123467890
invalid:
goto 586_cpu object-oriented
US$ true écu
NAMES
■29
䊐Valid Names
Within a program names are used to designate variables and functions. The following
rules apply when creating names, which are also known as identifiers:
■a name contains a series of letters, numbers, or underscore characters ( _ ). Ger-
man umlauts and accented letters are invalid. C++ is case sensitive; that is,
upper- and lowercase letters are different.
■the first character must be a letter or underscore
■there are no restrictions on the length of a name and all the characters in the
name are significant
■C++ keywords are reserved and cannot be used as names.
The opposite page shows C++ keywords and some examples of valid and invalid names.
The C++ compiler uses internal names that begin with one or two underscores fol-
lowed by a capital letter. To avoid confusion with these names, avoid use of the under-
score at the beginning of a name.
Under normal circumstances the linker only evaluates a set number of characters, for
example, the first 8 characters of a name. For this reason names of global objects, such as
functions, should be chosen so that the first eight characters are significant.
䊐Conventions
In C++ it is standard practice to use small letters for the names of variables and func-
tions. The names of some variables tend to be associated with a specific use.
EXAMPLES:
c, ch for characters
i, j, k, l, m, n for integers, in particular indices
x, y, z for floating-point numbers
To improve the readability of your programs you should choose longer and more self-
explanatory names, such as start_index or startIndex for the first index in a range
of index values.
In the case of software projects, naming conventions will normally apply. For exam-
ple, prefixes that indicate the type of the variable may be assigned when naming vari-
ables.
30 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
Both strings and all other values of fundamental types can be output with cout. Integers are printed in
decimal format by default.
✓HINT
■VARIABLES
Sample program
Screen output
Value of gVar1: 0
Value of gVar2: 2
Character in ch: A
Value of sum: 8
// Definition and use of variables
#include <iostream>
using namespace std;
int gVar1; // Global variables,
int gVar2 = 2; // explicit initialization
int main()
{
char ch('A'); // Local variable being initialized
// or: char ch = 'A';
cout << "Value of gVar1: " << gVar1 << endl;
cout << "Value of gVar2: " << gVar2 << endl;
cout << "Character in ch: " << ch << endl;
int sum, number = 3; // Local variables with
// and without initialization
sum = number + 5;
cout << "Value of sum: " << sum << endl;
return 0;
}
VARIABLES
■31
Data such as numbers, characters, or even complete records are stored in variables to
enable their processing by a program. Variables are also referred to as objects, particularly
if they belong to a class.
䊐Defining Variables
A variable must be defined before you can use it in a program. When you define a vari-
able the type is specified and an appropriate amount of memory reserved. This memory
space is addressed by reference to the name of the variable. A simple definition has the
following syntax:
SYNTAX: typ name1 [name2 ... ];
This defines the names of the variables in the list name1 [, name2 ...] as variables
of the type type. The parentheses [ ... ] in the syntax description indicate that this
part is optional and can be omitted. Thus, one or more variables can be stated within a
single definition.
EXAMPLES: char c;
int i, counter;
double x, y, size;
In a program, variables can be defined either within the program’s functions or out-
side of them. This has the following effect:
■a variable defined outside of each function is global, i.e. it can be used by all func-
tions
■a variable defined within a function is local, i.e. it can be used only in that func-
tion.
Local variables are normally defined immediately after the first brace—for example at
the beginning of a function. However, they can be defined wherever a statement is per-
mitted. This means that variables can be defined immediately before they are used by the
program.
䊐Initialization
A variable can be initialized, i.e. a value can be assigned to the variable, during its defini-
tion. Initialization is achieved by placing the following immediately after the name of
the variable:
■an equals sign ( =) and an initial value for the variable or
■round brackets containing the value of the variable.
EXAMPLES: char c = 'a';
float x(1.875);
Any global variables not explicitly initialized default to zero. In contrast, the initial
value for any local variables that you fail to initialize will have an undefined initial value.
32 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
// Circumference and area of a circle with radius 2.5
#include <iostream>
using namespace std;
const double pi = 3.141593;
int main()
{
double area, circuit, radius = 1.5;
area = pi * radius * radius;
circuit = 2 * pi * radius;
cout << "\nTo Evaluate a Circle\n" << endl;
cout << "Radius: " << radius << endl
<< "Circumference: " << circuit << endl
<< "Area: " << area << endl;
return 0;
}
By default cout outputs a floating-point number with a maximum of 6 decimal places without trailing
zeros.
✓NOTE
■THE KEYWORDS const AND volatile
Sample program
Screen output
To Evaluate a Circle
Radius: 1.5
Circumference: 9.42478
Area: 7.06858
THE KEYWORDS CONST AND VOLATILE
■33
A type can be modified using the const and volatile keywords.
䊐Constant Objects
The const keyword is used to create a “read only” object. As an object of this type is
constant, it cannot be modified at a later stage and must be initialized during its defini-
tion.
EXAMPLE: const double pi = 3.1415947;
Thus the value of pi cannot be modified by the program. Even a statement such as the
following will merely result in an error message:
pi = pi + 2.0; // invalid
䊐Volatile Objects
The keyword volatile, which is rarely used, creates variables that can be modified not
only by the program but also by other programs and external events. Events can be initi-
ated by interrupts or by a hardware clock, for example.
EXAMPLE: volatile unsigned long clock_ticks;
Even if the program itself does not modify the variable, the compiler must assume that
the value of the variable has changed since it was last accessed. The compiler therefore
creates machine code to read the value of the variable whenever it is accessed instead of
repeatedly using a value that has been read at a prior stage.
It is also possible to combine the keywords const and volatile when declaring a
variable.
EXAMPLE: volatile const unsigned time_to_live;
Based on this declaration, the variable time_to_live cannot be modified by the pro-
gram but by external events.
exercises
34 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
I
"RUSH"
\TO\
AND
/FRO/
■EXERCISES
Screen output for exercise 2
For exercise 3
Defining and initializing variables:
int a(2.5); const long large;
int b = '?'; char c('\'');
char z(500); unsigned char ch = '\201';
int big = 40000; unsigned size(40000);
double he's(1.2E+5); float val = 12345.12345;
EXERCISES
■35
Exercise 1
The sizeof operator can be used to determine the number of bytes occupied
in memory by a variable of a certain type. For example, sizeof(short) is
equivalent to 2.
Write a C++ program that displays the memory space required by each
fundamental type on screen.
Exercise 2
Write a C++ program to generate the screen output shown on the opposite
page.
Exercise 3
Which of the variable definitions shown on the opposite page is invalid or does
not make sense?
Exercise 4
Write a C++ program that two defines variables for floating-point numbers and
initializes them with the values
123.456 and 76.543
Then display the sum and the difference of these two numbers on screen.
solutions
36 ■
CHAPTER 2 FUNDAMENTAL TYPES, CONSTANTS, AND VARIABLES
■SOLUTIONS
Exercise 1
#include <iostream>
using namespace std;
int main()
{
cout << "\nSize of Fundamental Types\n"
<< " Type Number of Bytes\n"
<< "----------------------------------" << endl;
cout << " char: " << sizeof(char) << endl;
cout << " short: " << sizeof(short)<< endl;
cout << " int: " << sizeof(int) << endl;
cout << " long: " << sizeof(long) << endl;
cout << " float: " << sizeof(float)<< endl;
cout << " double: " << sizeof(double)<<endl;
cout << " long double: " << sizeof(long double)
<< endl;
return 0;
}
Exercise 2
// Usage of escape sequences
#include <iostream>
using namespace std;
int main()
{
cout << "\n\n\t I" // Instead of tabs
"\n\n\t\t \"RUSH\"" // you can send the
"\n\n\t\t\t \\TO\\" // suited number
"\n\n\t\t AND" // of blanks to
"\n\n\t /FRO/" << endl; // the output.
return 0;
}
SOLUTIONS
■37
Exercise 3
Incorrect:
int a(2.5); // 2.5 is not an integer value
const long large; // Without initialization
char z(500); // The value 500 is too large
// to fit in a byte
int big = 40000; // Attention! On 16-bit systems
// int values are <= 32767
double he's(1.2E+5); // The character ' is not
// allowed in names
float val = 12345.12345; // The accuracy of float
// is only 6 digits
Exercise 4
// Defining and initializing variables
#include <iostream>
using namespace std;
int main()
{
float x = 123.456F, // or double
y = 76.543F,
sum;
sum = x + y;
cout << "Total: "
<< x << " + " << y << " = " << sum << endl;
cout << "Difference: "
<< x << " — " << y << " = " << (x — y) << endl;
return 0;
}
This page intentionally left blank
39
Using Functions and
Classes
This chapter describes how to
■declare and call standard functions and
■use standard classes.
This includes using standard header files. In addition, we will be working
with string variables, i.e. objects belonging to the standard class string
for the first time.
Functions and classes that you define on your own will not be
introduced until later in the book.
chapter
3
40 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
■DECLARING FUNCTIONS
Example of a function prototype
The prototype above yields the following information to the compiler:
■func is the function name
■the function is called with two arguments: the first argument is of type int, the
second of type double
■the return value of the function is of type long.
Mathematical standard functions
Function name
Function type
= type of return value
Types of arguments
long func (int, double);
double sin (double);
double cos (double);
double tan (double);
double atan (double);
double cosh (double);
double sqrt (double);
double pow (double, double);
double exp (double);
double log (double);
double log10 (double);
// Sine
// Cosine
// Tangent
// Arc tangent
// Hyperbolic Cosine
// Square Root
// Power
// Exponential Function
// Natural Logarithm
// Base-ten Logarithm
DECLARING FUNCTIONS
■41
䊐Declarations
Each name (identifier) occurring in a program must be known to the compiler or it will
cause an error message. That means any names apart from keywords must be declared, i.e.
introduced to the compiler, before they are used.
Each time a variable or a function is defined it is also declared. But conversely, not
every declaration needs to be a definition. If you need to use a function that has already
been introduced in a library, you must declare the function but you do not need to rede-
fine it.
䊐Declaring Functions
A function has a name and a type, much like a variable. The function’s type is defined by
its return value, that is, the value the function passes back to the program. In addition,
the type of arguments required by a function is important. When a function is declared,
the compiler must therefore be provided with information on
■the name and type of the function and
■the type of each argument.
This is also referred to as the function prototype.
Examples: int toupper(int);
double pow(double, double);
This informs the compiler that the function toupper() is of type int, i.e. its return
value is of type int, and it expects an argument of type int. The second function
pow() is of type double and two arguments of type double must be passed to the
function when it is called. The types of the arguments may be followed by names, how-
ever, the names are viewed as a comment only.
Examples: int toupper(int c);
double pow(double base, double exponent);
From the compiler’s point of view, these prototypes are equivalent to the prototypes
in the previous example. Both junctions are standard junctions.
Standard function prototypes do not need to be declared, nor should they be, as they
have already been declared in standard header files. If the header file is included in the
program’s source code by means of the #include directive, the function can be used
immediately.
Example: #include <cmath>
Following this directive, the mathematical standard functions, such as sin(),cos(),
and pow(), are available. Additional details on header files can be found later in this
chapter.
42 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
// Calculating powers with
// the standard function pow()
#include <iostream> // Declaration of cout
#include <cmath> // Prototype of pow(), thus:
// double pow( double, double);
using namespace std;
int main()
{
double x = 2.5, y;
// By means of a prototype, the compiler generates
// the correct call or an error message!
// Computes x raised to the power 3:
y = pow("x", 3.0); // Error! String is not a number
y = pow(x + 3.0); // Error! Just one argument
y = pow(x, 3.0); // ok!
y = pow(x, 3); // ok! The compiler converts the
// int value 3 to double.
cout << "2.5 raised to 3 yields: "
<< y << endl;
// Calculating with pow() is possible:
cout << "2 + (5 raised to the power 2.5) yields: "
<< 2.0 + pow(5.0, x) << endl;
return 0;
}
■FUNCTION CALLS
Sample program
Screen output
2.5 raised to the power 3 yields: 15.625
2 + (5 raised to the power 2.5) yields: 57.9017
䊐Function Calls
Afunction call is an expression of the same type as the function and whose value corre-
sponds to the return value. The return value is commonly passed to a suitable variable.
Example: y = pow( x, 3.0);
In this example the function pow()is first called using the arguments xand 3.0, and
the result, the power x3, is assigned to y.
As the function call represents a value, other operations are also possible. Thus, the
function pow() can be used to perform calculations for double values.
Example: cout << 2.0 + pow( 5.0, x);
This expression first adds the number 2.0 to the return value of pow(5.0,x), then
outputs the result using cout.
Any expression can be passed to a function as an argument, such as a constant or an
arithmetical expression. However, it is important that the types of the arguments corre-
spond to those expected by the function.
The compiler refers to the prototype to check that the function has been called cor-
rectly. If the argument type does not match exactly to the type defined in the prototype,
the compiler performs type conversion, if possible.
Example: y = pow( x, 3); // also ok!
The value 3 of type int is passed to the function as a second argument. But since the
function expects a double value, the compiler will perform type conversion from int
to double.
If a function is called with the wrong number of arguments, or if type conversion
proves impossible, the compiler generates an error message. This allows you to recognize
and correct errors caused by calling functions at the development stage instead of causing
runtime errors.
Example: float x = pow(3.0 + 4.7); // Error!
The compiler recognizes that the number of arguments is incorrect. In addition, the
compiler will issue a warning, since a double, i.e. the return value of pow(), is assigned
to a float type variable.
FUNCTION CALLS
■43
44 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
■TYPE void FOR FUNCTIONS
Sample program
// Outputs three random numbers
#include <iostream> // Declaration of cin and cout
#include <cstdlib> // Prototypes of srand(), rand():
// void srand( unsigned int seed );
// int rand( void );
using namespace std;
int main()
{
unsigned int seed;
int z1, z2, z3;
cout << " --- Random Numbers --- \n" << endl;
cout << "To initialize the random number generator, "
<< "\n please enter an integer value: ";
cin >> seed; // Input an integer
srand( seed); // and use it as argument for a
// new sequence of random numbers.
z1 = rand(); // Compute three random numbers.
z2 = rand();
z3 = rand();
cout << "\nThree random numbers: "
<< z1 << " " << z2 << " " << z3 << endl;
return 0;
}
The statement cin >> seed; reads an integer from the keyboard, because seed is of the
unsigned int type.
✓NOTE
Sample screen output
--- Random Numbers ---
To initialize the random number generator,
please enter an integer value: 7777
Three random numbers: 25435 6908 14579
TYPE VOID FOR FUNCTIONS
■45
䊐Functions without Return Value
You can also write functions that perform a certain action but do not return a value to
the function that called them. The type void is available for functions of this type,
which are also referred to as procedures in other programming languages.
Example: void srand( unsigned int seed );
The standard function srand() initializes an algorithm that generates random num-
bers. Since the function does not return a value, it is of type void. An unsigned value
is passed to the function as an argument to seed the random number generator. The
value is used to create a series of random numbers.
䊐Functions without Arguments
If a function does not expect an argument, the function prototype must be declared as
void or the braces following the function name must be left empty.
Example: int rand( void ); // or int rand();
The standard function rand() is called without any arguments and returns a random
number between 0 and 32767. A series of random numbers can be generated by repeating
the function call.
䊐Usage of srand() and rand()
The function prototypes for srand() and rand() can be found in both the cstdlib
and stdlib.h header files.
Calling the function rand() without previously having called srand() creates the
same sequence of numbers as if the following statement would have been proceeded:
srand(1);
If you want to avoid generating the same sequence of random numbers whenever the
program is executed, you must call srand() with a different value for the argument
whenever the program is run.
It is common to use the current time to initialize a random number generator. See
Chapter 6 for an example of this technique.
46 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
■HEADER FILES
Using header files
iostream
Header file
Copy
Copy
myheader.h
Header file
application.cpp
Source file
// Declaration
// of cin, cout,
// . . .
#include <iostream>
#include "myheader.h"
int main()
{
int a;
. . .
cin >> a;
cout << myfunc (a);
. . .
return 0;
}
// Declaration
// of self-defined
// functions
// and classes
long myfunc(int);
HEADER FILES
■47
䊐Using Header Files
Header files are text files containing declarations and macros. By using an #include
directive these declarations and macros can be made available to any other source file,
even in other header files.
Pay attention to the following points when using header files:
■header files should generally be included at the start of a program before any
other declarations
■you can only name one header file per #include directive
■the file name must be enclosed in angled brackets < ... > or double quotes
" ... ".
䊐Searching for Header Files
The header files that accompany your compiler will tend to be stored in a folder of their
own—normally called include. If the name of the header file is enclosed by angled
brackets < ... >, it is common to search for header files in the include folder only.
The current directory is not searched to increase the speed when searching for header
files.
C++ programmers commonly write their own header files and store them in the cur-
rent project folder. To enable the compiler to find these header files, the #include
directive must state the name of the header files in double quotes.
Example: #include "project.h"
The compiler will then also search the current folder. The file suffix .h is normally used
for user-defined header files.
䊐Standard Class Definitions
In addition to standard function prototypes, the header files also contain standard class
definitions. When a header file is included, the classes defined and any objects declared
in the file are available to the program.
Example: #include <iostream>
using namespace std;
Following these directives, the classes istream and ostream can be used with the cin
and cout streams. cin is an object of the istream class and cout an object of the
ostream class.
48 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
Some IDE’s put the old-fashioned iostream.h and iomanip.h header files at your disposal. Within
these header files the identifiers of iostream and iomanip are not contained in the std namespace
but are declared globally.
✓NOTE
■STANDARD HEADER FILES
Header files of the C++ standard library
Header files of the C standard library
algorithm ios map stack
bitset iosfwd memory stdexcept
complex iostream new streambuf
dequeue istream numeric string
exception iterator ostream typeinfo
fstream limits queue utility
functional list set valarray
iomanip locale sstream vector
assert.h limits.h stdarg.h time.h
ctype.h locale.h stddef.h wchar.h
errno.h math.h stdio.h wctype.h
float.h setjmp.h stdlib.h
iso646.h signal.h string.h
STANDARD HEADER FILES
■49
The C++ standard library header files are shown opposite. They are not indicated by the
file extension .h and contain all the declarations in their own namespace, std. Name-
spaces will be introduced in a later chapter. For now, it is sufficient to know that identi-
fiers from other namespaces cannot be referred to directly. If you merely stipulate the
directive
Example: #include <iostream>
the compiler would not be aware of the cin and cout streams. In order to use the iden-
tifiers of the std namespace globally, you must add a using directive.
Example: #include <iostream>
#include <string>
using namespace std;
You can then use cin and cout without any additional syntax. The header file
string has also been included. This makes the string class available and allows user-
friendly string manipulations in C++. The following pages contain further details on this
topic.
䊐Header Files in the C Programming Language
The header files standardized for the C programming language were adopted for the C++
standard and, thus, the complete functionality of the standard C libraries is available to
C++ programs.
Example: #include <math.h>
Mathematical functions are made available by this statement.
The identifiers declared in C header files are globally visible. This can cause name
conflicts in large programs. For this reason each C header file, for example name.h, is
accompanied in C++ by a second header file, cname, which declares the same identifiers
in the std namespace. Including the file math.h is thus equivalent to
Example: #include <cmath>
using namespace std;
The string.h or cstring files must be included in programs that use standard func-
tions to manipulate C strings. These header files grant access to the functionality of the
C string library and are to be distinguished from the string header file that defines the
string class.
Each compiler offers additional header files for platform dependent functionalities.
These may be graphics libraries or database interfaces.
50 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
// To use strings.
#include <iostream> // Declaration of cin, cout
#include <string> // Declaration of class string
using namespace std;
int main()
{
// Defines four strings:
string prompt("What is your name: "),
name, // An empty
line( 40, '-'), // string with 40 '-'
total = "Hello "; // is possible!
cout << prompt; // Request for input.
getline( cin, name); // Inputs a name in one line
total = total + name; // Concatenates and
// assigns strings.
cout << line << endl // Outputs line and name
<< total << endl;
cout << " Your name is " // Outputs length
<< name.length() << " characters long!" << endl;
cout << line << endl;
return 0;
}
Both the operators +and += for concatenation and the relational operators <,<=,>, >=, ==, and
!= are defined for objects of class string. Strings can be printed with cout and the operator <<.
The class string will be introduced in detail later on.
✓NOTE
■USING STANDARD CLASSES
Sample program using class string
Sample screen output
What is your name: Rose Summer
---------------------------------------
Hello Rose Summer
Your name is 11 characters long!
---------------------------------------
USING STANDARD CLASSES
■51
Several classes are defined in the C++ standard library. These include stream classes for
input and output, but also classes for representing strings or handling error conditions.
Each class is a type with certain properties and capacities. As previously mentioned,
the properties of a class are defined by its data members and the class’s capacities are
defined by its methods. Methods are functions that belong to a class and cooperate with
the members to perform certain operations. Methods are also referred to as member func-
tions.
䊐Creating Objects
An object is a variable of a class type, also referred to as an instance of the class. When an
object is created, memory is allocated to the data members and initialized with suitable
values.
Example: string s("I am a string");
In this example the object s, an instance of the standard class string (or simply a
string), is defined and initialized with the string constant that follows. Objects of the
string class manage the memory space required for the string themselves.
In general, there are several ways of initializing an object of a class. A string can thus
be initialized with a certain number of identical characters, as the example on the oppo-
site page illustrates.
䊐Calling Methods
All the methods defined as public within the corresponding class can be called for an
object. In contrast to calling a global function, a method is always called for one particular
object. The name of the object precedes the method and is separated from the method by
a period.
Example: s.length(); // object.method();
The method length() supplies the length of a string, i.e. the number of characters in a
string. This results in a value of 13 for the string sdefined above.
䊐Classes and Global Functions
Globally defined functions exist for some standard classes. These functions perform certain
operations for objects passed as arguments. The global function getline(), for exam-
ple, stores a line of keyboard input in a string.
Example: getline(cin, s);
The keyboard input is terminated by pressing the return key to create a new-line charac-
ter, '\n', which is not stored in the string.
exercises
52 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
Number Square Root
42
12.25 3.5
0.0121 0.11
■EXERCISES
Screen output for exercise 1
Listing for exercise 2
// A program containing errors!
# include <iostream>, <string>
# include <stdlib>
# void srand( seed);
int main()
{
string message "\nLearn from your mistakes!";
cout << message << endl;
int len = length( message);
cout << "Length of the string: " << len << endl;
// And a random number in addition:
int a, b;
a = srand(12.5);
b = rand( a );
cout << "\nRandom number: " << b << endl;
return 0;
}
EXERCISES
■53
Exercise 1
Create a program to calculate the square roots of the numbers
4 12.25 0.0121
and output them as shown opposite.Then read a number from the keyboard and
output the square root of this number.
To calculate the square root, use the function sqrt(), which is defined by the
following prototype in the math.h (or cmath ) header file:
double sqrt( double x);
The return value of the sqrt() function is the square root of x.
Exercise 2
The program on the opposite page contains several errors! Correct the errors
and ensure that the program can be executed.
Exercise 3
Create a C++ program that defines a string containing the following character
sequence:
I have learned something new again!
and displays the length of the string on screen.
Read two lines of text from the keyboard. Concatenate the strings using " * "
to separate the two parts of the string. Output the new string on screen.
solutions
54 ■
CHAPTER 3 USING FUNCTIONS AND CLASSES
■SOLUTIONS
Exercise 1
// Compute square roots
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
double x1 = 4.0, x2 = 12.25, x3 = 0.0121;
cout << "\n Number \t Square Root" << endl;
cout << "\n " << x1 << " \t " << sqrt(x1)
<< "\n " << x2 << " \t " << sqrt(x2)
<< "\n " << x3 << " \t " << sqrt(x3) << endl;
cout << "\nType a number whose square root is to be"
" computed. ";
cin >> x1;
cout << "\n Number \t Square Root" << endl;
cout << "\n " << x1 << " \t " << sqrt(x1) << endl;
return 0;
}
Exercise 2
// The corrected program:
#include <iostream> // Just one header file in a line
#include <string>
#include <cstdlib> // Prototypes of functions
// void srand( unsigned int seed);
// int rand(void);
// or:
// #include <stdlib.h>
using namespace std; // Introduces all names of namespace
// std into the global scope.
int main()
{
string message = "\nLearn from your mistakes!";...// =
cout << message << endl;
SOLUTIONS
■55
int len = message.length();
// instead of: length(message);
cout << "Length of the string: " << len << endl;
// And another random number:
int b; // Variable a is not needed.
srand(12); // instead of: a = srand(12.5);
b = rand(); // instead of: b = rand(a);
cout << "\nRandom number: " << b << endl;
return 0;
}
Exercise 3
#include <iostream> // Declaration of cin, cout
#include <string> // Declaration of class string
using namespace std;
int main()
{
string message("I have learned something new again!\n"),
prompt("Please input two lines of text:"),
str1, str2, sum;
cout << message << endl; // Outputs the message
cout << prompt << endl; // Request for input
getline( cin, str1); // Reads the first
getline( cin, str2); // and the second line of text
sum = str1 + " * " + str2; // Concatenates, assigns
cout << sum << endl; // and outputs strings.
return 0;
}
This page intentionally left blank
57
Input and Output with
Streams
This chapter describes the use of streams for input and output, focusing
on formatting techniques.
chapter
4
58 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
ios
istream ostream
iostream
■STREAMS
Stream classes for input and output
The four standard streams
■cin Object of class istream to control standard input
■cout Object of class ostream to control standard output
■cerr Object of class ostream to control unbuffered error output
■clog Object of class ostream to control buffered error output
STREAMS
■59
䊐I/O Stream Classes
During the development of C++ a new class-based input/output system was imple-
mented. This gave rise to the I/O stream classes, which are now available in a library of
their own, the so-called iostream library.
The diagram on the opposite page shows how a so-called class hierarchy develops due
to inheritance. The class ios is the base class of all other stream classes. It contains the
attributes and abilities common to all streams. Effectively, the ios class
■manages the connection to the physical data stream that writes your program’s
data to a file or outputs the data on screen
■contains the basic functions needed for formatting data. A number of flags that
determine how character input is interpreted have been defined for this purpose.
The istream and ostream classes derived from ios form a user-friendly interface
for stream manipulation. The istream class is used for reading streams and the
ostream class is used for writing to streams. The operator >> is defined in istream
and << is defined in ostream, for example.
The iostream class is derived by multiple inheritance from istream and ostream
and thus offers the functionality of both classes.
Further stream classes, a file management class, for example, are derived from the
classes mentioned above. This allows the developer to use the techniques described for
file manipulation. These classes, which also contain methods for opening and closing
files, will be discussed in a later chapter.
䊐Standard Streams
The streams cin and cout, which were mentioned earlier, are instances of the
istream or ostream classes. When a program is launched these objects are automati-
cally created to read standard input or write to standard output.
Standard input is normally the keyboard and standard output the screen. However,
standard input and output can be redirected to files. In this case, data is not read from
the keyboard but from a file, or data is not displayed on screen but written to a file.
The other two standard streams cerr and clog are used to display messages when
errors occur. Error messages are displayed on screen even if standard output has been
redirected to a file.
60 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
Here the manipulator showpos is called.
↓
cout << showpos << 123; // Output: +123
The above statement is equivalent to
cout.setf( ios::showpos);
cout << 123;
The other positive numbers are printed with their sign as well:
cout << 22; // Output: +22
The output of a positive sign can be canceled by the manipulator
noshowpos:
cout << noshowpos << 123; // Output: 123
The last statement is equivalent to
cout.unsetf( ios::showpos);
cout << 123;
■The operators >> and << format the input and/or output according to how the flags in the base class
ios are set
■The manipulator showpos is a function that calls the method cout.setf(ios::showpos);,
ios::showpos being the flag showpos belonging to the ios class
■Using manipulators is easier than directly accessing flags. For this reason, manipulators are described in
the following section, whereas the methods setf() and unsetf() are used only under exceptional
circumstances.
■Old compilers only supply some of the manipulators. In this case, you have to use the methods setf()
and unsetf().
✓HINTS
■FORMATTING AND MANIPULATORS
Example: Calling a manipulator
FORMATTING AND MANIPULATORS
■61
䊐Formatting
When reading keyboard input, a valid input format must be used to determine how input
is to be interpreted. Similarly, screen output adheres to set of rules governing how, for
example, floating-point numbers are displayed.
The stream classes istream and ostream offer various options for performing these
tasks. For example, you can display a table of numeric values in a simple way.
In previous chapters we have looked at the cin and cout streams in statements such
as:
cout << "Please enter a number: ";
cin >> x;
The following sections systematically describe the abilities of the stream classes. This
includes:
■the >> and << operators for formatted input and output. These operators are
defined for expressions with fundamental types—that is, for characters, boolean
values, numbers and strings.
■manipulators, which can be inserted into the input or output stream. Manipula-
tors can be used to generate formats for subsequent input/output. One manipula-
tor that you are already familiar with is endl, which generates a line feed at the
end of a line.
■other methods for determining or modifying the state of a stream and unformat-
ted input and output.
䊐Flags and Manipulators
Formatting flags defined in the parent class ios determine how characters are input or
output. In general, flags are represented by individual bits within a special integral vari-
able. For example, depending on whether a bit is set or not, a positive number can be
output with or without a plus sign.
Each flag has a default setting. For example, integral numbers are output as decimals by
default, and positive numbers are output without a plus sign.
It is possible to modify individual formatting flags. The methods setf() and
unsetf() can be used for this purpose. However, the same effect can be achieved sim-
ply by using so-called manipulators, which are defined for all important flags. Manipula-
tors are functions that can be inserted into the input or output stream and thus be called.
62 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// Reads integral decimal values and
// generates octal, decimal, and hexadecimal output.
#include <iostream> // Declarations of cin, cout and
using namespace std; // manipulators oct, hex, ...
int main()
{
int number;
cout << "Please enter an integer: ";
cin >> number;
cout << uppercase // for hex-digits
<< " octal \t decimal \t hexadecimal\n "
<< oct << number << " \t "
<< dec << number << " \t "
<< hex << number << endl;
return 0;
}
■FORMATTED OUTPUT OF INTEGERS
Manipulators formatting integers
Sample program
Manipulator Effects
Octal base
Hexadecimal base
Decimal base (by default)
Generates a + sign in non-negative numeric
output.
Generates capital letters in hexadecimal
output.
Generates non-negative numeric output
without a + sign (by default).
Generates lowercase letters in hexadecimal
output (by default).
oct
hex
dec
showpos
noshowpos
uppercase
nouppercase
FORMATTED OUTPUT OF INTEGERS
■63
䊐Formatting Options
The << operator can output values of type short,int,long or a corresponding
unsigned type. The following formatting options are available:
■define the numeric system in which to display the number: decimal, octal, or
hexadecimal
■use capitals instead of small letters for hexadecimals
■display a sign for positive numbers.
In addition, the field width can be defined for the above types. The field width can
also be defined for characters, strings, and floating-point numbers, and will be discussed
in the following sections.
䊐Numeric System
Integral numbers are displayed as decimals by default. The manipulators oct,hex, and
dec can be used for switching from and to decimal display mode.
Example: cout << hex << 11; // Output: b
Hexadecimals are displayed in small letters by default, that is, using a,b, ..., f. The
manipulator uppercase allows you to use capitals.
Example: cout << hex << uppercase << 11; //Output: B
The manipulator nouppercase returns the output format to small letters.
䊐Negative Numbers
When negative numbers are output as decimals, the output will always include a sign.
You can use the showpos manipulator to output signed positive numbers.
Example: cout << dec << showpos << 11; //Output: +11
You can use noshowpos to revert to the original display mode.
When octal or hexadecimal numbers are output, the bits of the number to be output are
always interpreted as unsigned! In other words, the output shows the bit pattern of a
number in octal or hexadecimal format.
Example: cout << dec << -1 << " " << hex << -1;
This statement causes the following output on a 32-bit system:
-1 ffffffff
64 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
#include <iostream>
using namespace std;
int main()
{
double x = 12.0;
cout.precision(2); // Precision 2
cout << " By default: " << x << endl;
cout << " showpoint: " << showpoint << x << endl;
cout << " fixed: " << fixed << x << endl;
cout << " scientific: " << scientific << x << endl;
return 0;
}
The key word const within the prototype of precision() signifies that the method performs only
read operations.
✓NOTE
■FORMATTED OUTPUT OF FLOATING-POINT NUMBERS
Manipulators formatting floating-point numbers
Methods for precision
Sample program
Manipulator Effects
Sets the precision to n.
Returns the used precision.
int precision (int n);
int precision() const;
Manipulator Effects
Generates a decimal point character
shown in floating-point output. The
number of digits after the decimal point
corresponds to the used precision.
Output in fixed point notation
Output in scientific notation
Sets the precision to n.
Trailing zeroes after the decimal point
are not printed.
If there are no digits after the decimal
point, the decimal point is not printed
(by default).
showpoint
noshowpoint
fixed
scientific
setprecision (int n)
FORMATTED OUTPUT OF FLOATING-POINT NUMBERS
■65
䊐Standard Settings
Floating-points are displayed to six digits by default. Decimals are separated from the
integral part of the number by a decimal point. Trailing zeroes behind the decimal point
are not printed. If there are no digits after the decimal point, the decimal point is not
printed (by default).
Examples: cout << 1.0; // Output: 1
cout << 1.234; // Output: 1.234
cout << 1.234567; // Output: 1.23457
The last statement shows that the seventh digit is not simply truncated but rounded.
Very large and very small numbers are displayed in exponential notation.
Example: cout << 1234567.8; // Output: 1.23457e+06
䊐Formatting
The standard settings can be modified in several ways. You can
■change the precision, i.e. the number of digits to be output
■force output of the decimal point and trailing zeroes
■stipulate the display mode (fixed point or exponential).
Both the manipulator setprecision()and the method precision() can be used to
redefine precision to be used.
Example: cout << setprecision(3); // Precision: 3
// or: cout.precision(3);
cout << 12.34; // Output: 12.3
Note that the header file iomanip must be included when using the manipulator set-
precision(). This also applies to all standard manipulators called with at least one
argument.
The manipulator showpoint outputs the decimal point and trailing zeroes. The
number of digits being output (e.g. 6) equals the current precision.
Example: cout << showpoint << 1.0; // Output: 1.00000
However, fixed point output with a predetermined number of decimal places is often more
useful. In this case, you can use the fixed manipulator with the precision defining the
number of decimal places. The default value of 6 is assumed in the following example.
Example: cout << fixed << 66.0; // Output: 66.000000
In contrast, you can use the scientific manipulator to specify that floating-point
numbers are output as exponential expressions.
66 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
The manipulators setw() and setfill() are declared in the header file iomanip .
✓NOTE
■OUTPUT IN FIELDS
Element functions for output in fields
Manipulators for output in fields
Examples
#include <iostream> // Obligatory
#include <iomanip> // declarations
using namespace std;
1st Example:cout << '|' << setw(6) << 'X' << '|';
Output:| X| // Field width 6
2nd Example:cout << fixed << setprecision(2)
<< setw(10) << 123.4 << endl
<< "1234567890" << endl;
Output:123.40 // Field width 10
1234567890
Method Effects
Returns the minimum field width used
Sets the minimum field width to n
Returns the fill character used
Sets the fill character to ch
int width() const;
int width(int n);
int fill() const;
int fill(int ch);
Manipulator Effects
Sets the minimum field width to n
Sets the fill character to ch
Left-aligns output in fields
Right-aligns output in fields
Left-aligns output of the sign and
right-aligns output of the numeric
value
setw(int n)
setfill(int ch)
left
right
internal
OUTPUT IN FIELDS
■67
The << operator can be used to generate formatted output in fields. You can
■specify the field width
■set the alignment of the output to right- or left-justified
■specify a fill-character with which to fill the field.
䊐Field Width
The field width is the number of characters that can be written to a field. If the output
string is larger than the field width, the output is not truncated but the field is extended.
The output will always contain at least the number of digits specified as the field width.
You can either use the width() method or the setw() manipulator to define field
width.
Example: cout.width(6); // or: cout << setw(6);
One special attribute of the field width is the fact that this value is non-permanent:
the field width specified applies to the next output only, as is illustrated by the examples
on the opposite page. The first example outputs the character 'X' to a field with width
of 6, but does not output the '|' character.
The default field width is 0. You can also use the width() method to get the current
field width. To do so, call width() without any other arguments.
Example: int fieldwidth = cout.width();
䊐Fill Characters and Alignment
If a field is larger than the string you need to output, blanks are used by default to fill the
field. You can either use the fill() method or the setfill() manipulator to specify
another fill character.
Example: cout << setfill('*') << setw(5) << 12;
// Output: ***12
The fill character applies until another character is defined.
As the previous example shows, output to fields is normally right-aligned. The other
options available are left-aligned and internal, which can be set by using the manipula-
tors left and internal. The manipulator internal left-justifies the sign and right-
justifies the number within a field.
Example: cout.width(6); cout.fill('0');
cout << internal << -123; // Output: -00123
68 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// Enters a character and outputs its
// octal, decimal, and hexadecimal code.
#include <iostream> // Declaration of cin, cout
#include <iomanip> // For manipulators being called
// with arguments.
#include <string>
using namespace std;
int main()
{
int number = ' ';
cout << "The white space code is as follows: "
<< number << endl;
char ch;
string prompt =
"\nPlease enter a character followed by "
" <return>: ";
cout << prompt;
cin >> ch; // Read a character
number = ch;
cout << "The character " << ch
<< " has code" << number << endl;
cout << uppercase // For hex-digits
<< " octal decimal hexadecimal\n "
<< oct << setw(8) << number
<< dec << setw(8) << number
<< hex << setw(8) << number << endl;
return 0;
}
■OUTPUT OF CHARACTERS, STRINGS, AND BOOLEAN VALUES
Sample program
OUTPUT OF CHARACTERS, STRINGS, AND BOOLEAN VALUES
■69
䊐Outputting Characters and Character Codes
The >> operator interprets a number of type char as the character code and outputs the
corresponding character:
Example: char ch = '0';
cout << ch << ' ' << 'A';
// Outputs three characters: 0 A
It is also possible to output the character code for a character. In this case the character
code is stored in an int variable and the variable is then output.
Example: int code = '0';
cout << code; // Output: 48
The '0' character is represented by ASCII Code 48. The program on the opposite page
contains further examples.
䊐Outputting Strings
You can use the >> operator both to output string literals, such as "Hello", and string
variables, as was illustrated in previous examples. As in the case of other types, strings
can be positioned within output fields.
Example: string s("spring flowers ");
cout << left // Left-aligned
<< setfill('?') // Fill character ?
<< setw(20) << s ; // Field width 20
This example outputs the string "spring flowers??????". The manipulator
right can be used to right-justify the output within the field.
䊐Outputting Boolean Values
By default the << operator outputs boolean values as integers, with the value 0represent-
ing false and 1 true. If you need to output the strings true or false instead, the
flag ios::boolalpha must be set. To do so, use either the setf() method or the
manipulator boolalpha.
Example: bool ok = true;
cout << ok << endl // 1
<< boolalpha << ok << endl; // true
You can revert this setting using the noboolalpha manipulator.
70 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// Inputs an article label and a price
#include <iostream> // Declarations of cin, cout,...
#include <iomanip> // Manipulator setw()
#include <string>
using namespace std;
int main()
{
string label;
double price;
cout << "\nPlease enter an article label: ";
// Input the label (15 characters maximum):
cin >> setw(16); // or: cin.width(16);
cin >> label;
cin.sync(); // Clears the buffer and resets
cin.clear(); // any error flags that may be set
cout << "\nEnter the price of the article: ";
cin >> price; // Input the price
// Controlling output:
cout << fixed << setprecision(2)
<< "\nArticle:"
<< "\n Label: " << label
<< "\n Price: " << price << endl;
// ... The program to be continued
return 0;
}
The input buffer is cleared and error flags are reset by calling the sync() and clear() methods. This
ensures that the program will wait for new input for the price, even if more than 15 characters have
been entered for the label.
✓NOTE
■FORMATTED INPUT
Sample program
FORMATTED INPUT
■71
The >> operator, which belongs to the istream class, takes the current number base
and field width flags into account when reading input:
■the number base specifies whether an integer will be read as a decimal, octal, or
hexadecimal
■the field width specifies the maximum number of characters to be read for a
string.
When reading from standard input, cin is buffered by lines. Keyboard input is thus
not read until confirmed by pressing the <Return> key. This allows the user to press the
backspace key and correct any input errors, provided the return key has not been pressed.
Input is displayed on screen by default.
䊐Input Fields
The >> operator will normally read the next input field, convert the input by reference to
the type of the supplied variable, and write the result to the variable. Any white space
characters (such as blanks, tabs, and new lines) are ignored by default.
Example: char ch;
cin >> ch; // Enter a character
When the following keys are pressed
<return> <tab> <blank> <X> <return>
the character 'X' is stored in the variable ch.
An input field is terminated by the first white space character or by the first character
that cannot be processed.
Example: int i;
cin >> i;
Typing 123FF<Return> stores the decimal value 123 in the variable i. However, the
characters that follow, FF and the newline character, remain in the input buffer and will
be read first during the next read operation.
When reading strings, only one word is read since the first white space character will
begin a new input field.
Example: string city;
cin >> city; // To read just one word!
If Lao Kai is input, only Lao will be written to the city string. The number of charac-
ters to be read can also be limited by specifying the field width. For a given field width of
n, a maximum of n–1 characters will be read, as one byte is required for the null charac-
ter. Any initial white space will be ignored. The program on the opposite page illustrates
this point and also shows how to clear the input buffer.
72 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// Enter hexadecimal digits and a floating-point number
//
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int number = 0;
cout << "\nEnter a hexadecimal number: "
<< endl;
cin >> hex >> number; // Input hex-number
cout << "Your decimal input: " << number << endl;
// If an invalid input occurred:
cin.sync(); // Clears the buffer
cin.clear(); // Reset error flags
double x1 = 0.0, x2 = 0.0;
cout << "\nNow enter two floating-point values: "
<< endl;
cout << "1. number: ";
cin >> x1; // Read first number
cout << "2. number: ";
cin >> x2; // Read second number
cout << fixed << setprecision(2)
<< "\nThe sum of both numbers: "
<< setw(10) << x1 + x2 << endl;
cout << "\nThe product of both numbers: "
<< setw(10) << x1 * x2 << endl;
return 0;
}
■FORMATTED INPUT OF NUMBERS
Sample program
FORMATTED INPUT OF NUMBERS
■73
䊐Inputting Integers
You can use the hex,oct, and dec manipulators to stipulate that any character
sequence input is to processed as a hexadecimal, octal, or decimal number.
Example: int n;
cin >> oct >> n;
An input value of 10 will be interpreted as an octal, which corresponds to a decimal
value of 8.
Example: cin >> hex >> n;
Here, any input will be interpreted as a hexadecimal, enabling input such as f0a or -F7.
䊐Inputting Floating-Point Numbers
The >> operator interprets any input as a decimal floating-point number if the variable is
a floating-point type, i.e. float,double, or long double. The floating-point num-
ber can be entered in fixed point or exponential notation.
Example: double x;
cin >> x;
The character input is converted to a double value in this case. Input, such as 123,
-22.0, or 3e10 is valid.
䊐Input Errors
But what happens if the input does not match the type of variable defined?
Example: int i, j; cin >> i >> j;
Given input of 1A5 the digit 1will be stored in the variable i. The next input field
begins with A. But since a decimal input type is required, the input sequence will not be
processed beyond the letter A. If, as in our example, no type conversion is performed, the
variable is not written to and an internal error flag is raised.
It normally makes more sense to read numerical values individually, and clear the
input buffer and any error flags that may have been set after each entry.
Chapter 6, “Control Flow,” and Chapter 28, “Exception Handling,” show how a pro-
gram can react to input errors.
74 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// Reads a text with the operator >>
// and the function getline().
#include <iostream>
#include <string>
using namespace std;
string header =
" --- Demonstrates Unformatted Input ---";
int main()
{
string word, rest;
cout << header
<< "\n\nPress <return> to go on" << endl;
cin.get(); // Read the new line
// without saving.
cout << "\nPlease enter a sentence with several words!"
<< "\nEnd with <!> and <return>."
<< endl;
cin >> word; // Read the first word
getline( cin, rest, '!'); // and the remaining text
// up to the character !
cout << "\nThe first word: " << word
<< "\nRemaining text: " << rest << endl;
return 0;
}
1. A text of more than one line can be entered.
2. The sample program requires that at least one word and a following white space are entered.
✓NOTE
■UNFORMATTED INPUT/OUTPUT
Sample program
UNFORMATTED INPUT/OUTPUT
■75
Unformatted input and output does not use fields, and any formatting flags that have
been set are ignored. The bytes read from a stream are passed to the program “as is.”
More specifically, you should be aware that any white space characters preceding the
input will be processed.
䊐Reading and Writing Characters
You can use the methods get() and put() to read or write single characters. The
get() method reads the next character from a stream and stores it in the given char
variable.
Example: char ch;
cin.get(ch);
If the character is a white space character, such as a newline, it will still be stored in the
ch variable. To prevent this from happening you can use
cin >> ch;
to read the first non-white space character.
The get() method can also be called without any arguments. In this case, get()
returns the character code of type int.
Example: int c = cin.get();
The put() method can be used for unformatted output of a character. The character to
be output is passed to put() as an argument.
Example: cout.put('A');
This statement is equivalent to cout << 'A'; , where the field width is undefined or
has been set to 1.
䊐Reading a Line
The >> operator can only be used to read one word into a string. If you need to read a
whole line of text, you can use the global function getline(), which was introduced
earlier in this chapter.
Example: getline(cin, text);
This statement reads characters from cin and stores them in the string variable text
until a new line character occurs. However, you can specify a different delimiting charac-
ter by passing the character to the getline() function as a third argument.
Example: getline(cin, s, '.');
The delimiting character is read, but not stored in the string. Any characters subsequent
to the first period will remain in the input buffer of the stream.
exercises
76 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
// A program with resistant mistakes
#include <iostream>
using namespace std;
int main()
{
char ch;
string word;
cin >> "Let's go! Press the return key: " >> ch;
cout << "Enter a word containing
three characters at most: ";
cin >> setprecision(3) >> word;
cout >> "Your input: " >> ch >> endl;
return 0;
}
■EXERCISES
Screen output for exercise 3
Article Number Number of Pieces Price per piece
....... ...... ...... Dollar
Program listing for exercise 5
EXERCISES
■77
The variable type defines whether a character or a number is to be read or output.
✓TIP
Exercise 1
What output is generated by the program on the page entitled “Formatted output
of floating-point numbers” in this chapter?
Exercise 2
Formulate statements to perform the following:
a. Left-justify the number 0.123456 in an output field with a width of 15.
b. Output the number 23.987 as a fixed point number rounded to two dec-
imal places, right-justifying the output in a field with a width of 12.
c. Output the number –123.456 as an exponential and with four decimal
spaces. How useful is a field width of 10?
Exercise 3
Write a C++ program that reads an article number, a quantity, and a unit price
from the keyboard and outputs the data on screen as displayed on the opposite
page.
Exercise 4
Write a C++ program that reads any given character code (a positive integer)
from the keyboard and displays the corresponding character and the character
code as a decimal, an octal, and a hexadecimal on screen.
Why do you think the character P is output when the number 336 is entered?
Exercise 5
Correct the mistakes in the program on the opposite page.
solutions
78 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
■SOLUTIONS
Exercise 1
Output of a sample program formatting floating-point numbers:
By default: 12
showpoint: 12.
fixed: 12.00
scientific: 1.20e+001
Exercise 2
#include <iostream>
#include <iomanip> // For setw() and setprecision()
using namespace std;
int main()
{
double x1 = 0.123456, x2 = 23.987, x3 = -123.456;
// a)
cout << left << setw(15) << x1 << endl;
// b)
cout << fixed << setprecision(2) << right << setw(12)
<< x2 << endl;
// c)
cout << scientific << setprecision(4) << x3 << endl;
// Output: -1.2346e+002
// A field width of 12 or more would be convenient!
return 0;
}
Exercise 3
// Input and formatted output of article characteristics.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
long number = 0;
int count = 0;
double price = 0.0;
// Input:
cout << "\nPlease enter article characteristics.\n";
cout << "Article number: ";
cin >> number;
SOLUTIONS
■79
cout << "Number of pieces: ";
cin >> count;
cout << "Price per piece: ";
cin >> price;
// Output:
cout <<
"\n\tArticle Number Quantity Price per piece ";
cout << "\n\t"
<< setw(8) << number
<< setw(16) << count
<< fixed << setprecision(2)
<< setw(16) << price << " Dollar" << endl;
return 0;
}
Exercise 4
#include <iostream>
#include <iomanip> // Manipulator setw()
using namespace std;
int main()
{
unsigned char c = 0;
unsigned int code = 0;
cout << "\nPlease enter a decimal character code: ";
cin >> code;
c = code; // Save for output
cout << "\nThe corresponding character: " << c << endl;
code = c; // Character code. Is only
// necessary, if input is > 255.
cout << "\nCharacter codes"
<< "\n decimal: " << setw(3) << dec << code
<< "\n octal: " << setw(3) << oct << code
<< "\n hexadecimal: " << setw(3) << hex << code
<< endl;
return 0;
}
80 ■
CHAPTER 4 INPUT AND OUTPUT WITH STREAMS
When entering 336, the value 80 is stored in the low byte of variable code
(336 = 256 + 80).Thus after the assignment, the variable ccontains the value
80, representing the character P.
Exercise 5
The corrected program:
// Corrections are commented.
//
#include <iostream>
#include <iomanip> // Manipulator setw()
#include <string> // Class string
using namespace std;
int main()
{
string word; // To read a word.
// char ch; is not needed.
// cout << ...instead of cin >> .
cout << "Let's go! Press the return key: ";
cin.get(); // Input newline character
cout << " Enter a word " // "
"containing three characters at the most: ";// "
cin >> setw(3) >> word; // setw(3) instead of
// setprecision(3)
cout << "Your input: " // <<
<< word << endl; // instead of >> ch
return 0;
}
81
Operators for
Fundamental Types
In this chapter, operators needed for calculations and selections are
introduced. Overloading and other operators, such as those needed for
bit manipulations, are introduced in later chapters.
chapter
5
82 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
#include <iostream>
using namespace std;
int main()
{
double x, y;
cout << "\nEnter two floating-point values: ";
cin >> x >> y;
cout << "The average of the two numbers is: "
<< (x + y)/2.0 << endl;
return 0;
}
■BINARY ARITHMETIC OPERATORS
Binary operator and operands
The binary arithmetic operators
Sample program
Sample output for the program
Enter two floating-point values: 4.75 12.3456
The average of the two numbers is: 8.5478
Operator
Left operand Right operand
a+b
+
-
*
/
%
Operator Significance
Addition
Subraction
Multiplication
Division
Remainder
BINARY ARITHMETIC OPERATORS
■83
If a program is to be able to process the data input it receives, you must define the opera-
tions to be performed for that data. The operations being executed will depend on the
type of data — you could add, multiply, or compare numbers, for example. However, it
would make no sense at all to multiply strings.
The following sections introduce you to the most important operators that can be
used for arithmetic types. A distinction is made between unary and binary operators. A
unary operator has only one operand, whereas a binary operator has two.
䊐Binary Arithmetic Operators
Arithmetic operators are used to perform calculations. The opposite page shows an
overview. You should be aware of the following:
■Divisions performed with integral operands will produce integral results; for exam-
ple, 7/2 computes to 3. If at least one of the operands is a floating-point number,
the result will also be a floating-point number; e.g., the division 7.0/2 produces
an exact result of 3.5.
■Remainder division is only applicable to integral operands and returns the remain-
der of an integral division. For example, 7%2 computes to 1.
䊐Expressions
In its simplest form an expression consists of only one constant, one variable, or one
function call. Expressions can be used as the operands of operators to form more complex
expressions. An expression will generally tend to be a combination of operators and
operands.
Each expression that is not a void type returns a value. In the case of arithmetic
expressions, the operands define the type of the expression.
Examples: int a(4); double x(7.9);
a * 512 // Type int
1.0 + sin(x) // Type double
x – 3 // Type double, since one
// operand is of type double
An expression can be used as an operand in another expression.
Example: 2 + 7 * 3 // Adds 2 and 21
Normal mathematical rules (multiplication before addition) apply when evaluating an
expression, i.e. the *,/,%operators have higher precedence than +and -. In our exam-
ple, 7*3 is first calculated before adding 2. However, you can use parentheses to apply a
different precedence order.
Example: (2 + 7) * 3 // Multiplies 9 by 3.
84 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
#include <iostream>
using namespace std;
int main()
{
int i(2), j(8);
cout << i++ << endl; // Output: 2
cout << i << endl; // Output: 3
cout << j-- << endl; // Output: 8
cout << --j << endl; // Output: 6
return 0;
}
■UNARY ARITHMETIC OPERATORS
The unary arithmetic operators
Precedence of arithmetic operators
Effects of prefix and postfix notation
+-
++
--
Operator Significance
Unary sign operators
Increment operator
Decrement operator
Precedence Operator Grouping
High
Low
++ --
++ --
+ -
*/%
+
-
(postfix) left to right
left to right
left to right
right to left
(prefix)
(sign)
(addition)
(subtraction)
UNARY ARITHMETIC OPERATORS
■85
There are four unary arithmetic operators: the sign operators +and -, the increment
operator ++, and the decrement operator --.
䊐Sign Operators
The sign operator –returns the value of the operand but inverts the sign.
Example: int n = –5; cout << -n; // Output: 5
The sign operator +performs no useful operation, simply returning the value of its
operand.
䊐Increment / Decrement Operators
The increment operator ++ modifies the operand by adding 1 to its value and cannot be
used with constants for this reason.
Given that iis a variable, both i++ (postfix notation) and ++i (prefix notation) raise
the value of iby 1. In both cases the operation i=i+1is performed.
However, prefix ++ and postfix ++ are two different operators. The difference
becomes apparent when you look at the value of the expression; ++i means that the
value of ihas already been incremented by 1, whereas the expression i++ retains the
original value of i. This is an important difference if ++i or i++ forms part of a more
complex expression:
++i i is incremented first and the new value of iis then applied,
i++ the original value of iis applied before iis incremented.
The decrement operator -- modifies the operand by reducing the value of the
operand by 1. As the sample program opposite shows, prefix or postfix notation can be
used with --.
䊐Precedence
How is an expression with multiple operators evaluated?
Example: float val(5.0); cout << val++ – 7.0/2.0;
Operator precedence determines the order of evaluation, i.e. how operators and
operands are grouped. As you can see from the table opposite, ++ has the highest prece-
dence and /has a higher precedence than -. The example is evaluated as follows:
(val++) –(7.0/2.0). The result is 1.5, as val is incremented later.
If two operators have equal precedence, the expression will be evaluated as shown in
column three of the table.
Example: 3 * 5 % 2 is equivalent to (3 * 5) % 2
86 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
// Demonstration of compound assignments
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
float x, y;
cout << "\n Please enter a starting value: ";
cin >> x;
cout << "\n Please enter the increment value: ";
cin >> y;
x += y;
cout << "\n And now multiplication! ";
cout << "\n Please enter a factor: ";
cin >> y;
x *= y;
cout << "\n Finally division.";
cout << "\n Please supply a divisor: ";
cin >> y;
x /= y;
cout << "\n And this is "
<< "your current lucky number: "
// without digits after
// the decimal point:
<< fixed << setprecision(0)
<< x << endl;
return 0;
}
■ASSIGNMENTS
Sample program
ASSIGNMENTS
■87
䊐Simple Assignments
Asimple assignment uses the assignment operator =to assign the value of a variable to an
expression. In expressions of this type the variable must be placed on the left and the
assigned value on the right of the assignment operator.
Examples: z = 7.5;
y = z;
x = 2.0 + 4.2 * z;
The assignment operator has low precedence. In the case of the last example, the right
side of the expression is first evaluated and the result is assigned to the variable on the
left.
Each assignment is an expression in its own right, and its value is the value assigned.
Example: sin(x = 2.5);
In this assignment the number 2.5 is assigned to xand then passed to the function as an
argument.
Multiple assignments, which are always evaluated from right to left, are also possible.
Example: i = j = 9;
In this case the value 9is first assigned to jand then to i.
䊐Compound Assignments
In addition to simple assignment operators there are also compound assignment opera-
tors that simultaneously perform an arithmetic operation and an assignment, for exam-
ple.
Examples. i += 3; is equivalent to i = i + 3;
i *= j + 2; is equivalent to i = i * (j+2);
The second example shows that compound assignments are implicitly placed in paren-
theses, as is demonstrated by the fact that the precedence of the compound assignment is
just as low as that of the simple assignment.
Compound assignment operators can be composed from any binary arithmetic opera-
tor (and, as we will see later, with bit operators). The following compound operators are
thus available: +=,-=,*=,/=, and %=.
You can modify a variable when evaluating a complex expression by means of an
assignment or the ++,-- operators. This technique is referred to as a side effect. Avoid
use of side effects if possible, as they often lead to errors and can impair the readability of
your programs.
88 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
■RELATIONAL OPERATORS
The relational operators
Precedence of relational operators
Examples for comparisons:
Operator Significance
less than
less than or equal to
greater than
geater than or equal to
equal
unequal
<
>
<=
>=
==
!=
arithmetic operators
< <= > >=
== !=
assignment operators
Precedence Operator
High
Low
5 >= 6 false
true
false
true
1.7 < 1.8
4 + 2 == 5
2 * 4 != 7
Comparison Result
RELATIONAL OPERATORS
■89
䊐The Result of Comparisons
Each comparison in C++ is a bool type expression with a value of true or false,
where true means that the comparison is correct and false means that the compari-
son is incorrect.
Example: length == circuit // false or true
If the variables length and circuit contain the same number, the comparison is
true and the value of the relational expression is true. But if the expressions contain
different values, the value of the expression will be false.
When individual characters are compared, the character codes are compared. The
result therefore depends on the character set you are using. The following expression
results in the value true when ASCII code is used.
Example: 'A' < 'a' // true, since 65 < 97
䊐Precedence of Relational Operators
Relational operators have lower precedence than arithmetic operators but higher prece-
dence than assignment operators.
Example: bool flag = index < max – 1;
In our example, max –1is evaluated first, then the result is compared to index, and the
value of the relational expression (false or true) is assigned to the flag variable.
Similarly, in the following
Example: int result;
result = length + 1 == limit;
length + 1 is evaluated first, then the result is compared to limit, and the value of
the relational expression is assigned to the result variable. Since result is an int
type, a numerical value is assigned instead of false or true, i.e. 0for false and 1for
true.
It is quite common to assign a value before performing a comparison, and parentheses
must be used in this case.
Example: (result = length + 1) == limit
Our example stores the result of length + 1 in the variable result and then compares
this expression with limit.
You cannot use the assignment operator =to compare two expressions. The compiler will not generate
an error message if the value on the left is a variable. This mistake has caused headaches for lots of
beginners when troubleshooting their programs.
✓NOTE
90 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
A numeric value, such as xor x+1, is interpreted as “false” if its value is 0. Any value other than 0 is
interpreted as “true.”
✓NOTE
■LOGICAL OPERATORS
“Truth” table for logical operators
Examples for logical expressions
true
true true
false
false
true
false
false
false
true
true
false
false
true
true
false
A B A && B A || B
true
false true
false
A!A
1
0
-1
0
-1
0
0
1
false
true
true
false
x <= y || y >=0
x > -2 && y == 0
x && !y
!(x+1) || y - 1 > 0
x y ResultLogical Expression
LOGICAL OPERATORS
■91
The logical operators comprise the boolean operators && (AND), || (OR), and !(NOT).
They can be used to create compound conditions and perform conditional execution of a
program depending on multiple conditions.
A logical expression results in a value false or true, depending on whether the log-
ical expression is correct or incorrect, just like a relational expression.
䊐Operands and Order of Evaluation
The operands for boolean type operators are of the bool type. However, operands of any
type that can be converted to bool can also be used, including any arithmetic types. In
this case the operand is interpreted as false, or converted to false, if it has a value of
0. Any other value than 0is interpreted as true.
The OR operator || will return true only if at least one operand is true, so the value
of the expression
Example: (length < 0.2) || (length > 9.8)
is true if length is less than 0.2 or greater than 9.8.
The AND operator && will return true only if both operands are true, so the logical
expression
Example: (index < max) && (cin >> number)
is true, provided index is less than max and a number is successfully input. If the con-
dition index < max is not met, the program will not attempt to read a number! One
important feature of the logical operators && and || is the fact that there is a fixed order
of evaluation. The left operand is evaluated first and if a result has already been ascer-
tained, the right operand will not be evaluated!
The NOT operator !will return true only if its operand is false. If the variable flag
contains the value false (or the value 0), !flag returns the boolean value true.
䊐Precedence of Boolean Operators
The && operator has higher precedence than ||. The precedence of both these operators
is higher than the precedence of an assignment operator, but lower than the precedence
of all previously used operators. This is why it was permissible to omit the parentheses in
the examples earlier on in this chapter.
The !operator is a unary operator and thus has higher precedence. Refer to the table
of precedence in the Appendix for further details.
exercises
92 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
// Evaluating operands in logical expressions.
#include <iostream>
using namespace std;
int main()
{
cout << boolalpha; // Outputs boolean values
// as true or false
bool res = false;
int y = 5;
res = 7 || (y = 0);
cout << "Result of (7 || (y = 0)): " << res
<< endl;
cout << "Value of y: " << y << endl;
int a, b, c;
a = b = c = 0;
res = ++a || ++b && ++c;
cout << '\n'
<< " res = " << res
<< ", a = " << a
<< ", b = " << b
<< ", c = " << c << endl;
a = b = c = 0;
res = ++a && ++b || ++c;
cout << " res = " << res
<< ", a = " << a
<< ", b = " << b
<< ", c = " << c << endl;
return 0;
}
■EXERCISES
Program listing for exercise 4
EXERCISES
■93
Exercise 1
What values do the following arithmetic expressions have?
a. 3/10 b. 11%4 c. 15/2.0
d. 3 + 4 % 5 e. 3 * 7 % 4 f. 7 % 4 * 3
Exercise 2
a. How are operands and operators in the following expression associated?
x = –4 * i++ – 6 % 4;
Insert parentheses to form equivalent expressions.
b. What value will be assigned in part a to the variable xif the variable ihas a
value of –2?
Exercise 3
The int variable xcontains the number 7. Calculate the value of the following
logical expressions:
a. x < 10 && x >= –1
b. !x && x >= 3
c. x++ == 8 || x == 7
Exercise 4
What screen output does the program on the opposite page generate?
solutions
94 ■
CHAPTER 5 OPERATORS FOR FUNDAMENTAL TYPES
■SOLUTIONS
Exercise 1
a. 0b. 3c. 7.5
d. 7e. 1f. 9
Exercise 2
a. x = ( ((–4) * (i++)) – (6 % 4) )
b. The value 6will be assigned to the variable x.
Exercise 3
a. true
b. false
c. false
Exercise 4
Result of (7 || (y = 0)): true
Value of y: 5
res = true, a = 1, b = 0, c = 0
res = true, a = 1, b = 1, c = 0
95
Control Flow
This chapter introduces the statements needed to control the flow of a
program.These are
■loops with while,do-while, and for
■selections with if-else,switch, and the conditional operator
■jumps with goto,continue, and break.
chapter
6
96 ■
CHAPTER 6 CONTROL FLOW
// average.cpp
// Computing the average of numbers
#include <iostream>
using namespace std;
int main()
{
int x, count = 0;
float sum = 0.0;
cout << "Please enter some integers:\n"
"(Break with any letter)"
<< endl;
while( cin >> x )
{
sum += x;
++count;
}
cout << "The average of the numbers: "
<< sum / count << endl;
return 0;
}
As long as the expression is true
statement
■THE while STATEMENT
Structogram for while
Sample program
Sample output from the above program
Please enter some integers:
(Break with any letter)
9 10 12q
The average of the numbers: 10.3333
THE WHILE STATEMENT
■97
Loops are used to perform a set of instructions repeatedly. The set of instructions to be
iterated is called the loop body. C++ offers three language elements to formulate iteration
statements: while,do-while, and for. The number of times a loop is repeated is
defined by a controlling expression. In the case of while and for statements this expres-
sion is verified before the loop body is executed, whereas a do-while loop is performed
once before testing.
The while statement takes the following format:
Syntax: while( expression )
statement // loop body
When entering the loop, the controlling expression is verified, i.e. the expression is
evaluated. If this value is true, the loop body is then executed before the controlling
expression is evaluated once more.
If the controlling expression is false, i.e. expression evaluates to false, the pro-
gram goes on to execute the statement following the while loop.
It is common practice to place the loop body in a new line of the source code and to
indent the statement to improve the readability of the program.
Example: int count = 0;
while( count < 10)
cout << ++count << endl;
As this example illustrates, the controlling expression is normally a boolean expression.
However, the controlling expression might be any expression that can be converted to
the bool type including any arithmetic expressions. As we already learned from the sec-
tion on boolean operators, the value 0converts to false and all other values convert to
true.
䊐Building Blocks
If you need to repeat more than one statement in a program loop, you must place the
statements in a block marked by parentheses { }. A block is syntactically equivalent to a
statement, so you can use a block wherever the syntax requires a statement.
The program on the opposite page calculates the average of a sequence of integers
input via the keyboard. Since the loops contains two statements, the statements must be
placed in a block.
The controlling expression cin >> x is true provided the user inputs an integer.
The result of converting the expression cin >> x to a bool type will be true for any
valid input and false in any other case. Invalid input, if the user types a letter instead
of an integer, for example, terminates the loop and executes the next statement.
98 ■
CHAPTER 6 CONTROL FLOW
// Euro1.cpp
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
double rate = 1.15; // Exchange rate:
// one Euro to one Dollar
cout << fixed << setprecision(2);
cout << "\tEuro \tDollar\n";
for( int euro = 1; euro <= 5; ++euro)
cout << "\t " << euro
<< "\t " << euro*rate << endl;
return 0;
}
■THE for STATEMENT
Structogram for for
Sample program
Screen output
Euro Dollar
1 0.95
2 1.90
3 2.85
4 3.80
5 4.75
expression1
statement
expression3
As long as expression2 is true
THE FOR STATEMENT
■99
䊐Initializing and Reinitializing
A typical loop uses a counter that is initialized, tested by the controlling expression and
reinitialized at the end of the loop.
Example: int count = 1; // Initialization
while( count <= 10) // Controlling
{ // expression
cout << count
<< ". loop" << endl;
++count; // Reinitialization
}
In the case of a for statement the elements that control the loop can be found in the
loop header. The above example can also be expressed as a for loop:
Example: int count;
for( count = 1; count <= 10; ++count)
cout << count
<< ". loop" << endl;
Any expression can be used to initialize and reinitialize the loop. Thus, a for loop has
the following form:
Syntax: for( expression1; expression2; expression3 )
statement
expression1 is executed first and only once to initialize the loop. expression2 is
the controlling expression, which is always evaluated prior to executing the loop body:
■if expression2 is false, the loop is terminated
■if expression2 is true, the loop body is executed. Subsequently, the loop is
reinitialized by executing expression3 and expression2 is re-tested.
You can also define the loop counter in expression1. Doing so means that the
counter can be used within the loop, but not after leaving the loop.
Example: for( int i = 0; i < 10; cout << i++ )
;
As this example illustrates, the loop body can be an empty statement. This is always the
case if the loop header contains all necessary statements. However, to improve readabil-
ity, even the empty statement should occupy a line of its own.
100 ■
CHAPTER 6 CONTROL FLOW
// EuroDoll.cpp
// Outputs a table of exchange: Euro and US-$
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
long euro, maxEuro; // Amount in Euros
double rate; // Exchange rate Euro <-> $
cout << "\n* * * TABLE OF EXCHANGE "
<< " Euro – US-$ * * *\n\n";
cout << "\nPlease give the rate of exchange: "
" one Euro in US-$: ";
cin >> rate;
cout << "\nPlease enter the maximum euro: ";
cin >> maxEuro;
// --- Outputs the table ---
// Titles of columns:
cout << '\n'
<< setw(12) << "Euro" << setw(20) << "US-$"
<< "\t\tRate: " << rate << endl;
// Formatting US-$:
cout << fixed << setprecision(2) << endl;
long lower, upper, // Lower and upper limit
step; // Step width
// The outer loop determines the actual
// lower limit and the step width:
for( lower=1, step=1; lower <= maxEuro;
step*= 10, lower = 2*step)
// The inner loop outputs a "block":
for( euro = lower, upper = step*10;
euro <= upper && euro <= maxEuro; euro+=step)
cout << setw(12) << euro
<< setw(20) << euro*rate << endl;
return 0;
}
■THE for STATEMENT (CONTINUED)
Sample program
THE FOR STATEMENT (CONTINUED)
■101
Any of the three expressions in a for statement can be omitted, however, you must type
at least two semicolons. The shortest loop header is therefore:
Example: for(;;)
This statement causes an infinite loop, since the controlling expression is assumed to be
true if expression2 is missing. In the following
Example: for( ; expression; )
the loop header is equivalent to while(expression). The loop body is executed as
long as the test expression is true.
䊐The Comma Operator
You can use the comma operator to include several expressions where a single expression
is syntactically correct. For example, several variables can be initialized in the loop
header of a for statement. The following syntax applies for the comma operator
Syntax: expression1, expression2 [, expression3 ...]
The expressions separated by commas are evaluated from left to right.
Example: int x, i, limit;
for( i=0, limit=8; i < limit; i += 2)
x = i * i, cout << setw(10) << x;
The comma operator separates the assignments for the variables iand limit and is
then used to calculate and output the value of xin a single statement.
The comma operator has the lowest precedence of all operators — even lower than
the assignment operators. This means you can leave out the parentheses in the above
example.
Like any other C++ expression, an expression containing the comma operator has a
value and belongs to a certain type. The type and value are defined by the last expression
in a statement separated by commas.
Example: x = (a = 3, b = 5, a * b);
In this example the statements in brackets are executed before the value of the product
of a * b is assigned to x.
102 ■
CHAPTER 6 CONTROL FLOW
As long as the expression is true
statement
// tone.cpp
#include <iostream>
using namespace std;
const long delay = 10000000L;
int main()
{
int tic;
cout << "\nHow often should the tone be output? ";
cin >> tic;
do
{
for( long i = 0; i < delay; ++i )
;
cout << "Now the tone!\a" << endl;
}
while( --tic > 0 );
cout << "End of the acoustic interlude!\n";
return 0;
}
■THE do-while STATEMENT
Structogram for do-while
Sample program
THE DO-WHILE STATEMENT
■103
In contrast to while and for loops, which are controlled by their headers, the do-
while loop is controlled by its footer, i.e. the controlling expression is evaluated after
executing the first loop. This results in the loop body being performed at least once.
Syntax: do
statement
while( expression);
When a do-while loop is executed, the loop body is processed first. Only then is the
controlling expression evaluated. The loop body is iterated again if the result is
true, otherwise the loop is terminated.
The do-while loop must be followed by a semicolon.
✓NOTE
䊐Nesting Loops
Loops can be nested, that is, the loop body can also contain a loop. The ANSI standard
stipulates a maximum depth of 256 nested loops.
The program on the opposite page outputs a number of tones with the number being
defined by user input.
The program contains two loops — one of which is nested in the other. Each time the
outer do-while loop is repeated a short break occurs. The break is caused by the inner
for loop where the variable iis incremented from 0to the value of delay.
Text and a tone are subsequently output. The tone is generated by outputting the
control character BELL (ASCII code 7), which is represented by the escape sequence
\a.
Since a do-while statement is used, the program outputs a tone even if the user
types 0or a negative number.
104 ■
CHAPTER 6 CONTROL FLOW
true false
statement1 statement2
if (expression)
// if_else.cpp
// Demonstrates the use of if-else statements
#include <iostream>
using namespace std;
int main()
{
float x, y, min;
cout << "Enter two different numbers:\n";
if( cin >> x && cin >> y) // If both inputs are
{ // valid, compute
if( x < y ) // the lesser.
min = x;
else
min = y;
cout << "\nThe smaller number is: " << min << endl;
}
else
cout << "\nInvalid Input!" << endl;
return 0;
}
■SELECTIONS WITH if-else
Structogram for the if-else statement
Sample program
Sample output for this program
Enter two different numbers:
7.5 5.7
The smaller number is: 5.7
SELECTIONS WITH IF-ELSE
■105
The if-else statement can be used to choose between two conditional statements.
Syntax: if( expression )
statement1
[ else
statement2 ]
When the program is run, expression is first evaluated and the program control
branches accordingly. If the result is true,statement1 is executed and statement2
is executed in all other cases, provided an else branch exists. If there is no else and
expression is false, the control jumps to the statement following the if statement.
䊐Nested if-else Statements
As the program opposite illustrates, multiple if-else statements can be nested. But
not every if statement has an else branch. To solve the resulting problem, an else
branch is always associated with the nearest preceding if statement that does not have
an else branch.
Example: if( n > 0 )
if( n%2 == 1 )
cout << " Positive odd number ";
else
cout << "Positive even number";
In this example, the else branch belongs to the second if, as is indicated by the fact
that the statement has been indented. However, you can use a code block to redefine the
association of an else branch.
Example: if( n > 0 )
{ if( n%2 == 1 )
cout << " Positive odd number \n";
}
else
cout << " Negative number or zero\n";
䊐Defining Variables in if Statements
You can define and initialize a variable within an if statement. The expression is true if
converting the variable’s value to a bool type yields true. In this case the variable is
available within the if statement.
Example: if( int x = func() )
{ . . . } // Here to work with x.
The return value of the function, func(), is used to initialize the variable x. If this
value is not 0, the statements in the next block are executed. The variable xno longer
exists after leaving the if statement.
106 ■
CHAPTER 6 CONTROL FLOW
if(expression)
if(expression)
if(expression)
true false
true
true false
false
statement1
statement(n) statement(n+1)
statement2
. . .
// speed.cpp
// Output the fine for driving too fast.
#include <iostream>
using namespace std;
int main()
{
float limit, speed, toofast;
cout << "\nSpeed limit: ";
cin >> limit;
cout << "\nSpeed: ";
cin >> speed;
if( (toofast = speed – limit ) < 10)
cout << "You were lucky!" << endl;
else if( toofast < 20)
cout << "Fine payable: 40,-. Dollars" << endl;
else if( toofast < 30)
cout << "Fine payable: 80,-. Dollars" << endl;
else
cout << "Hand over your driver's license!" << endl;
return 0;
}
■Else-if CHAINS
Structogram for an else-if chain
Sample program
ELSE-IF CHAINS
■107
䊐Layout and Program Flow
You can use an else-if chain to selectively execute one of several options. An else-
if chain implies a series of embedded if-else statements whose layout is normally as
follows:
if ( expression1 )
statement1
else if( expression2 )
statement2
.
.
.
else if( expression(n) )
statement(n)
[ else statement(n+1)]
When the else-if chain is executed, expression1,expression2,... are
evaluated in the order in which they occur. If one of the expressions proves to be true,
the corresponding statement is executed and this terminates the else-if chain.
If none of the expressions are true, the else branch of the last if statement is exe-
cuted. If this else branch is omitted, the program executes the statement following the
else-if chain.
䊐The Sample Program
The program opposite uses an else-if chain to evaluate the penalty for driving too fast
and outputs the fine on screen.
The speed limit and the actual speed are read from the keyboard. If the user types 60
as the speed limit and 97.5 as the actual speed, the first three expressions are not true,
and the last else branch is executed. This outputs the message "Hand over your
driver's license!" on a new line.
108 ■
CHAPTER 6 CONTROL FLOW
true false
expression1 expression2
expression
// greater.cpp
#include <iostream>
using namespace std;
int main()
{
float x, y;
cout << "Type two different numbers:\n";
if( !(cin >> x && cin >> y) ) // If the input was
{ // invalid.
cout << "\nInvalid input!" << endl;
}
else
{
cout << "\nThe greater value is: "
<< (x > y ? x : y) << endl;
}
return 0;
}
■CONDITIONAL EXPRESSIONS
Structogram for a conditional expression
Sample program
Sample output for this program
Type two different numbers:
173.2
216.7
The greater value is: 216.7
CONDITIONAL EXPRESSIONS
■109
䊐Conditional Operator
The conditional operator ?: is used to form an expression that produces either of two
values, depending on the value of some condition. Because the value produced by such
an expression depends on the value of a condition, it is called conditional expression.
In contrast to the if-else statement the selection mechanism is based on expres-
sions: one of two possible expressions is selected. Thus, a conditional expression is often
a concise alternative to an if-else statement.
Syntax: expression ? expression1 : expression2
expression is evaluated first. If the result is true,expression1 is evaluated; if not
expression2 is executed. The value of the conditional expression is therefore either
the value of expression1 or expression2.
Example: z = (a >= 0) ? a : -a;
This statement assigns the absolute value of ato the variable z. If ahas a positive value
of 12, the number 12 is assigned to z. But if ahas a negative value, for example –8, the
number 8is assigned to z.
Since this sample program stores the value of the conditional expression in the vari-
able z, the statement is equivalent to
if( a > 0 )
z = a;
else
z = -a;
䊐Precedence
The conditional operator is the only C++ operator with three operands. Its precedence is
higher than that of the comma and assignment operators but lower than all other opera-
tors. In other words, you could omit the brackets in the first example.
You can use the result of a conditional evaluation without assigning it, as the sample
program on the opposite page shows. In this example, xis printed on screen if xis
greater than y, and yis printed otherwise.
However, you should assign the result of complex expressions to a variable explicitly
to improve the readability of your program.
110 ■
CHAPTER 6 CONTROL FLOW
case Const1:
case Const2:
statements
break
statements
break
statements
break
default:
. . .
switch(expression)
// Evaluates given input.
int command = menu(); // The function menu() reads
// a command.
switch( command ) // Evaluate command.
{
case 'a':
case 'A':
action1(); // Carry out 1st action.
break;
case 'b':
case 'B':
action2(); // Carry out 2nd action.
break;
default:
cout << '\a' << flush; // Beep on
} // invalid input
■SELECTING WITH switch
Structogram for the switch statement
Example
SELECTING WITH SWITCH
■111
䊐The switch Statement
Just like the else-if chain, the switch statement allows you to choose between mul-
tiple alternatives. The switch statement compares the value of one expression with
multiple constants.
switch( expression )
{
case const1: [ statement ]
[ break; ]
case const2: [ statement ]
[ break; ]
.
.
.
[default: statement ]
}
First, the expression in the switch statement is evaluated. It must be an integral
type. The result is then compared to the constants, const1,const2,..., in the case
labels. The constants must be different and can only be integral types (boolean values
and character constants are also integral types).
If the value of an expression matches one of the case constants, the program
branches to the appropriate case label. The program then continues and the case labels
lose their significance.
You can use break to leave the switch statement unconditionally. The statement is
necessary to avoid executing the statements contained in any case labels that follow.
If the value of the expression does not match any of the case constants, the program
branches to the default label, if available. If you do not define a default label, noth-
ing happens. The default does not need to be the last label; it can be followed by addi-
tional case labels.
䊐Differences between switch and else-if Chains
The else-if chain is more versatile than the switch statement. Every selection can
be programmed using an else-if chain. But you will frequently need to compare the
value of an integral expression with a series of possible values. In this case (and only this
case), you can use a switch statement.
As the example opposite shows, a switch statement is more easily read than an
equivalent else-if chain, so use the switch statement whenever possible.
112 ■
CHAPTER 6 CONTROL FLOW
As long as expression is true
break;
statement, which follows the loop.
// ascii.cpp : To output an ASCII Code Table
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int ac = 32; // To begin with ASCII Code 32
// without control characters.
while(true)
{ cout << "\nCharacter Decimal Hexadecimal\n\n";
int upper;
for( upper =ac + 20; ac < upper && ac < 256; ++ac)
cout << " " << (char)ac // as character
<< setw(10) << dec << ac
<< setw(10) << hex << ac << endl;
if( upper >= 256) break;
cout <<"\nGo on -> <return>,Stop -> <q>+<return>";
char answer;
cin.get(answer);
if( answer == 'q' || answer == 'Q' )
break;
cin.sync(); // Clear input buffer
}
return 0;
}
The expression (char)ac yields the value ac of type char.
✓NOTE
■JUMPS WITH break,continue, AND goto
Structogram for break within a while statement
Sample program containing a break statement
JUMPS WITH BREAK, CONTINUE, AND GOTO
■113
䊐break
The break statement exits from a switch or loop immediately. You can use the break
keyword to jump to the first statement that follows the switch or loop.
The program on the opposite page, which outputs a group of 20 ASCII characters and
their corresponding codes, uses the break keyword in two places. The first break exits
from an infinite while(true) { ... } loop when a maximum value of 256 has been
reached. But the user can also opt to continue or terminate the program. The second
break statement is used to terminate the while loop and hence the program.
䊐continue
The continue statement can be used in loops and has the opposite effect to break,
that is, the next loop is begun immediately. In the case of a while or do-while loop
the program jumps to the test expression, whereas a for loop is reinitialized.
Example: for( int i = 0; i < 100; i++ )
{
. . . // Processes all integers.
if( i % 2 == 1)
continue;
. . . // Process even
// numbers only.
}
䊐goto and Labels
C++ also offers a goto statement and labels. This allows you to jump to any given point
marked by a label within a function. For example, you can exit from a deeply embedded
loop construction immediately.
Example: for( . . . )
for( . . . )
if (error) goto errorcheck;
. . .
errorcheck: . . . // Error handling
Alabel is a name followed by a colon. Labels can precede any statement.
Any program can do without goto statements. If you need to use a goto statement,
do so to exit from a code block, but avoid entering code blocks by this method.
exercises
114 ■
CHAPTER 6 CONTROL FLOW
12345678910
1
2
3
4
5
6
7
8
9
10
1
2
10
2
4
20
3
6
30
.
.
.
.
.
.
.
.
.
10
20
100
.
.
.
. . . . . .
. . . . . .
****** ******
MULTIPLICATION TABLE
■EXERCISES
Screen output for exercise 2
Note on exercise 4
Use the function time() to initialize the random number generator:
#include <time.h> // Prototype of time()
#include <stdlib.h> // Prototypes of srand()
// and rand()
long sec;
time( &sec ); // Take the number of seconds and
srand( (unsigned)sec ); // use it to initialize.
EXERCISES
■115
Use the system time to seed the random number generator as shown opposite. The time() function
returns the number of seconds since 1/1/1970, 0:0. The long value of the sec variable is converted to
unsigned by unsigned(sec) and then passed to the srand() function.
✓NOTE
Exercise 1
Rewrite the EuroDoll.cpp program in this chapter to replace both the for
loops with while loops.
Exercise 2
Write a C++ program that outputs a complete multiplication table (as shown
opposite) on screen.
Exercise 3
Write a C++ program that reads an integer between 0 and 65535 from the
keyboard and uses it to seed a random number generator.Then output 20
random numbers between 1 and 100 on screen.
Exercise 4
Write a program for the following numerical game:
The computer stores a random number between 1 and 15 and the player
(user) attempts to guess it.The player has a total of three attempts.After each
wrong guess, the computer tells the user if the number was too high or too low.
If the third attempt is also wrong, the number is output on screen.
The player wins if he or she can guess the number within three attempts.
The player is allowed to repeat the game as often as he or she wants.
solutions
116 ■
CHAPTER 6 CONTROL FLOW
■SOLUTIONS
Exercise 1
The for loops of program EuroDoll.cpp are equivalent to the following while
loops:
// The outer loop sets the lower
// limit and the step width used:
lower=1, step=1;
while( lower <= maxEuro)
{
// The inner loop outputs a block:
euro = lower;
upper = step*10;
while( euro <= upper && euro <= maxEuro)
{
cout << setw(12) << euro
<< setw(20) << euro*rate << endl;
euro += step;
}
step *= 10, lower = 2*step;
}
Exercise 2
// MultTable.cpp
// Outputs a multiplication table.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
int factor1, factor2;
cout << "\n\n "
<< " ****** MULTIPLICATION TABLE ******"
<< endl;
// Outputs the first and second line:
cout << "\n\n\n "; // 1. line
for( factor2 = 1 ; factor2 <= 10 ; ++factor2 )
cout << setw(5) << factor2;
cout << "\n " // 2. line
<< "-------------------------------------------"
<< endl;
SOLUTIONS
■117
// Outputs the remaining lines of the table:
for( factor1 = 1 ; factor1 <= 10 ; ++factor1 )
{
cout << setw(6) << factor1 << " |";
for( factor2 = 1 ; factor2 <= 10 ; ++factor2 )
cout << setw(5) << factor1 * factor2;
cout << endl;
}
cout << "\n\n\n"; // To shift up the table
return 0;
}
Exercise 3
// random.cpp
// Outputs 20 random numbers from 1 to 100.
#include <stdlib.h> // Prototypes of srand() and rand()
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
unsigned int i, seed;
cout << "\nPlease type an integer between "
"0 and 65535: ";
cin >> seed; // Reads an integer.
srand( seed); // Seeds the random
// number generator.
cout << "\n\n "
"****** RANDOM NUMBERS ******\n\n";
for( i = 1 ; i <= 20 ; ++i)
cout << setw(20) << i << ". random number = "
<< setw(3) << (rand() % 100 + 1) << endl;
return 0;
}
118 ■
CHAPTER 6 CONTROL FLOW
Exercise 4
// NumGame.cpp : A numerical game against the computer
#include <cstdlib> // Prototypes of srand() and rand()
#include <ctime> // Prototype of time()
#include <iostream>
using namespace std;
int main()
{
int number, attempt;
char wb = 'r'; // Repeat or finish.
long sec;
time( &sec); // Get the time in seconds.
srand((unsigned)sec); // Seeds the random
// number generator
cout << "\n\n "
<< " ******* A NUMERICAL GAME *******" << endl;
cout << "\n\nRules of the game:" << endl;
while( wb == 'r')
{
cout << "I have a number between 1 and 15 in mind \n"
<< "You have three chances to guess correctly!\n"
<< endl;
number = (rand() % 15) + 1;
bool found = false; int count = 0;
while( !found && count < 3 )
{
cin.sync(); // Clear input buffer
cin.clear();
cout << ++count << ". attempt: ";
cin >> attempt;
if(attempt < number) cout << "too small!"<< endl;
else if(attempt > number) cout <<"too big!"<< endl;
else found = true;
}
if( !found)
cout << "\nI won!"
<< " The number in question was: "
<< number << endl;
else
cout << "\nCongratulations! You won!" << endl;
cout << "Repeat —> <r> Finish —> <f>\n";
do
cin.get(wb);
while( wb != 'r' && wb != 'f');
}
return 0;
}
119
Symbolic Constants and
Macros
This chapter introduces you to the definition of symbolic constants and
macros illustrating their significance and use. In addition, standard macros
for character handling are introduced.
chapter
7
120 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
// sintab.cpp
// Creates a sine function table
#include <iostream>
#include <iomanip>
#include <cmath>
using namespace std;
#define PI 3.1415926536
#define START 0.0 // Lower limit
#define END (2.0 * PI) // Upper limit
#define STEP (PI / 8.0) // Step width
#define HEADER (cout << \
" ***** Sine Function Table *****\n\n")
int main()
{
HEADER; // Title
// Table Head:
cout << setw(16) << "x" << setw(20) << "sin(x)\n"
<< " -----------------------------------------"
<< fixed << endl;
double x;
for( x = START; x < END + STEP/2; x += STEP)
cout << setw(20) << x << setw(16) << sin(x)
<< endl;
cout << endl << endl;
return 0;
}
■MACROS
Sample program
Screen output
****** Table for the Sine Function ******
x sin(x)
--------------------------------------------
0.000000 0.000000
0.392699 0.382683
0.785398 0.707107
. .
. .
. .
MACROS
■121
C++ has a simple mechanism for naming constants or sequences of commands, that is for
defining macros. You simply use the preprocessor’s #define directive.
Syntax: #define name substitutetext
This defines a macro called name. The preprocessor replaces name with substitute-
text throughout the subsequent program. For example, in the program on the opposite
page, the name PI is replaced by the number 3.1415926536 throughout the program
in the first phase of compilation.
There is one exception to this general rule: substitution does not take place within
strings. For example, the statement
cout << "PI";
outputs only PI and not the numerical value of PI.
䊐Symbolic Constants
Macros that are replaced by constants, such as the PI macro, are also known as symbolic
constants. You should note that neither an equals sign nor a semicolon is used, as these
would become part of the substitute text.
You can use any macros you have previously defined in subsequent #define direc-
tives. The program opposite uses the symbolic constant PI to define other constants.
䊐More about Working with Macros
Any preprocessor directive, and this includes the #define directive, must be placed in a
line of its own. If the substitute text is longer than one line, you can terminate the line
with a backslash \and continue the substitute text in the next line, as is illustrated by
the macro HEADER on the opposite page.
The rules that apply to naming variables also apply to naming macros. However, it is
standard practice to capitalize symbolic constants to distinguish them from the names of
variables in a program.
Using macros makes a C++ program more transparent and flexible. There are two
main advantages:
1. good readability: You can name a macro to indicate the use of the macro
2. easy to modify: If you need to change the value of a constant throughout a pro-
gram, you simply change the value of the symbolic constant in the #define
directive.
122 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
// ball1.cpp
// Simulates a bouncing ball
// ---------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
#define DELAY 10000000L // Output delay
#define CLS (cout << "\033[2J") // Clear screen
#define LOCATE(z,s) (cout <<"\033["<< z <<';'<< s <<'H')
// Position the cursor in row z and column s
void main()
{
int x = 2, y = 3, dx = 1, speed = 0;
string floor(79, '-'),
header = "**** JUMPING BALL ****";
CLS;
LOCATE(1,25); cout << header;
LOCATE(25,1); cout << floor;
while(true) // Let the ball "always" bounce
{ // Terminate by interrupt key (^C)
LOCATE(y,x); cout << 'o' << endl; // Show the ball
for( long wait = 0; wait < DELAY; ++wait)
;
if(x == 1 || x == 79) dx = -dx; // Bounce off
// a wall?
if( y == 24 ) // On the floor?
{
speed = - speed;
if( speed == 0 ) speed = -7; // Restart
}
speed += 1; // Acceleration = 1
LOCATE(y,x); cout << ' '; // Clear output
y += speed; x += dx; // New Position
}
}
■MACROS WITH PARAMETERS
Sample program
MACROS WITH PARAMETERS
■123
1These escape sequences are valid for all standard UNIX terminals. The driver ansi.sys must be
loaded for DOS or a DOS box in Win95 or Win98. For Win NT and Win 2000, corresponding
functions based on system calls are offered for download.
It is possible to call macros with arguments. To do so, you must supply the appropriate
parameters when defining the macro. The parameters are replaced by valid arguments at
run time.
Example: #define SQUARE(a) ((a) * (a))
This defines a macro called SQUARE() with a parameter a. The name of the macro must
be followed immediately by a left bracket. When the macro is called, for example
Example: z = SQUARE(x+1);
the preprocessor inserts the substitute text with the current arguments, which will be
expanded as follows, in this case
z = ((x+1) * (x+1));
This example also shows that you must be careful when using brackets to indicate param-
eters for macros. Omitting the brackets in the previous example, SQUARE, would cause
the expression to be expanded as follows z = x + 1 * x + 1.
The outer brackets in the definition ensure that even when the macro is used in a
complex expression, the square is calculated before the result can be used for any further
calculations.
䊐Macros for Screen Control
The program opposite uses macros to change the appearance of the screen. Peripheral
devices, such as the screen or printers, can be controlled by special character sequences
that normally begin with the ESC character (decimal 27, octal 033) and are thus known
as escape sequences. A number of ANSI standard escape sequences exists for screen con-
trol.1See the appendix on Escape Sequences for Screen Control for an overview of the
most important sequences.
CLS is a macro without any parameters that uses the escape sequence \033[2J to
clear the screen. LOCATE is just one example of a macro with two parameters. LOCATE
uses the escape sequence \033[z;sH to place the cursor at the position of the next
screen output. The values zfor the line and sfor the column require decimal input with
z=1,s = 1 representing the top left corner of the screen or window.
The ball is “thrown in” at the coordinates x = 2,y = 3 and bounces off the “floor”
and the “walls.” In direction x (horizontally) the ball has a constant speed of dx = 1 or
-1. In direction y (vertically) the ball is subject to a constant acceleration of 1,
expressed as speed += 1.
124 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
#include "proj.h"
.
.
.
#include "proj.h"
.
.
.
#include "proj.h"
.
.
.
...
Header file proj.h
Macros
Classes
and other type
definitions
Prototypes of
global functions
Source
file 1
Source
file 2
Source
file n
■WORKING WITH THE #define DIRECTIVE
Using macros in different source files
WORKING WITH THE #DEFINE DIRECTIVE
■125
You can place the #define directive in any line of a program as long as it is placed prior
to using the macro. However, it is recommended to place all definitions at the beginning
of the source file for ease of location and modification.
If you need to use the same macros in different source files, it makes sense to create a
header file. You can then include the header file in your source files. This method also
lends itself to large-scale software projects. The programmers working on the project
then have access to the same set of macro definitions and other declarations. This con-
cept is illustrated opposite using the header file proj.h as an example.
Macros with parameters can be called just like functions. You should note the follow-
ing important differences, however:
■Macros: A macro definition must be visible to the compiler. The substitute text is
inserted and re-compiled each time the macro is called. For this reason, a macro
should contain only a few statements to avoid inflating the object file each time
the macro is called. The speed of program execution will, however, improve since
the program does not need to branch to sub-routines in contrast to normal func-
tion calls. This can become apparent when macros are used within loops, for
example.
Side effects of macros are possible if the substitute text contains multiple
instances of a parameter. The statement SQUARE( ++x ) expands to ((++x)
* (++x)), for example. The variable xis incremented twice and the product
does not represent the square of the incremented number.
■Functions: Functions are compiled independently. The linker then links them
into the executable file. The program branches to the function whenever it is
called, and this can reduce the speed of execution in comparison to a macro.
However, the executable file will be shorter as it contains only one instance of
the function code.
The compiler checks the argument types, and the side effects seen with
macros cannot occur.
Inline functions, which are introduced in the chapter on functions, are an alterna-
tive to macros.
126 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
Header file
basis.h
Source file
application.cpp
Header file
statist.h
Header file
graph.h
#include <iostream>
#include "basis.h"
#ifndef_BASIS_H_
#define_BASIS_H_
#define BSIZE 1000
#endif
//content of basis,
//ex.
. . .
. . .
#include <iostream>
#include "basis.h"
. . .
#include "statist.h"
#include "graph.h"
int main()
{
return 0;
}
. . .
■CONDITIONAL INCLUSION
Multiple inclusions of header files
CONDITIONAL INCLUSION
■127
䊐Redefining Macros
A macro cannot simply be redefined. A macro definition is valid until it is removed by
using an #undef directive. However, you do not need to supply the parameter list of a
macro with parameters.
Example: #define MIN(a,b) ((a)<(b)? (a) : (b))
. . . // Here MIN can be called
#undef MIN
The macro MIN cannot be used after this point, but it can be defined again, possibly with
a different meaning, using the #define directive.
䊐Conditional Inclusion
You can use the preprocessor directives #ifdef and #ifndef to allow the compiler to
check whether a macro has been defined.
Syntax: #ifdef name
. . . // Block, which will be compiled
// if name is defined.
#endif
In the case of the #ifndef directive, the code block is compiled up to the next #endif
only if the macro name has not been previously defined.
On conditional inclusion else branching and nesting is also permissible. See Pre-
processor Directives in the appendix for further information.
A macro definition does not need to include a substitute text to be valid.
Example: #define MYHEADER
A symbol without a substitute text is often used to identify header files and avoid multi-
ple inclusion.
If you have a header file named "article.h", you can identify the header by defin-
ing a symbol, such as _ARTICLE_, within that file.
Example: #ifndef _ARTICLE_
#define _ARTICLE_
. . . // Content of the header file
#endif
If you have already included the header, _ARTICLE_ will already be defined, and the
contents of the header file need not be compiled. This technique is also employed by
standard header files.
128 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
// toupper.cpp: A filter that converts to capitals.
// ---------------------------------------------------
#include <iostream>
#include <cctype>
using namespace std;
int main()
{
char c;
long nChar = 0, // Counts all characters
nConv = 0; // and converted characters
while ( cin.get(c) ) // As long as a character
{ ++nChar; // can be read, to increment.
if( islower(c)) // Lowercase letter?
{ c = toupper(c); // Converts the character
++nConv; // and counts it.
}
cout.put(c); // Outputs the character.
}
clog << "\nTotal of characters: " << nChar
<< "\nTotal of converted characters: " << nConv
<< endl;
return 0;
}
The program reads characters from a file until end-of-file. When reading keyboard input, end-of-file is
simulated by Ctrl+Z (DOS) or Ctrl+D (UNIX).
✓NOTE
■STANDARD MACROS FOR CHARACTER MANIPULATION
Sample program
Macros for character classification
isalpha(c)
islower(c)
isupper(c)
isdigit(c)
isalnum(c)
isspace(c)
isprint(c)
cis a letter
cis a small letter
cis a capital letter
cis a decimal digit
cis a letter or a digit
cis a space letter
cis a printable letter
Macro Return value true means:
STANDARD MACROS FOR CHARACTER MANIPULATION
■129
The following section introduces macros that classify or convert single characters. The
macros are defined in the header files ctype.h and cctype.
䊐Case Conversion
You can use the macro toupper to convert lowercase letters to uppercase. If c1 and c2
are variables of type char or int where c1 contains the code for a lowercase letter, you
can use the following statement
Example: c2 = toupper(c1);
to assign the corresponding uppercase letter to the variable c2. However if c1 is not a
lowercase letter, toupper(c1) returns the character “as is.”
The sample program on the opposite page reads standard input, converts any letters
from lower- to uppercase, and displays the letters. As toupper only converts the letters
of the English alphabet by default, any national characters, such as accentuated charac-
ters in other languages, must be dealt with individually. A program of this type is known
as a filter and can be applied to files. Refer to the next section for details.
The macro tolower is available for converting uppercase letters to lowercase.
䊐Testing Characters
A number of macros, all of which begin with is..., are available for classifying charac-
ters. For example, the macro islower(c) checks whether ccontains a lowercase letter
returning the value true, in this case, and false in all other cases.
Example: char c; cin >> c; // Reads and
// classifies
if( !isdigit(c) ) // a character.
cout << "The character is no digit \n";
The following usage of islower() shows a possible definition of the toupper()
macro:
Example: #define toupper(c) \
(islower(c) ? ((c)-'a'+'A') : (c))
This example makes use of the fact that the codes of lower- and uppercase letters differ
by a constant, as is the case for all commonly used character sets such as ASCII and
EBCDIC.
The opposite page contains an overview of macros commonly used to classify char-
acters.
130 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
// lines.cpp
// A filter that numbers lines.
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
int main()
{
string line;
int number = 0;
while( getline( cin, line)) // As long as a line
{ // can be read.
cout << setw(5) << ++number << ": "
<< line << endl;
}
return 0;
}
■REDIRECTING STANDARD INPUT AND OUTPUT
Sample program
How to call the program
1. Redirecting the standard input:
lines < text.dat | more
This outputs the text file text.dat with line numbers. In addition, the data
stream is sent to the standard filter more, which pauses screen output if the page
is full.
2. Redirecting the standard output:
lines > new.dat
Here the program reads from the keyboard and adds the output to the new file
new.dat. Please note, if the file already exists, it will be overwritten!
You can use
lines >> text.dat
to append program output to the file text.dat. If the file text.dat does not
already exist, it will be created.
Type Ctrl+Z (DOS) or Ctrl+D (UNIX) to terminate keyboard input.
REDIRECTING STANDARD INPUT AND OUTPUT
■131
These examples assume that the compiled program lines.exe is either in the current directory or in
a directory defined in your system’s PATH variable.
✓NOTE
䊐Filter Programs
The previous program, toupper.cpp, reads characters from standard input, processes
them, and sends them to standard output. Programs of this type are known as filters.
In the program toupper.cpp, the loop
while( cin.get(c)) { ... }
is repeated while the test expression cin.get(c) yields the value true, that is, as long
as a valid character can be read for the variable c. The loop is terminated by end-of-file
or if an error occurs since the test expression cin.get(c) will then be false.
The program on the opposite page, lines.cpp, is also a filter that reads a text and
outputs the same text with line numbers. But in this case standard input is read line by
line.
while( getline(cin,line)) { ... }
The test expression getline(cin,line) is true while a line can be read.
䊐Using Filter Programs
Filter programs are extremely useful since various operating systems, such as DOS,
Win**, WinNT, and UNIX are capable of redirecting standard input and output. This
allows easy data manipulation.
For example, if you need to output text.dat with line numbers on screen, you can
execute the program lines by typing the following command:
Example: lines < text.dat
This syntax causes the program to read data from a file instead of from the keyboard. In
other words, the standard input is redirected.
The opposite page contains additional examples. You can redirect input and output
simultaneously:
Example: lines < text.dat > new.dat
In this example the contents of text.dat and the line numbers are stored in new.dat.
The program does not generate any screen output.
exercises
132 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
When a function key, such as F1, F2, ..., Ins, Del, etc. was pressed, the function
getch() initially returns 0. A second call yields the key number.
✓NOTE
■EXERCISES
Hints for Exercise 2
You can use the function kbhit() to test whether the user has pressed a key. If
so, the function getch() can be used to read the character.This avoids
interrupting the program when reading from the keyboard.
These functions have not been standardized by ANSI but are available on
almost every system. Both functions use operating system routines and are
declared in the header file conio.h.
The function kbhit()
Prototype:int kbhit();
Returns:0, if no key was pressed, otherwise != 0.
When a key has been pressed, the corresponding character can be read by
getch().
The function getch()
Prototype:int getch();
Returns: The character code.There is no special return value on reaching
end-of-file or if an error occurs.
In contrast to cin.get(),getch() does not use an input buffer when
reading characters, that is, when a character is entered, it is passed directly to
the program and not printed on screen. Additionally, control characters, such as
return ( = 13), Ctrl+Z ( = 26), and Esc ( = 27), are passed to the program “as is.”
Example: int c;
if( kbhit() != 0) // Key was pressed?
{
c = getch(); // Yes -> Get character
if( c == 27 ) // character == Esc?
// . . .
}
EXERCISES
■133
Since the program must not immediately output a single character following a control character, you will
need to store the predecessor of this character. You may want to use two counters to count the
number of characters and control characters in the current string.
✓NOTE
Exercise 1
Please write
a. the macro ABS, which returns the absolute value of a number,
b. the macro MAX, which determines the greater of two numbers.
In both cases use the conditional operator ?: .
Add these macros and other macros from this chapter to the header file
myMacros.h and then test the macros.
If your system supports screen control macros, also add some screen control
macros to the header. For example, you could write a macro named
COLOR(f,b) to define the foreground and background colors for the following
output.
Exercise 2
Modify the program ball1.cpp to
a. display a white ball on a blue background,
b. terminate the program when the Esc key is pressed,
c. increase the speed of the ball with the + key and decrease the speed
with the – key.
You will need the functions kbhit() and getch() (shown opposite) to solve
parts b and c of this problem.
Exercise 3
Write a filter program to display the text contained in any given file.The
program should filter any control characters out of the input with the exception
of the characters \n (end-of-line) and \t (tabulator), which are to be treated as
normal characters for the purpose of this exercise. Control characters are
defined by codes 0to 31.
A sequence of control characters is to be represented by a single space
character.
A single character, that is, a character appearing between two control
characters, is not to be output!
solutions
134 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// myMacros.h
// Header file contains the Macros
// ABS, MIN, MAX, CLS, LOCATE, COLOR, NORMAL, INVERS
// and symbolic constants for colors.
// ------------------------------------------------------
#ifndef _MYMACROS_
#define _MYMACROS_
#include <iostream>
using namespace std;
// ------------------------------------------------------
// Macro ABS
// Call: ABS( val)
// Returns the absolute value of val
#define ABS(a) ( (a) >= 0 ? (a) : -(a))
// ------------------------------------------------------
// Macro MIN
// Call: MIN(x,y)
// Returns the minimum of x and y
#define MIN(a,b) ( (a) <= (b) ? (a) : (b))
// ------------------------------------------------------
// Macro MAX
// Call: MAX(x,y)
// Returns the maximum of x and y
#define MAX(a,b) ( (a) >= (b) ? (a) : (b))
// ------------------------------------------------------
// Macros for controlling the screen
// ------------------------------------------------------
// Macro CLS
// Call: CLS;
// Clears the screen
#define CLS (cout << "\033[2J")
// ------------------------------------------------------
// Macro LOCATE
// Call: LOCATE(row, column);
// Positions the cursor to (row,column).
// (1,1) is the upper left corner.
#define LOCATE(r,c) (cout <<"\033["<< (r) <<';'<<(c)<<'H')
SOLUTIONS
■135
// ------------------------------------------------------
// Macro COLOR
// Call: COLOR(foreground, background);
// Sets the foreground and background color
// for the following output.
#define COLOR( f, b) (cout << "\033[1;3"<< (f) \
<<";4"<< (b) <<'m' << flush)
// 1: light foreground
// 3x: foreground x
// 4x: background x
// Color values for the macro COLOR
// To call ex.: COLOR( WHITE,BLUE);
#define BLACK 0
#define RED 1
#define GREEN 2
#define YELLOW 3
#define BLUE 4
#define MAGENTA 5
#define CYAN 6
#define WHITE 7
// ------------------------------------------------------
// Macro INVERS
// Call: INVERS;
// The following output is inverted.
#define INVERS (cout << "\033[7m")
// ------------------------------------------------------
// Macro NORMAL
// Call: NORMAL;
// Sets the screen attributes on default values.
#define NORMAL (cout << "\033[0m")
#endif // _MYMACROS_
Exercise 2
// ---------------------------------------------------
// ball2.cpp
// Simulates a bouncing ball
// ---------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
#include <conio.h> // For kbhit() and getch()
#include "myMacros.h"
136 ■
CHAPTER 7 SYMBOLIC CONSTANTS AND MACROS
#define ESC 27 // ESC terminates the program
unsigned long delay = 5000000; // Delay for output
int main()
{
int x = 2, y = 2, dx = 1, speed = 0;
bool end = false;
string floor(80, '-'),
header = "**** BOUNCING BALL ****",
commands = "[Esc] = Terminate "
"[+] = Speed up [-] = Slow down";
COLOR(WHITE,BLUE); CLS;
LOCATE(1,25); cout << header;
LOCATE(24,1); cout << floor;
LOCATE(25,10); cout << commands;
while( !end) // As long as the flag is not set
{
LOCATE(y,x); cout << 'o'; // Show the ball
for( long wait = 0; wait < delay; ++wait)
;
if(x == 1 || x == 79) dx = -dx; // Bounce off a wall?
if( y == 23 ) // On the floor?
{
speed = - speed;
if( speed == 0 ) speed = -7; // Kick
}
speed += 1; // Speed up = 1
LOCATE(y,x); cout << ' '; // Clear screen
y += speed; x += dx; // New position
if( kbhit() != 0 ) // Key pressed?
{
switch(getch()) // Yes
{
case '+': delay -= delay/5; // Speed up
break;
case '-': delay += delay/5; // Slow down
break;
case ESC: end = true; // Terminate
}
}
}
NORMAL; CLS;
return 0;
}
SOLUTIONS
■137
Exercise 3
// ---------------------------------------------------
// NoCtrl.cpp
// Filter to ignore control characters
// To call e.g.: NoCtrl < file
// ---------------------------------------------------
#include <iostream>
using namespace std;
#define isCtrl(c) ( c >= 0 && c <= 31 \
&& c != '\n' && c != '\t')
int main()
{
char c, prec = 0; // Character and predecessor
long nCtrl = 0, nChar = 0; // Number of the following
// control characters or
// other characters
while( cin.get(c))
{
if( isCtrl(c)) // Control characters
{
++nCtrl;
nChar = 0;
}
else // Normal character
{
if( nCtrl > 0)
{
cout.put(' ');
nCtrl = 0;
}
switch( ++nChar)
{
case 1: break;
case 2: cout.put(prec); // Predecessor and
default: cout.put(c); // current character
}
prec = c;
}
}
return 0;
}
This page intentionally left blank
139
Converting Arithmetic
Types
This chapter introduces implicit type conversions, which are performed in
C++ whenever different arithmetic types occur in expressions.
Additionally, an operator for explicit type conversion is introduced.
chapter
8
140 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
■IMPLICIT TYPE CONVERSIONS
Integer promotions
bool
short
char, signed char, unsigned char int
int
unsigned int
unsigned short
if int equals long
if int equals short
Type hierarchy
Example
short size(512); double res, x = 1.5;
res = size / 10 * x; // short -> int -> double
int
long double
double
float
unsigned long
long
unsigned int
int
not-existent, if int
equals long
IMPLICIT TYPE CONVERSIONS
■141
C++ allows you to mix arithmetic types in a single expression — in other words, the
operands of an operator can belong to different types. The compiler automatically per-
forms implicit type conversion, where a common type, which allows the operation in ques-
tion to be performed, is assigned for the values of both operands. You can generally
assume that the “smaller” type will be converted to the “larger” type. The assignment
operator is an exception to this rule and will be discussed separately.
The result of an arithmetic operation belongs to the common type used to perform
the calculation. However, comparison expressions will be bool types no matter what
type of operands are involved.
䊐Integer Promotion
Integer promotion is first performed for any expression:
■bool,char,signed char,unsigned char, and short are converted to
int
■unsigned short is also converted to int if the int type is greater than
short, and to unsigned int in all other cases.
This type conversion is performed so as to preserve the original values. The boolean
value false is converted to 0and true is converted to 1.
Thus, C++ will always use int type values or greater when performing calculations.
Given a char variable c, the values of cand 'a' in the expression
Example: c < 'a'
will be converted to int before being compared.
䊐Usual Arithmetic Type Conversions
If operands of different arithmetic types still occur after integer promotion, further
implicit type conversions along the lines of the hierarchy on the opposite page will be
necessary. In this case, the type of the operand with the highest rank in the hierarchy is
applied. These type conversions and integer promotions are collectively known as usual
arithmetic type conversions.
In our example, size/10 * x, the value of size is first promoted to int before an
integer division size/10 is performed. The interim result 50 is then converted to dou-
ble and multiplied by x.
Usual arithmetic type conversions are performed for all binary operators and the con-
ditional operator ?: provided the operands belong to an arithmetic type, the only excep-
tions being the assignment operator and the logical operators && and ||.
142 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
Sign bit(= 0 ↔ not negative)
Sign bit(= 0 ↔ not negative)
00001010
Extension to int (here 16 bit)
The value 10 is preserved.
000000000000 1010
26
214 213 28272120
252423222120
• • •• • • • •
Binary representaion of the integer 10
as value of type signed char (8 bits):
•
•
Sign bit(= 1 ↔ negative)
Sign bit(= 1 ↔ negative)
11110110
Extension to int (here 16 bit)
The value –10 is preserved.
111111111111 0110
26
214 213 28272120
252423222120
••••••••
Binary representaion of the integer –10
as value of type signed char (8 bits):
•
•
The value of a negative number changes if the bit pattern is interpreted as unsigned. The bit pattern
1111 0110 of –10, for example, corresponds to the unsigned char value
246 == 0*20+ 1*21+ 1*22+ 0*23+ 1*24+ 1*25+ 1*26+ 1*27
✓NOTE
■PERFORMING USUAL ARITHMETIC TYPE CONVERSIONS
Converting signed integers
a) Converting a positive number
b) Converting a negative number
The bit pattern of –10 is computed by starting with the bit pattern of 10 and generat-
ing the binary complement (see Binary Representation of Numbers in the appendix).
PERFORMING USUAL ARITHMETIC TYPE CONVERSIONS
■143
Usual arithmetic type conversions retain the value of a number provided it can be repre-
sented by the new type. The procedure for type conversion depends on the types
involved:
1. Conversion of an unsigned type to a larger integral type
Examples: unsigned char to int or unsigned int
Zero extension is performed first. During this process, the bit pattern of the num-
ber to be converted is expanded to match the length of the new type by adding
zeros from the left.
2. Conversion of a signed type to a larger integral type
■The new type is also signed
Examples: char to int,short to long
Signed integers are represented by generating the binary complement. The
value is retained by performing sign extension. As shown in the example on the
opposite page, the original bit pattern is expanded to match the length of the
new type by padding the sign bit from the left.
■The new type is unsigned
Examples: char to unsigned int,long to unsigned long
In this case the value of negative numbers is not retained. If the new type is of
the same length, the bit pattern is retained. However, the bit pattern will be
interpreted differently. The sign bit loses its significance (see the note oppo-
site).
If the new type is longer, sign extension is performed first and the new bit
pattern is then interpreted as unsigned.
3. Conversion of an integral type to a floating-point type
Examples: int to double,unsigned long to float
The number is converted to an exponential floating-point type and the value
retained. When converting from long or unsigned long to float, some
rounding may occur.
4. Conversion of a floating-point type to a larger floating-point type
Examples: float to double,double to long double
The value is retained during this type conversion.
144 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
■IMPLICIT TYPE CONVERSIONS IN ASSIGNMENTS
Example 1:
int i = 100;
long lg = i + 50; // Result of type int is
// converted to long.
Example 2:
long lg = 0x654321; short st;
st = lg; //0x4321 is assigned to st.
Example 3:
int i = –2; unsigned int ui = 2;
i = i * ui;
// First the value contained in i is converted to
// unsigned int (preserving the bit pattern) and
// multiplied by 2 (overflow!).
// While assigning the bit pattern the result
// is interpreted as an int value again,
// i.e. –4 is stored in i.
Example 4:
double db = –4.567;
int i; unsigned int ui;
i = db; // Assigning –4.
i = db – 0.5; // Assigning –5.
ui = db; // –4 is incompatible with ui.
Example 5:
double d = 1.23456789012345;
float f;
f = d; // 1.234568 is assigned to f.
IMPLICIT TYPE CONVERSIONS IN ASSIGNMENTS
■145
Arithmetic types can also be mixed in assignments. The compiler adjusts the type of the
value on the right of the assignment operator to match the type of the variable on the
left.
In the case of compound assignments, calculations using normal arithmetic type con-
versions are performed first before type conversion is performed following the rule for
simple assignments.
Two different cases can occur during type conversion in assignments:
1. If the type of the variable is larger than the type of the value to be assigned, the
type of the value must be promoted. The rules for usual arithmetic type conver-
sions are applied in this case (see Example 1).
2. If the type of the value to be assigned is larger, this type must be “demoted.” The
following procedures are followed depending on individual circumstances:
a. Conversion of an integral type to a smaller type:
■the type is converted to a smaller type by removing the most significant
byte(s). The bit pattern that remains will be interpreted as unsigned, if the
new type is also unsigned, and as signed in all other cases. The value can
only be retained if it can be represented by the new type (see Example 2).
■when converting an unsigned type to a signed type of the same scale,
the bit pattern is retained and will be interpreted as signed (see Example
3).
b. Conversion of a floating-point type to an integral type
The decimal part of the floating-point number is removed. For example, 1.9
converts to the integer 1. Rounding can be achieved by adding 0.5 to a posi-
tive floating-point number or subtracting 0.5 from a negative floating-point
number. This would allow for converting (1.9 + 0.5) to 2.
If the resulting integer is too large or too small for the new type, the result
is unpredictable. This particularly applies to converting negative floating-
point numbers to unsigned integers (see Example 4).
c. Conversion of a floating-point type to a smaller type
If the floating-point number falls within the range of the new type, the value
will be retained, although the accuracy may be compromised. If the value is
too large to be represented by the new type, the result is unpredictable (see
Example 5).
146 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
// Ellipse.cpp
// The program draws an ellipse.
// The points (x,y) on an ellipse with center (0,0)
// and axes A and B satisfy:
// x = A*cos(t), y = B*sint(t) for 0 <= t <= 2*PI .
//------------------------------------------------------
#include <iostream>
#include <cmath> // Prototypes of sin() and cos()
using namespace std;
#define CLS (cout << "\033[2J")
#define LOCATE(z,s) (cout <<"\033["<<(z)<<';'<<(s)<<'H')
#define DOT(x,y) (LOCATE(y,x) << '*')
#define PI 3.1416
#define Mx 40 // The point (Mx, My) is the
#define My 12 // center of the ellipse.
#define A 25 // Length of main axis
#define B 10 // Length of subsidiary axis
int main()
{
int x, y; // Screen coordinates.
CLS;
// 0 <= t <= PI/2 is a 1/4-circle:
for( double t = 0.0 ; t <= PI/2 ; t += 0.03)
{
x = (int) (A * cos(t) + 0.5);
y = (int) (B * sin(t) + 0.5);
DOT( x+Mx, y+My);
DOT( x+Mx,-y+My);
DOT(-x+Mx, y+My);
DOT(-x+Mx,-y+My);
}
LOCATE(24,0);
return 0;
}
■MORE TYPE CONVERSIONS
Sample program
MORE TYPE CONVERSIONS
■147
䊐Implicit Type Conversions in Function Calls
In the case of function calls, arguments with arithmetic types are converted to the types of
the corresponding parameters, similarly to conversions in assignments.
Example: void func( short, double); // Prototype
int size = 1000;
// . . .
func( size, 77); // Call
The function func() has two parameters belonging to the short and double types.
However, the function is called using two int arguments. This leads to implicit conver-
sion of the value of size to short and the integer 77 to double.
When an int is converted to short the compiler issues a warning, since some data
loss may occur. You can use explicit type conversion to avoid warnings during type con-
version.
䊐Explicit Type Conversion
It is possible to convert the type of an expression explicitly using the cast operator
(type).
Syntax: (type) expression
This converts the value of an expression to the given type. Explicit type conversion is
also known as casting.
The cast operator (type) is a unary operator and thus has a higher precedence than
the arithmetic operators.
Example: int a = 1, b = 4;
double x;
x = (double)a/b;
In this example the value of ais explicitly converted to a double. Following the con-
ventions of usual implicit type conversion, bis also converted to double and a floating-
point division is performed. The exact result, 0.25, is assigned to the variable x.
Without casting, an integer division with a result of 0would have occurred.
C++ has additional operators for explicit type conversion—the cast operator
dynamic_cast<>, for example. These operators, which are described in later chapters,
are required for special circumstances, for example, to perform type checking at runtime
when converting classes.
exercises
148 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
// Convert.cpp —> Demonstrates type conversions.
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
char v_char = 'A';
cout << "v_char: " << setw(10) << v_char
<< setw(10) << (int)v_char
<< endl;
short v_short = –2;
cout << "v_short: " << dec << setw(10) << v_short
<< hex << setw(10) << v_short
<< endl;
unsigned short v_ushort = v_short;
cout << "v_ushort: " << dec << setw(10) << v_ushort
<< hex << setw(10) << v_ushort
<< endl;
unsigned long v_ulong = v_short;
cout << "v_ulong: " << hex << setw(20) << v_ulong
<< endl;
float v_float = –1.99F;
cout << "v_float: " << setw(10) << v_float << endl;
cout << "(int)v_float: " << setw(10)
<< dec << (int)v_float << endl;
return 0;
}
■EXERCISES
Program listing for exercise 3
Graphic for exercise 4
------- The Sine Function -------
Ÿsin(x)
Ő
Ő1 *******
Ő*** ***
Ő** **
Ő**
Ő** **
Ő**
Ő** **
Ő** 2PIx
ņŐņņņņņņņ*ņņņņņņņŐņņņņņņņŐņņņņņņņŐņņņņņņņ*ņņņņņņņŐņņņņņņņŐņņņņņņņŐņņņņņņņ*ņņņņŹ
Ő**
Ő** **
Ő**
Ő** **
Ő**
Ő** **
Ő*** ***
Ő-1 *******
Ő
EXERCISES
■149
1. Plot one point of the curve in columns 10, 10+1, ..., 10+64 respectively. This leads to a
step value of 2*PI/64 for x.
2. Use the following extended ASCII code characters to draw the axes:
Example: cout << '\020'; // up arrowhead
✓NOTE
Character Decimal Octal
–
+
196
197
16
30
304
305
020
036
Exercise 1
A function has the following prototype
void func( unsigned int n);
What happens when the function is called with –1 as an argument?
Exercise 2
How often is the following loop executed?
unsigned int limit = 1000;
for (int i = –1; i < limit; i++)
// . . .
Exercise 3
What is output when the program opposite is executed?
Exercise 4
Write a C++ program to output the sine curve on screen as in the graphic
shown on the opposite page.
solutions
150 ■
CHAPTER 8 CONVERTING ARITHMETIC TYPES
■SOLUTIONS
Exercise 1
When called, the value –1 is converted to parameter n, i.e. to unsigned int.
The pattern of –1 is interpreted as unsigned, which yields the greatest unsigned
value.
On a 32-bit system, –1 has the bit pattern 0xFFFFFFFF, which, when
interpreted as unsigned, corresponds to the decimal value 4 294 967 295.
Exercise 2
The statement within the loop is not executed at all! In the expression
i < limit
the value of variable i, –1, is implicitly converted to unsigned int and thus it
represents the greatest unsigned value (see Exercise 1).
Exercise 3
The screen output of the program
v_char: A 65
v_short: -2 fffe
v_ushort: 65534 fffe
v_ulong: fffffffe
v_float: -1.99
(int)v_float: -1
Exercise 4
// -----------------------------------------------------
// sinCurve.cpp
// Outputs a sine curve
// -----------------------------------------------------
#include <iostream>
#include <cmath> // Prototypes of sin()
using namespace std;
#define CLS (cout << "\033[2J")
#define LOCATE(z,s) (cout <<"\033["<<(z)<<';'<<(s)<<'H')
#define PI 3.1415926536
#define START 0.0 // Lower limit
#define END (2.0 * PI) // Upper limit
SOLUTIONS
■151
#define PNT 64 // Number of points on the curve
#define STEP ((END-START)/PNT)
#define xA 14 // Row of x-axis
#define yA 10 // Column of y-axis
int main()
{
int row, column;
CLS;
LOCATE(2,25);
cout << "------- The Sine Function -------";
// --- Draws the coordinate system: ---
LOCATE(xA,1); // x-axis
for( column = 1 ; column < 78 ; ++column)
{
cout << ((column - yA) % 8 ? '\304' : '\305');
}
cout << '\020'; // top
LOCATE(xA-1, yA+64); cout << "2PI x";
for( row = 5 ; row < 24 ; ++row) // y-axis
{
LOCATE(row, yA); cout << '\305';
}
LOCATE( 4, yA); cout << "\036 sin(x)"; // top
LOCATE( xA-8, yA+1); cout << " 1";
LOCATE( xA+8, yA+1); cout << " -1";
// --- Displays the sine function: ---
int begpt = yA,
endpt = begpt + PNT;
for( column = begpt ; column <= endpt ; ++column)
{
double x = (column-yA) * STEP;
row = (int)(xA - 8 * sin(x) + 0.5);
LOCATE( row, column); cout << '*';
}
LOCATE(25,1); // Cursor to the last row
return 0;
}
This page intentionally left blank
153
The Standard Class
string
This chapter introduces the standard class string, which is used to
represent strings. Besides defining strings we will also look at the various
methods of string manipulation.These include inserting and erasing,
searching and replacing, comparing, and concatenating strings.
chapter
9
154 ■
CHAPTER 9 THE STANDARD CLASS STRING
'G' 'o' 'o' 'o' 'r' 'n' 'n' '!'g''i''d' ' ' 'M'
13
String
literal:
Length:
String message in memory:
// string1.cpp: Using strings
#include <iostream>
#include <string>
using namespace std;
string prompt("Enter a line of text: "), // Global
line( 50, '*'); // strings
int main()
{
string text; // Empty string
cout << line << endl << prompt << endl;
getline( cin, text); // Reads a line of text
cout << line << endl
<< "Your text is " << text.size()
<< " characters long!" << endl;
// Two new strings:
string copy(text), // a copy and the
start(text,0,10); // first 10 characters
// starting with
// position 0.
cout << "Your text:\n" << copy << endl;
text = "1234567890"; // Assignment
cout << line << endl
<< "The first 10 characters:\n" << start << endl
<< text << endl;
return 0;
}
■DEFINING AND ASSIGNING STRINGS
Initializing
string message = "Good Morning!";
Sample program
Objects of class string do not necessarily contain the string terminating character '\0', as is the case
with C strings.
✓NOTE
DEFINING AND ASSIGNING STRINGS
■155
C++ uses the standard class string to represent and manipulate strings allowing for
comfortable and safe string handling. During string operations the required memory
space is automatically reserved or modified. The programmer does not need to concern
himself or herself with internal memory allocation.
The string class is defined in the string header file and was mentioned in Chap-
ter 3 as an example for the use of classes. Several operators are overloaded for strings,
that is, they were also defined for the string class. This allows for easy copying, con-
catenation, and comparison. Additionally, various methods for string manipulation such
as insertion, erasing, searching, and replacing are available.
䊐Initializing Strings
A string, that is, an object belonging to the string class, can be initialized when you
define it using
■a predefined string constant
■a certain number of characters
■a predefined string or part of a string.
If a string is not initialized explicitly, an empty string with a length of 0is created.
The length of a string, that is, the current number of characters in the string, is stored
internally and can be accessed using the length() method or its equivalent size().
Example: string message("Good morning!");
cout << message.length(); // Output: 13
䊐String Assignments
When you assign a value to a string, the current contents are replaced by a new character
sequence. You can assign the following to a string object:
■another string
■a string constant or
■a single character.
The memory space required is adjusted automatically.
The program on the opposite page uses the function getline(), which was intro-
duced in an earlier chapter, to store a line of text from the keyboard in a string. In con-
trast, the >> operator reads only one word, ignoring any leading white space. In both
cases the original content of the string is lost.
156 ■
CHAPTER 9 THE STANDARD CLASS STRING
// string2.cpp: Reads several lines of text and
// outputs in reverse order.
#include <iostream>
#include <string>
using namespace std;
string prompt("Please enter some text!\n"),
line( 50, '-');
int main()
{
prompt+="Terminate the input with an empty line.\n ";
cout << line << '\n' << prompt << line << endl;
string text, line; // Empty strings
while( true)
{
getline( cin, line); // Reads a line of text
if( line.length() == 0) // Empty line?
break; // Yes ->end of the loop
text = line + '\n' + text; // Inserts a new
// line at the beginning.
}
// Output:
cout << line << '\n'
<< "Your lines of text in reverse order:"
<< '\n' << line << endl;
cout << text << endl;
return 0;
}
■CONCATENATING STRINGS
Sample program
Sample output for this program
---------------------------------------
Please enter some text!
Terminate the input with an empty line.
---------------------------------------
Babara, Bobby, and Susan
will go to the movies today
---------------------------------------
Your lines of text in reverse order:
---------------------------------------
will go to the movies today
Babara, Bobby, and Susan
CONCATENATING STRINGS
■157
At least one operand must be a string class object. The expression "Good morning " +
"mister X" would be invalid!
✓NOTE
Within the string class the operators +and += are defined for concatenating, and the
operators ==,!=,<,<=,>, and >= are defined for comparing strings. Although these
operators are being applied to strings, the well-known rules apply: the +has precedence
over the comparative operators, and these in turn have higher precedence than the
assignment operators =and +=.
䊐Using +to Concatenate Strings
You can use the +operator to concatenate strings, that is, to join those strings together.
Example: string sum, s1("sun"), s2("flower");
sum = s2 + s3;
This example concatenates the strings s1 and s2. The result, "sunflower" is then
assigned to sum.
Two strings concatenated using the +operator will form an expression of the string
type. This expression can in turn be used as an operand in a more complex expression.
Example: string s1("sun"),s2("flower"),s3("seed");
cout << s1 + s2 + s3;
Since the +operator has precedence over the << operator, the strings are concatenated
before the “sum” is output. Concatenation takes place from left to right. String constants
and single characters are also valid as operands in expressions containing strings:
Example: string s("Good morning ");
cout << s + "mister X" + '!';
䊐Using += to Concatenate Strings
Strings can also be concatenated by first performing concatenation and then assigning
the result.
Example: string s1("Good "),s2("luck!");
s1 = s1 + s2; // To concatenate s2 and s1
This example creates a temporary object as a result of s1 + s2 and then assigns the
result to s1. However, you can obtain the same result using the assignment operator += ,
which is far more efficient.
Example: s1 += s2; // To concatenate s2 and s1.
s1 += "luck!"; // Also possible
This adds the content of the second string directly to s1. Thus, the += operator is prefer-
able to a combination of the +and =operators.
158 ■
CHAPTER 9 THE STANDARD CLASS STRING
// string3.cpp: Inputs and compares lines of text.
#include <iostream>
#include <string>
using namespace std;
string prompt = "Please enter two lines of text!\n",
line( 30, '-');
int main()
{
string line1, line2, key = "y";
while( key == "y" || key == "Y")
{
cout << line << '\n' << prompt << line << endl;
getline( cin, line1); // Read the first
getline( cin, line2); // and second line.
if( line1 == line2)
cout << " Both lines are the same!" << endl;
else
{
cout << "The smaller line is:\n\t";
cout << (line1 < line2 ? line1 : line2)
<< endl;
int len1 = line1.length(),
len2 = line2.length();
if( len1 == len2)
cout << "Both lines have the same length! \n";
else
{ cout << "The shorter line is:\n\t";
cout << (len1 < len2 ? line1 : line2)
<< endl;
}
}
cout << "\nRepeat? (y/n) ";
do
getline( cin, key);
while( key != "y" && key != "Y"
&& key != "n" && key != "N");
}
return 0;
}
The relational operators yield the desired result for strings only if at least one operand is an object of
class string. See Chapter 17, Pointers and Arrays, for more information.
✓NOTE
■COMPARING STRINGS
Sample program
COMPARING STRINGS
■159
The comparative operators
== != < <= > >=
were overloaded in the string class to allow easy comparison of strings. This also
allows you to use strings to formulate the conditions for branches and loops.
Example: // str1 and str2 are objects of type string
if( str1 < str2) // str1 is less than str2?
. . .
䊐Results of Comparisons
Strings are compared lexicographically, that is character by character, beginning at the
first character. To decide whether a single character is smaller, greater, or identical to
another character, the character codes of the character set are compared. Thus, if you are
using the ASCII character set, the letter 'A' (ASCII code 65) is smaller than the letter
'a' (ASCII code 97).
A comparison results in a bool type value. Given two strings s1 and s2:
s1 == s2 is true only if both strings are identical; this requires that both strings
are exactly the same length.
s1 < s2 is true only if the first character in s1 that differs from the correspon-
ding character in s2 is smaller than the corresponding character in s2,
or if s2 is simply an extension of s1.
All other comparative operations can be deduced from the above rules. For example, the
expression s1 > s2 is true only if s2 < s1 is also true.
In an expression comparing strings, one operand can again be a string constant or a
single character.
Example: while( key == 'y' ) { . . . }
This example compares the string key with the single character 'y'. This is an alterna-
tive method of expressing the comparison key == "y".
String comparisons can also be combined to form more complex expressions.
Example: while( key == "y" || key == "Y")
{ . . . }
The controlling expression is valid if the string key contains only the letter 'Y' or 'y'.
Due to the higher precedence of the comparative operator versus the || operator, no
parentheses are required in this example.
160 ■
CHAPTER 9 THE STANDARD CLASS STRING
Position:
String s1
012345678910
'M' 'i' 's' 's' ' ' 'S' 'u' 'e' 'r''m' 'm'
'A' 's' 'h' 'l' 'e' 'y' ' '
Position:
String s
before
String s
afterwards
01 2 3 4 5 67 8 910
'T' 'h' 'e' ' ' 's' 'u' 'm' 'm' 'e' 'r' '-' 't' 'i' 'm' 'e'
'T' 'h' 'e' 'i' 'e''t' 'm'' '
11 12 13 14
■INSERTING AND ERASING IN STRINGS
䊐Inserting a string
string s1("Miss Summer");
s1.insert(5, "Ashley "); // Insert at position: 5
Effect of the statement:
Erasing a substring
string s("The summer-time");
s.erase(4,7); // Start position: 4, Quantity: 7
Effect of the statement:
INSERTING AND ERASING IN STRINGS
■161
The string class contains numerous methods for performing string manipulations. A
method exists for each operation, such as inserting, erasing, searching, and replacing.
These methods generally allow passing a string constant instead of a second string. A sin-
gle character can also be used wherever appropriate.
䊐Insertion
The method insert() inserts a string at a certain position of another string. The posi-
tion is passed as the first argument and defines the character before which to insert the
string. The first character in a string occupies position 0, the second character position 1,
and so on.
Example: string s1("Miss Summer");
s1.insert(5, "Ashley ");
The string "Ashley " is inserted into the string s1 at position 5, that is in front of the
'S' character in "Summer". Following this, the string "Miss Ashley Summer" is
assigned to s1.
If you need to insert only part of a string into another string, you can pass two addi-
tional arguments to the insert() method, the starting position and the length of the
string.
Example: string s1("Ashley is a devil"),
s2(" sweetheart");
s1.insert(12, s2, 0, 12);
This example inserts the first 12 characters from the string s2 at position 13 in string s1.
String s1 then contains the string “Ashley is a sweetheart".
䊐Erasing
You can use the erase() method to delete a given number of characters from a string.
The starting position is supplied as the first argument and the number of characters to be
erased is the second argument.
Example: string s("The summer-time");
s.erase(4,6); // Result: "The time"
This statement deletes 7 characters from string sstarting at position 4. The erase()
method can also be called without specifying a length and will then delete all the charac-
ters in the string up to the end of the string.
Example: string s("winter-story");
s.erase(6); // s now contains "winter"
You can also call erase() without any arguments to delete all the characters in a
string.
162 ■
CHAPTER 9 THE STANDARD CLASS STRING
■SEARCHING AND REPLACING IN STRINGS
䊐Replacing substrings
a. Example “Bob and Bill”
string s1("There they go again!"),
s2("Bob and Bill");
s1.replace(6, 4, s2);
Effect of the statement:
b. Example “my love”
string s1("Here comes Mike!"), s2("my love?");
s1.replace(11, 4, s2, 0, 7);
Effect of the statement:
012345678910111213141516171819
s1
s2
'T'
'B' 'B''o' 'b' 'a' 'i' 'l' 'l''d''n'' ' ' '
'h' 'h''e' 'r' 'e' 'e' 'y' 'o' 'a' 'i' 'n' '!''a''g' 'g'' ' ' ' '''t'
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
s1
s2
'H' 'e' 'r' 'e' ' ' 'c' 'o' 'm' 'e' 's' ' ' 'M' 'i' 'k' 'e' '!'
'm' 'y' ' ' 'l' 'o' 'v' 'e' '?'
SEARCHING AND REPLACING IN STRINGS
■163
䊐Searching
You can search strings to find the first or last instance of a substring. If the string con-
tains the required substring, the position of the substring found by the search is returned.
If not, a pseudo-position npos, or –1, is returned. Since the npos constant is defined in
the string class, you can reference it as string::npos.
The find() method returns the position at which a substring was first found in the
string. The method requires the substring to be located as an argument.
Example: string youth("Bill is so young, so young");
int first = youth.find("young");
The variable first has a value of 11 in this example.
You can use the “right find” method rfind() to locate the last occurrence of a sub-
string in a string. This initializes the variable last with a value of 21 in our example.
Example: int last = youth.rfind("young");
䊐Replacing
When replacing in strings, a string overwrites a substring. The string lengths need not be
identical.
You can use the replace() method to perform this operation. The first two argu-
ments supply the starting position and the length of the substring to be replaced. The
third argument contains the replacement string.
Example: string s1("There they go again!"),
s2("Bob and Bill");
int pos = s1.find("they"); // pos == 6
if( pos != string::npos )
s1.replace(pos, 2, s2);
This example uses the string s2 to replace 4 characters, "they", starting at position 6 in
s1. After this operation s1 contains the string "There Bob and Bill go
again!".
If you only need to insert part of a string, you can use the fourth argument to define
the starting position and the fifth to define the length of the substring.
Example: string s1("Here comes Mike!"),
s2("my love?");
s1.replace(11, 4, s2, 0, 7);
The string s1 is changed to "Here comes my love!".
164 ■
CHAPTER 9 THE STANDARD CLASS STRING
// string4.cpp
// The program counts words and white space characters.
// (A word is the maximum sequence of characters
// containing no white space characters.)
//---------------------------------------------------------------
#include <iostream>
#include <string>
#include <cctype> // Macro isspace()
using namespace std;
int main()
{
string header(" **** Counts words ****\n"),
prompt("Enter a text and terminate"
" with a period and return:"),
line( 60, '-'),
text; // Empty string
cout << header << endl << prompt << endl
<< line << endl;
getline( cin, text, '.'); // Reads a text up to
// the first '.'
// Counts words and white space characters
int i, // Index
nSpace = 0, // Number of white spaces
nWord = 0; // Number of words
bool fSpace = true; // Flag for white space
for( i = 0; i < text.length(); ++i)
{
if( isspace( text[i]) ) // white space?
{
++nSpace; fSpace = true;
}
else if( fSpace) // At the beginning of a word?
{
++nWord; fSpace = false;
}
}
cout << line // Outputs the result.
<< "\nYour text contains (without periods)"
<< "\n characters: " << text.length()
<< "\n words: " << nWord
<< "\n white spaces: " << nSpace
<< endl;
return 0;
}
■ACCESSING CHARACTERS IN STRINGS
Sample program
ACCESSING CHARACTERS IN STRINGS
■165
When manipulating strings it is often important to access the individual characters that
form the string. C++ has the operator [] and the method at() for this purpose. An
individual character is always identified by its index, also referred to as subscript, that is,
its position in the string. The first character will always have an index value of 0, the
second an index of 1, and so on.
䊐Subscript Operator
The easiest way to access a single character in the string is to use the subscript operator
[]. If you define a string as follows,
Example: string s = "Let";
the individual characters in the string are:
s[0] == 'L', s[1] == 'e', s[2] == 't'
The last character in a string always has an index of s.length() – 1. You can use
the subscript operator to read any character in a string and also to overwrite a character,
provided the string was not defined as a constant.
Example: char c = s[0];
This statement copies the first character from sto the variable c. In contrast
Example: s[s.length() –1] = 'g';
overwrites the last character in the string s. Following this, swill contain the string
"Leg".
䊐Invalid Indices
Any integral expression can be used as an index. However, no error message occurs if the
boundaries of a valid index are overstepped.
Example: cout << s[5]; // Error
Your program’s reaction to an invalid index is undefined; this requires careful atten-
tion by the programmer! You can call the at() method if you need to perform range
checks.
䊐The at() method
You can also use the at()method to access a single character.
Example: s.at(i) = 'X'; is equivalent to s[i] = 'X';
In contrast to the subscript operator, the at() method performs range checking. If an
invalid index is found an exception occurs and the program will normally be terminated at
this point. However, you can specify how a program should react to an exception.
exercises
166 ■
CHAPTER 9 THE STANDARD CLASS STRING
// timeStr.cpp
// Demonstrates operations on a string containing
// the present time.
#include <iostream>
#include <string>
#include <ctime> // For time(), ctime(), ...
using namespace std;
int main()
{
long sec;
time( &sec); // Reads the present time
// (in seconds) into sec.
string tm = ctime( &sec); // Converts the
// seconds to a string.
cout << "Date and time: " << tm << endl;
string hr(tm, 11, 2); // Substring of tm starting at
// position 11, 2 characters long.
string greeting("Have a wonderful ");
if( hr < "10") // Compares strings
greeting += "Morning!";
else if( hr < "17")
greeting += "Day!";
else
greeting += "Evening!";
cout << greeting << endl;
return 0;
}
■EXERCISES
For exercise 3
EXERCISES
■167
The function time() returns the current time as the number of seconds since 1/1/1970, 0:0. The
number of seconds is stored in the variable sec, whose address was supplied as &sec when the
function was called.
The function ctime() converts the number of seconds to a string with a date and time and returns
this string. The string comprises exactly 26 characters including the null character \0 and has the
following format:
Weekday Month Day Hr:Min:Sec Year\n\0
Example: Wed Jan 05 02:03:55 2000\n\0
✓NOTE
Exercise 1
Write a C++ program to
■initialize a string s1 with the string "As time by ..." and a second
string s2 with the string "goes",
■insert string s2 in front of "by" in string s1,
■erase the remainder of string s1 after the substring "by",
■replace the substring "time" in s1 with "Bill".
In each case, your program should determine the position of the substring.
Output string s1 on screen at the beginning of the program and after every
modification.
Exercise 2
Write a C++ program that reads a word from the keyboard, stores it in a string,
and checks whether the word is a palindrome.A palindrome reads the same
from left to right as from right to left.The following are examples of
palindromes:“OTTO, ” “deed, ” and “level.”
Use the subscript operator []. Modify the program to continually read and
check words.
Exercise 3
Write down the screen output for the program on the opposite page.
solutions
168 ■
CHAPTER 9 THE STANDARD CLASS STRING
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// strDemo.cpp: Insert, search, and replace in strings.
// ------------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
string header = "Demonstrating the use of strings\n",
s1 = "As time by ...",
s2 = "goes ";
int main()
{
int pos = 0;
cout << header << endl;
cout << "s1 : " << s1 << endl;
// To insert:
cout << "\nInserting in string \"" << s2 <<"\""<< endl;
pos = s1.find("by");
if( pos != string::npos )
s1.insert(pos,s2);
cout << "s1 : " << s1 << endl; // Result
// To erase:
cout << "\nTo erase remaining characters behind \"by\":"
<< endl;
pos = s1.find("by");
if( pos != string::npos )
s1.erase(pos + 3);
cout << "s1 : " << s1 << endl; // Result
// To replace:
cout << "\nTo replace \"time\" by \"Bill\":"
<< endl;
pos = s1.find("time");
if( pos != string::npos )
s1.replace(pos, 4, "Bill");
cout << "s1 : " << s1 << endl; // Result
return 0;
}
SOLUTIONS
■169
Exercise 2
// -----------------------------------------------------
// palindrome.cpp: Reads and compares lines of text.
// -----------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
string header = " * * * Testing palindromes * * * ",
prompt = "Enter a word: ",
line( 50, '-');
int main()
{
string word; // Empty string
char key = 'y';
cout << "\n\t" << header << endl;
while( key == 'y' || key == 'Y')
{
cout << '\n' << line << '\n'
<< prompt;
cin >> word;
// Compares the first and last character,
// the second and the second to last etc.
int i = 0, j = word.length() - 1;
for( ; i <= j ; ++i, --j)
if( word[i] != word[j] )
break;
if( i > j) // All characters equal?
cout << "\nThe word " << word
<< " is a P A L I N D R O M E !" << endl;
else
cout << "\nThe word " << word
<< " is not a palindrome" << endl;
cout << "\nRepeat? (y/n) ";
do
cin.get(key);
while( key != 'y' && key != 'Y'
&& key != 'n' && key != 'N');
cin.sync();
}
return 0;
}
170 ■
CHAPTER 9 THE STANDARD CLASS STRING
Exercise 3
The program outputs the date and time first.Then a greeting is printed
according the time of day. For example:
Date and time: Thu Nov 28 09:01:37 2001
Have a wonderful morning!
171
Functions
This chapter describes how to write functions of your own. Besides the
basic rules, the following topics are discussed:
■passing arguments
■definition of inline functions
■overloading functions and default arguments
■the principle of recursion.
chapter
10
172 ■
CHAPTER 10 FUNCTIONS
C++ program
Core elements of
C++
(built-in types,
operators,
control structures)
Functions and
classes of the
standard library
Self-defined
functions and
classes and
other libraries
■SIGNIFICANCE OF FUNCTIONS IN C++
Elements of a C++ program
SIGNIFICANCE OF FUNCTIONS IN C++
■173
C++ supports efficient software development on the lines of the top-down principle. If
you are looking to provide a solution for a more complex problem, it will help to divide
the problem into smaller units. After identifying objects you will need to define classes
that describe these objects. You can use available classes and functions to do so. In addi-
tion, you can make use of inheritance to create specialized classes without needing to
change any existing classes.
When implementing a class you must define the capacities of those objects, that is,
the member functions, in your program. However, not every function is a member func-
tion.
Functions can be defined globally, such as the function main() for example. Func-
tions of this type do not belong to any particular class but normally represent algorithms
of a more general nature, such as the search or sort functions of the standard library.
䊐Libraries
You will not need to program each “building block” yourself. Many useful global func-
tions and classes are available from the C++ standard library. In addition, you can use
other libraries for special purposes. Often a compiler package will offer commercial class
libraries or graphical user interfaces. Thus, a C++ program will be made up of
■language elements of the C++ core
■global functions and classes from the C++ standard library
■functions and classes you have programmed yourself and other libraries.
Classes and functions that belong together are normally compounded to form separate
source files, which can be compiled and tested independently. Using software compo-
nents that you have already tested makes programming a complex solution much easier
and improves the reliability of your programs. You can enhance the reusability of your
source code by compiling your own libraries, but be sure to include comments for ease of
readability.
Compiled source files, also known as modules, are compounded by the linker to an
executable file by reference to the libraries you include. If you modify a source file, you
may also need to recompile other files. In large scale projects it is recommended to use
the MAKE utility for module management. An integrated developer environment will
offer the functionality of this utility when you create a new project. This includes your
own source files, the libraries used, and the compiler/linker settings for program com-
pilation.
174 ■
CHAPTER 10 FUNCTIONS
// func1.cpp
#include <iostream>
using namespace std;
void test( int, double ); // Prototype
int main()
{
cout << "\nNow function test() will be called.\n";
test( 10, -7.5); // Call
cout << "\nAnd back again in main()." << endl;
return 0;
}
void test(int arg1, double arg2 ) // Definition
{
cout << "\nIn function test()."
<< "\n 1. argument: " << arg1
<< "\n 2. argument: " << arg2 << endl;
}
[type] name([declaration_list]) // Function header
{ // Beginning
.
.
What will be done // Function block
.
.
} // End
■DEFINING FUNCTIONS
Example of a function definition
General form of a function
DEFINING FUNCTIONS
■175
The following section describes how to program global functions. Chapter 13, Defining
Classes, describes the steps for defining member functions.
䊐Definition
Functions can be defined in any order, however, the first function is normally main.
This makes the program easier to understand, since you start reading at the point where
the program starts to execute.
The function test() is shown opposite as an example and followed by the general
form of a function. The example can be read as follows:
type is the function type, that is, the type of the return value.
name is the function name, which is formed like a variable name
and should indicate the purpose of the function.
declaration_list contains the names of the parameters and declares their
types. The list can be empty, as for the function main(),
for example. A list of declarations that contains only the
word void is equivalent to an empty list.
The parameters declared in a list are no more than local variables. They are created
when the function is called and initialized by the values of the arguments.
Example: When test( 10, -7.5); is called, the parameter
arg1 is initialized with a value of 10 and arg2 with -7.5.
The left curved bracket indicates the start of a function block, which contains the state-
ments defining what the function does.
䊐Prototype and Definition
In a function definition the function header is similar in form to the prototype of a func-
tion. The only difference when a function is defined is that the name and declaration list
are not followed by a semicolon but by a function code block.
The prototype is the declaration of the function and thus describes only the formal
interface of that function. This means you can omit parameter names from the proto-
type, whereas compiling a function definition will produce machine code.
176 ■
CHAPTER 10 FUNCTIONS
// area.cpp
// Example for a simple function returning a value.
//-----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
double area(double, double); // Prototype
int main()
{
double x = 3.5, y = 7.2, res;
res = area( x, y+1); // Call
// To output to two decimal places:
cout << fixed << setprecision(2);
cout << "\n The area of a rectangle "
<< "\n with width " << setw(5) << x
<< "\n and length " << setw(5) << y+1
<< "\n is " << setw(5) << res
<< endl;
return 0;
}
// Defining the function area():
// Computes the area of a rectangle.
double area( double width, double len)
{
return (width * len); // Returns the result.
}
■RETURN VALUE OF FUNCTIONS
Defining and calling the function area()
Screen output:
The area of a rectangle
with width 3.50
and length 8.20
is 28.70
RETURN VALUE OF FUNCTIONS
■177
The program opposite shows how the function area() is defined and called. As previ-
ously mentioned, you must declare a function before calling it. The prototype provides
the compiler with all the information it needs to perform the following actions when a
function is called:
■check the number and type of the arguments
■correctly process the return value of the function.
A function declaration can be omitted only if the function is defined within the same
source file immediately before it is called. Even though simple examples often define and
call a function within a single source file, this tends to be an exception. Normally the
compiler will not see a function definition as it is stored in a different source file.
When a function is called, an argument of the same type as the parameter must be
passed to the function for each parameter. The arguments can be any kind of expressions,
as the example opposite with the argument y+1 shows. The value of the expression is
always copied to the corresponding parameter.
䊐Return Statement
When the program flow reaches a return statement or the end of a function code block, it
branches back to the function that called it. If the function is any type other than void,
the return statement will also cause the function to return a value to the function that
called it.
Syntax: return [expression]
If expression is supplied, the value of the expression will be the return value. If the
type of this value does not correspond to the function type, the function type is con-
verted, where possible. However, functions should always be written with the return
value matching the function type.
The function area() makes use of the fact that the return statement can contain
any expression. The return expression is normally placed in parentheses if it contains
operators.
If the expression in the return statement, or the return statement itself, is miss-
ing, the return value of the function is undefined and the function type must be void.
Functions of the void type, such as the standard function srand(), will perform an
action but not return any value.
178 ■
CHAPTER 10 FUNCTIONS
On call
“push”
On return
“pop”
Stack
further local objects
return address
first parameter
last parameter
• • •
• • •
■PASSING ARGUMENTS
Calling function and called function
Stack content after calling a function
long func2(int, double); // Prototype
// . . .
void func1()
{
int x = 1.1;
double y;
. . .
long a = func2(x,y); // Call of func2().
. . .
} // Pass by value
long func2(int a, double b) // Definition
{
double x = 2.2;
long result;
. // Here the result
. // is computed.
.
return result;
}
PASSING ARGUMENTS
■179
䊐Passing by Value
Passing values to a function when the function is called is referred to as passing by value.
Of course the called function cannot change the values of the arguments in the calling
function, as it uses copies of the arguments.
However, function arguments can also be passed by reference. In this case, the function
is passed a reference to an object as an argument and can therefore access the object
directly and modify it.
An example of passing by reference was provided in the example containing the func-
tion time(). When time(&sek); is called, the address of the variable sek is passed
as an argument, allowing the function to store the result in the variable. We will see how
to create functions of this type later.
Passing by value does, however, offer some important advantages:
■function arguments can be any kind of expression, even constants, for example
■the called function cannot cause accidental modifications of the arguments in
the calling function
■the parameters are available as suitable variables within the functions. Additional
indirect memory access is unnecessary.
However, the fact that copying larger objects is difficult can be a major disadvantage,
and for this reason vectors are passed by reference to their starting address.
䊐Local Objects
The scope of function parameters and the objects defined within a function applies only
to the function block. That is, they are valid within the function only and not related to
any objects or parameters of the same name in any other functions.
For example, the program structure opposite contains a variable ain the function
func1() and in the function func2(). The variables do not collide because they refer-
ence different memory addresses. This also applies to the variables xin func1() and
func2().
A function’s local objects are placed on the stack—the parameters of the function are
placed first and in reverse order. The stack is an area of memory that is managed accord-
ing to the LIFO (last in first out) principle. A stack of plates is a good analogy. The last
plate you put on the stack has to be taken off first. The LIFO principle ensures that the
last local object to be created is destroyed first.
180 ■
CHAPTER 10 FUNCTIONS
Program Function
Branching
// 1st Call
// 2nd Call
func();
void func()
func();
{
}
Program Inline function
// 1st Call
// 2nd Call
func(); inline void func()
func();
{
}
Copy
The executable file only contains one instance of the function’s machine code.
✓HINT
The machine code of the function is stored in the executable file wherever the function is called.
✓HINT
■INLINE FUNCTIONS
Call to a function not defined as inline
Call to an inline function
INLINE FUNCTIONS
■181
䊐Jumping to Sub-Routines
When a function is called, the program jumps to a sub-routine, which is executed as fol-
lows:
■the function parameters are placed on the stack and initialized with appropriate
arguments
■the so-called return address, that is, the place where the function was called, is
stored on the stack and the program flow branches to the function
■after executing the function the program uses the return address it stored previ-
ously to return to the calling function. The part of the stack occupied by the
function is then released.
All this jumping back and forth can affect the run time of your program, especially if the
function contains only a few instructions and is called quite often. The time taken to
branch to a small function can be greater than the time needed to execute the function
itself. However, you can define inline functions to avoid this problem.
䊐Inline Definition
The compiler inserts the code of an inline function at the address where the function is
called and thus avoids jumping to a sub-routine. The definition of an inline function is
introduced by the inline keyword in the function header.
Example: inline int max( int x, int y)
{ return (x >= y ? x : y ); }
The program code will expand each time an inline function is called. This is why
inline functions should contain no more than one or two instructions. If an inline
function contains too many instructions, the compiler may ignore the inline keyword
and issue a warning.
An inline function must be defined in the source file in which it is called. You can-
not simply supply a prototype of the function. The code containing the instructions must
also be available to the compiler. It therefore makes sense to define inline functions in
header files, in contrast to “normal” functions. This means the function will be available
in several source files.
䊐Inline Functions and Macros
Inline functions are an alternative to macros with parameters. When a macro is called,
the preprocessor simply replaces a block of text. In contrast, an inline function
behaves like a normal function, although the program flow is not interrupted by the
function branching. The compiler performs a type check, for example.
182 ■
CHAPTER 10 FUNCTIONS
// Computes the final capital with interest and
// compound interest.
// Formula: capital = k0 * (1.0 + p/100)n
// where k0 = start capital, p = rate, n = run time
// ----------------------------------------------------
#include <math.h>
double capital( double k0, double p, double n)
{
return (k0 * pow(1.0+p/100, n));
}
// Function capital() with two default arguments
// Prototype:
double capital( double k0, double p=3.5, double n=1.0);
double endcap;
endcap = capital( 100.0, 3.5, 2.5); // ok
endcap = capital( 2222.20, 4.8); // ok
endcap = capital( 3030.00); // ok
endcap = capital( ); // not ok
// The first argument has no default value.
endcap = capital( 100.0, , 3.0); // not ok
// No gap!
endcap = capital( , 5.0); // not ok
// No gap either.
A function defined with default arguments is always called with the full number of arguments. For
reasons of efficiency it may be useful to define several versions of the same function.
✓NOTE
■DEFAULT ARGUMENTS
Defining the function capital()
Possible calls
DEFAULT ARGUMENTS
■183
So-called default arguments can be defined for functions. This allows you to omit some
arguments when calling the function. The compiler simply uses the default values for any
missing arguments.
䊐Defining Default Arguments
The default values of a function’s arguments must be known when the function is called.
In other words, you need to supply them when you declare the function.
Example: void moveTo( int x = 0, int y = 0);
Parameter names can be omitted, as usual.
Example: void moveTo( int = 0, int = 0);
The function moveTo() can then be called with or without one or two arguments.
Example: moveTo (); moveTo (24); moveTo(24, 50);
The first two calls are equivalent to moveTo(0,0); or moveTo(24,0); .
It is also possible to define default arguments for only some of the parameters. The fol-
lowing general rules apply:
■the default arguments are defined in the function prototype. They can also be
supplied when the function is defined, if the definition occurs in the same source
file and before the function is called
■if you define a default argument for a parameter, all following parameters must
have default arguments
■default arguments must not be redefined within the prototype scope (the next
chapter gives more details on this topic).
䊐Possible Calls
When calling a function with default arguments you should pay attention to the follow-
ing points:
■you must first supply any arguments that do not have default values
■you can supply arguments to replace the defaults
■if you omit an argument, you must also omit any following arguments.
You can use default arguments to call a function with a different number of arguments
without having to write a new version of the function.
184 ■
CHAPTER 10 FUNCTIONS
// random.cpp
// To generate and output random numbers.
//-----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstdlib> // For rand(), srand()
#include <ctime> // For time()
using namespace std;
bool setrand = false;
inline void init_random() // Initializes the random
{ // number generator with the
// present time.
if( !setrand )
{ srand((unsigned int)time(NULL));
setrand = true;
}
}
inline double myRandom() // Returns random number x
{ // with 0.0 <= x <= 1.0
init_random();
return (double)rand() / (double)RAND_MAX;
}
inline int myRandom(int start, int end) // Returns the
{ // random number n with
init_random(); // start <= n <= end
return (rand() % (end+1 - start) + start);
}
// Testing myRandom() and myRandom(int,int):
int main()
{
int i;
cout << "5 random numbers between 0.0 and 1.0 :"
<< endl;
for( i = 0; i < 5; ++i)
cout << setw(10) << myRandom();
cout << endl;
cout << "\nAnd now 5 integer random numbers "
"between -100 and +100 :" << endl;
for( i = 0; i < 5; ++i)
cout << setw(10) << myRandom(-100, +100);
cout << endl;
return 0;
}
■OVERLOADING FUNCTIONS
Sample program
OVERLOADING FUNCTIONS
■185
Functions in traditional programming languages, such as C, which perform the same task
but have different arguments, must have different names. To define a function that cal-
culated the maximum value of two integers and two floating-point numbers, you would
need to program two functions with different names.
Example: int int_max( int x, int y);
double dbl_max( double x, double y);
Of course this is detrimental to efficient naming and the readability of your program—
but luckily, this restriction does not apply to C++.
䊐Overloading
C++ allows you to overload functions, that is, different functions can have the same
name.
Example: int max( int x, int y);
double max( double x, double y);
In our example two different function share the same name, max. The function max()
was overloaded for int and double types. The compiler uses a function’s signature to
differentiate between overloaded functions.
䊐Function Signatures
A function signature comprises the number and type of parameters. When a function is
called, the compiler compares the arguments to the signature of the overloaded functions
and simply calls the appropriate function.
Example: double maxvalue, value = 7.9;
maxvalue = max( 1.0, value);
In this case the double version of the function max() is called.
When overloaded functions are called, implicit type conversion takes place. However,
this can lead to ambiguities, which in turn cause a compiler error to be issued.
Example: maxvalue = max( 1, value); // Error!
The signature does not contain the function type, since you cannot deduce the type by
calling a function. It is therefore impossible to differentiate between overloaded func-
tions by type.
Example: int search(string key);
string search(string name);
Both functions have the same signature and cannot be overloaded.
186 ■
CHAPTER 10 FUNCTIONS
// recursive.cpp
// Demonstrates the principle of recursion by a
// function, which reads a line from the keyboard
// and outputs it in reverse order.
// ----------------------------------------------------
#include <iostream>
using namespace std;
void getput(void);
int main()
{
cout << "Please enter a line of text:\n";
getput();
cout << "\nBye bye!" << endl;
return 0;
}
void getput()
{
char c;
if( cin.get(c) && c != '\n')
getput();
cout.put(c);
}
■RECURSIVE FUNCTIONS
Using a recursive function
Program flow after typing ok<return>
1st Execution 2nd Execution 3rd Execution
main() getput() getput() getput()
{ { { {
... ... ... ...
// c = 'o' // c = 'k' // c = '\n'
getput(); getput(); getput(); // No call of
// getput()
} cout.put(c); cout.put(c); cout.put(c);
} } }
RECURSIVE FUNCTIONS
■187
䊐Recursion
A function that calls itself is said to be recursive. This process can also be performed indi-
rectly if the function first calls another function or multiple functions before it is called
once more. But a break criterion is always necessary to avoid having the function call
itself infinitely.
The concept of local objects makes it possible to define recursive functions in C++.
Recursion requires local objects to be created each time the function is called, and these
objects must not have access to any other local objects from other function calls. What
effectively happens is that the local objects are placed on the stack, and thus the object
created last is destroyed first.
䊐A Sample Program
Let’s look at the principle of recursion by referring to the sample program opposite. The
program contains the recursive function getput() that reads a line of text from the
keyboard and outputs it in reverse order.
The function getput() is first called by main() and reads a character from the key-
board, storing it in the local variable c. If the character is not '\n', the function get-
put() calls itself again and thus reads a further character from the keyboard before
storing it in the local variable c.
The chain of recursive function calls is terminated by the user pressing the Return
key. The last character to be read, '\n' (line feed), is output and the program flow
branches to the previous getput() instance. This outputs the second to last character,
and so on. When the first character to have been read has finally been output, the pro-
gram flow is handed back to main().
䊐Practical Usage
The logic of various solutions to common problems results in a recursive structure, for
example, browsing directory trees, using binary trees for data management, or some sort-
ing algorithms, such as the quick sort algorithm. Recursive functions allow you to formu-
late this kind of logic in an efficient and elegant manner. However, always make sure
that sufficient memory is available for the stack.
exercises
188 ■
CHAPTER 10 FUNCTIONS
■EXERCISES
Hint for exercise 1
Working with several source files:
Within an integrated development environment a project, containing all source
files of the program, first has to be created.This ensures that all the source files
will be compiled and linked automatically.
However, when calling the compiler/linker from the command line, it is
sufficient to declare the source files, for example:
cc sum_t.cpp sum.cpp
Screen output for exercise 3
n Factorial of n
0
1
2
3
4
5
6
7
.
.
.
19
20
1
1
2
6
24
120
720
5040
. . .
. . . .
. . . . . . .
121645100408832000
2432902008176640000
EXERCISES
■189
Exercise 1
a. Write the function sum() with four parameters that calculates the argu-
ments provided and returns their sum.
Parameters: Four variables of type long.
Returns: The sum of type long.
Use the default argument 0 to declare the last two parameter of the
function sum().Test the function sum() by calling it by all three possible
methods. Use random integers as arguments.
b. Now restructure your program to store the functions main() and
sum() in individual source files, for example, sum_t.cpp and sum.cpp .
Exercise 2
a. Write an inline function, Max(double x, double y), which returns
the maximum value of xand y. (Use Max instead of max to avoid a colli-
sion with other definitions of max.) Test the function by reading values
from the keyboard.
Can the function Max() also be called using arguments of the types
char,int, or long?
b. Now overload Max() by adding a further inline function Max(char x,
char y) for arguments of type char .
Can the function Max() still be called with two arguments of type
int?
Exercise 3
The factorial n! of a positive integer nis defined as
n! = 1*2*3 . . . * (n-1) * n
Where 0! = 1
Write a function to calculate the factorial of a number.
Argument: A number nof type unsigned int.
Returns: The factorial n! of type long double.
Formulate two versions of the function, where the factorial is
a. calculated using a loop
b. calculated recursively
Test both functions by outputting the factorials of the numbers 0 to 20 as shown
opposite on screen.
190 ■
CHAPTER 10 FUNCTIONS
1. The power x0is defined as 1.0 for a given number x.
2. The power xnis defined as (1/x)-n for a negative exponent n.
3. The power 0nwhere n> 0will always yield 0.0 .
The power 0nis not defined for n < 0. In this case, your function should return the
value HUGE_VAL. This constant is defined in math.h and represents a large double
value. Mathematical functions return HUGE_VAL when the result is too large for a
double.
✓NOTE
Exercise 4
Write a function pow(double base, int exp) to calculate integral powers of
floating-point numbers.
Arguments: The base of type double and the exponent of type int.
Returns: The power baseexp of type double.
For example, calling pow(2.5, 3) returns the value
2.53= 2.5 * 2.5 * 2.5 = 15.625
This definition of the function pow()means overloading the standard function
pow(), which is called with two double values.
Test your function by reading one value each for the base and the exponent
from the keyboard. Compare the result of your function with the result of the
standard function.
solutions
SOLUTIONS
■191
■SOLUTIONS
Exercise 1
// -----------------------------------------------------
// sum_t.cpp
// Calls function sum() with default arguments.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <ctime>
#include <cstdlib>
using namespace std;
long sum( long a1, long a2, long a3=0, long a4=0);
int main() // Several calls to function sum()
{
cout << " **** Computing sums ****\n"
<< endl;
srand((unsigned int)time(NULL)); // Initializes the
// random number generator.
long res, a = rand()/10, b = rand()/10,
c = rand()/10, d = rand()/10;
res = sum(a,b);
cout << a << " + " << b << " = " << res << endl;
res = sum(a,b,c);
cout << a << " + " << b << " + " << c
<< " = " << res << endl;
res = sum(a,b,c,d);
cout << a << " + " << b << " + " << c << " + " << d
<< " = " << res << endl;
return 0;
}
// -----------------------------------------------------
// sum.cpp
// Defines the function sum()
// -----------------------------------------------------
long sum( long a1, long a2, long a3, long a4)
{
return (a1 + a2 + a3 + a4);
}
192 ■
CHAPTER 10 FUNCTIONS
Exercise 2
// -----------------------------------------------------
// max.cpp
// Defines and calls the overloaded functions Max().
// ------------------------------------------------------
// As long as just one function Max() is defined, it can
// be called with any arguments that can be converted to
// double, i.e. with values of type char, int or long.
// After overloading no clear conversion will be possible.
#include <iostream>
#include <string>
using namespace std;
inline double Max(double x, double y)
{
return (x < y ? y : x);
}
inline char Max(char x, char y)
{
return (x < y ? y : x);
}
string header(
"To use the overloaded function Max().\n"),
line(50,'-');
int main() // Several different calls to function Max()
{
double x1 = 0.0, x2 = 0.0;
line += '\n';
cout << line << header << line << endl;
cout << "Enter two floating-point numbers:"
<< endl;
if( cin >> x1 && cin >> x2)
{
cout << "The greater number is " << Max(x1,x2)
<< endl;
}
else
cout << "Invalid input!" << endl;
cin.sync(); cin.clear(); // Invalid input
// was entered.
SOLUTIONS
■193
cout << line
<< "And once more with characters!"
<< endl;
cout << "Enter two characters:"
<< endl;
char c1, c2;
if( cin >> c1 && cin >> c2)
{
cout << "The greater character is " << Max(c1,c2)
<< endl;
}
else
cout << "Invalid input!" << endl;
cout << "Testing with int arguments." << endl;
int a = 30, b = 50;
cout << Max(a,b) << endl; // Error! Which
// function Max()?
return 0;
}
Exercise 3
// -----------------------------------------------------
// factorial.cpp
// Computes the factorial of an integer iteratively,
// i.e. using a loop, and recursively.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#define N_MAX 20
long double fact1(unsigned int n); // Iterative solution
long double fact2(unsigned int n); // Recursive solution
int main()
{
unsigned int n;
// Outputs floating-point values without
// decimal places:
cout << fixed << setprecision(0);
194 ■
CHAPTER 10 FUNCTIONS
// --- Iterative computation of factorial ---
cout << setw(10) << "n" << setw(30) << "Factorial of n"
<< " (Iterative solution)\n"
<< " -----------------------------------------"
<< endl;
for( n = 0; n <= N_MAX; ++n)
cout << setw(10) << n << setw(30) << fact1(n)
<< endl;
cout << "\nGo on with <return>";
cin.get();
// --- Recursive computation of factorial ----
cout << setw(10) << "n" << setw(30) << "Factorial of n"
<< " (Recursive solution)\n"
<< " -----------------------------------------"
<< endl;
for( n = 0; n <= N_MAX; ++n)
cout << setw(10) << n << setw(30) << fact2(n)
<< endl;
cout << endl;
return 0;
}
long double fact1(unsigned int n) // Iterative
{ // solution.
long double result = 1.0;
for( unsigned int i = 2; i <= n; ++i)
result *= i;
return result;
}
long double fact2(unsigned int n) // Recursive
{ // solution.
if( n <= 1)
return 1.0;
else
return fact2(n-1) * n;
}
SOLUTIONS
■195
Exercise 4
// -----------------------------------------------------
// power.cpp
// Defines and calls the function pow() to
// compute integer powers of a floating-point number.
// Overloads the standard function pow().
// -----------------------------------------------------
#include <iostream>
#include <cmath>
using namespace std;
double pow(double base, int exp);
int main() // Tests the self-defined function pow()
{
double base = 0.0;
int exponent = 0;
cout << " **** Computing Integer Powers ****\n"
<< endl;
cout << "Enter test values.\n"
<< "Base (floating-point): "; cin >> base;
cout << "Exponent (integer): "; cin >> exponent;
cout << "Result of " << base << " to the power of "
<< exponent << " = " << pow( base, exponent)
<< endl;
cout << "Computing with the standard function: "
<< pow( base, (double)exponent) << endl;
return 0;
}
double pow(double base, int exp)
{
if( exp == 0) return 1.0;
if( base == 0.0)
if( exp > 0) return 0.0;
else return HUGE_VAL;
if( exp < 0)
{
base = 1.0 / base;
exp = -exp;
}
double power = 1.0;
for( int n = 1; n <= exp; ++n)
power *= base;
return power;
}
This page intentionally left blank
197
Storage Classes and
Namespaces
This chapter begins by describing storage classes for objects and
functions.The storage class is responsible for defining those parts of a
program where an object or function can be used. Namespaces can be
used to avoid conflicts when naming global identifiers.
chapter
11
198 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
file scope
block scope
program scope
Function
Module 1
Module 2
file scope
block scope
Function
block scope
Function
■STORAGE CLASSES OF OBJECTS
䊐Availability of Objects
C++ program
䊐Storage Class Specifiers
The storage class of an object is determined by
■the position of its declaration in the source file
■the storage class specifier, which can be supplied optionally.
The following storage class specifiers can be used
extern static auto register
STORAGE CLASSES OF OBJECTS
■199
When an object is declared, not only are the object’s type and name defined but also its
storage class. The storage class specifies the lifetime of the object, that is, the period of
time from the construction of the object until its destruction. In addition, the storage
class delimits the part of the program in which the object can be accessed directly by its
name, the so-called object scope.
Essentially, an object is only available after you have declared it within a translation
unit. A translation unit, also referred to as module, comprises the source file you are com-
piling and any header files you have included.
As a programmer, you can define an object with:
■block scope The object is only available in the code block in which it was
defined. The object is no longer visible once you have left
the code block.
■file scope The object can be used within a single module. Only the
functions within this module can reference the object. Other
modules cannot access the object directly.
■program scope The object is available throughout the program, providing a
common space in memory that can be referenced by any pro-
gram function. For this reason, these objects are often
referred to as global.
Access to an object as defined by the object’s storage class is independent of any
access controls for the elements of a class. Namespaces that subdivide program scope and
classes will be introduced at a later stage.
䊐Lifetime
Objects with block scope are normally created automatically within the code block that
defines them. Such objects can only be accessed by statements within that block and are
called local to that block. The memory used for these objects is freed after leaving the
code block. In this case, the lifetime of the objects is said to be automatic.
However, it is possible to define objects with block scope that are available through-
out the runtime of a program. The lifetime of these objects is said to be static. When the
program flow re-enters a code block, any pre-existing conditions will apply.
Objects with program and file scope are always static. These objects are created when
a program is launched and are available until the program is terminated.
Four storage classes are available for creating objects with the scope and lifetime you
need. These storage classes will be discussed individually in the following sections.
200 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// Cutline1.cpp
// A filter to remove white-space characters
// at the ends of lines.
// --------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
void cutline( void ); // Prototype
string line; // Global string
int main()
{
while( getline(cin, line)) // As long as a line
{ // can be read.
cutline(); // Shorten the line.
cout << line << endl; // Output the line.
}
return 0;
}
// Cutline2.cpp
// Containing the function cutline(), which removes
// tabulator characters at the end of the string line.
// The string line has to be globally defined in another
// source file.
// --------------------------------------------------
#include <string>
using namespace std;
extern string line; // extern declaration
void cutline()
{
int i = line.size(); // Position after the
// last character.
while( i-- >= 0 )
if( line[i] != ' ' // If no blank and
&& line[i] != '\t' ) // no tab ->
break; // stop the loop.
line.resize(++i); // Fix new length.
}
■THE STORAGE CLASS extern
Source file 1
Source file 2
THE STORAGE CLASS extern
■201
䊐Defining Global Objects
If an object is not defined within a function, it belongs to the extern storage class.
Objects in this storage class have program scope and can be read and, provided they have
not been defined as const, modified at any place in the program. External objects thus
allow you to exchange information between any functions without passing any argu-
ments. To demonstrate this point, the program on the opposite page has been divided
into two separate source files. The string line, which has a global definition, is used to
exchange data.
Global objects that are not explicitly initialized during definition receive an initial
value of 0(that is, all bits = 0) by default. This also applies to objects belonging to class
types, if not otherwise stipulated by the class.
䊐Using Global Objects
An object belonging to the extern storage class is initially only available in the source
file where it was defined. If you need to use an object before defining it or in another
module, you must first declare the object. If you do not declare the object, the compiler
issues a message stating that the object is unknown. The declaration makes the name and
type of the object known to the compiler.
In contrast to a definition, the storage class identifier extern precedes the object
name in a declaration.
Example: extern long position; // Declaration
This statement declares position as an external object of type long. The extern
declaration thus allows you to “import” an object from another source file.
A global object must be defined once, and once only, in a program. However, it can
be declared as often as needed and at any position in the program. You will normally
declare the object before the first function in a source file or in a header file that you can
include when needed. This makes the object available to any functions in the file.
Remember, if you declare the object within a code block, the object can only be used
within the same block.
An extern declaration only refers to an object and should therefore not be used to
initialize the object. If you do initialize the object, you are defining that object!
Global objects affect the whole program and should be used sparingly. Large programs in particular
should contain no more than a few central objects defined as extern.
✓NOTE
202 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// Passw1.cpp
// The functions getPassword() and timediff()
// to read and examine a password.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
#include <ctime>
using namespace std;
long timediff(void); // Prototype
static string secret = "ISUS"; // Password
static long maxcount = 3, maxtime = 60; // Limits
bool getPassword() // Enters and checks a password.
{ // Return value: true, if password is ok.
bool ok_flag = false; // For return value
string word; // For input
int count = 0, time = 0;
timediff(); // To start the stop watch
while( ok_flag != true &&
++count <= maxcount) // Number of attempts
{
cout << "\n\nInput the password: ";
cin.sync(); // Clear input buffer
cin >> setw(20) >> word;
time += timediff();
if( time >= maxtime ) // Within time limit?
break; // No!
if( word != secret)
cout << "Invalid password!" << endl;
else
ok_flag = true; // Give permission
}
return ok_flag; // Result
}
long timediff() // Returns the number of
{ // seconds after the last call.
static long sec = 0; // Time of last call.
long oldsec = sec; // Saves previous time.
time( &sec); // Reads new time.
return (sec - oldsec); // Returns the difference.
}
■THE STORAGE CLASS static
THE STORAGE CLASS static
■203
In contrast to objects with an extern definition, the name of an external static object is unknown to
the linker and thus retains its private nature within a module.
✓NOTE
䊐Static Objects
If an object definition is preceded by the static keyword, the object belongs to the
static storage class.
Example: static int count;
The most important characteristic of static objects is their static (or permanent) lifetime.
Static objects are not placed on the stack, but are stored in the data area of a program
just like external objects.
However, in contrast to external objects, access to static objects is restricted. Two
conditions apply, depending on where the object is defined:
1. Definition external to all program functions
In this case, the object is external static, that is, the object can be designated using
its name within the module only, but will not collide with any objects using the
same name in other modules.
2. Definition within a code block
This means that the object is internal static, that is, the object is only visible
within a single block. However, the object is created only once and is not
destroyed on leaving the block. On re-entering the block, you can continue to
work with the original object.
The same rules apply to initializing static objects as they do to external objects. If the
object is not initialized explicitly, a default value of 0applies.
䊐Notes on the Sample Programs Opposite
The function getPassword() checks a password that is entered. Permission is refused
following three unsuccessful attempts or when 60 seconds have elapsed. You could use
the following instructions to call the function in another source file:
Example: if( !getPassword() )
cout << "No authorization!\n"; exit(1);
The string secret and the thresholds maxcount and maxtime are external static,
whereas the variable sec in the function timediff() is internal static. Its value is zero
only when the function is first called.
It makes sense to add a further function to these source files providing for password
changes.
204 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// StrToL.cpp
// The function strToLong() converts a string containing
// a leading integer into an integer of type long.
// Argument: A string.
// Return value: An integer of type long.
// --------------------------------------------------
// The digits are interpreted with base 10. White spaces
// and a sign can precede the sequence of digits.
// The conversion terminates when the end of the string is
// reached or when a character that cannot be converted is
// reached.
// --------------------------------------------------
#include <string> // Type string
#include <cctype> // isspace() and isdigit()
using namespace std;
long strToLong( string str)
{
register int i = 0; // Index
long vz = 1, num = 0; // Sign and number
// Ignore leading white spaces.
for(i=0; i < str.size() && isspace(str[i]); ++i)
;
// Is there a sign?
if( i < str.size())
{
if( str[i] == '+' ) { vz = 1; ++i; }
if( str[i] == '-' ) { vz = ---1; ++i; }
}
// Sequence of digits -> convert to integer
for( ; i < str.size() && isdigit(str[i]); ++i)
num = num * 10 + (str[i] - '0');
return vz * num;
}
■THE SPECIFIERS auto AND register
Sample function with a register variable
THE SPECIFIERS auto AND register
■205
䊐auto Objects
The storage class auto (automatic) includes all those objects defined within a function
but without the static keyword. The parameters of a function are also auto objects.
You can use the auto keyword during a definition.
Example: auto float radius; // Equivalent to:
// float radius;
When the program flow reaches the definition, the object is created on the stack, but in
contrast to a static type object, the object is destroyed on leaving the block.
auto objects have no specific initial value if they are not initialized explicitly. How-
ever, objects belonging to a class type are normally initialized with default values, which
can be specified in the class definition.
䊐Using CPU Registers
To increase the speed of a program, commonly used auto variables can be stored in
CPU registers instead of on the stack. In this case, the register keyword is used to
declare the object.
A register is normally the size of an int variable. In other words, it only makes sense
to define register variables if the variable is not too large, as in the case of types such
as char,short,int or pointers. If you omit the type when defining a register variable,
an int is assumed.
However, the compiler can ignore the register keyword. The number of registers
available for register variables depends on your hardware, although two registers are nor-
mally available. If a program defines too many register variables in a code block, the
superfluous variables are placed in the auto storage class.
䊐Sample Function
The function strToLong() illustrates an algorithm that converts a sequence of digits
to a binary number. This is useful if you need to perform calculations with a number con-
tained in a string.
The algorithm using the string "37" and the long variable num:
Step 0: num = 0;
Step 1: 1st character '3' →number 3 = ('3'-'0')
num = num * 10 + 3; // = 3
Step 2: 2nd character '7' →number 7 = ('7'-'0')
num = num * 10 + 7; // = 37
This pattern is followed for every number in a longer string.
206 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
extern bool getPassword(void); // Prototype
int main()
{
// The function permission(),
// but not the function timediff()
// can be called here.
.
.
.
}
static long timediff(void); // Prototype
bool getPassword(void) // Definition
{
// timediff() can be called here.
.
.
.
}
static long timediff(void) // Definition
{
.
.
.
}
■THE STORAGE CLASSES OF FUNCTIONS
䊐Example of a Program Structure
Source file 1
Source file 2
THE STORAGE CLASSES OF FUNCTIONS
■207
Only two storage classes are available for functions: extern and static. Functions
with block scope are invalid: you cannot define a function within another function.
The storage class of a function defines access to the function, as it does for an object.
External functions have program scope, whereas static functions have file scope.
䊐External Functions
If the keyword static is not used when defining a function, the function must belong
to the extern storage class.
In a similar manner to external objects, external functions can be used at any position
in a program. If you need to call a function before defining it, or in another source file,
you will need to declare that function.
Example: extern bool getPassword(void); // Prototype
As previously seen, you can omit the extern keyword, since functions belong to the
extern storage class by default.
䊐Static Functions
To define a static function, simply place the keyword static before the function
header.
Example: static long timediff()
{ . . . }
Functions in the static storage class have “private” character: they have file scope,
just like external static objects. They can only be called in the source file that defines
them. The name of a static function will not collide with objects and functions of the
same name in other modules.
If you need to call a static function before defining it, you must first declare the
function in the source file.
Example: static long timediff( void );
The program structure opposite takes up the example with the functions
getPassword() and timediff() once more. The function timediff() is an aux-
iliary function and not designed to be called externally. The function is declared as
static for this reason.
208 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// namesp1.cpp
// Defines and tests namespaces.
// ----------------------------------------------------
#include <string> // Class string defined within
// namespace std
namespace MySpace
{
std::string mess = "Within namespace MySpace";
int count = 0; // Definition: MySpace::count
double f( double); // Prototype: MySpace::f()
}
namespace YourSpace
{
std::string mess = "Within namespace YourSpace";
void f( ) // Definition of
{ // YourSpace::f()
mess += '!';
}
}
namespace MySpace // Back in MySpace.
{
int g(void); // Prototype of MySpace::g()
double f( double y) // Definition of
{ // MySpace::f()
return y / 10.0;
}
}
int MySpace::g( ) // Separate definition
{ // of MySpace::g()
return ++count;
}
#include <iostream> // cout, ... within namespace std
int main()
{
std::cout << "Testing namespaces!\n\n"
<< MySpace::mess << std::endl;
MySpace::g();
std::cout << "\nReturn value g(): " << MySpace::g()
<< "\nReturn value f(): " << MySpace::f(1.2)
<< "\n---------------------" << std::endl;
YourSpace::f();
std::cout << YourSpace::mess << std::endl;
return 0;
}
■NAMESPACES
Defining namespaces
NAMESPACES
■209
Using global names in large-scale software projects can lead to conflicts, especially when
multiple class libraries are in operation.
C++ provides for the use of namespaces in order to avoid naming conflicts with global
identifiers. Within a namespace, you can use identifiers without needing to check
whether they have been defined previously in an area outside of the namespace. Thus,
the global scope is subdivided into isolated parts.
A normal namespace is identified by a name preceded by the namespace keyword.
The elements that belong to the namespace are then declared within braces.
Example: namespace myLib
{
int count;
double calculate(double, int);
// . . .
}
This example defines the namespace myLib that contains the variable count and the
function calculate().
Elements belonging to a namespace can be referenced directly by name within the
namespace. If you need to reference an element from outside of the namespace, you must
additionally supply the namespace. To do so, place the scope resolution operator, ::,
before the element name.
Example: myLib::count = 7; // Outside of myLib
This allows you to distinguish between identical names in different namespaces. You can
also use the scope resolution operator :: to reference global names, that is, names
declared outside of any namespaces. To do so, simply omit the name of the namespace.
This technique is useful when you need to access a global name that is hidden by an
identical name defined in the current namespace.
Example: ::demo(); // Not belonging to any namespace
Be aware of the following when using namespaces:
■namespaces do not need to be defined contiguously. You can reopen and expand
a namespace you defined previously at any point in the program
■namespaces can be nested, that is, you can define a namespace within another
namespace.
Global identifiers belonging to the C++ standard library automatically belong to the
standard namespace std.
210 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// namesp2.cpp
// Demonstrates the use of using-declarations and
// using-directives.
// ----------------------------------------------------
#include <iostream> // Namespace std
void message() // Global function ::message()
{
std::cout << "Within function ::message()\n";
}
namespace A
{
using namespace std; // Names of std are visible here
void message() // Function A::message()
{
cout << "Within function A::message()\n";
}
}
namespace B
{
using std::cout; // Declaring cout of std.
void message(void); // Function B::message()
}
void B::message(void) // Defining B::message()
{
cout << "Within function B::message()\n";
}
int main()
{
using namespace std; // Names of namespace std
using B::message; // Function name without
// braces!
cout << "Testing namespaces!\n";
cout << "\nCall of A::message()" << endl;
A::message();
cout << "\nCall of B::message()" << endl;
message(); // ::message() is hidden because
// of the using-declaration.
cout << "\nCall of::message()" << endl;
::message(); // Global function
return 0;
}
■THE KEYWORD using
Sample program
THE KEYWORD using
■211
You can simplify access to the elements of a namespace by means of a using declaration or
using directive. In this case, you do not need to repeatedly quote the namespace. Just like
normal declarations, using declarations and using directives can occur at any part of
the program.
䊐using Declarations
Ausing declaration makes an identifier from a namespace visible in the current scope.
Example: using myLib::calculate; // Declaration
You can then call the function calculate() from the myLib namespace.
double erg = calculate( 3.7, 5);
This assumes that you have not previously used the name calculate in the same
scope.
䊐using Directive
The using directive allows you to import all the identifiers in a namespace.
Example: using namespace myLib;
This statement allows you to reference the identifiers in the myLib namespace directly.
If myLib contains an additional namespace and a using directive, this namespace is
also imported.
If identical identifiers occur in the current namespace and an imported namespace,
the using directive does not automatically result in a conflict. However, referencing an
identifier can lead to ambiguities. In this case, you should use the scope resolution opera-
tor to resolve the situation.
C++ header files without file extensions are used to declare the global identifiers in
the standard namespace std. The using directive was used in previous examples to
import any required identifiers to the global scope:
Example: #include <string>
using namespace std;
When developing large-scale programs or libraries, it is useful to declare the elements of
any proprietary namespaces in header files. Normal source files are used to define these
elements.
exercises
212 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// scope.cpp
// Accessing objects with equal names
// ---------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
int var = 0;
namespace Special { int var = 100; }
int main()
{
int var = 10;
cout << setw(10) << var; // 1.
{
int var = 20;
cout << setw(10) << var << endl; // 2.
{
++var;
cout << setw(10) << var; // 3.
cout << setw(10) << ++ ::var; // 4.
cout << setw(10) << Special::var * 2 // 5.
<< endl;
}
cout << setw(10) << var - ::var; // 6.
}
cout << setw(10) << var << endl; // 7.
return 0;
}
■EXERCISES
Program listing for exercise 1
EXERCISES
■213
Exercise 1
In general, you should use different names for different objects. However, if you
define a name for an object within a code block and the name is also valid for
another object, you will reference only the new object within the code block.
The new declaration hides any object using the same name outside of the block.
When you leave the code block, the original object once more becomes visible.
The program on the opposite page uses identical variable names in different
blocks.What does the program output on screen?
Exercise 2
You are developing a large-scale program and intend to use two commercial
libraries, tool1 and tool2.The names of types, functions, macros, and so on are
declared in the header files tool1.h and tool2.h for users of these libraries.
Unfortunately, the libraries use the same global names in part. In order to use
both libraries, you will need to define namespaces.
Write the following program to simulate this situation:
■Define an inline function called calculate() that returns the sum of two
numbers for the header file tool1.h.The function interface is as follows:
double calculate(double num1, double num2);
■Define an inline function called calculate() that returns the product of
two numbers for a second header file tool2.h.This function has the
same interface as the function in tool1.h.
■Then write a source file containing a main function that calls both func-
tions with test values and outputs the results.
To resolve potential naming conflicts, define the namespaces TOOL1 and
TOOL2 that include the relevant header files.
214 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
// static.cpp
// Tests an internal static variable
// ---------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
double x = 0.5,
fun(void);
int main()
{
while( x < 10.0 )
{
x += fun();
cout << " Within main(): "
<< setw(5) << x << endl;
}
return 0;
}
double fun()
{
static double x = 0;
cout << " Within fun():"
<< setw(5) << x++;
return x;
}
Program listing for exercise 3
EXERCISES
■215
The modified password is only available during runtime as it is not stored permanently.
✓NOTE
Exercise 3
Test your knowledge of external and static variables by reference to the
program on the opposite page.What screen output does the program generate?
Exercise 4
a. The function getPassword(), which checks password input, was intro-
duced previously as an example of the use of static variables. Modify the
source file Passw1.cpp, which contains the function getPassword(),by
adding the function changePassword().This function allows the user to
change his or her password. Save the modified source file as
Passw2.cpp.
b. A large-scale program with several users is used to perform bookings.
Only authorized users, that is, users that have access to the password, are
allowed to perform bookings.
In the initial stages of program development, you need to test the
functionality of the source file, Passw2.cpp.To do so, create a new
source file with a main function that contains only the following menu
items in its main loop:
B = Booking
E = End of program
When Bis typed, the password is first checked. If the user enters the
correct password, he or she can change the password.The program does
not need to perform any real bookings.
solutions
216 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
■SOLUTIONS
Exercise 1
Screen output of the program
10 20
21 1 200
20 10
Exercise 2
// ----------------------------------------------------
// tool1.h
// Defining first function calculate() inline.
// ----------------------------------------------------
#ifndef _TOOL1_H_
#define _TOOL1_H_
inline double calculate( double num1, double num2)
{
return num1 + num2;
}
#endif // End of _TOOL1_H_
// ----------------------------------------------------
// tool2.h
// Defining second function calculate() inline.
// ----------------------------------------------------
#ifndef _TOOL2_H_
#define _TOOL2_H_
inline double calculate( double num1, double num2)
{
return num1 * num2;
}
#endif // End of _TOOL2_H_
SOLUTIONS
■217
// --------------------------------------------------------
// tool_1_2.cpp
// Uses two "libraries" and tests name lookup conflicts.
// --------------------------------------------------------
#include <iostream>
namespace TOOL1
{
#include "tool1.h"
}
namespace TOOL2
{
#include "tool2.h"
}
#include <iostream>
int main()
{
using namespace std;
double x = 0.5, y = 10.5, res = 0.0;
cout << "Calling function of Tool1!" << endl;
res = TOOL1::calculate( x, y);
cout << "Result: " << res
<< "\n---------------------------------" << endl;
cout << "Calling function of Tool2!" << endl;
res = TOOL2::calculate( x, y);
cout << "Result: " << res << endl;
return 0;
}
Exercise 3
Screen output of the program
In fun(): 0 In main(): 1.5
In fun(): 1 In main(): 3.5
In fun(): 2 In main(): 6.5
In fun(): 3 In main(): 10.5
218 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
Exercise 4
// -----------------------------------------------------
// Passw2.cpp
// Defines the functions getPassword(), timediff() and
// changePassword() to examine and change a password.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
#include <ctime>
using namespace std;
static long timediff(void); // Prototype
static string secret = "guest"; // Password
static long maxcount = 3, maxtime = 60; // Limits
bool getPassword() // Read and verify a password.
{
// As before.
// . . .
}
// Auxiliary function timediff() --> defining static
static long timediff() // Returns the number of seconds
{ // since the last call.
// As before.
// . . .
}
bool changePassword() // Changes password.
{ // Returns: true, if the
// password has been changed
string word1,word2; // For input
// To read a new password
cout <<"\nEnter a new password (2 - 20 characters): ";
cin.sync(); // Discards former input
cin >> setw(20) >> word1;
SOLUTIONS
■219
if( word1.size() > 1)
{
cout << "\nEnter the password once more: ";
cin >> setw(20) >> word2;
if( word1 == word2) // Password confirmed?
{ // Yes!
secret = word1;
return true;
}
}
return false; // No new password
}
// -----------------------------------------------------
// Password.cpp
// Testing the functions getPassword() and
// changePassword().
//
// After entering the password correctly (max. three
// attempts within 60 seconds), the user can change it.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
#include <cctype>
using namespace std;
bool getPassword(void); // Read a password.
bool changePassword(void); // Change a password.
// Inline functions:
inline void cls() { cout << "\033[2J"; }
inline void go_on()
{
cout << "\n\nGo on with return! ";
cin.sync(); cin.clear(); // Only new input
while( cin.get() != '\n')
;
}
inline char getYesOrNo() // Read character Y or N.
{
char c = 0;
cin.sync(); cin.clear(); // Just new input
do
{
cin.get(c);
c = toupper(c); // Permitting lower case letters also.
}
while( c != 'Y' && c != 'N');
return c;
}
220 ■
CHAPTER 11 ST0RAGE CLASSES AND NAMESPACES
static string header =
"\n\n **** Test password handling ****\n\n";
static string menu =
"\n\n B = Booking "
"\n\n E = End of program"
"\n\n Your choice: ";
int main()
{
char choice = 0;
while( choice != 'E')
{
cls(); cout << header << menu; // Header and Menu
cin.get(choice); choice = toupper(choice);
cls(); cout << header << endl; // Header
switch( choice)
{
case 'B': // Booking
if( !getPassword() )
{
cout << "Access denied!" << endl;
go_on();
}
else
{ cout << "Welcome!\n\n"
<< "Do you want to change the password? (y/n)";
if( getYesOrNo() == 'Y')
{
if( changePassword() )
cout << "Password changed!" << endl;
else
cout << "Password unchanged!" << endl;
go_on();
}
// Place statements for booking here.
}
break;
case 'E':
cls(); cout << "\n Bye Bye!" << endl;
break;
}
} // End of while
return 0;
}
221
References and Pointers
This chapter describes how to define references and pointers and how
to use them as parameters and/or return values of functions. In this
context, passing by reference and read-only access to arguments are
introduced.
chapter
12
222 ■
CHAPTER 12 REFERENCES AND POINTERS
// Ref1.cpp
// Demonstrates the definition and use of references.
// ---------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
float x = 10.7F; // Global
int main()
{
float &rx = x; // Local reference to x
// double &ref = x; // Error: different type!
rx *= 2;
cout << " x = " << x << endl // x = 21.4
<< " rx = " << rx << endl; // rx = 21.4
const float& cref = x; // Read-only reference
cout << "cref = " << cref << endl; // ok!
// ++cref; // Error: read-only!
const string str = "I am a constant string!";
// str = "That doesn't work!"; // Error: str constant!
// string& text = str; // Error: str constant!
const string& text = str; // ok!
cout << text << endl; // ok! Just reading.
return 0;
}
Object names: The object in
memory
10.7
x, rx
. . .
. . .
■DEFINING REFERENCES
Example
float x = 10.7, &rx = x;
Sample program
DEFINING REFERENCES
■223
A reference is another name, or alias, for an object that already exists. Defining a refer-
ence does not occupy additional memory. Any operations defined for the reference are
performed with the object to which it refers. References are particularly useful as parame-
ters and return values of functions.
䊐Definition
The ampersand character, &, is used to define a reference. Given that Tis a type, T&
denotes a reference to T.
Example: float x = 10.7;
float& rx = x; // or: float &rx = x;
rx is thus a different way of expressing the variable xand belongs to the type “reference
to float”. Operations with rx, such as
Example: --rx; // equivalent to --x;
will automatically affect the variable x. The &character, which indicates a reference,
only occurs in declarations and is not related to the address operator &! The address
operator returns the address of an object. If you apply this operator to a reference, it
returns the address of the referenced object.
Example: &rx // Address of x, thus is equal to &x
A reference must be initialized when it is declared, and cannot be modified subse-
quently. In other words, you cannot use the reference to address a different variable at a
later stage.
䊐Read-Only References
A reference that addresses a constant object must be a constant itself, that is, it must be
defined using the const keyword to avoid modifying the object by reference. However,
it is conversely possible to use a reference to a constant to address a non-constant object.
Example: int a; const int& cref = a; // ok!
The reference cref can be used for read-only access to the variable a, and is said to be a
read-only identifier.
A read-only identifier can be initialized by a constant, in contrast to a normal refer-
ence:
Example: const double& pi = 3.1415927;
Since the constant does not take up any memory space, the compiler creates a
temporary object which is then referenced.
224 ■
CHAPTER 12 REFERENCES AND POINTERS
// Ref2.cpp
// Demonstrating functions with parameters
// of reference type.
// --------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
// Prototypes:
bool getClient( string& name, long& nr);
void putClient( const string& name, const long& nr);
int main()
{
string clientName;
long clientNr;
cout << "\nTo input and output client data \n"
<< endl;
if( getClient( clientName, clientNr)) // Calls
putClient( clientName, clientNr);
else
cout << "Invalid input!" << endl;
return 0;
}
bool getClient( string& name, long& nr) // Definition
{
cout << "\nTo input client data!\n"
<< " Name: ";
if( !getline( cin, name)) return false;
cout << " Number: ";
if( !( cin >> nr)) return false;
return true;
}
// Definition
void putClient( const string& name, const long& nr)
{ // name and nr can only be read!
cout << "\n-------- Client Data ---------\n"
<< "\n Name: "; cout << name
<< "\n Number: "; cout << nr << endl;
}
■REFERENCES AS PARAMETERS
Sample program
REFERENCES AS PARAMETERS
■225
䊐Passing by Reference
Apass by reference can be programmed using references or pointers as function parame-
ters. It is syntactically simpler to use references, although not always permissible.
A parameter of a reference type is an alias for an argument. When a function is called,
a reference parameter is initialized with the object supplied as an argument. The function
can thus directly manipulate the argument passed to it.
Example: void test( int& a) { ++a; }
Based on this definition, the statement
test( var); // For an int variable var
increments the variable var. Within the function, any access to the reference aauto-
matically accesses the supplied variable, var.
If an object is passed as an argument when passing by reference, the object is not
copied. Instead, the address of the object is passed to the function internally, allowing
the function to access the object with which it was called.
䊐Comparison to Passing by Value
In contrast to a normal pass by value an expression, such as a+b, cannot be used as an
argument. The argument must have an address in memory and be of the correct type.
Using references as parameters offers the following benefits:
■arguments are not copied. In contrast to passing by value, the run time of a pro-
gram should improve, especially if the arguments occupy large amounts of mem-
ory
■a function can use the reference parameter to return multiple values to the calling
function. Passing by value allows only one result as a return value, unless you
resort to using global variables.
If you need to read arguments, but not copy them, you can define a read-only reference
as a parameter.
Example: void display( const string& str);
The function display() contains a string as an argument. However, it does not gener-
ate a new string to which the argument string is copied. Instead, str is simply a refer-
ence to the argument. The caller can rest assured that the argument is not modified
within the function, as str is declared as a const.
226 ■
CHAPTER 12 REFERENCES AND POINTERS
// Ref3.cpp
// Demonstrates the use of return values with
// reference type.
// --------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
// Returns a
double& refMin( double&, double&); // reference to
// the minimum.
int main()
{
double x1 = 1.1, x2 = x1 + 0.5, y;
y = refMin( x1, x2); // Assigns the minimum to y.
cout << "x1 = " << x1 << " "
<< "x2 = " << x2 << endl;
cout << "Minimum: " << y << endl;
++refMin( x1, x2); // ++x1, as x1 is minimal
cout << "x1 = " << x1 << " " // x1 = 2.1
<< "x2 = " << x2 << endl; // x2 = 1.6
++refMin( x1, x2); // ++x2, because x2 is
// the minimum.
cout << "x1 = " << x1 << " " // x1 = 2.1
<< "x2 = " << x2 << endl; // x2 = 2.6
refMin( x1, x2) = 10.1; // x1 = 10.1, because
// x1 is the minimum.
cout << "x1 = " << x1 << " " // x1 = 10.1
<< "x2 = " << x2 << endl; // x2 = 2.6
refMin( x1, x2) += 5.0; // x2 += 5.0, because
// x2 is the minimum.
cout << "x1 = " << x1 << " " // x1 = 10.1
<< "x2 = " << x2 << endl; // x2 = 7.6
return 0;
}
double& refMin( double& a, double& b) // Returns a
{ // reference to
return a <= b ? a : b; // the minimum.
}
The expression refMin(x1,x2) represents either the object x1 or the object x2, that is, the object
containing the smaller value.
✓NOTE
■REFERENCES AS RETURN VALUE
Sample program
REFERENCES AS RETURN VALUE
■227
䊐Returning References
The return type of a function can also be a reference type. The function call then repre-
sents an object, and can be used just like an object.
Example: string& message() // Reference!
{
static string str = "Today only cold cuts!";
return str;
}
This function returns a reference to a static string, str. Pay attention to the following
point when returning references and pointers:
The object referenced by the return value must exist after leaving the function.
It would be a critical error to declare the string str as a normal auto variable in the
function message(). This would destroy the string on leaving the function and the ref-
erence would point to an object that no longer existed.
䊐Calling a Reference Type Function
The function message() (mentioned earlier in this section) is of type “reference to
string.” Thus, calling
message()
represents a string type object, and the following statements are valid:
message() = "Let's go to the beer garden!";
message() += " Cheers!";
cout << "Length: " << message().length();
In these examples, a new value is first assigned to the object referenced by the function
call. Then a new string is appended before the length of the referenced string is output in
the third statement.
If you want to avoid modifying the referenced object, you can define the function type
as a read-only reference.
Example: const string& message(); // Read-only!
References are commonly used as return types when overloading operators. The oper-
ations that an operator has to perform for a user-defined type are always implemented by
an appropriate function. Refer to the chapters on overloading operators later in this book
for more details. However, examples with operators from standard classes can be pro-
vided at this point.
228 ■
CHAPTER 12 REFERENCES AND POINTERS
Reference to cout
cout << "Good morning"
<<'!';
// Ref4.cpp
// Expressions with reference type exemplified by
// string assignments.
// --------------------------------------------------
#include <iostream>
#include <string>
#include <cctype> // For toupper()
using namespace std;
void strToUpper( string& ); // Prototype
int main()
{
string text("Test with assignments \n");
strToUpper(text);
cout << text << endl;
strToUpper( text = "Flowers");
cout << text << endl;
strToUpper( text += " cheer you up!\n");
cout << text << endl;
return 0;
}
void strToUpper( string& str) // Converts the content
{ // of str to uppercase.
int len = str.length();
for( int i=0; i < len; ++i)
str[i] = toupper( str[i]);
}
■EXPRESSIONS WITH REFERENCE TYPE
Example: Operator << of class ostream
cout << "Good morning" << '!';
Sample assignments of class string
EXPRESSIONS WITH REFERENCE TYPE
■229
Every C++ expression belongs to a certain type and also has a value, if the type is not
void. Reference types are also valid for expressions.
䊐The Stream Class Shift Operators
The << and >> operators used for stream input and output are examples of expressions
that return a reference to an object.
Example: cout << " Good morning "
This expression is not a void type but a reference to the object cout, that is, it repre-
sents the object cout. This allows you to repeatedly use the << on the expression:
cout << "Good morning" << '!'
The expression is then equivalent to
(cout << " Good morning ") << '!'
Expressions using the << operator are composed from left to right, as you can see from
the table of precedence contained in the appendix.
Similarly, the expression cin >> variable represents the stream cin. This allows
repeated use of the >> operator.
Example: int a; double x;
cin >> a >> x; // (cin >> a) >> x;
䊐Other Reference Type Operators
Other commonly used reference type operators include the simple assignment operator =
and compound assignments, such as += and *=. These operators return a reference to the
operand on the left. In an expression such as
a = b or a += b
amust therefore be an object. In turn, the expression itself represents the object a. This
also applies when the operators refer to objects belonging to class types. However, the
class definition stipulates the available operators. For example, the assignment operators
=and += are available in the standard class string.
Example: string name("Jonny ");
name += "Depp"; //Reference to name
Since an expression of this type represents an object, the expression can be passed as
an argument to a function that is called by reference. This point is illustrated by the
example on the opposite page.
230 ■
CHAPTER 12 REFERENCES AND POINTERS
100
456FD4ptr
var
456FD0
456FD4
Address
(hexadecimal)
Variable Value of the Variable
. . .
. . .
// pointer1.cpp
// Prints the values and addresses of variables.
// --------------------------------------------------
#include <iostream>
using namespace std;
int var, *ptr; // Definition of variables var and ptr
int main() // Outputs the values and addresses
{ // of the variables var and ptr.
var = 100;
ptr = &var;
cout << " Value of var: " << var
<< " Address of var: " << &var
<< endl;
cout << " Value of ptr: " << ptr
<< " Address of ptr: " << &ptr
<< endl;
return 0;
}
■DEFINING POINTERS
Sample program
Sample screen output
Value of var: 100 Address of var: 00456FD4
Value of ptr: 00456FD4 Address of ptr: 00456FD0
The variables var and ptr in memory
DEFINING POINTERS
■231
Efficient program logic often requires access to the memory addresses used by a program’s
data, rather than manipulation of the data itself. Linked lists or trees whose elements are
generated dynamically at runtime are typical examples.
䊐Pointers
Apointer is an expression that represents both the address and type of another object.
Using the address operator, &, for a given object creates a pointer to that object. Given
that var is an int variable,
Example: &var // Address of the object var
is the address of the int object in memory and thus a pointer to var. A pointer points
to a memory address and simultaneously indicates by its type how the memory address
can be read or written to. Thus, depending on the type, we refer to pointers to char,point-
ers to int, and so on, or use an abbreviation, such as char pointer, int pointer, and so on.
䊐Pointer Variables
An expression such as &var is a constant pointer; however, C++ allows you to define
pointer variables, that is, variables that can store the address of another object.
Example: int *ptr; // or: int* ptr;
This statement defines the variable ptr, which is an int* type (in other words, a pointer
to int). ptr can thus store the address of an int variable. In a declaration, the star char-
acter *always means “pointer to.”
Pointer types are derived types. The general form is T*, where Tcan be any given type.
In the above example Tis an int type.
Objects of the same base type Tcan be declared together.
Example: int a, *p, &r = a; // Definition of a, p, r
After declaring a pointer variable, you must point the pointer at a memory address. The
program on the opposite page does this using the statement
ptr = &var;.
䊐References and Pointers
References are similar to pointers: both refer to an object in memory. However, a pointer
is not merely an alias but an individual object that has an identity separate from the
object it references. A pointer has its own memory address and can be manipulated by
pointing it at a new memory address and thus referencing a different object.
232 ■
CHAPTER 12 REFERENCES AND POINTERS
Address
of px
Address of x
= value of px
Value of x
px
&px &x
px
x
*px
x
double x, y, *px;
px = &x; // Let px point to x.
*px = 12.3; // Assign the value 12.3 to x
*px += 4.5; // Increment x by 4.5.
y = sin(*px); // To assign sine of x to y.
■THE INDIRECTION OPERATOR
Using the indirection operator
Address and value of the variables xand px
Notes on addresses in a program
■Each pointer variable occupies the same amount of space, independent of the
type of object it references. That is, it occupies as much space as is necessary to
store an address. On a 32-bit computer, such as a PC, this is four bytes.
■The addresses visible in a program are normally logic addresses that are allocated
and mapped to physical addresses by the system. This allows for efficient storage
management and the swapping of currently unused memory blocks to the hard
disk.
■C++ guarantees that any valid address will not be equal to 0. Thus, the special
value 0 is used to indicate an error. For pointers, the symbolic constant NULL is
defined as 0 in standard header files. A pointer containing the value NULL is
also called NULL pointer.
THE INDIRECTION OPERATOR
■233
䊐Using Pointers to Access Objects
The indirection operator *is used to access an object referenced by a pointer:
Given a pointer, ptr,*ptr is the object referenced by ptr.
As a programmer, you must always distinguish between the pointer ptr and the
addressed object *ptr.
Example: long a = 10, b, // Definition of a, b
*ptr; // and pointer ptr.
ptr = &a; // Let ptr point to a.
b = *ptr;
This assigns the value of ato b, since ptr points to a. The assignment b=a;would
return the same result. The expression *ptr represents the object a, and can be used
wherever acould be used.
The star character *used for defining pointer variables is not an operator but merely
imitates the later use of the pointer in expressions. Thus, the definition
long *ptr;
has the following meaning: ptr is a long* (pointer to long) type and *ptr is a long
type.
The indirection operator *has high precedence, just like the address operator &. Both
operators are unary, that is, they have only one operand. This also helps distinguish the
redirection operator from the binary multiplication operator *, which always takes two
operands.
䊐L-values
An expression that identifies an object in memory is known as an L-value in C++. The
term L-value occurs commonly in compiler error messages and is derived from the assign-
ment. The left operand of the =operator must always designate a memory address.
Expressions other than an L-value are often referred to as R-values.
A variable name is the simplest example of an L-value. However, a constant or an
expression, such as x + 1, is an R-value. The indirection operator is one example of an
operator that yields L-values. Given a pointer variable p, both pand *p are L-values, as
*p designates the object to which ppoints.
234 ■
CHAPTER 12 REFERENCES AND POINTERS
■POINTERS AS PARAMETERS
Sample function
// pointer2.cpp
// Definition and call of function swap().
// Demonstrates the use of pointers as parameters.
// ----------------------------------------------------
#include <iostream>
using namespace std;
void swap( float *, float *); // Prototype of swap()
int main()
{
float x = 11.1F;
float y = 22.2F;
.
.
.
swap( &x, &y );
.
. // p2 = &y
.
} // p1 = &x
void swap( float *p1, float *p2)
{
float temp; // Temporary variable
temp = *p1; // At the above call p1 points
*p1 = *p2; // to x and p2 to y.
*p2 = temp;
}
POINTERS AS PARAMETERS
■235
䊐Objects as Arguments
If an object is passed as an argument to a function, two possible situations occur:
■the parameter in question is the same type as the object passed to it. The func-
tion that is called is then passed a copy of the object (passing by value)
■the parameter in question is a reference. The parameter is then an alias for the
argument, that is, the function that is called manipulates the object passed by the
calling function (passing by reference).
In the first case, the argument passed to the function cannot be manipulated by the
function. This is not true for passing by reference. However, there is a third way of pass-
ing by reference—passing pointers to the function.
䊐Pointers as Arguments
How do you declare a function parameter to allow an address to be passed to the function
as an argument? The answer is quite simple: The parameter must be declared as a pointer
variable.
If, for example, the function func() requires the address of an int value as an argu-
ment, you can use the following statement
Example: long func( int *iPtr )
{
// Function block
}
to declare the parameter iPtr as an int pointer. If a function knows the address of an
object, it can of course use the indirection operator to access and manipulate the object.
In the program on the opposite page, the function swap() swaps the values of the
variables xand yin the calling function. The function swap() is able to access the vari-
ables since the addresses of these variables, that is &x and &y, are passed to it as argu-
ments.
The parameters p1 and p2 in swap() are thus declared as float pointers. The
statement
swap( &x, &y);
initializes the pointers p1 and p2 with the addresses of xor y. When the function
manipulates the expressions *p1 and *p2, it really accesses the variables xand yin the
calling function and exchanges their values.
exercises
236 ■
CHAPTER 12 REFERENCES AND POINTERS
// A version of swap() with incorrect logic.
// Find the error!
void swap(float *p1, float *p2)
{
float *temp; // Temporary variable
temp = p1;
p1 = p2;
p2 = temp;
}
■EXERCISES
Listing for exercise 3
Solutions of quadratic equations
The quadratic equation: a*x2+ b*x + c = 0 has real solutions:
x12 = (-b ± √(b2- 4ac)) / 2a
if the discriminant satisfies: b2-4ac >= 0
If the value of (b2- 4ac) is negative, no real solution exists.
Test values
Quadratic Equation Solutions
2x2- 2x - 1.5 = 0 x1= 1.5, x2= -0.5
x2- 6x + 9 = 0 X1= 3.0, x2= 3.0
2x2+ 2 = 0 none
EXERCISES
■237
Given a circle with radius r:
Area = π* r * r and circumference = 2 * π* r where π= 3.1415926536
✓NOTE
Exercise 1
What happens if the parameter in the sample function strToUpper() is
declared as a string& instead of a string?
Exercise 2
Write a void type function called circle()to calculate the circumference and
area of a circle.The radius and two variables are passed to the function, which
therefore has three parameters:
Parameters: A read-only reference to double for the radius and two
references to double that the function uses to store the area
and circumference of the circle.
Test the function circle() by outputting a table containing the radius, the
circumference, and the area for the radii 0.5,1.0,1.5,. . . ,10.0.
Exercise 3
a. The version of the function swap() opposite can be compiled without
producing any error messages. However, the function will not swap the
values of xand ywhen swap(&x,&y); is called.What is wrong?
b. Test the correct pointer version of the function swap() found in this
chapter.Then write and test a version of the function swap() that uses
references instead of pointers.
Exercise 4
Create a function quadEquation() that calculates the solutions to quadratic
equations.The formula for calculating quadratic equations is shown opposite.
Arguments: The coefficients a,b,cand two pointers to both solutions.
Returns:false, if no real solution is available, otherwise true.
Test the function by outputting the quadratic equations on the opposite page
and their solutions.
solutions
238 ■
CHAPTER 12 REFERENCES AND POINTERS
■SOLUTIONS
Exercise 1
The call to function strToUpper() is left unchanged. But instead of passing by
reference, a passing by value occurs, i.e., the function manipulates a local copy.
Thus, only a local copy of the string is changed in the function, but the string in
the calling function remains unchanged.
Exercise 2
// ----------------------------------------------------
// circle.cpp
// Defines and calls the function circle().
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
// Prototype of circle():
void circle( const double& rad, double& um, double& fl);
const double startRadius = 0.5, // Start, end and
endRadius = 10.0, // step width of
step = 0.5; // the table
string header = "\n ***** Computing Circles ***** \n",
line( 50, '-');
int main()
{
double rad, circuit, plane;
cout << header << endl;
cout << setw(10) << "Radius"
<< setw(20) << "Circumference"
<< setw(20) << "Area\n" << line << endl;
cout << fixed; // Floating point presentation
for( rad = startRadius;
rad < endRadius + step/2; rad += step)
{
circle( rad, circuit, plane);
cout << setprecision(1)<< setw(8) << rad
<< setprecision(5)<< setw(22) << circuit
<< setw(20) << plane <<endl;
}
return 0;
}
SOLUTIONS
■239
// Function circle(): Compute circumference and area.
void circle( const double& r, double& u, double& f)
{
const double pi = 3.1415926536;
u = 2 * pi * r;
f = pi * r * r;
}
Exercise 3
// ----------------------------------------------------
// swap.cpp
// Definition and call of the function swap().
// 1. version: parameters with pointer type,
// 2. version: parameters with reference type.
// ----------------------------------------------------
#include <iostream>
using namespace std;
void swap( float*, float*); // Prototypes of swap()
void swap( float&, float&);
int main()
{
float x = 11.1F;
float y = 22.2F;
cout << "x and y before swapping: "
<< x << " " << y << endl;
swap( &x, &y); // Call pointer version.
cout << "x and y after 1. swapping: "
<< x << " " << y << endl;
swap( x, y); // Call reference version.
cout << "x and y after 2. swapping: "
<< x << " " << y << endl;
return 0;
}
void swap(float *p1, float *p2) // Pointer version
{
float temp; // Temporary variable
temp = *p1; // Above call points p1
*p1 = *p2; // to x and p2 to y.
*p2 = temp;
}
240 ■
CHAPTER 12 REFERENCES AND POINTERS
void swap(float& a, float& b) // Reference version
{
float temp; // Temporary variable
temp = a; // For above call
a = b; // a equals x and b equals y
b = temp;
}
Exercise 4
// ----------------------------------------------------
// quadEqu.cpp
// Defines and calls the function quadEquation(),
// which computes the solutions of quadratic equations
// a*x*x + b*x + c = 0
// The equation and its solutions are printed by
// the function printQuadEquation().
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
#include <cmath> // For the square root sqrt()
using namespace std;
string header =
" *** Solutions of Quadratic Equations ***\n",
line( 50, '-');
// ----- Prototypes -----
// Computing solutions:
bool quadEquation( double a, double b, double c,
double* x1Ptr, double* x2Ptr);
// Printing the equation and its solutions:
void printQuadEquation( double a, double b, double c);
int main()
{
cout << header << endl;
printQuadEquation( 2.0, -2.0, -1.5);
printQuadEquation( 1.0, -6.0, 9.0);
printQuadEquation( 2.0, 0.0, 2.0);
return 0;
}
SOLUTIONS
■241
// Prints the equation and its solutions:
void printQuadEquation( double a, double b, double c)
{
double x1 = 0.0, x2 = 0.0; // For solutions
cout << line << '\n'
<< "\nThe quadratic equation:\n\t "
<< a << "*x*x + " << b << "*x + " << c << " = 0"
<< endl;
if( quadEquation( a, b, c, &x1, &x2) )
{
cout << "has real solutions:"
<< "\n\t x1 = " << x1
<< "\n\t x2 = " << x2 << endl;
}
else
cout << "has no real solutions!" << endl;
cout << "\nGo on with return. \n\n";
cin.get();
}
bool quadEquation( double a, double b, double c,
double* x1Ptr, double* x2Ptr)
// Computes the solutions of the quadratic equation:
// a*x*x + b*x + c = 0
// Stores the solutions in the variables to which
// x1Ptr and x2Ptr point.
// Returns: true, if a solution exists,
// otherwise false.
{
bool return_flag = false;
double help = b*b - 4*a*c;
if( help >= 0) // There are real solutions.
{
help = sqrt( help);
*x1Ptr = (-b + help) / (2*a);
*x2Ptr = (-b - help) / (2*a);
return_flag = true;
}
return return_flag;
}
This page intentionally left blank
243
Defining Classes
This chapter describes how classes are defined and how instances of
classes, that is, objects, are used. In addition, structs and unions are
introduced as examples of special classes.
chapter
13
244 ■
CHAPTER 13 DEFINING CLASSES
Real World
A Car
Abstraction
Instantiation
Class CAR
Objects
Properties (Data Members):
Date when built
Capacity (PS)
Serial number
. . .
Properties:
Date when built = 1990
Capacity = 100
Chassis number = 11111
. . .
Methods
. . .
Methods (Member functions):
to run, to brake,
to park, to turn off
. . .
car1
Properties:
Date when built = 2000
Capacity = 200
Chassis number = 22222
. . .
Methods
. . .
car2
• • •
■THE CLASS CONCEPT
THE CLASS CONCEPT
■245
Classes are the language element in C++ most important to the support object-oriented
programming (OOP). A class defines the properties and capacities of an object.
䊐Data Abstraction
Humans use abstraction in order to manage complex situations. Objects and processes are
reduced to basics and referred to in generic terms. Classes allow more direct use of the
results of this type of abstraction in software development.
The first step towards solving a problem is analysis. In object-oriented programming,
analysis comprises identifying and describing objects and recognizing their mutual rela-
tionships. Object descriptions are the building blocks of classes.
In C++, a class is a user-defined type. It contains data members, which describe the
properties of the class, and member functions, or methods, which describe the capacities of
the objects. Classes are simply patterns used to instantiate, or create, objects of the class
type. In other words, an object is a variable of a given class.
䊐Data Encapsulation
When you define a class, you also specify the private members, that is, the members that
are not available for external access, and the public members of that class. An applica-
tion program accesses objects by using the public methods of the class and thus activat-
ing its capacities.
Access to object data is rarely direct, that is, object data is normally declared as pri-
vate and then read or modified by methods with public declarations to ensure correct
access to the data.
One important aspect of this technique is the fact that application programs need not
be aware of the internal structure of the data. If needed, the internal structure of the pro-
gram data can even be modified. Provided that the interfaces of the public methods
remain unchanged, changes like these will not affect the application program. This
allows you to enhance an application by programming an improved class version without
changing a single byte of the application.
An object is thus seen to encapsulate its private structure, protecting itself from exter-
nal influences and managing itself by its own methods. This describes the concept of data
encapsulation concisely.
246 ■
CHAPTER 13 DEFINING CLASSES
class Demo
{
private:
// Private data members and methods here
public:
// Public data members and methods here
};
// account.h
// Defining the class Account.
// ---------------------------------------------------
#ifndef _ACCOUNT_ // Avoid multiple inclusions.
#define _ACCOUNT_
#include <iostream>
#include <string>
using namespace std;
class Account
{
private: // Sheltered members:
string name; // Account holder
unsigned long nr; // Account number
double balance; // Account balance
public: //Public interface:
bool init( const string&, unsigned long, double);
void display();
};
#endif // _ACCOUNT_
■DEFINING CLASSES
Definition scheme
Example of a class
DEFINING CLASSES
■247
A class definition specifies the name of the class and the names and types of the class
members.
The definition begins with the keyword class followed by the class name. The data
members and methods are then declared in the subsequent code block. Data members
and member functions can belong to any valid type, even to another previously defined
class. At the same time, the class members are divided into:
■private members, which cannot be accessed externally
■public members, which are available for external access.
The public members form the so-called public interface of the class.
The opposite page shows a schematic definition of a class. The private section gen-
erally contains data members and the public section contains the access methods for
the data. This provides for data encapsulation.
The following example includes a class named Account used to represent a bank
account. The data members, such as the name of the account holder, the account num-
ber, and the account balance, are declared as private. In addition, there are two public
methods, init() for initialization purposes and display(), which is used to display
the data on screen.
The labels private: and public: can be used at the programmer’s discretion
within a class:
■you can use the labels as often as needed, or not at all, and in any order. A sec-
tion marked as private: or public: is valid until the next public: or pri-
vate: label occurs
■the default value for member access is private. If you omit both the private
and public labels, all the class members are assumed to be private.
䊐Naming
Every piece of software uses a set of naming rules. These rules often reflect the target
platform and the class libraries used. For the purposes of this book, we decided to keep to
standard naming conventions for distinguishing classes and class members. Class names
begin with an uppercase letter and member names with a lowercase letter.
Members of different classes can share the same name. A member of another class
could therefore also be named display().
248 ■
CHAPTER 13 DEFINING CLASSES
// account.cpp
// Defines methods init() and display().
// ---------------------------------------------------
#include "account.h" // Class definition
#include <iostream>
#include <iomanip>
using namespace std;
// The method init() copies the given arguments
// into the private members of the class.
bool Account::init(const string& i_name,
unsigned long i_nr,
double i_balance)
{
if( i_name.size() < 1) // No empty name
return false;
name = i_name;
nr = i_nr;
balance = i_balance;
return true;
}
// The method display() outputs private data.
void Account::display()
{
cout << fixed << setprecision(2)
<< "--------------------------------------\n"
<< "Account holder: " << name << '\n'
<< "Account number: " << nr << '\n'
<< "Account balance: " << balance << '\n'
<< "--------------------------------------\n"
<< endl;
}
■DEFINING METHODS
Methods of class Account
DEFINING METHODS
■249
A class definition is not complete without method definitions. Only then can the objects
of the class be used.
䊐Syntax
When you define a method, you must also supply the class name, separating it from the
function name by means of the scope resolution operator ::.
Syntax: type class_name::function_name(parameter_list)
{ . . . }
Failure to supply the class name results in a global function definition.
Within a method, all the members of a class can be designated directly using their
names. The class membership is automatically assumed. In particular, methods belonging
to the same class can call each other directly.
Access to private members is only possible within methods belonging to the same
class. Thus, private members are completely controlled by the class.
Defining a class does not automatically allocate memory for the data members of that
class. To allocate memory, you must define an object. When a method is called for a
given object, the method can then manipulate the data of this object.
䊐Modular Programming
A class is normally defined in several source files. In this case, you will need to place the
class definition in a header file. If you place the definition of the class Account in the
file Account.h, any source file including the header file can use the class Account.
Methods must always be defined within a source file. This would mean defining the
methods for the class Account in a source file named Account.cpp, for example.
The source code of the application program, for example, the code containing the
function main, is independent of the class and can be stored in separate source files. Sep-
arating classes from application programs facilitates re-use of classes.
In an integrated development environment, a programmer will define a project to help
manage the various program modules by inserting all the source files into the project.
When the project is compiled and linked, modified source files are automatically re-com-
piled and linked to the application program.
250 ■
CHAPTER 13 DEFINING CLASSES
"Cheers, Mary"
1234567
2002.22
"Dylan, Bob"
87654321
–1300.13
current
name
nr
balance
savings
name
nr
balance
■DEFINING OBJECTS
The objects current and savings in memory
DEFINING OBJECTS
■251
Defining a class also defines a new type for which variables, that is, objects, can be
defined. An object is also referred to as an instance of a class.
䊐Defining Objects
An object is defined in the usual way by supplying the type and the object name.
Syntax: class_name object_name1 [, object_name2,...]
The following statement defines an object current of type Account:
Example: Account current; // or: class Account ...
Memory is now allocated for the data members of the current object. The current
object itself contains the members name,nr, and balance.
䊐Objects in Memory
If multiple objects of the same class type are declared, as in
Example: Account current, savings;
each object has its own data members. Even the object savings contains the members
name,nr, and balance. However, these data members occupy a different position in
memory than the data members belonging to current.
The same methods are called for both objects. Only one instance of the machine code
for a method exists in memory—this applies even if no objects have been defined for the
class.
A method is always called for a particular instance and then manipulates the data
members of this object. This results in the memory content as shown on the opposite
page, when the method init() is called for each object with the values shown.
䊐Initializing Objects
The objects belonging to the Account class were originally defined but not initialized.
Each member object is thus defined but not explicitly initialized. The string name, is
empty, as it is thus defined in the class string. The initial values of the members nr
and balance are unknown, however. As is the case for other variables, these data mem-
bers will default to 0if the object is declared global or static.
You can define exactly how an object is created and destroyed. These tasks are per-
formed by constructors and destructors. Constructors are specifically responsible for initial-
izing objects—more details are given later.
252 ■
CHAPTER 13 DEFINING CLASSES
// account_t.cpp
// Uses objects of class Account.
// ---------------------------------------------------
#include "Account.h"
int main()
{
Account current1, current2;
current1.init("Cheers, Mary", 1234567, -1200.99);
current1.display();
// current1.balance += 100; // Error: private member
current2 = current1; // ok: Assignment of
// objects is possible.
current2.display(); // ok
// New values for current2
current2.init("Jones, Tom", 3512347, 199.40);
current2.display();
// To use a reference:
Account& mtr = current1; // mtr is an alias name
// for object current1.
mtr.display(); // mtr can be used just
// as object current1.
return 0;
}
■USING OBJECTS
Sample program
USING OBJECTS
■253
䊐Class Member Access Operator
An application program that manipulates the objects of a class can access only the pub-
lic members of those objects. To do so, it uses the class member access operator (in short:
dot operator).
Syntax: object.member
Where member is a data member or a method.
Example: Account current;
current.init("Jones, Tom",1234567,-1200.99);
The expression current.init represents the public method init of the Account
class. This method is called with three arguments for current.
The init() call cannot be replaced by direct assignments.
Example: current.name = "Dylan, Bob"; // Error:
current.nr = 1234567; // private
current.balance = -1200.99; // members
Access to the private members of an object is not permissible outside the class. It is
therefore impossible to display single members of the Account class on screen.
Example: cout << current.balance; // Error
current.display(); // ok
The method display() displays all the data members of current. A method such as
display() can only be called for one object. The statement
display();
would result in an error message, since there is no global function called display().
What data would the function have to display?
䊐Assigning Objects
The assignment operator =is the only operator that is defined for all classes by default.
However, the source and target objects must both belong to the same class. The assign-
ment is performed to assign the individual data members of the source object to the cor-
responding members of the target object.
Example: Account current1, current2;
current2.init("Marley, Bob",350123, 1000.0);
current1 = current2;
This copies the data members of current2 to the corresponding members of
current1.
254 ■
CHAPTER 13 DEFINING CLASSES
// ptrObj.cpp
// Uses pointers to objects of class Account.
// ---------------------------------------------------
#include "Account.h" // Includes <iostream>, <string>
bool getAccount( Account *pAccount); // Prototype
int main()
{
Account current1, current2, *ptr = &current1;
ptr->init("Cheer, Mary", // current1.init(...)
3512345, 99.40);
ptr->display(); // current1.display()
ptr = &current2; // Let ptr point to current2
if( getAccount( ptr)) // Input and output a new
ptr->display(); // account.
else
cout << "Invalid input!" << endl;
return 0;
}
// --------------------------------------------------
// getAccount() reads data for a new account
// and adds it into the argument.
bool getAccount( Account *pAccount )
{
string name, line(50,'-'); // Local variables
unsigned long nr;
double startcapital;
cout << line << '\n'
<< "Enter data for a new account: \n"
<< "Account holder: ";
if( !getline(cin,name) || name.size() == 0)
return false;
cout << "Account number: ";
if( !(cin >> nr)) return false;
cout << "Starting capital: ";
if( !(cin >> startcapital)) return false;
// All input ok
pAccount->init( name, nr, startcapital);
return true;
}
■POINTERS TO OBJECTS
Sample program
POINTERS TO OBJECTS
■255
An object of a class has a memory address—just like any other object. You can assign this
address to a suitable pointer.
Example: Account savings("Mac, Rita",654321, 123.5);
Account *ptrAccount = &savings;
This defines the object savings and a pointer variable called ptrAccount. The
pointer ptrAccount is initialized so that it points to the object savings. This makes
*ptrAccount the object savings itself. You can then use the statement
Example: (*ptrAccount).display();
to call the method display() for the object savings. Parentheses must be used in
this case, as the operator .has higher precedence than the *operator.
䊐Arrow Operator
You can use the class member access operator -> (in short: arrow operator) instead of a
combination of *and ..
Syntax: objectPointer->member
This expression is equivalent to
(*objectPointer).member
The operator -> is made up of a minus sign and the greater than sign.
Example: ptrAccount->display();
This statement calls the method display() for the object referenced by ptrAccount,
that is, for the object savings. The statement is equivalent to the statement in the pre-
vious example.
The difference between the class member access operators .and -> is that the left
operand of the dot operator must be an object, whereas the left operand of the arrow
operator must be a pointer to an object.
䊐The Sample Program
Pointers to objects are often used as function parameters. A function that gets the
address of an object as an argument can manipulate the referenced object directly. The
example on the opposite page illustrates this point. It uses the function getAccount()
to read the data for a new account. When called, the address of the account is passed:
getAccount(ptr) // or: getAccount(&current1)
The function can then use the pointer ptr and the init() method to write new data
to the referenced object.
256 ■
CHAPTER 13 DEFINING CLASSES
// structs.cpp
// Defines and uses a struct.
// ---------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
struct Representative // Defining struct Representative
{
string name; // Name of a representative.
double sales; // Sales per month.
};
inline void print( const Representative& v)
{
cout << fixed << setprecision(2)
<< left << setw(20) << v.name
<< right << setw(10) << v.sales << endl;
}
int main()
{
Representative rita, john;
rita.name = "Strom, Rita";
rita.sales = 37000.37;
john.name = "Quick, John";
john.sales = 23001.23;
rita.sales += 1700.11; // More Sales
cout << " Representative Sales\n"
<< "-------------------------------" << endl;
print( rita);
print( john);
cout << "\nTotal of sales: "
<< rita.sales + john.sales << endl;
Representative *ptr = &john; // Pointer ptr.
// Who gets the
if( john.sales < rita.sales) // most sales?
ptr = &rita;
cout << "\nSalesman of the month: "
<< ptr->name << endl; // Representative's name
// pointed to by ptr.
return 0;
}
■structs
Sample program
structs
■257
䊐Records
In a classical, procedural language like C, multiple data that belong together logically are
put together to form a record. Extensive data such as the data for the articles in an auto-
mobile manufacturer’s stocks can be organized for ease of viewing and stored in files.
From the viewpoint of an object-oriented language, a record is merely a class contain-
ing only public data members and no methods. Thus, you can use the class keyword to
define the structure of a record in C++.
Example: class Date
{ public: short month, day, year; };
However, it is common practice to use the keyword struct, which is also available
in the C programming language, to define records. The above definition of Date with
the members day,month, and year is thus equivalent to:
Example: struct Date { short month, day, year; };
䊐The Keywords class and struct
You can also use the keyword struct to define a class, such as the class Account.
Example: struct Account {
private: // . . . as before
public: // . . .
};
The keywords class and struct only vary with respect to data encapsulation; the
default for access to members of a class defined as a struct is public. In contrast to a
class defined using the class keyword, all the class members are public unless a pri-
vate label is used. This allows the programmer to retain C compatibility.
Example: Date future;
future.year = 2100; // ok! Public data
Records in the true sense of the word, that is, objects of a class containing only pub-
lic members, can be initialized by means of a list during definition.
Example: Date birthday = { 1, 29, 1987};
The first element in the list initializes the first data member of the object, and so on.
258 ■
CHAPTER 13 DEFINING CLASSES
w (16 bit word)
Low byte b[0]
High byte b[1]
// unions.cpp
// Defines and uses a union.
// ---------------------------------------------------
#include <iostream>
using namespace std;
union WordByte
{
private:
unsigned short w; // 16 bits
unsigned char b[2]; // Two bytes: b[0], b[1]
public: // Word- and byte-access:
unsigned short& word() { return w; }
unsigned char& lowByte() { return b[0]; }
unsigned char& highByte(){ return b[1]; }
};
int main()
{
WordByte wb;
wb.word() = 256;
cout << "\nWord: " << (int)wb.word();
cout << "\nLow-byte: " << (int)wb.lowByte()
<< "\nHigh-byte: " << (int)wb.highByte()
<< endl;
return 0;
}
■UNIONS
An object of union WordByte in memory
Defining and using union WordByte
Screen output of the program
Word: 256
Low-Byte: 0
High-Byte: 1
UNIONS
■259
䊐Memory Usage
In normal classes, each data member belonging to an object has its own separate memory
space. However, a union is a class whose members are stored in the same memory space.
Each data member has the same starting address in memory. Of course, a union cannot
store various data members at the same address simultaneously. However, a union does
provide for more versatile usage of memory space.
䊐Definition
Syntactically speaking, a union is distinguished from a class defined as a class or
struct only by the keyword union.
Example: union Number
{
long n;
double x;
};
Number number1, number2;
This example defines the union Number and two objects of the same type. The union
Number can be used to store either integral or floating-point numbers.
Unless a private label is used, all union members are assumed to be public. This
is similar to the default setting for structures. This allows direct access to the members n
and xin the union Number.
Example: number1.n = 12345; // Storing an integer
number1.n *= 3; // and multiply by 3.
number2.x = 2.77; // Floating point number
The programmer must ensure that the current content of the union is interpreted cor-
rectly. This is normally achieved using an additional type field that identifies the current
content.
The size of a union type object is derived from the longest data member, as all data
members begin at the same memory address. If we look at our example, the union
Number, this size is defined by the double member, which defaults to 8 ==
sizeof(double) byte.
The example opposite defines the union WordByte that allows you to read or write
to a 16-bit memory space byte for byte or as a unit.
exercise
260 ■
CHAPTER 13 DEFINING CLASSES
struct tm
{
int tm_sec; // 0 - 59(60)
int tm_min; // 0 - 59
int tm_hour; // 0 - 23
int tm_mday; // Day of month: 1 - 31
int tm_mon; // Month: 0 - 11 (January == 0)
int tm_year; // Years since 1900 (Year - 1900)
int tm_wday; // Weekday: 0 - 6 (Sunday == 0)
int tm_yday; // Day of year: 0 - 365
int tm_isdst; // Flag for summer-time
};
#include <iostream>
#include <ctime>
using namespace std;
struct tm *ptr; // Pointer to struct tm.
time_t sec; // For seconds.
. . .
time(&sec); // To get the present time.
ptr = localtime(&sec); // To initialize a struct of
// type tm and return a
// pointer to it.
cout << "Today is the " << ptr->tm_yday + 1
<< ". day of the year " << ptr->tm_year
<< endl;
. . .
■EXERCISE
Struct tm in header file ctime
Sample calls to functions time( ) and localtime( )
EXERCISE
■261
Use the functions declared in ctime
time_t time(time_t *ptrSec)
struct tm *localtime(const time_t *ptrSec);
✓NOTE
Exercise
A program needs a class to represent the date.
■Define the class Date for this purpose using three integral data members
for day, month, and year.Additionally, declare the following methods:
void init( int month, int day, int year);
void init(void);
void print(void);
Store the definition of the class Date in a header file.
■Implement the methods for the class Date in a separate source file:
1. The method print() outputs the date to standard output using the
format Month-Day-Year.
2. The method init() uses three parameters and copies the values
passed to it to corresponding members. A range check is not required
at this stage, but will be added later.
3. The method init() without parameters writes the current date to the
corresponding members.
The structure tm and sample calls to this function are included oppo-
site.The type time_t is defined as long in ctime.
The function time() returns the system time expressed as a num-
ber of seconds and writes this value to the variable referenced by ptr-
Sec. This value can be passed to the function localtime() that
converts the number of seconds to the local type tm date and returns
a pointer to this structure.
■Test the class Date using an application program that once more is stored
in a separate source file.To this end, define two objects for the class and
display the current date. Use object assignments and—as an additional
exercise—references and pointers to objects.
solution
262 ■
CHAPTER 13 DEFINING CLASSES
■SOLUTION
// ----------------------------------------------------
// date.h
// First Definition of class Date.
// ----------------------------------------------------
#ifndef _DATE_ // Avoid multiple inclusion.
#define _DATE_
class Date
{
private: // Sheltered members:
short month, day, year;
public: // Public interface:
void init(void);
void init( int month, int day, int year);
void print(void);
};
#endif // _DATE_
// ----------------------------------------------------
// date.cpp
// Implementing the methods of class Date.
// ----------------------------------------------------
#include "date.h"
#include <iostream>
#include <ctime>
using namespace std;
// ---------------------------------------------------
void Date::init(void) // Get the present date and
{ // assign it to data members.
struct tm *ptr; // Pointer to struct tm.
time_t sec; // For seconds.
time(&sec); // Get the present date.
ptr = localtime(&sec); // Initialize a struct of
// type tm and return a
// pointer to it.
month = (short) ptr->tm_mon + 1;
day = (short) ptr->tm_mday;
year = (short) ptr->tm_year + 1900;
}
SOLUTION
■263
// ---------------------------------------------------
void Date::init( int m, int d, int y)
{
month = (short) m;
day = (short) d;
year = (short) y;
}
// ---------------------------------------------------
void Date::print(void) // Output the date
{
cout << month << '-' << day << '-' << year
<< endl;
}
// ----------------------------------------------------
// date_t.cpp
// Using objects of class Date.
// ----------------------------------------------------
#include "date.h"
#include <iostream>
using namespace std;
int main()
{
Date today, birthday, aDate;
today.init();
birthday.init( 12, 11, 1997);
cout << "Today's date: ";
today.print();
cout << "\n Felix' birthday: ";
birthday.print();
cout << "----------------------------------\n"
"Some testing outputs:" << endl;
aDate = today; // Assignment ok
aDate.print();
Date *pDate = &birthday; // Pointer to birthday
pDate->print();
Date &holiday = aDate; // Reference to aDate.
holiday.init( 1, 5, 2000); // Writing to aDate.
aDate.print(); // holiday.print();
return 0;
}
This page intentionally left blank
265
Methods
This chapter describes
■how constructors and destructors are defined to create and
destroy objects
■how inline methods, access methods, and read-only methods
can be used
■the pointer this, which is available for all methods, and
■what you need to pay attention to when passing objects as
arguments or returning objects.
chapter
14
266 ■
CHAPTER 14 METHODS
// account.h
// Defining class Account with two constructors.
// ---------------------------------------------------
#ifndef _ACCOUNT_
#define _ACCOUNT_
#include <string>
using namespace std;
class Account
{
private: // Sheltered members:
string name; // Account holder
unsigned long nr; // Account number
double state; // State of the account
public: // Public interface:
Account( const string&, unsigned long, double );
Account( const string& );
bool init( const string&, unsigned long, double);
void display();
};
#endif // _ACCOUNT_
// Within file account.cpp:
Account::Account( const string& a_name,
unsigned long a_nr, double a_state)
{
nr = a_nr;
name = a_name;
state = a_state;
}
Account::Account( const string& a_name )
{
name = a_name;
nr = 1111111; state = 0.0;
}
■CONSTRUCTORS
Class Account with constructors
Defining the constructors
CONSTRUCTORS
■267
䊐The Task of a Constructor
Traditional programming languages only allocate memory for a variable being defined.
The programmer must ensure that the variable is initialized with suitable values.
An object of the class Account, as described in the previous chapter, does not possess
any valid values until the method init() is called. Non-initialized objects can lead to
serious runtime errors in your programs.
To avoid errors of this type, C++ performs implicit initialization when an object is
defined. This ensures that objects will always have valid data to work on. Initialization is
performed by special methods known as constructors.
䊐Declaration
Constructors can be identified by their names. In contrast to other member functions,
the following applies:
■the name of the constructor is also the class name
■a constructor does not possess a return type—not even void.
Constructors are normally declared in the public section of a class. This allows you to
create objects wherever the class definition is available.
Constructors can be overloaded, just like other functions. Constructors belonging to a
class must be distinguishable by their signature (that is, the number, order, and type of
parameters). This allows for different methods of object initialization. The example
opposite shows an addition to the Account class. The class now has two constructors.
䊐Definition
Since a constructor has the same name as its class, the definition of a constructor always
begins with
Class_name::Class_name
In the definition itself, the arguments passed can be checked for validity before they are
copied to the corresponding data members. If the number of arguments is smaller than
the number of data members, the remaining members can be initialized using default val-
ues.
Constructors can also perform more complex initialization tasks, such as opening files,
allocating memory, and configuring interfaces.
268 ■
CHAPTER 14 METHODS
// account2_t.cpp
// Using the constructors of class Account.
// ---------------------------------------------------
#include "account.h"
int main()
{
Account giro("Cheers, Mary", 1234567, -1200.99 ),
save("Lucky, Luke");
Account depot; // Error: no default constructor
// defined.
giro.display(); // To output
save.display();
Account temp("Funny, Susy", 7777777, 1000000.0);
save = temp; // ok: Assignment of
// objects possible.
save.display();
// Or by the presently available method init():
save.init("Lucky, Luke", 7654321, 1000000.0);
save.display();
return 0;
}
■CONSTRUCTOR CALLS
Sample program
CONSTRUCTOR CALLS
■269
Unlike other methods, constructors cannot be called for existing objects. For this reason,
a constructor does not have a return type. Instead, a suitable constructor is called once
only when an object is created.
䊐Initialization
When an object is defined, initial values can follow the object name in parentheses.
Syntax: class object( initializing_list);
During initialization the compiler looks for a constructor whose signature matches the
initialization list. After allocating sufficient memory for the object, the constructor is
called. The values in the initialization list are passed as arguments to the constructor.
Example: account nomoney("Poor, Charles");
This statement calls the constructor with one parameter for the name. The other data
members will default to standard values.
If the compiler is unable to locate a constructor with a suitable signature, it will not
create the object but issue an error message.
Example: account somemoney("Li, Ed",10.0); // Error!
The class Account does not contain a constructor with two parameters.
If a constructor with only one parameter is defined in the class, the statement can be
written with an equals sign =.
Example: account nomoney = "Poor, Charles";
This statement is equivalent to the definition in the example before last. Initialization
with parentheses or the = sign was introduced previously for fundamental types. For
example, int i(0); is equivalent to int i =0;.
䊐Default Constructor
A constructor without parameters is referred to as a default constructor. The default con-
structor is only called if an object definition does not explicitly initialize the object. A
default constructor will use standard values for all data members.
If a class does not contain a constructor definition, the compiler will create a minimal
version of the default constructor as a public member. However, this constructor will
not perform initialization. By contrast, if a class contains at least one constructor, a
default constructor must be defined explicitly, if it is needed. The definition of the
Account class does not specify a default constructor; thus a new account object can be
created with initialization only.
270 ■
CHAPTER 14 METHODS
// demo.cpp
// Outputs constructor and destructor calls.
// ---------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
int count = 0; // Number of objects.
class Demo
{
private: string name;
public: Demo( const string& ); // Constructor
~Demo(); // Destructor
};
Demo::Demo( const string& str)
{
++count; name = str;
cout << "I am the constructor of "<< name << '\n'
<< "This is the " << count << ". object!\n"
}
Demo:: ~Demo() // Defining the destructor
{
cout << "I am the destructor of " << name << '\n'
<< "The " << count << ". object "
<< "will be destroyed " << endl;
--count;
}
// -- To initialize and destroy objects of class Demo --
Demo globalObject("the global object");
int main()
{
cout << "The first statement in main()." << endl;
Demo firstLocalObject("the 1. local object");
{
Demo secLocalObject("the 2. local object");
static Demo staticObject("the static object");
cout << "\nLast statement within the inner block"
<< endl;
}
cout << "Last statement in main()." << endl;
return 0;
}
■DESTRUCTORS
Sample program
DESTRUCTORS
■271
䊐Cleaning Up Objects
Objects that were created by a constructor must also be cleaned up in an orderly manner.
The tasks involved in cleaning up include releasing memory and closing files.
Objects are cleaned up by a special method called a destructor, whose name is made up
of the class name preceded by ∼(tilde).
䊐Declaration and Definition
Destructors are declared in the public section and follow this syntax:
Syntax: ∼class_name(void);
Just like the constructor, a destructor does not have a return type. Neither does it
have any parameters, which makes the destructor impossible to overload. Each class thus
has one destructor only.
If the class does not define a destructor, the compiler will create a minimal version of
a destructor as a public member, called the default destructor.
It is important to define a destructor if certain actions performed by the constructor
need to be undone. If the constructor opened a file, for example, the destructor should
close that file. The destructor in the Account class has no specific tasks to perform. The
explicit definition is thus:
Account::∼Account(){} // Nothing to do
The individual data members of an object are always removed in the order opposite of
the order in which they were created. The first data member to be created is therefore
cleaned up last. If a data member is also a class type object, the object’s own destructor
will be called.
䊐Calling Destructors
A destructor is called automatically at the end of an object’s lifetime:
■for local objects except objects that belong to the static storage class, at the
end of the code block defining the object
■for global or static objects, at the end of the program.
The sample program on the opposite page illustrates various implicit calls to constructors
and destructors.
272 ■
CHAPTER 14 METHODS
// account.h
// New definition of class Account with inline methods
// ----------------------------------------------------
#ifndef _ACCOUNT_
#define _ACCOUNT_
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class Account
{
private: // Sheltered members:
string name; // Account holder
unsigned long nr; // Account number
double state; // State of the account
public: //Public interface:
// Constructors: implicit inline
Account( const string& a_name = "X",
unsigned long a_nr = 1111111L,
double a_state = 0.0)
{
name = a_name; nr = a_nr; state = a_state;
}
~Account(){ } // Dummy destructor: implicit inline
void display();
};
// display() outputs data of class Account.
inline void Account::display() // Explicit inline
{
cout << fixed << setprecision(2)
<< "--------------------------------------\n"
<< "Account holder: " << name << '\n'
<< "Account number: " << nr << '\n'
<< "Account state: " << state << '\n'
<< "--------------------------------------\n"
<< endl;
}
#endif // _ACCOUNT_
■INLINE METHODS
Sample class Account
INLINE METHODS
■273
A class typically contains multiple methods that fulfill simple tasks, such as reading or
updating data members. This is the only way to ensure data encapsulation and class func-
tionality.
However, continually calling “short” methods can impact a program’s runtime. In
fact, saving a re-entry address and jumping to the called function and back into the call-
ing function can take more time than executing the function itself. To avoid this over-
head, you can define inline methods in a way similar to defining inline global
functions.
䊐Explicit and Implicit inline Methods
Methods can be explicitly or implicitly defined as inline. In the first case, the method
is declared within the class, just like any other method. You simply need to place the
inline keyword before the method name in the function header when defining the
method.
Example: inline void Account::display()
{
. . .
}
Since the compiler must have access to the code block of an inline function, the
inline function should be defined in the header containing the class definition.
Short methods can be defined within the class. Methods of this type are known as
implicit inline methods, although the inline keyword is not used.
Example: // Within class Account:
bool isPositive(){ return state > 0; }
䊐Constructors and Destructors with inline Definitions
Constructors and destructors are special methods belonging to a class and, as such, can
be defined as inline. This point is illustrated by the new definition of the Account
class opposite. The constructor and the destructor are both implicit inline. The con-
structor has a default value for each argument, which means that we also have a default
constructor. You can now define objects without supplying an initialization list.
Example: Account temp;
Although we did not explicitly supply values here, the object temp was correctly initial-
ized by the default constructor we defined.
274 ■
CHAPTER 14 METHODS
// account.h
// Class Account with set- and get-methods.
// ----------------------------------------------------
#ifndef _ACCOUNT_
#define _ACCOUNT_
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class Account
{
private: // Sheltered members:
string name; // Account holder
unsigned long nr; // Account number
double state; // State of the account
public: //Public interface:
// constructors, destructor:
Account( const string& a_name = "X",
unsigned long a_nr = 1111111L,
double a_state = 0.0)
{ name = a_name; nr = a_nr; state = a_state; }
∼Account(){ }
// Access methods:
const string& getName() { return name; }
bool setName( const string& s)
{
if( s.size() < 1) // No empty name
return false;
name = s;
return true;
}
unsigned long getNr() { return nr; }
void setNr( unsigned long n) { nr = n; }
double getState() { return state; }
void setState(double x) { state = x; }
void display();
};
// inline definition of display() as before.
#endif // _ACCOUNT_
■ACCESS METHODS
Access methods for the Account class
ACCESS METHODS
■275
䊐Accessing Private Data Members
An object’s data members are normally found in the private section of a class. To
allow access to this data, you could place the data members in the public section of the
class; however, this would undermine any attempt at data encapsulation.
Access methods offer a far more useful way of accessing the private data members.
Access methods allow data to be read and manipulated in a controlled manner. If the
access methods were defined as inline, access is just as efficient as direct access to the
public members.
In the example opposite, several access methods have been added to the Account
class. You can now use the
getName(), getNr(), getState()
methods to read the individual data members. As is illustrated in getName(), references
should be read-only when used as return values. Direct access for write operations could
be possible otherwise. To manipulate data members, the following methods can be used:
setName(), setNr(), setState().
This allows you to define a new balance, as follows:
Example: save.setState( 2199.0);
䊐Access Method Benefits
Defining access methods for reading and writing to each data member may seem like a
lot of work—all that typing, reams of source code, and the programmer has to remember
the names and tasks performed by all those methods.
So, you may be asking yourself how you benefit from using access methods. There are
two important issues:
■Access methods can prevent invalid access attempts at the onset by performing
sanity checks. If a class contains a member designed to represent positive num-
bers only, an access method can prevent processing negative numbers.
■Access methods also hide the actual implementation of a class. It is therefore pos-
sible to modify the internal structure of your data at a later stage. If you detect
that a new data structure will allow more efficient data handling, you can add
this modification to a new version of the class. Provided the public interface to
the class remains unchanged, an application program can be leveraged by the
modification without needing to modify the application itself. You simply re-
compile the application program.
276 ■
CHAPTER 14 METHODS
// account.h
// Account class with read-only methods.
// ----------------------------------------------------
#ifndef _ACCOUNT_
#define _ACCOUNT_
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class Account
{
private: // Sheltered members
// Data members: as before
public: // Public interface
// Constructors and destructor
. . . // as before
// Get-methods:
const string& getName() const { return name; }
unsigned long getNr() const { return nr; }
double getState() const { return state; }
// Set-methods:
. . . // as before
// Additional methods:
void display() const;
};
// display() outputs the data of class Account.
inline void Account::display() const
{
cout << fixed << setprecision(2)
<< "--------------------------------------\n"
<< "Account holder: " << name << '\n'
<< "Account number: " << nr << '\n'
<< "Account state: " << state << '\n'
<< "--------------------------------------\n"
<< endl;
}
#endif // _ACCOUNT_
■const OBJECTS AND METHODS
Read-only methods in the Account class
const OBJECTS AND METHODS
■277
䊐Accessing const Objects
If you define an object as const, the program can only read the object. As mentioned
earlier, the object must be initialized when you define it for this reason.
Example: const Account inv("YMCA, FL", 5555, 5000.0);
The object inv cannot be modified at a later stage. This also means that methods such
as setName() cannot be called for this object. However, methods such as getName or
display() will be similarly unavailable although they only perform read access with
the data members.
The reason for this is that the compiler cannot decide whether a method performs
write operations or only read operations with data members unless additional informa-
tion is supplied.
䊐Read-Only Methods
Methods that perform only read operations and that you need to call for constant objects
must be identified as read-only. To identify a method as read-only, append the const
keyword in the method declaration and in the function header for the method defini-
tion.
Example: unsigned long getNr() const;
This declares the getNr() method as a read-only method that can be used for constant
objects.
Example: cout << "Account number: " << inv.getNr();
Of course, this does not prevent you from calling a read-only method for a non-constant
object.
The compiler issues an error message if a read-only method tries to modify a data
member. This also occurs when a read-only method calls another method that is not
defined as const.
䊐const and Non-const Versions of a Method
Since the const keyword is part of the method’s signature, you can define two versions
of the method: a read-only version, which will be called for constant objects by default,
and a normal version, which will be called for non-const objects.
278 ■
CHAPTER 14 METHODS
// stdMeth.cpp
// Using standard methods.
// ---------------------------------------------------
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class CD
{private:
string interpret, title;
long seconds; // Time duration of a song
public:
CD( const string& i="", const string& t="", long s = 0L)
{
interpret = i; title = t; seconds = s;
}
const string& getInterpret() const{ return interpret; }
const string& getTitle() const { return title; }
long getSeconds() const { return seconds; }
};
// Generate objects of class CD and output it in tabular form
void printLine( CD cd) ; // A row of the table
int main()
{
CD cd1( "Mister X", "Let's dance", 30*60 + 41),
cd2( "New Guitars", "Flamenco Collection", 2772 ),
cd3 = cd1, // Copy constructor!
cd4; // Default constructor.
cd4 = cd2; // Assignment!
string line( 70,'-'); line += '\n';
cout << line << left
<< setw(20) << "Interpreter" << setw(30) << "Title"
<< "Length (Min:Sec)\n" << line << endl;
printLine(cd3); // Call by value ==>
printLine(cd4); // Copy constructor!
return 0;
}
void printLine( CD cd)
{ cout << left << setw(20) << cd.getInterpret()
<< setw(30) << cd.getTitle()
<< right << setw(5) << cd.getSeconds() / 60
<< ':' << setw(2) << cd.getSeconds() % 60 << endl;
}
■STANDARD METHODS
Sample program
STANDARD METHODS
■279
Every class automatically contains four standard methods:
■the default constructor
■the destructor
■the copy constructor and
■the assignment.
You can use your own definitions to replace these standard methods. As illustrated by
the sample class Account, the compiler only uses the pre-defined default constructor if
no other constructor is available.
The default constructor and the implicit, minimal version of a destructor were intro-
duced earlier.
䊐Copy Constructor
The copy constructor initializes an object with another object of the same type. It is
called automatically when a second, already existing object is used to initialize an object.
Example: Account myAccount("Li, Ed", 2345, 124.80);
Account yourAccount(myAccount);
In this example, the object yourAccount is initialized by calling the copy constructor
with the myAccount object. Each member is copied individually, that is, the following
initialization process takes place:
yourAccount.name = myAccount.name;
yourAccount.nr = myAccount.nr;
yourAccount.state = myAccount.state;
The copy constructor is also called when an object is passed to a function by value.
When the function is called, the parameter is created and initialized with the object used
as an argument.
䊐Assignment
Assignment has been used in several previous examples. An object can be assigned to
another object of the same type.
Example: hisAccount = yourAccount;
The data members of the yourAccount object are copied to the corresponding mem-
bers of hisAccount in this case also. In contrast to initialization using the copy con-
structor, assignment requires two existing objects.
Later in the book, you will be introduced to situations where you need to define the
copy constructor or an assignment yourself, and the necessary techniques will be dis-
cussed.
280 ■
CHAPTER 14 METHODS
// DayTime.h
// The class DayTime represents the time in
// hours, minutes and seconds.
// ---------------------------------------------------
#ifndef _DAYTIME_
#define _DAYTIME_
class DayTime
{
private:
short hour, minute, second;
bool overflow;
public:
DayTime( int h = 0, int m = 0, int s = 0)
{
overflow = false;
if( !setTime( h, m, s)) // this->setTime(...)
hour = minute = second = 0; // hour is equivalent
} // to this->hour etc.
bool setTime(int hour, int minute, int second = 0)
{
if( hour >= 0 && hour < 24
&& minute >= 0 && minute < 60
&& second >= 0 && second < 60 )
{
this->hour = (short)hour;
this->minute = (short)minute;
this->second = (short)second;
return true;
}
else
return false;
}
int getHour() const { return hour; }
int getMinute() const { return minute; }
int getSecond() const { return second; }
int asSeconds() const // daytime in seconds
{ return (60*60*hour + 60*minute + second); }
bool isLess( DayTime t) const // compare *this and t
{
return asSeconds() < t.asSeconds();
} // this->asSeconds() < t.asSeconds();
};
#endif // _DAYTIME_
■this POINTER
Sample class DayTime
this POINTER
■281
䊐Accessing the Current Object
A method can access any member of an object without the object name being supplied
in every case. A method will always reference the object with which it was called.
But how does a method know which object it is currently working with? When a
method is called, it is passed a hidden argument containing the address of the current
object.
The address of the current object is available to the method via the constant pointer
this. Given that actObj is the current object of type Class_id, for which a method
was called, the pointer this has the following declaration:
Class_id* const this = &actObj;
The name this is a keyword. As this is a constant pointer, it cannot be redirected.
In other words, the pointer this allows you to access the current object only.
䊐Using the this Pointer
You can use the this pointer within a method to address an object member as follows:
Example: this->data // Data member: data
this->func() // Calling member function
The compiler implicitly creates an expression of this type if only a member of the current
object is supplied.
Example: data = 12; // Corresponds to this->data=12;
Write operations of this type are permissible since the pointer this is a constant, but
the referenced object is not. However, the above statement would be invalid for a read-
only method.
The this pointer can be used explicitly to distinguish a method’s local variables from
class members of the same name. This point is illustrated by the sample method
setTime() on the opposite page.
The this pointer is always necessary to access the current object, *this, collec-
tively. This situation often occurs when the current object needs to be returned as a copy
or by reference. Then the return statement is as follows:
return *this;
282 ■
CHAPTER 14 METHODS
#include "DayTime.h"
. . .
DayTime depart1( 11, 11, 11), depart2;
. . .
depart2.setTime(12, 0, 0);
if( depart1.isLess( depart2) )
cout << "\nThe 1st plane takes off earlier" << endl;
. . .
#include "DayTime.h"
// Defines the global function swap():
void swap( DayTime& t1, DayTime& t2) // Two
{ // parameters!
DayTime temp(t1); t1 = t2; t2 = temp; // To swap
} // t1 and t2.
// A call (e.g. in function main()):
DayTime arrival1( 14, 10), arrival2( 15, 20);
. . .
swap( arrival1, arrival2); // To swap
. . .
// Defines the method swap():
class DayTime // With a new method swap()
{ . . .
public:
void swap( DayTime& t) // One parameter!
{ // To swap *this and t:
DayTime temp(t); t = *this; *this = temp;
}
};
// A call (e.g. in function main()):
#include "DayTime.h"
DayTime arrival1( 10, 10), arrival2( 9, 50);
. . .
arrival1.swap(arrival2);
. . .
■PASSING OBJECTS AS ARGUMENTS
Calling methods setTime() and isLess()
Global function swap()
Implementing swap() as a method
PASSING OBJECTS AS ARGUMENTS
■283
䊐Passing by Value
As you already know, passing by value copies an object that was passed as an argument to
the corresponding parameter of a function being called. The parameter is declared as an
object of the class in question.
Example: bool isLess( DayTime t) const;
When the method isLess() is called, the copy constructor executes and initializes the
created object, t, with the argument.
depart1.isLess( depart2) // Copy constructor
The function uses a copy of the object depart2. The copy is cleaned up when leaving
the function, that is, the destructor is called.
䊐Passing by Reference
The overhead caused by creating and cleaning up objects can be avoided by passing argu-
ments by reference. In this case, the parameter is declared as a reference or pointer.
Example: bool isLess( const DayTime& t) const;
This new declaration of the isLess() method is preferable to the previous declaration.
There is no formal difference to the way the method is called. However, isLess() no
longer creates an internal copy, but accesses directly the object being passed. Of course,
the object cannot be changed, as the parameter was declared read-only.
䊐Methods Versus Global Functions
Of course, it is possible to write a global function that expects one object as an argument.
However, this rarely makes sense since you would normally expect an object’s function-
ality to be defined in the class itself. Instead, you would normally define a method for the
class and the method would perform the task in hand. In this case, the object would not
be passed as an argument since the method would manipulate the members of the cur-
rent object.
A different situation occurs where operations with at least two objects need to be per-
formed, such as comparing or swapping. For example, the method isLess() could be
defined as a global function with two parameters. However, the function could only
access the public interface of the objects. The function swap()on the opposite page
additionally illustrates this point.
The major advantage of a globally defined function is its symmetry. The objects
involved are peers, since both are passed as arguments. This means that conversion rules
are applied to both arguments when the function is called.
284 ■
CHAPTER 14 METHODS
#include "DayTime.h"
#include <ctime> // Functions time(), localtime()
using namespace std;
const DayTime& currentTime() // Returns the
{ // present time.
static DayTime curTime;
time_t sec; time(&sec); // Gets the present time.
// Initializes the struct
struct tm *time = localtime(&sec); // tm with it.
curTime.setTime( time->tm_hour, time->tm_min,
time->tm_sec );
return curTime;
}
// DayTim_t.cpp
// Tests class DayTime and function currentTime()
// ---------------------------------------------------
#include "DayTime.h" // Class definition
#include <iostream>
using namespace std;
const DayTime& currentTime(); // The current time.
int main()
{
DayTime cinema( 20,30);
cout << "\nThe movie starts at ";
cinema.print();
DayTime now(currentTime()); // Copy constructor
cout << "\nThe current time is ";
now.print();
cout << "\nThe movie has ";
if( cinema.isLess( now) )
cout << "already begun!\n" << endl;
else
cout << "not yet begun!\n" << endl;
return 0;
}
■RETURNING OBJECTS
Global function currentTime()
Sample program
RETURNING OBJECTS
■285
A function can use the following ways to return an object as a return value: It can create
a copy of the object, or it can return a reference or pointer to that object.
䊐Returning a Copy
Returning a copy of an object is time-consuming and only makes sense for small-scale
objects.
Example: DayTime startMeeting()
{
DayTime start;
. . . // Everyone has time at 14:30:
start.setTime( 14, 30);
return( start);
}
On exiting the function, the local object start is destroyed. This forces the compiler to
create a temporary copy of the local object and return the copy to the calling function.
䊐Returning a Reference
Of course, it is more efficient to return a reference to an object. But be aware that the
lifetime of the referenced object must not be local.
If this is the case, the object is destroyed on exiting the function and the returned ref-
erence becomes invalid. If you define the object within a function, you must use a
static declaration.
The global function currentTime() on the opposite page exploits this option by
returning a reference to the current time that it reads from the system each time the
function is called. The sample program that follows this example uses the current time to
initialize the new object now and then outputs the time. In order to output the time, an
additional method, print(), was added to the class.
䊐Using Pointers as Return Values
Instead of returning a reference, a function can also return a pointer to an object. In this
case too, you must ensure that the object still exists after exiting the function.
Example: const DayTime* currentTime() // Read-only pointer
{ // to the current time
. . . // Unchanged
return &curTime;
}
exercises
286 ■
CHAPTER 14 METHODS
Private members:
Type
Article number: long
Article name: string
Sales price: double
Public members:
Article(long, const string&, double);
~Article();
void print(); // Formatted output
set- and get-methods for any data member
An object of type Article . . . is created.
This is the . . .. Article.
The object of type Article . . . is destroyed.
There are still . . . articles.
■EXERCISES
Class Article
Output from constructor
Output from destructor
EXERCISES
■287
Exercise 1
A warehouse management program needs a class to represent the articles in
stock.
■Define a class called Article for this purpose using the data members
and methods shown opposite. Store the class definition for Article in a
separate header file. Declare the constructor with default arguments for
each parameter to ensure that a default constructor exists for the class.
Access methods for the data members are to be defined as inline. Neg-
ative prices must not exist. If a negative price is passed as an argument,
the price must be stored as 0.0.
■Implement the constructor, the destructor, and the method print() in a
separate source file. Also define a global variable for the number of
Article type objects.
The constructor must use the arguments passed to it to initialize the data
members, additionally increment the global counter, and issue the message
shown opposite.
The destructor also issues a message and decrements the global counter.
The method print() displays a formatted object on screen.After
outputting an article, the program waits for the return key to be pressed.
■The application program (again use a separate source file) tests the Arti-
cle class. Define four objects belonging to the Article class type:
1. A global object and a local object in the main function.
2. Two local objects in a function test() that is called twice by main().
One object needs a static definition.The function test() displays
these objects and outputs a message when it is terminated.
Use articles of your own choice to initialize the objects.Additionally, call
the access methods to modify individual data members and display the
objects on screen.
■Test your program. Note the order in which constructors and destruc-
tors are called.
Supplementary question: Suppose you modify the program by declaring a function
called test() with a parameter of type Article and calling the function with
an article type object.The counter for the number of objects is negative after
running the program.Why?
288 ■
CHAPTER 14 METHODS
Public Methods:
Date();
Date( int month, int day, int year);
void setDate();
bool setDate( int mn, int da, int yr);
int getMonth() const;
int getDay() const;
int getYear() const;
bool isEqual( const Date&) const;
bool isLess( const Date&) const;
const string& asString() const;
void print() const;
// Example: Converting a number to a string.
#include <sstream> // Class stringstream
#include <iomanip> // Manipulators
double x = 12.3; // Number
string str; // Destination string
stringstream iostream; // For conversion
// number -> string.
iostream << setw(10) << x; // Add to the stream.
iostream >> str; // Read from the stream.
Methods for class Date
Converting a number to a string
The class stringstream offers the same functionality for reading and writing to
character buffer as the classes istream and ostream do.Thus, the operators >>
and << , just as all manipulators, are available.
Notices for exercise 3
■A year is a leap year if it is divisible by 4 but not by 100. In addition, all
multiples of 400 are leap years. February has 29 days in a leap year.
■Use a switch statement to examine the number of days for months con-
taining less than 31 days.
EXERCISES
■289
Exercise 2
In the exercises for chapter 13, an initial version of the Date class containing
members for day, month, and year was defined. Now extend this class to add
additional functionality.The methods are shown on the opposite page.
■The constructors and the method setDate() replace the init method
used in the former version.The default constructor uses default values,
for example, 1.1.1, to initialize the objects in question.The setDate()
method without any parameters writes the current date to the object.
■The constructor and the setDate() method with three parameters do
not need to perform range checking.This functionality will be added in
the next exercise.
■The methods isEqual() and isLess() enable comparisons with a date
passed to them.
■The method asString() returns a reference to a string containing the
date in the format mm-dd-year, e.g. 03-19-2006.You will therefore need
to convert any numerical values into their corresponding decimal strings.
This operation is performed automatically when you use the << operator
to output a number to the standard output cout. In addition to the cin
and cout streams, with which you are already familiar, so-called string
streams with the same functionality also exist. However, a string stream
does not read keyboard input or output data on screen. Instead, the tar-
get, or source, is a buffer in main memory.This allows you to perform for-
matting and conversion in main memory.
■Use an application program that calls all the methods defined in the class
to test the Date class.
Exercise 3
The Date class does not ensure that an object represents a valid date.To avoid
this issue, add range checking functionality to the class. Range checking is
performed by the constructor and the setDate() method with three
parameters.
■First, write a function called isLeapYear() that belongs to the bool
type and checks whether the year passed to it is a leap year. Define the
function as a global inline function and store it in the header file
Date.h.
■Modify the setDate() method to allow range checking for the date
passed to it.The constructor can call setDate().
■Test the new version of the Date class.To do so, and to test set-
Date(...), read a date from the keyboard.
solutions
290 ■
CHAPTER 14 METHODS
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// article.h
// Defines a simple class, Article.
// ----------------------------------------------------
#ifndef _ARTICLE_
#define _ARTICLE_
#include <string>
using namespace std;
class Article
{
private:
long nr; // Article number
string name; // Article name
double sp; // Selling price
public:
Article( long nr=0, const string& name="noname",
double sp=0.0);
~Article();
void print();
const string& getName() const { return name; }
long getNr() const { return nr; }
double getSP() const { return sp; }
bool setName( const string& s)
{
if( s.size() < 1) // No empty name
return false;
name = s;
return true;
}
void setNr( long n) { nr = n; }
void setSP(double v)
{ // No negative price
sp = v > 0.0 ? v : 0.0;
}
};
#endif // _ARTICLE_
SOLUTIONS
■291
// ------------------------------------------------------
// article.cpp
// Defines those methods of Article, which are
// not defined inline.
// Screen output for constructor and destructor calls.
// ------------------------------------------------------
#include "Article.h" // Definition of the class
#include <iostream>
#include <iomanip>
using namespace std;
// Global counter for the objects:
int count = 0;
// ------------------------------------------------------
// Define constructor and destructor:
Article::Article( long nr, const string& name, double sp)
{
setNr(nr); setName(name); setSP(sp);
++count;
cout << "Created object for the article " << name
<< ".\n"
<< "This is the " << count << ". article!\n"
}
Article::~Article()
{
cout << "Cleaned up object for the article " << name
<< ".\n"
<< "There are still " << --count << " articles!"
<< endl;
}
// ------------------------------------------------------
// The method print() outputs an article.
void Article::print()
{
long savedFlags = cout.flags(); // To mark the
// flags of cout.
cout << fixed << setprecision(2)
<< "-----------------------------------------\n"
<< "Article data:\n"
<< " Number ....: " << nr << '\n'
<< " Name ....: " << name << '\n'
<< " Sales price: " << sp << '\n'
<< "-----------------------------------------"
<< endl;
cout.flags(savedFlags); // To restore
// old flags.
cout << " --- Go on with return --- ";
cin.get();
}
292 ■
CHAPTER 14 METHODS
// ------------------------------------------------------
// article_t.cpp
// Tests the Article class.
// ------------------------------------------------------
#include "Article.h" // Definition of the class
#include <iostream>
#include <string>
using namespace std;
void test();
// -- Creates and destroys objects of Article class --
Article Article1( 1111,"volley ball", 59.9);
int main()
{
cout << "\nThe first statement in main().\n" << endl;
Article Article2( 2222,"gym-shoes", 199.99);
Article1.print();
Article2.print();
Article& shoes = Article2; // Another name
shoes.setNr( 2233);
shoes.setName("jogging-shoes");
shoes.setSP( shoes.getSP() - 50.0);
cout << "\nThe new values of the shoes object:\n";
shoes.print();
cout << "\nThe first call to test()." << endl;
test();
cout << "\nThe second call to test()." << endl;
test();
cout << "\nThe last statement in main().\n" << endl;
return 0;
}
void test()
{
Article shirt( 3333, "T-Shirt", 29.9);
shirt.print();
static Article net( 4444, "volley ball net", 99.0);
net.print();
cout << "\nLast statement in function test()"
<< endl;
}
SOLUTIONS
■293
Answer to the supplementary question:
The copy constructor is called on each “passing by value,” although this
constructor has not been defined explicitly. In other words, the implicitly defined
copy constructor is used and of course does not increment the object counter.
However, the explicitly defined destructor, which decrements the counter, is still
called for each object.
Exercises 2 and 3
// ------------------------------------------------------
// Date.h
// Defining class Date with optimized
// functionality, e.g. range check.
// ------------------------------------------------------
#ifndef _DATE_ // Avoids multiple inclusions.
#define _DATE_
#include <string>
using namespace std;
class Date
{
private:
short month, day, year;
public:
Date() // Default constructor
{ month = day = year = 1; }
Date( int month, int day, int year)
{
if( !setDate( month, day, year) )
month = day = year = 1; // If date is invalid
}
void setDate(); // Sets the current date
bool setDate( int month, int day, int year);
int getMonth() const { return month; }
int getDay() const { return day; }
int getYear() const { return year; }
bool isEqual( const Date& d) const
{
return month == d.month && day == d.day
&& year == d.year ;
}
bool isLess( const Date& d) const;
const string& asString() const;
void print(void) const;
};
294 ■
CHAPTER 14 METHODS
inline bool Date::isLess( const Date& d) const
{
if( year != d.year) return year < d.year;
else if( month != d.month) return month < d.month;
else return day < d.day;
}
inline bool isLeapYear( int year)
{
return (year%4 == 0 && year%100 != 0) || year%400 == 0;
}
#endif // _DATE_
// ------------------------------------------------------
// Date.cpp
// Implements those methods of Date class,
// which are not defined inline.
// ------------------------------------------------------
#include "Date.h" // Class definition
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>
#include <ctime>
using namespace std;
// -----------------------------------------------------
void Date::setDate() // Get the present date and
{ // assign it to the data members.
struct tm *dur; // Pointer to struct tm.
time_t sec; // For seconds.
time(&sec); // Get the present time.
dur = localtime(&sec); // Initialize a struct of
// type tm and return a
// pointer to it.
day = (short) dur->tm_mday;
month = (short) dur->tm_mon + 1;
year = (short) dur->tm_year + 1900;
}
SOLUTIONS
■295
// -----------------------------------------------------
bool Date::setDate( int mn, int da, int yr)
{
if( mn < 1 || mn > 12 ) return false;
if( da < 1 || da > 31 ) return false;
switch(mn) // Month with less than 31 days
{
case 2: if( isLeapYear(yr))
{
if( da > 29)
return false;
}
else if( da > 28)
return false;
break;
case 4:
case 6:
case 9:
case 11:
if( da > 30) return false;
}
month = (short) mn;
day = (short) da;
year = (short) yr;
return true;
}
// -----------------------------------------------------
void Date::print() const // Output a date
{
cout << asString() << endl;
}
// -----------------------------------------------------
const string& Date::asString() const // Return a date
{ // as string.
static string dateString;
stringstream iostream; // For conversion
// number -> string
iostream << setfill('0') // and formatting.
<< setw(2) << month << '-'
<< setw(2) << day << '-' << year;
iostream >> dateString;
return dateString;
}
296 ■
CHAPTER 14 METHODS
// ------------------------------------------------------
// date_t.cpp
// Using objects of class Date.
// ------------------------------------------------------
#include "Date.h"
#include <iostream>
using namespace std;
int main()
{
Date today, birthday( 1, 29, 1927);
const Date d2010(1,1,2010);
cout << "\n Brigit's birthday: "
<< birthday.asString() << endl;
today.setDate();
cout << "\nToday's date: " << today.asString()
<< endl;;
if( today.isLess( d2010))
cout << " Good luck for this decade \n"
<< endl;
else
cout << " See you next decade \n" << endl;
Date holiday;
int month, day, year; char c;
cout << "\nWhen does your next vacation begin?\n"
<< "Enter in Month-Day-Year format: ";
if( !(cin >> month >> c >> day >> c >> year) )
cerr << "Invalid input!\n" << endl;
else if ( !holiday.setDate( month, day, year))
cerr << "Invalid date!\n" << endl;
else
{
cout << "\nYour first vacation: ";
holiday.print();
if( today.getYear() < holiday.getYear())
cout << "You should go on vacation this year!\n"
<< endl;
else
cout << "Have a nice trip!\n" << endl;
}
return 0;
}
297
Member Objects and
Static Members
The major topics discussed in this chapter are
■member objects and how they are initialized
■data members that are created once only for all the objects in a
class.
In addition, this chapter describes constant members and enumerated
types.
chapter
15
298 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
// result.h
// Class Result to represent a measurement
// and the time of measurement.
// ---------------------------------------------------
#ifndef _RESULT_
#define _RESULT_
#include "DayTime.h" // Class DayTime
class Result
{
private:
double val;
DayTime time;
public:
Result(); // Default constructor
Result(double w, const DayTime& z = currentTime());
Result(double w, int hr, int min, int sec);
double getVal() const { return val; }
void setVal( double w ) { val = w; }
const DayTime& getTime() const { return time; }
void setTime( const DayTime& z) { time = z; }
bool setTime(int hr, int min, int sec)
{ return time.setTime( hr, min, sec); }
void print() const; // Output result and time.
};
#endif // _RESULT_
#include "result.h"
Result::Result() { val = 0.0; }
Result::Result( double w, const DayTime& z)
{
val = w; time = z;
}
Result::Result( double w, int hr, int min, int sec)
{ val = w;
time = DayTime(hr, min, sec); // Assign a temporary
// object of type
} // DayTime to time.
■MEMBER OBJECTS
A class representing measurement results
A first implementation of constructors
MEMBER OBJECTS
■299
䊐“Has-A” Relationship
Data members belonging to one class can be objects of a different class. In our example,
the Account class, we already made use of this feature. The name of the account holder
is stored in a string type data member. An object of the Account class therefore has a
string type member sub-object, or member object for short.
If a class contains a data member whose type is of a different class, the relationship
between the classes is referred to as a “Has-A” relationship.
䊐Calling Constructors
When an object containing member objects is initialized, multiple constructor calls are
to be executed. One of these is the constructor for the complete object, and the others
are constructors for the member objects. The order of the constructor calls is significant in
this case. First, the member objects are created and initialized; this then allows the con-
structor to create the whole object. Unless otherwise stated, the default constructor will
be called for each member object.
䊐The Constructors for the Sample Class Result
The example on the opposite page defines a sample class called Result. In addition to a
double type measurement, the time of each measurement is recorded. For ease of read-
ing, the constructors were defined separately, rather than as inline.
The default constructor only sets the value of the measurement to 0. However, initial-
ization is complete since the default constructor is called for the member object time.
Example: Result current;
The default constructor for the member object time first sets the hours, minutes and
seconds to 0. Then the constructor for the Result class is called and a value of 0.0 is
assigned to val.
The other constructors can be used to initialize an object explicitly.
Example: Result temperature1(15.9); // Current Time
Result temperature2(16.7, 14, 30, 35);
Since the compiler has no information on the relation of initial values and member
objects, it first calls the default constructor for the member object time. Subsequently
the instructions for the Result class constructor can be executed, and values are
assigned to the data members.
300 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
#include "result.h"
Result::Result() : val(0.0) { /* ... */ }
Result::Result( double w, const DayTime& z)
: val(w), time(z)
{ /* ... */ }
Result::Result( double w, int hr, int min, int sec)
: val(w), time(hr, min, sec)
{
/* ... */
}
You can replace the comment /* ... */ with statements, if needed. However, in the case of the
Result class there is nothing to do at present.
✓NOTE
// result_t.cpp
// Tests constructors of class Result
// ---------------------------------------------------
#include "Result.h"
#include <iostream>
using namespace std;
int main() // Some air temperature measurements
{
DayTime morning(6,30);
Result t1, // Default constructor
t2( 12.5, morning),
t3( 18.2, 12,0,0),
t4(17.7); // at current time
cout << "Default values: "; t1.print();
cout << "\n Temperature Time \n"
<< "-------------------------" << endl;
t2.print();
t3.print();
t4.print();
cout << endl;
return 0;
}
■MEMBER INITIALIZERS
New implementation of constructors
Sample program
MEMBER INITIALIZERS
■301
䊐Initializing Member Objects
Calling default constructors to create member objects raises several issues:
■A member object is initialized first with default values. Correct values are
assigned later. This additional action can impact your program’s performance.
■Constant objects or references cannot be declared as member objects since it is
impossible to assign values to them later.
■Classes that do not have a default constructor definition cannot be used as types
for member objects.
When defining a constructor, you can use member initializers to ensure general and
efficient use of member objects.
䊐Syntax for Member Initializers
A member initializer contains the name of the data member, followed by the initial val-
ues in parentheses.
Example: time(hr,min,sec) // Member initializer
Multiple member initializers are separated by commas. A list of member initializers
defined in this way follows the constructor header and is separated from the header by a
colon.
Example: Result::Result( /* Parameters */ )
: val(w), time(hr, min, sec)
{ /* Function block */ }
This ensures that a suitable constructor will be called for data members with member ini-
tializers and avoids calls to the default constructor with subsequent assignments. As the
example shows, you can also use member initializers for data members belonging to fun-
damental types.
The argument names of the member initializers are normally constructor parameters.
This helps pass the values used to create an object to the right member object.
Member initializers can only be stated in a constructor definition. The constructor declaration remains
unchanged.
✓NOTE
302 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
// result2.h
// The class Result with a constant data member.
// ---------------------------------------------------
#ifndef _RESULT_
#define _RESULT_
#include "DayTime.h" // Class DayTime
class Result
{
private:
double val;
const DayTime time;
public:
Result(double w, const DayTime& z = currentTime());
Result(double w, int hr, int min, int sec);
double getVal() const { return val; }
void setVal( double w ) { val = w; }
const DayTime& getTime() const { return time; }
void print() const;
};
#endif // _RESULT_
// result2_t.cpp : Tests the new class Result.
// ---------------------------------------------------
#include "result2.h"
#include <iostream>
using namespace std;
int main()
{
DayTime start(10,15);
Result m1( 101.01, start),
m2( m1), // Copy constructor ok!
m3( 99.9); // At current time.
// m2 = m3; // Error! Standard assignment incorrect.
m2.setVal(100.9); // Corrected value for m2
cout << "\n Result Time \n"
<< "-------------------------" << endl;
m1.print(); m2.print(); m3.print();
return 0;
}
■CONSTANT MEMBER OBJECTS
New version of class Result
Using the new class Result
CONSTANT MEMBER OBJECTS
■303
䊐Declaring const Member Objects
If a class contains data members that need to keep their initial values, you can define
these members as const. For example, you could set the time for a measurement once
and not change this time subsequently. However, you need to be able to edit the meas-
urement value to correct systematic errors. In this case, the member object time can be
declared as follows:
Example: const DayTime time;
Since the const member object time cannot be modified by a later assignment, the cor-
rect constructor must be called to initialize the object. In other words, when you define a
constructor for a class, you must also define a member initializer for each const member
object.
䊐The Sample Class Result
If the member object time is const, the first version of the constructors are invalid
since they modify time by means of a later assignment.
Example: time = DayTime(st, mn, sk); // Error!
However, the later versions of these constructors are ok. The member initializer ensures
that the desired initial values are used to create the member object time.
One further effect of the const member object is the fact that the setTime(...)
methods can no longer be applied. The compiler will issue an error message at this point
and for any statement in the current program that attempts to modify the static member,
time. This means that a programmer cannot accidentally overwrite a member declared
as a const.
The new version of the Result class no longer contains a default constructor, since a
default value for the time of the measurement does not make sense.
䊐Example with Fundamental Type
Data members with fundamental types can also be defined as const. The class Client
contains a number, nr, which is used to identify customers. Since the client number
never changes, it makes sense to define the number as const. The constructor for
Client would then read as follows:
Example: Client::Client( /*...*/ ) : nr(++id)
{ /*...*/ }
The member initializer nr(++id) initializes the const data member nr with the
global value id, which is incremented prior to use.
304 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
// result3.h
// The class Result with static data members.
// ---------------------------------------------------
#ifndef _RESULT_
#define _RESULT_
#include "DayTime.h" // Class DayTime
class Result
{
private:
double val;
const DayTime time;
// Declaration of static members:
static double min, max; // Minimum, maximum
static bool first; // true, if it is the first value.
void setMinMax(double w); // private function
public:
Result(double w, const DayTime& z = currentTime());
Result(double w, int hr, int min, int sec);
// ... The other member functions as before
};
#endif // _RESULT_
// result3.cpp
// Defining static data members and
// methods, which are not defined inline.
// ---------------------------------------------------
#include "result3.h"
double Result::min = 0.0;
double Result::max = 0.0;
bool Result::first = true;
void Result::setMinMax(double w) // Help function
{ if(first) { min = max = w; first = false; }
else if( w < min) min = w;
else if( w > max) max = w;
}
// Constructors with member initializer.
Result::Result( double w, const DayTime& z)
: val(w), time(z)
{ setMinMax(w); }
Result::Result( double w, int hr, int min, int sec)
: val(w), time(hr, min, sec)
{ setMinMax(w); }
// Implements the other member functions.
■STATIC DATA MEMBERS
Class Result with static members
Implementation and initialization
STATIC DATA MEMBERS
■305
䊐Class-Specific Data
Every object has its own characteristics. This means that the data members of two differ-
ent objects will be stored at different memory addresses.
However, sometimes it is useful to keep some common data that can be accessed by all
the objects belonging to a class, for example:
■figures such as exchange rates, interest rates or time limits which have the same
value for every object
■status information, such as the number of objects, current minimum or maximum
threshold values, or pointers to some objects; for example, a pointer to an active
window in a window class.
This kind of data needs to be stored once only, no matter how many objects exist.
Since a programmer will also need to manage the data from within the class, it should be
represented within the class rather than globally. Static data members can be used for this
purpose. In contrast to normal data members, static data members occur only once in
memory.
䊐Declaration
Static data members are declared within a class, that is, the keyword static is used to
declare members of this type. On the opposite page, the following statement
Example: static double min, max; // Declaration
defines two static data members called min and max that record the minimum and maxi-
mum values for the measurements.
䊐Definition and Initialization
Static data members occupy memory space even if no objects of the class in question
have been created. Just like member functions, which occur only once, static data mem-
bers must be defined and initialized in an external source file. The range operator :: is
then used to relate the data members to the class.
Example: double Result::min = 0.0; // Definition
As the example illustrates, the static keyword is not used during the definition. Static
data members and member functions belonging to the same class are normally defined in
one source file.
306 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
class Result
{
private:
double val;
const DayTime time;
static double min, max; // Minimum, Maximum
static bool first; // true, if first result
static void setMinMax(double w); // Help function
public:
// ... Member functions as before, plus:
static double getMin() { return min; }
static double getMax() { return max; }
};
// result3_t.cpp
// Uses the new class Result.
// ---------------------------------------------------
#include "result3.h"
#include <iostream>
using namespace std;
int main() //Some air temperature measurements
{
DayTime morning(6,45);
Result temp1( 6.45, morning),
temp2( 11.2, 12,0,0);
double temp = 0.0;
cout << "\nWhat is the air temperature now? ";
cin >> temp;
Result temp3(temp); // At current time.
cout << "\n Temperature Time \n"
<< "-------------------------" << endl;
temp1.print(); temp2.print(); temp3.print();
cout
<< "\n Minimum Temperature: " << Result::getMin()
<< "\n Maximum Temperature: " << Result::getMax()
<< endl;
return 0;
}
■ACCESSING STATIC DATA MEMBERS
Class Result with static methods
Application program
ACCESSING STATIC DATA MEMBERS
■307
䊐Static Data Members and Encapsulation
The normal rules for data encapsulation also apply to static data members. A static data
member declared as public is therefore directly accessible to any object.
If the static data members min and max in the Result class are declared public
rather than private, and given that temperature is an object belonging to the class,
the following statement
Example: cout << temperature.max;
outputs the maximum measured value. You can also use the range operator:
Example: cout << Result::max;
This syntax is preferable to the previous example, since it clearly shows that a static data
member exists independently of any objects.
䊐Static Member Functions
Of course, you can use class methods to access a static data member with a private
declaration. However, normal methods can be used for class objects only. Since static
data members are independent of any objects, access to them should also be independent
of any objects. Static member functions are used for this purpose. For example, you can call
a static member function for a class even though no objects exist in that class.
The static keyword is used to define static member functions.
Example: static double getMin(); // Within class.
As the Result class, which was modified to include the static member functions
getMin(),setMin(), etc. shows, an inline definition is also permissible. Defini-
tions outside of the class do not need to repeat the static keyword.
A static member function can be called using any object belonging to the class or,
preferably, using a range operator.
Example: temperature.setMax(42.4); // Equivalent
Result::setMax(42.4); // Calls.
Calling a static member function does not bind the function to any class object. The
this pointer is therefore unavailable, in contrast to normal member functions. This also
means that static member functions cannot access data members and methods that are
not static themselves.
308 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
// enum.cpp
// Uses enum-constants within a class.
// ---------------------------------------------------
#include <iostream>
using namespace std;
class Lights
{
public: // Enumeration for class Lights
enum State { off, red, green, amber };
private:
State state;
public:
Lights( State s = off) : state(s) {}
State getState() const { return state; }
void setState( State s)
{ switch(s)
{case off: cout << " OFF "; break;
case red: cout << " RED "; break;
case green: cout << " GREEN "; break;
case amber: cout << " AMBER "; break;
default: return;
}
state = s;
}
};
int main()
{
cout << "Some statements with objects "
<< "of type Lights!\n"
Lights A1, A2(Lights::red);
Lights::State as;
as = A2.getState();
if( as == Lights::red)
{
A1.setState( Lights::red);
A2.setState( Lights::amber);
}
cout << endl;
return 0;
}
■ENUMERATION
Sample program
ENUMERATION
■309
䊐Definition
An enumeration is a user-definable, integral type. An enumeration is defined using the
enum keyword. A range of values and a name for these values are also defined at the
same time.
Example: enum Shape{ Line, Rectangle, Ellipse};
This statement defines the enumerated type Shape. The names quoted in the list iden-
tify integral constants. Their values can be deduced from the list order. The first constant
has a value of 0, and each subsequent constant has a value that is one higher than its
predecessor.
In the previous example, Line thus represents a value of 0,Rectangle a value of 1,
and Ellipse a value of 2. A Shape type variable can only assume one of these values.
Example: Shape shape = Rectangle; // Variable shape
// ...
switch(shape) // To evaluate shape
{
case Line: // ... etc.
However, you can also define the values of the constants explicitly.
Example: enum Bound { Lower = -100, Upper = 100};
You can leave out the type name, if you only need to define the constants.
Example: enum { OFF, OUT=0, ON, IN=1 };
This statement defines the constants OFF and OUT, setting their value to 0, and the con-
stants ON and IN with a value of 1. The values for OFF and ON are implicit.
䊐Class-Specific Constants
Enumeration can be used to define integral symbolic constants in a simple way. In con-
trast to #define directives, which merely replace text strings, enum constants are part
of a declaration and thus have a valid range. This allows you to define constants that are
visible within a namespace or class only.
The example on the opposite page shows the enumerated type State, which was
defined within the Lights class. This means that the type and enum constant are only
available for direct use within the class. The enumeration itself is declared as public,
however, and access from outside the class is therefore possible.
Example: if(Lights.getState() == Lights::red)
// ...
exercises
310 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
Article( const Article& );
■EXERCISES
Copy constructor of class Article
The copy constructor creates a copy of an existing object.The parameter is thus
a read-only reference to the object that needs to be copied.The copy
constructor in the Article class is thus declared as follows:
Declaration of copy constructor:
The default copy constructor simply transfers the data members to the new
object.
The Member Class
int
string
const Date
Member Number
Name
Birthday
//Possibly more information, such as an address, telephone number, ...
Constructor with one parameter for each data member
Access methods for each data member. The birthday is read-only.
A method for formatted screen output of all data members
Private Data Members
Public Methods
Type
EXERCISES
■311
Exercise 1
In the first exercise of the last chapter you defined a simple class called
Article.This involved using a global counter to log object creation and
destruction. Improve and extend the Article class as follows:
■Use a static data member instead of a global variable to count the current
number of objects.
■Declare a static access method called getCount()for the Article class.
The method returns the current number of objects.
■Define a copy constructor that also increments the object counter by 1
and issues a message.This ensures that the counter will always be accu-
rate.
Tip: Use member initializers.
■Test the new version of the class.To do so, call the function test() by
passing an article type object to the function.
Exercise 2
A sports club needs a program to manage its members.Your task is to define
and test a class called Member for this purpose.
■Define the Member class using the data members shown opposite. Use
the Date class defined in the last chapter for your definition. Since a
member’s birthday will not change, the data member for birthdays must
be defined as a const.
Overload the constructor to allow for entering a date as an object as
well as three values for day, month, and year.
■Implement the necessary methods.
■Test the new Member class by creating at least two objects with the data
of your choice and calling the methods you defined.
■Add a static member called ptrBoss to the class.This pointer indicates
the member who has been appointed as chairperson. If no chairperson
has been appointed, the pointer should point to NULL.
■Additionally, define the static access methods getBoss() and setBoss().
Use a pointer to set and return the object in question.
■Test the enhanced Member class by reading the number of an existing
member, making the member the new chairperson and displaying the
chairperson using getBoss().
312 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
Simulation of two traffic lights!
Terminate this program with <Ctrl>+<C>!
1. Light 2. Light
---------------------------
RED AMBER
GREEN
AMBER
AMBER RED
GREEN
AMBER
RED AMBER
GREEN
// . . .
Sample output
Hints for implementing the function wait()
1. The function time() is declared in the header file ctime. The call
time(NULL) determines the number of seconds of type time_t since
1.1.1970, 0:0 hours.The type time_t is defined as long.
2. Instead of calling the function time() in a loop, you can use the function
Sleep() for Windows or the function sleep() for Unix. These system
calls are not standardized, yet they are much more effective because they
send a process to sleep instead of using a waiting loop.
EXERCISES
■313
Exercise 3
Create a program to simulate the signal positions for two sets of traffic lights at
a junction. Use the class Lights as defined in this chapter for your program.
■Each set of lights is switched through the phases red, amber, green, amber,
red, and so on.You must ensure that one set of lights can be only in the
amber or green state when the other set of lights is red.
■The lights operate in an infinite loop that can be terminated by interrupt-
ing the program.You can use the key combination <Ctrl>+<C> for DOS
and Windows and the Interrupt key, i.e., normally the <Del> key, for
UNIX.
■The status of the lights is constant for a certain number of seconds. For
example, the green phase can take 20 seconds and the amber phase 1
second.These values can be different for each set of lights. Define an
auxiliary function
inline void wait( int sec)
The function returns after the stipulated number of seconds.To do so,
you can call the standard function time() in a loop. Don’t forget to read
the notes on the opposite page.
solutions
314 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// article.h
// Defines a simple class - Article.
// ----------------------------------------------------
#ifndef _ARTICLE_H_
#define _ARTICLE_H_
#include <string>
using namespace std;
class Article
{
private:
long nr; // Article number
string name; // Article name
double sp; // Sales price
// Static data member:
static int countObj; // Number of objects
public:
Article( long nr=0, const string& name="noname",
double sp=0.0);
// Copy constructor:
Article( const Article& anArticle);
~Article();
void print();
// Access methods:
const string& getName() const { return name; }
long getNr() const { return nr; }
double getSP() const { return sp; }
static int getCount() { return countObj; }
bool setName( const string& s)
{
if( s.size() < 1) // No empty Name
return false;
name = s;
return true;
}
void setNr( long n) { nr = n; }
void setSP(double v)
{ // No negative price
sp = v > 0.0 ? v : 0.0;
}
};
#endif // _ARTICLE_
SOLUTIONS
■315
// ---------------------------------------------------
// article.cpp
// Methods of Article, which are not defined as inline.
// Constructor and destructor output when called.
// ---------------------------------------------------
#include "article.h" // Definition of the class
#include <iostream>
#include <iomanip>
using namespace std;
// Defining the static data member:
int Article::countObj = 0; // Number of objects
// Defining the constructor and destructor:
Article::Article( long nr, const string& name, double sp)
{
setNr(nr); setName(name); setSP(sp);
++countObj;
cout << "An article \"" << name
<< "\" is created.\n"
<< "This is the " << countObj << ". article!"
<< endl;
}
// Defining the copy constructor:
Article::Article( const Article& art)
:nr(art.nr), name(art.name), sp(art.sp)
{
++countObj;
cout << "A copy of the article \"" << name
<< "\" is generated.\n"
<< "This is the " << countObj << ". article!"
<< endl;
}
Article::~Article()
{
cout << "The article \"" << name
<< "\" is destroyed.\n"
<< "There are still " << --countObj << " articles!"
<< endl;
}
// The method print() outputs an article.
void Article::print()
{
// As before! Compare to the solutions of chapter 14.
}
316 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
// ------------------------------------------------------
// article_t.cpp
// Tests the class Article including a copy constructor.
// ------------------------------------------------------
#include "article.h" // Definition of the class
#include <iostream>
#include <string>
using namespace std;
void test( Article a); // Prototype
Article article1( 1111,"tent", 159.9); // Global
int main()
{
cout << "\nThe first statement in main().\n" << endl;
Article article2( 2222,"jogging shoes", 199.99);
cout << "\nThe first call of test()." << endl;
test(article1); // Passing by Value
cout << "\nThe second call of test()." << endl;
test(article2); // Passing by Value
cout << "\nThe last statement in main().\n"
<< "\nThere are still " << Article::getCount()
<< " objects\n" << endl;
return 0;
}
void test( Article a) // Calls the copy constructor
{
cout << "\nThe given object:" << endl;
a.print();
static Article bike( 3333, "bicycle", 999.0);
cout << "\nThe static object in function test():"
<< endl;
bike.print();
cout << "\nThe last statement in function test()"
<< endl;
}
SOLUTIONS
■317
Exercise 2
The Date class from the last chapter ( see files Date.h and Date.cpp ) can be
left unchanged. But it makes sense to define the function isLeapYear()as a
static member function of class Date rather than globally.
The other files:
// ------------------------------------------------------
// member.h
// Defines the Member class containing a constant
// and a static member.
// ------------------------------------------------------
#ifndef _MEMBER_H_
#define _MEMBER_H_
#include "Date.h"
#include <string>
using namespace std;
class Member
{
private:
int nr; // Member number
string name; // Name
const Date birth; // Birthday
// ... more data
static Member *ptrBoss; // Pointer to boss,
// NULL = no boss.
public:
Member( long m_nr, const string& m_name,
const Date& m_birth)
: nr(m_nr), birth(m_birth)
{
if( !setName(m_name)) name = "Unknown";
}
Member( long m_nr, const string& m_name,
int day, int month, int year)
: nr(m_nr), birth(day,month,year)
{
if( !setName(m_name)) name = "Unknown";
}
int getNr() const { return nr; }
const string& getName() const { return name; }
const Date& getBirthday() const { return birth; }
void setNr( int n) { nr = n; }
318 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
bool setName( const string& s)
{
if( s.size() < 1) // No empty name
return false;
name = s;
return true;
}
void display() const;
// static methods:
static Member* getBoss()
{
return ptrBoss;
}
static void setBoss( Member* ptrMem)
{
ptrBoss = ptrMem;
}
};
#endif // _MEMBER_H_
// ---------------------------------------------------
// member.cpp
// Members of class Member not defined inline.
// ---------------------------------------------------
#include "member.h" // Class definition
#include <iostream>
using namespace std;
// Pointer to the boss:
Member* Member::ptrBoss = NULL;
void Member::display() const
{
string line( 50, '-');
cout << line
<< "\n Member number: " << nr
<< "\n Member: " << name
<< "\n Birthday " << birth.asString()
<< '\n' << line << endl;
}
SOLUTIONS
■319
// ----------------------------------------------------
// member_t.cpp
// Using the class Member.
// ----------------------------------------------------
#include "member.h" // Class definition
#include <iostream>
#include <string>
using namespace std;
int main()
{
Date today; today.setDate();
cout << "Date: " << today.asString() << endl;
Member fran( 0, "Quick, Fran", 17,11,81),
kurt( 2222, "Rush, Kurt", Date(3,5,77) );
franzi.setNr(1111);
cout << "\nTwo members of the sports club:\n" << endl;
fran.display();
kurt.display();
cout << "\nSomething changed!" << endl;
fran.setName("Rush-Quick");
fran.display();
Member benny( 1122,"Rush, Benny", 1,1,2000);
cout << "The youngest member of the sports club: \n";
benny.display();
// Who is the boss?
int nr;
Member *ptr = NULL;
cout << "\nWho is the boss of the sports club?\n"
<< "Enter the member number: ";
if( cin >> nr)
{
if( nr == fran.getNr())
ptr = &fran;
else if( nr == kurt.getNr())
ptr = &kurt;
Member::setBoss( ptr);
}
cout << "\nThe Boss of the sports club:" << endl;
ptr = Member::getBoss();
if( ptr != NULL)
ptr->display();
else
cout << "No boss existing!" << endl;
return 0;
}
320 ■
CHAPTER 15 MEMBER OBJECTS AND STATIC MEMBERS
Exercise 3
The definition of class Lights from this chapter remains
unchanged.
// -----------------------------------------------------
// Lights_t.cpp : Simulates two traffic lights.
// -----------------------------------------------------
#include "lights.h" // Definition of class Lights
#include <iostream>
#include <ctime> // Standard function time()
using namespace hr;
inline void wait( int sec) // Wait sec seconds.
{ time_t end = time(NULL) + sec;
while( time(NULL) < end) ;
}
// Alternative for Windows:
// #include <windows.h>
// inline void wait( int sec) { Sleep( 1000 * sec); }
Lights A1, A2; // Traffic lights and
enum { greenTime1 = 10 , amberTime1 = 1, // time to wait.
greenTime2 = 14 , amberTime2 = 2 };
int main()
{ cout << "Simulating two traffic lights!\n\n"
<< "Terminate this program with <Ctrl>+<C>!\n"
<< endl;
cout << " 1. Light 2. Light\n"
<< "---------------------------" << endl;
while(true)
{ A1.setState( Lights::red); // A1 = red
A2.setState( Lights::amber); cout << endl;
wait( amberTime2);
cout << " ";
A2.setState( Lights::green); cout << endl;
wait(greenTime2);
cout << " ";
A2.setState( Lights::amber); cout << endl;
wait(amberTime2);
A1.setState( Lights::amber); // A2 = red
A2.setState( Lights::red); cout << endl;
wait(amberTime1);
A1.setState( Lights::green); cout << endl;
wait(greenTime1);
A1.setState( Lights::amber); cout << endl;
wait(amberTime1);
}
return 0;
}
321
Arrays
This chapter describes how to define and use arrays, illustrating one-
dimensional and multidimensional arrays, C strings and class arrays.
chapter
16
322 ■
CHAPTER 16 ARRAYS
// array.cpp
// To input numbers into an array and output after.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
const int MAXCNT = 10; // Constant
float arr[MAXCNT], x; // Array, temp. variable
int i, cnt; // Index, quantity
cout << "Enter up to 10 numbers \n"
<< "(Quit with a letter):" << endl;
for( i = 0; i < MAXCNT && cin >> x; ++i)
arr[i] = x;
cnt = i;
cout << "The given numbers:\n" << endl;
for( i = 0; i < cnt; ++i)
cout << setw(10) << arr[i];
cout << endl;
return 0;
}
arr[0]
arr[1]
arr[2]
arr[9]
.
.
.
. . .
■DEFINING ARRAYS
The array arr in memory
Sample program
DEFINING ARRAYS
■323
An array contains multiple objects of identical types stored sequentially in memory. The
individual objects in an array, referred to as array elements, can be addressed using a num-
ber, the so-called index or subscript. An array is also referred to as a vector.
䊐Defining Arrays
An array must be defined just like any other object. The definition includes the array
name and the type and number of array elements.
Syntax: type name[count]; // Array name
In the above syntax description, count is an integral constant or integral expression
containing only constants.
Example: float arr[10]; // Array arr
This statement defines the array arr with 10 elements of float type. The object arr
itself is of a derived type, an “array of float elements” or “float array.”
An array always occupies a contiguous memory space. In the case of the array arr, this
space is 10*sizeof(float) = 40 bytes.
䊐Index for Array Elements
The subscript operator [] is used to access individual array elements. In C++ an index
always begins at zero. The elements belonging to the array arr are thus
arr[0], arr[1] , arr[2], ... , arr[9]
The index of the last array element is thus 1 lower than the number of array elements.
Any int expression can be used as an index. The subscript operator [] has high prece-
dence, just like the class member operators .and -> .
No error message is issued if the index exceeds the valid index range. As a program-
mer, you need to be particularly careful to avoid this error! However, you can define a
class to perform range checking for indices.
You can create an array from any type with the exception of some special types, such
as void and certain classes. Class arrays are discussed later.
Example: short number[20]; // short array
for( int i=0; i < 20; i++ )
number[i] = (short)(i*10);
This example defines an array called number with 20 short elements and assigns the
values 0, 10, 20, ... , 190 to the elements.
324 ■
CHAPTER 16 ARRAYS
// fibo.cpp
// The program computes the first 20 Fibonacci
// numbers and the corresponding Fibonacci quotients.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cmath> // Prototype of sqrt()
#include <string>
using namespace std;
#define COUNT 20
long fib[COUNT + 1] = { 0, 1 };
string header =
" Index Fibonacci number Fibonacci quotient Deviation"
"\n of limit "
"\n---------------------------------------------------";
int main()
{
int i;
double q, lim;
for( i=1; i < COUNT; ++i ) // Computing the
fib[i+1] = fib[i] + fib[i-1]; // Fibonacci numbers
lim = ( 1.0 + sqrt(5.0)) / 2.0; // Limit
// Title and the first two Fibonacci numbers:
cout << header << endl;
cout << setw(5) << 0 << setw(15) << fib[0] << endl;
cout << setw(5) << 1 << setw(15) << fib[1] << endl;
// Rest of the table:
for( i=2; i <= COUNT; i++ )
{ // Quotient:
q = (double)fib[i] / (double)fib[i-1];
cout << setw(5) << i << setw(15) << fib[i]
<< setw(20) << fixed << setprecision(10) << q
<< setw(20) << scientific << setprecision(3)
<< lim - q << endl;
}
return 0;
}
■INITIALIZING ARRAYS
Sample program
INITIALIZING ARRAYS
■325
䊐Initialization List
Arrays can be initialized when you define them. A list containing the values for the indi-
vidual array elements is used to initialize the array:
Example: int num[3] = { 30, 50, 80 };
A value of 30 is assigned to num[0],50 to num[1], and 80 to num[2]. If you initialize
an array when you define it, you do not need to state its length.
Example: int num[] = { 30, 50, 80 };
In this case, the length of the array is equal to the number of initial values. If the array
length is explicitly stated in the definition and is larger than the number of initial values,
any remaining array elements are set to zero. If, in contrast, the number of initial values
exceeds the array length, the surplus values are ignored.
Locally defined arrays are created on the stack at program runtime. You should there-
fore be aware of the following issues when defining arrays:
■Arrays that occupy a large amount of memory (e.g., more than one kbyte) should
be defined as global or static.
■Unless they are initialized, the elements of a local array will not necessarily have
a definite value. Values are normally assigned by means of a loop.
You cannot assign a vector to another vector. However, you can overload the assign-
ment operator within a class designed to represent arrays. This topic will be discussed in
depth later.
䊐The Sample Program Opposite
The example on the opposite page contains the first twenty Fibonacci numbers and their
quotients. Fibonacci numbers are useful for representing natural growth. In computer sci-
ence, Fibonacci numbers are used for things like memory management and hashing.
Their definition is as follows:
■the first Fibonacci number is 0, the second is 1
■each subsequent Fibonacci number is the sum of its two immediate predecessors.
This results in the following sequence: 0, 1, 1, 2, 3, 5, 8, 13, ....
The quotient of a Fibonacci number and its predecessor is referred to as a Fibonacci
quotient. The sequence of Fibonacci quotients, 1/1,2/1,3/2, ..., converges towards
the threshold value (1 + √5)/2.
326 ■
CHAPTER 16 ARRAYS
String text
Index: 01234567891011
'H' 'e' 'l' 'l' 'o' ' ' 'E' 'v' 'e' '\0' . .
The array text has length of 40, whereas the string “Hello Eve" only occupies the first 9bytes.
✓NOTE
// C-string.cpp : Using C strings.
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstring>
using namespace std;
char header[] = "\n *** C Strings ***\n\n";
int main()
{
char hello[30] = "Hello ", name[20], message[80];
cout << header << "Your first name: ";
cin >> setw(20) >> name; // Enter a word.
strcat( hello, name); // Append the name.
cout << hello << endl;
cin.sync(); // No previous input.
cout << "\nWhat is the message for today?"
<< endl;
cin.getline( message, 80); // Enter a line with a
// max of 79 characters.
if( strlen( message) > 0) // If string length is
{ // longer than 0.
for( int i=0; message[i] != '\0'; ++i)
cout << message[i] << ' '; // Output with
cout << endl; // white spaces.
}
return 0;
}
■C STRINGS
䊐Initializing
char text[40] = "Hello Eve";
String text in memory:
Sample program
C STRINGS
■327
䊐char Arrays
Arrays whose elements are of char type are often used as data communication buffers.
Example: char buffer[10*512]; // 5 Kbyte buffer
However, their most common use is for string storage. One way of representing a
string is to store the string and the terminating null character '\0' in a char array.
When you define an array, you can use a string constant to initialize the array.
Example: char name[] = "Hugo";
This definition is equivalent to
char name[] = { 'H','u','g','o','\0' };
As you can see, the string name occupies five bytes, including an additional byte for the
null character. If you need to allocate more memory, you can state the size of the array
explicitly as shown opposite.
In the C language, strings are usually represented as char vectors with a terminating
null character. In C++, strings of this type are referred to as C strings to distinguish them
from objects of the string class.
䊐C Strings and the string Class
C strings are simple char arrays, which means that the functionality of the string
class is not available for them. Thus, for example, assignments and comparisons are not
defined.
Example: char str1[20], str2[20] = "A string";
str1 = str2; // Error!
strcpy( str1, str2); // ok!
The standard functions of the C language, such as strlen(),strcpy(),strcmp(),
and others, are available for C strings. These global functions all begin with the str pre-
fix.
As the program on the opposite page shows, I/O streams are overloaded for char
arrays, too. Input and output are as easily achieved as with string class objects. How-
ever, the program must make sure not to overrun the end of the char array when read-
ing data into the array. You can use the width() method or the setw() manipulator
for this purpose.
Example: cin >> setw(20) >> name; // 19 characters
C strings are preferable to the string class if only a few operations are needed and
you want to avoid unnecessary overheads.
328 ■
CHAPTER 16 ARRAYS
// AccountTab.cpp
// An array containing objects of class Account.
// ---------------------------------------------------
#include "account.h" // Definition of class Account
#include <iostream>
using namespace std;
Account giro("Lucky, Peter", 1234567, -1200.99 );
Account accountTab[] =
{
Account("Tang, Sarah", 123000, 2500.0),
Account("Smith, John", 543001),
Account(), // Default constructor
"Li, Zhang", // Account("Li, Zhang"),
giro // Account(giro)
};
int cnt = sizeof(accountTab) / sizeof(Account);
int main()
{
// To set some values:
accountTab[1].setState( 10000.00);
// Assignment ok:
accountTab[2] =Account("Pit, Dave", 727003, 200.00);
cout << "The accounts in the table:" << endl;
for( int i = 0; i < cnt; ++i)
{
accountTab[i].display();
if( i % 3 == 2)
{
cout << "Press return to go on!\n";
cin.get();
}
}
cout << endl;
return 0;
}
■CLASS ARRAYS
Sample program
CLASS ARRAYS
■329
䊐Declaring Class Arrays
Array elements can also be objects of a class type. The array is known as a class array in
this case. When you declare an array of this type, you only need to state the type of the
array elements.
Example: Result temperatureTab[24];
This statement defines the class array temperatureTab that stores 24 objects of type
Result. This class was introduced at the beginning of the last chapter.
As the statement does not initialize the array explicitly, the default constructor is
automatically called for each array element.
Thus, the previous example is only valid for the first version of the Result class as
this class contains a default constructor.
䊐Explicit Initialization
A class array is initialized as usual by an initialization list. The list contains a constructor
call for each array element.
Example: Result temperatureTab[24] =
{
Result( -2.5, 0,30,30),
Result( 3.5), // At present time
4.5, // Just so
Result( temp1), // Copy constructor
temp2 // Just so
};
The first five array elements are initialized by the constructor calls implicitly contained
in these statements. Instead of using a constructor with one argument, you can simply
supply the argument. The default constructor is then called for the remaining elements.
If the size of an array is not stated explicitly, the number of values in the initialization
list defines the size of the array.
The public interface of the objects in the array is available for use as usual.
Example: temperatureTab[2].setTime( 2,30,21);
No additional parentheses are needed in this statement since the subscript operator []
and the class member operator .are read from left to right, although they have the same
precedence.
Class arrays can only be defined without explicit initialization if a default constructor exists for the class.
✓NOTE
330 ■
CHAPTER 16 ARRAYS
// multidim.cpp
// Demonstrates multidimensional arrays.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
char representative[2][20] = {"Armstrong, Wendy",
"Beauty, Eve"};
// Each representative has five different
// articles available, having sold the following:
int articleCount[2][5] = { { 20, 51, 30, 17, 44},
{150, 120, 90, 110, 88}
};
int main()
{
for( int i=0; i < 2; i++ )
{
cout <<"\nRepresentative: " << representative[i];
cout << "\nNumber of items sold: ";
for( int j = 0; j < 5; j++ )
cout << setw(6) << articleCount[i][j];
cout << endl;
}
return 0;
}
■MULTIDIMENSIONAL ARRAYS
Sample program
Screen output:
Representative: Armstrong, Wendy
Items sold: 20 51 30 17 44
Representative: Beauty, Eve
Items sold: 150 120 90 110 88
MULTIDIMENSIONAL ARRAYS
■331
䊐Defining Multidimensional Arrays
In C++ you can define multidimensional arrays with any number of dimensions. The
ANSI standard stipulates a minimum of 256 dimensions but the total number of dimen-
sions is in fact limited by the amount of memory available.
The most common multidimensional array type is the two-dimensional array, the so-
called matrix.
Example: float number[3][10]; // 3 x 10 matrix
This defines a matrix called number that contains 3 rows and 10 columns. Each of the 30
(3 ⫻10) elements is a float type. The assignment
Example: number[0][9] = 7.2; // Row 0, column 9
stores the value 7.2 in the last element of the first row.
䊐Arrays as Array Elements
C++ does not need any special syntax to define multidimensional arrays. On the con-
trary, an n-dimensional array is no different than an array with only one dimension
whose elements are (n–1)-dimensional arrays.
The array number thus contains the following three elements:
number[0] number[1] number[2].
Each of these elements is a float array with a size of 10, which in turn forms the rows of
the two-dimensional array, number.
This means that the same rules apply to multidimensional arrays as to one-dimen-
sional arrays. The initialization list of a two-dimensional array thus contains the values of
the array elements, that is, the one-dimensional rows.
Examples: int arr[2][3] = { {5, 0, 0}, {7, 0, 0} };
int arr[][3] = { {5}, {7} };
These two definitions are equivalent. When you initialize an array, you can only omit
the size of the first dimension. It is necessary to define any other dimensions since they
define the size of array elements.
䊐The Example on the Opposite Page
The program opposite defines the two-dimensional arrays representative and
articleCount, which have two rows each. The representative[i] rows are
char arrays used for storing the names of the representatives. You can also use a one-
dimensional string array.
Example: string representative[2] = {"La..","Fo.."};
332 ■
CHAPTER 16 ARRAYS
// telList.h
// Class TelList to represent a list
// containing names and telephone numbers.
// ----------------------------------------------------
#ifndef _TelList_
#define _TelList_
#include <string>
using namespace std;
#define PSEUDO -1 // Pseudo position
#define MAX 100 // Maximal number of elements
// Type of a list element:
struct Element { string name, telNr; };
class TelList
{
private:
Element v[MAX]; // The array and the current
int count; // number of elements
public:
TelList(){ count = 0;}
int getCount() const { return count; }
Element *retrieve( int i )
{
return (i >= 0 && i < count)? &v[i] : NULL;
}
bool append( const Element& el )
{
return append( el.name, el.telNr);
}
bool append( const string& name,
const string& telNr);
bool erase( const string& name);
int search( const string& name);
void print();
int print( const string& name);
int getNewEntries();
};
#endif // _TelList_
■MEMBER ARRAYS
Class TelList
MEMBER ARRAYS
■333
䊐Encapsulating Arrays
A programmer often needs to handle objects of the same type, such as company employ-
ees, bank accounts, or the articles in stock. A class designed to perform this task can use
an array for ease of data management. An array allows you to access individual objects
directly and perform searches.
A class that encapsulates an array will provide methods for simple array operations,
such as inserting and deleting objects. When you design a class of this type, one aim will
be to perform automatic range checking. This helps avoid overrunning the end of an
array when performing read or write operations. The resulting class will contain a com-
fortable and safe interface for object data management.
䊐The Class TelList
The class TelList on the opposite page is designed to manage a simple telephone list.
Each entry in the list contains a dataset containing a name and a phone number. The
Element type, which comprises two strings, was defined for this purpose. The array v
can store up to MAX entries of the Element type. The data member count records the
number of elements currently stored in the array. When a phone list is created, this num-
ber will initially be 0. When an element is inserted or deleted, the number is modified
correspondingly.
The TelList class uses a single default constructor that sets the counter, count, to
zero. It is not necessary to provide an initial value for the MAX elements in the array v
since the default constructor of the string class is executed for all strings.
The tasks performed by the other methods are easily deduced from their names. The
retrieve() method returns to a given index a pointer to the corresponding element.
Using a pointer makes it possible to return a NULL pointer if the index is invalid.
The append() methods add a new entry to the list. The data passed to a method is
copied to the next free array element and the counter is incremented. If there is no space
available, the name field is empty, or the name is already in use, nothing happens. In this
case, the method returns false instead of true.
The exercises for this chapter contain further details on these methods. You can
implement the methods for the TelList yourself and go on to test them.
exercises
334 ■
CHAPTER 16 ARRAYS
Original array:
After the first loop:
After the second loop:
100 50 30 70 40
50 30 70 40 100
30 50 40 70 100
second largest element
largest element
0123456789 . . .
. . .
falseArray
Index
false true true false true false true false false
* * B R E A K * * * *
--- Press interrupt key to terminate (^C) ---
The output of a scrolling string has to be performed at the same cursor position.
The screen control characters make it possible to locate the cursor, and that
independent of the current compiler (see appendix).
✓NOTE
■EXERCISES
Example of a bubble sort algorithm
Sieve of Eratosthenes
For this task you can define an array of boolean values in which each element is
initially true.To eliminate a number nyou simply set the nth element in the array
to false.
Result:
Screen shot of exercise 4
EXERCISES
■335
Use the bubble sort algorithm to sort the array. This algorithm repeatedly accesses the array, comparing
neighboring array elements and swapping them if needed. The sorting algorithm terminates when there
are no more elements that need to be swapped. You use a flag to indicate that no elements have been
swapped.
✓NOTE
Exercise 1
Write a C++ program that reads a maximum of 100 integers from the keyboard,
stores them in a long array, sorts the integers in ascending order, and displays
sorted output. Input can be terminated by any invalid input, such as a letter.
Exercise 2
Chapter 14 introduced the sample class DayTime and the isLess() method.
Define and initialize an array with four DayTime class objects.
Then write a main function that first uses the print() method to display the
four elements. Finally, find the largest and smallest elements and output them on
screen.
Exercise 3
Write a program that outputs all prime numbers less than 1000.The program
should also count the number of prime numbers less than 1000.An integer >= 2
is a prime number if it is not divisible by any number except 1 and itself. Use the
Sieve of Eratosthenes:
To find primary numbers simply eliminate multiples of any primary numbers
you have already found, i.e.:
first eliminate any multiples of 2 ( 4, 6, 8, ... ),
then eliminate any multiples of 3 ( 6, 9, 12, ...),
then eliminate any multiples of 5 ( 10, 15, 20, ..) // 4 has already been eliminated
and so on.
Exercise 4
Write a C++ program to create the screen output shown opposite.The
following banner
* * * B R E A K * * *
is to be displayed in the center of the window and scrolled left.You can scroll
the banner by beginning string output with the first character, then the second,
and so on. Handle the string like a loop where the first letter follows the last
letter and output continues until the starting position is reached.
You can use a wait loop to modify the speed of the banner after each string is
output.
336 ■
CHAPTER 16 ARRAYS
bool append( const string& name,
const string& telNr);
bool erase( const string& name);
int search( const string& name);
void print();
int print( const string& name);
int getNewEntries();
***** Telephone List *****
D = Display all entries
F = Find a telephone number
A = Append an entry
E = Erase an entry
Q = Quit the program
Your choice:
Exercise 5
Methods to be implemented for the TelList class
Menu of the application program
EXERCISES
■337
The phone list will not be stored permanently in a file. This is just one of the enhancements (another
would be variable length) that will be added at a later stage.
✓NOTE
Exercise 5
The sample class TelList was introduced in this chapter; however, some
methods still need to be implemented and tested.
■Implement the TelList class methods shown opposite.
The name is used as an unambiguous key.This means the append()
method can only be used to append an entry provided the name is nei-
ther blank nor already in use.
The method erase() deletes an array element.The position of the ele-
ment to be deleted is first located using the search() method. If the ele-
ment does not exist, erase() returns a value of false. In any other case,
the last element in the array is used to overwrite the element that is to
be deleted and the counter count is decremented.
The search() method finds the position in the array that contains the
search name. If the search operation is unsuccessful, the value PSEUDO is
returned.
The print method without parameters outputs all available entries.You
can pass the first letter or letters of a name to the second method to
output any entries beginning with these letters. Use the method com-
pare() from the string class to help you with this task.
Example: str1.compare( 0, 5, str2) == 0
This expression is true if the five characters subsequent to position 0 in
the strings str1 and str2 are identical.
The getNewEntries() method is used to read new phone list entries
from the keyboard. Each new entry is appended using the append()
method. Reading should be terminated if the user types an empty string.
The method returns the number of new entries.
Write an application program that creates a phone list of type TelList
and displays the menu shown on the opposite page.
■The menu must be placed in a function of your own that can return the
command input.The menu must be called in the main loop of the pro-
gram. Depending on the command input, one of the methods defined in
the class TelList should be called. If the menu item “Erase” or “Search”
is chosen, you must also read a name or the first letters of a name from
the keyboard.
solutions
338 ■
CHAPTER 16 ARRAYS
■SOLUTIONS
Exercise 1
// ---------------------------------------------------
// bubble.cpp
// Inputs integers into an array,
// sorts in ascending order, and outputs them.
// ---------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#define MAX 100 // Maximum number
long number[MAX];
int main()
{
int i, cnt; // Index, quantity
cout << "\nS o r t i n g I n t e g e r s \n"
<< endl;
// To input the integers:
cout << "Enter up to 100 integers \n"
<< "(Quit with any letter):" << endl;
for( i = 0; i < MAX && cin >> number[i]; ++i)
;
cnt = i;
// To sort the numbers:
bool sorted = false; // Not yet sorted.
long help; // Swap.
int end = cnt; // End of a loop.
while( !sorted) // As long as not
{ // yet sorted.
sorted = true;
--end;
for( i = 0; i < end; ++i) // Compares
{ // adjacent integers.
if( number[i] > number[i+1])
{
sorted = false; // Not yet sorted.
help = number[i]; // Swap.
number[i] = number[i+1];
number[i+1]= help;
}
}
}
SOLUTIONS
■339
// Outputs the numbers
cout << "The sorted numbers:\n" << endl;
for( i = 0; i < cnt; ++i)
cout << setw(10) << number[i];
cout << endl;
return 0;
}
Exercise 2
// ----------------------------------------------------
// DayTime.h
// The class DayTime represents the time in hours,
// minutes and seconds.
// ----------------------------------------------------
#ifndef _DAYTIME_
#define _DAYTIME_
#include <iostream>
#include <iomanip>
using namespace std;
class DayTime
{
private:
short hour, minute, second;
bool overflow;
public:
DayTime( int h = 0, int m = 0, int s = 0)
{
overflow = false;
if( !setTime( h, m, s)) // this->setTime(...)
hour = minute = second = 0;
}
bool setTime(int hour, int minute, int second = 0)
{
if( hour >= 0 && hour < 24
&& minute >= 0 && minute < 60
&& second >= 0 && second < 60 )
{
this->hour = (short)hour;
this->minute = (short)minute;
this->second = (short)second;
return true;
}
else
return false;
}
340 ■
CHAPTER 16 ARRAYS
int getHour() const { return hour; }
int getMinute() const { return minute; };
int getSecond() const { return second; };
int asSeconds() const // Daytime in seconds
{
return (60*60*hour + 60*minute + second);
}
bool isLess( DayTime t) const // Compares
// *this and t.
{
return asSeconds() < t.asSeconds();
} // this->sSeconds() < t.asSeconds();
void print() const
{
cout << setfill('0')
<< setw(2) << hour << ':'
<< setw(2) << minute << ':'
<< setw(2) << second << " Uhr" << endl;
cout << setfill(' ');
}
void swap( DayTime& t) // Just one parameter!
{ // Swaps *this and t:
DayTime temp(t); t = *this; *this = temp;
}
};
#endif // _DAYTIME_
// -----------------------------------------------------
// TimeTab.cpp
// An array containing objects of class DayTime.
// -----------------------------------------------------
#include "DayTime.h" // Definition of class DayTime
#include <iostream>
using namespace std;
char header[] =
"\n\n *** Table with Daytimes ***\n\n";
int main()
{
DayTime timeTab[4] =
{ 18, DayTime(10,25), DayTime(14,55,30)};
int i;
timeTab[3].setTime( 8,40,50); // Last element.
cout << header << endl;
SOLUTIONS
■341
// Output:
for( i = 0; i < 4; ++i)
{
timeTab[i].print();
cout << endl;
}
// To compute shortest and longest time:
int i_min = 0, i_max = 0; // Indices for shortest
// and longest elements.
for( i = 1; i < 4; ++i)
{
if( timeTab[i].isLess( timeTab[i_min]) )
i_min = i;
if( timeTab[i_max].isLess( timeTab[i]) )
i_max = i;
}
cout << "\nShortest time: "; timeTab[i_min].print();
cout << "\nLongest time: "; timeTab[i_max].print();
return 0;
}
342 ■
CHAPTER 16 ARRAYS
Exercise 3
// ------------------------------------------------------
// sieve.cpp
// Identifies prime numbers using the Sieve of
// Eratosthenes.
// ------------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#define LIMIT 1000 // Upper limit
bool flags[LIMIT] = { false, false}; // Array with flags
int main()
{
register int i, j; // Indices
for( i = 2; i < LIMIT; ++i)
flags[i] = true; // Sets flags to true
// Sieving:
for( i = 2; i < LIMIT/2; ++i)
{
if( flags[i]) // Is i a prime number?
{ // Yes -> Delete multiples.
for( j = i+i; j < LIMIT; j += i)
flags[j] = false;
}
}
// To count:
int count = 0; // Counter
for( i = 2; i < LIMIT; ++i)
if(flags[i]) // If i is a prime number
++count; // -> count
// Output:
cout << "There are"<< count <<" prime numbers less than"
<< LIMIT << endl;
cout << "\nTo output prime numbers? (y/n) ";
char reply; cin.get(reply);
if( reply == 'y' || reply == 'Y')
{ for( i = 2; i < LIMIT; ++i)
if(flags[i]) // If i is a prime number
{ // -> to output it.
cout.width(8); cout << i;
}
}
cout << endl; return 0;
}
SOLUTIONS
■343
Exercise 4
// ----------------------------------------------------
// scroll.cpp
// Scrolling a message.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#define DELAY 10000000L // Output delay
inline void cls() // Clear screen
{
cout << "\033[2J\n";
}
inline void locate(int z, int s) // Put cursor in row z
{ // and column s
cout << "\033[" << z << ';' << s << 'H';
}
char msg[] = "* * * B R E A K * * * ";
int main()
{
int i, start = 0, len = strlen(msg);
cls(); locate(24, 20); // Row 24, column 20
cout << "--- Press interrupt key to terminate (^C) ---";
while( true )
{
locate( 12, 25); // Row 12, column 25
i = start; // Output from index start
do
{
cout << msg[i++];
i = i % len; // if( i == len) i = 0;
}
while( i != start);
cout << endl; // Outputs buffer to screen
// Wait in short
for( int count = 0; count < DELAY; ++count)
;
++start; // For next output
start %= len; // start = start % len;
}
cls();
return 0;
}
344 ■
CHAPTER 16 ARRAYS
Exercise 5
// ----------------------------------------------------
// telList.h
// The class TelList representing a list
// with names and telephone numbers.
// -----------------------------------------------------
//
// As before in this chapter.
// ----------------------------------------------------
// telList.cpp
// Implements the methods of class TelList.
// -----------------------------------------------------
#include "telList.h" // Definition of class TelList
#include <iostream>
#include <iomanip>
using namespace std;
bool TelList::append( const string& name,
const string& telNr)
{
if( count < MAX // Space available,
&& name.length() > 1 // 2 characters at least
&& search(name) == PSEUDO) // not yet existing
{
v[count].name = name;
v[count].telNr = telNr;
++count;
return true;
}
return false;
}
bool TelList::erase( const string& key )
{
int i = search(key);
if( i != PSEUDO )
{ // Copies the last
v[i] = v[count-1]; --count; // element to position i
return true;
}
return false;
}
SOLUTIONS
■345
int TelList::search(const string& key )
{
for( int i = 0; i < count; i++ ) // Searching.
if( v[i].name == key )
return i; // Found
return PSEUDO; // Not found
}
// Functions to support the output:
inline void tabHeader() // Title of the table
{
cout << "\n Name Telephone #\n"
"----------------------------------------------"
<< endl;
}
inline void printline( const Element& el)
{
cout << left << setw(30) << el.name.c_str()
<< left << setw(20) << el.telNr.c_str()
<< endl;
}
void TelList::print() // Outputs all entries
{
if( count == 0)
cout << "\nThe telephone list is empty!" << endl;
else
{
tabHeader();
for( int i = 0; i < count; ++i)
printline( v[i]);
}
}
int TelList::print( const string& name) const // Entries
{ // beginning with name.
int matches = 0, len = name.length();
for( int i = 0; i < count; ++i)
{
if( v[i].name.compare(0, len, name) == 0)
{
if( matches == 0) tabHeader(); // Title before
// first output.
++matches;
printline( v[i]);
}
}
if( matches == 0)
cout << "No corresponding entry found!" << endl;
return matches;
}
346 ■
CHAPTER 16 ARRAYS
int TelList::getNewEntries() // Input new entries
{
int inputCount = 0;
cout << "\nEnter new names and telephone numbers:"
"\n(Terminate by empty input) "
<< endl;
Element el;
while( true)
{
cout << "\nNew last name, first name: ";
cin.sync(); getline( cin, el.name);
if( el.name.empty())
break;
cout << "\nTelephone number: ";
cin.sync(); getline( cin, el.telNr);
if( !append( el))
{
cout << "Name has not been found!" << endl;
if( count == MAX)
{
cout << "The Table is full!" << endl;
break;
}
if( search( el.name) != PSEUDO)
cout << "Name already exists!" << endl;
}
else
{
++inputCount;
cout << "A new element has been inserted!"
<< endl;
}
}
return inputCount;
}
// --------------------------------------------------------
// telList_t.cpp
// Manages a telephone list.
// --------------------------------------------------------
#include "telList.h" // Definition of class TelList
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
inline void cls()
{ cout << "\033[2J\n";// Output only new-lines, if ANSI
} // control characters are not available.
SOLUTIONS
■347
inline void go_on()
{
cout << "\n\nGo on with return! ";
cin.sync(); cin.clear(); // No previous input
while( cin.get() != '\n')
;
}
int menu(); // Reads a command
char header[] =
"\n\n ***** Telephone List *****\n\n";
TelList myFriends; // A telephone list
int main()
{
int action = 0; // Command
string name; // Reads a name
myFriends.append("Lucky, Peter", "0203-1234567");
while( action != 'B')
{
action = menu();
cls();
cout << header << endl;
switch( action)
{
case 'D': // Show all
myFriends.print();
go_on();
break;
case 'F': // Search
cout <<
"\n--- To search for a phone number ---\n"
"\nEnter the beginning of a name: ";
getline( cin, name);
if( !name.empty())
{
myFriends.print( name);
go_on();
}
break;
case 'A': // Insert
myFriends.getNewEntries();
break;
348 ■
CHAPTER 16 ARRAYS
case 'E': // Delete
cout <<
"\n--- To delete a telephone entry. ---\n "
"\nEnter the complete name: ";
getline( cin, name);
if( !name.empty())
{
if( !myFriends.erase( name))
cout << name << " not found!"
<< endl;
else
cout << "Entry for " << name
<< " deleted!" << endl;
go_on();
}
break;
case 'T': cls(); // To terminate
break;
}
} // End of while
return 0;
}
int menu()
{
static char menuStr[] =
"\n\n D = Display all entries"
"\n\n F = Find a telephone number"
"\n\n A = Append a new entry "
"\n\n E = Erase an entry "
"\n\n Q = Quit the program"
"\n\n Your choice: ";
cls();
cout << header << menuStr;
char choice;
cin.sync(); cin.clear(); // No previous input
if( !cin.get(choice))
choice = 'B';
else
choice = toupper(choice);
cin.sync(); // Clear input buffer
return choice;
}
349
Arrays and Pointers
This chapter describes the relationship between pointers and arrays.This
includes:
■pointer arithmetic
■pointer version of functions
■pointers as return values and read-only pointers
■pointer arrays
Operations that use C strings illustrate how to use pointers for efficient
programming. String access via the command line of an application
program is used to illustrate pointer arrays.
chapter
17
350 ■
CHAPTER 17 ARRAYS AND POINTERS
// textPtr.cpp
// Using arrays of char and pointers to char
// -----------------------------------------------------
#include <iostream>
using namespace std;
int main()
{
cout << "Demonstrating arrays of char "
<< "and pointers to char.\n"
<< endl;
char text[] = "Good morning!",
name[] = "Bill!";
char *cPtr = "Hello "; // Let cPtr point
// to "Hello ".
cout << cPtr << name << '\n'
<< text << endl;
cout << "The text \"" << text
<< "\" starts at address " << (void*)text
<< endl;
cout << text + 6 // What happens now?
<< endl;
cPtr = name; // Let cPtr point to name, i.e. *cPtr
// is equivalent to name[0]
cout << "This is the " << *cPtr << " of " << cPtr
<< endl;
*cPtr = 'k';
cout << "Bill can not " << cPtr << "!\n" << endl;
return 0;
}
■ARRAYS AND POINTERS (1)
Sample program
Sample output:
Demonstrating arrays of char and pointers to char.
Hello Bill!
Good morning!
The text "Good morning!" starts at address 00451E40
morning!
This is the B of Bill!
Bill can not kill!
ARRAYS AND POINTERS (1)
■351
䊐Name and Address of an Array
In C++ the name of an array is also the starting address for that array. To be more pre-
cise, an array name is a pointer to the first array element.
Example: char town[] = "Beijing";
In this case, town is a char pointer to town[0], that is, a pointer to the memory
address that stores the 'B' character. Expressions town and &town[0] are thus equiva-
lent.
Example: cout << town; // or: cout << &town[0];
A pointer to the first character of the string town is passed. The characters forming the
string are read and displayed from this point onward until the terminating null character,
'\0', is reached.
䊐Pointer Variables and Arrays
An array name is not a pointer variable but a constant that cannot be modified. How-
ever, you can assign this constant to a pointer variable.
Example: char *cPtr;
cPtr = town; // or: cPtr = &town[0];
cout << cPtr; // To output "Beijing"
Now cPtr points to the array element town[0] just like town. But, in contrast to
town,cPtr is a variable that can be moved.
Example: cPtr = "Hello!";
After this statement, cPtr points to the ‘H' character. String constants such as
“Hello!" are also char arrays and thus represent the address of the first array element.
䊐Typeless Pointers
If you need to display the address rather than the string, you should pass a void* type
pointer rather than a char pointer.
Example: cout << (void *)town;
This casts the char pointer to a void * type pointer and passes it as an argument to
the << operator, which in turn outputs the address in hexadecimal format. The << oper-
ator belongs to the ostream class and is overloaded for void * types for this purpose.
Avoid * pointer represents a memory address without establishing a certain type.
void * pointers are also referred to as typeless pointers for this reason. When you use a
typeless pointer for memory access, you must therefore name the type being accessed
explicitly by means of type casting.
352 ■
CHAPTER 17 ARRAYS AND POINTERS
0
10
20
30
arr arr[0]
arr[1]
arr[0]
arr[3]
arr + 1
arr + 2
arr + 3
// arrPtr.cpp
// Outputs addresses and values of array elements.
// ---------------------------------------------------
#include <iostream>
using namespace std;
int arr[4] = { 0, 10, 20, 30 };
int main()
{
cout << "\nAddress and value of array elements:\n"
<< endl;
for( int i = 0; i < 4; i++ )
cout << "Address: " << (void*)(arr+i) // &arr[i]
<< " Value: " << *(arr+i) // arr[i]
<< endl;
return 0;
}
■ARRAYS AND POINTERS (2)
Sample program
Interrelation between pointers and array elements
ARRAYS AND POINTERS (2)
■353
䊐Addressing Array Elements
Access to individual array elements in C++ is very closely related to pointer arithmetic.
Now let’s look at an int array to illustrate this point.
Example: int arr[4] = { 0, 10, 20, 30 };
As you already know, the name of the array arr is an int pointer to arr[0].
Now it is possible to add or subtract pointers and integral values. The size of the
object referenced by the pointer is automatically taken into consideration.
Since arr is an int pointer to arr[0],arr+1 points to the next array element
arr[1], i.e., to an address that is sizeof(int) bytes higher in memory. The memory
space between the two entries will be two or four bytes, depending on the size of the type
int. Thus the following applies to any given number, i:
arr + i points to the array element arr[i] ,
*(arr + i) is the array element arr[i] ,
This technique can also be used to address memory spaces outside of the array. Thus,
arr - 1 addresses the word that precedes arr[0]. But generally this does not make
much sense, since you have no means of knowing what is stored at this memory address.
䊐Addressing with Pointer Variables
Array elements can also be addressed using pointer variables.
Example: int *ptr = arr; // ptr points to arr[0]
In this case, both ptr and arr are pointers to the array element arr[0]. Thus,
ptr + 1,ptr + 2,. . . point to the array elements arr[1],arr[2],... .
For any given integer, i, the following expressions are thus equivalent:
&arr[i] arr + i ptr + i
The following thus represent equivalent values:
arr[i] *(arr + i) *(ptr + i) ptr[i]
At first it might seem surprising that you can use the array notation ptr[i] for pointers.
The compiler translates arr[i] to *(arr + i)—in other words: “Start at address
arr, move iobjects up, and access the object!” This also applies to ptr[i].
354 ■
CHAPTER 17 ARRAYS AND POINTERS
float v[6] = { 0.0, 0.1, 0.2, 0.3, 0.4, 0.5 },
*pv, x;
pv = v + 4; // Let pv point to v[4].
*pv = 1.4; // Assign 1.4 to v[4].
pv -= 2; // Reset pv to v[2].
++pv; // Let pv point to v[3].
x = *pv++; // Assign v[3] to x and
// increment pv.
x += *pv--; // Increment x by v[4] and let
// pv point to v[3] again.
--pv; // Reset pv to v[2].
// Searches for a given account number in a table of
// accounts and outputs the found account.
// --------------------------------------------------
#include "account.h" // Definition of class Account.
Account accountTab[100]; // Table containing accounts.
int main()
{
int cnt; // Actual number of accounts.
Account *aPtr; // Pointer to Account-objects.
// To input data into accountTab and actualize cnt.
// To search for the account number 1234567:
bool found = false;
for( aPtr = accountTab; aPtr < accountTab+cnt;++aPtr)
if( aPtr->getNr() == 1234567 )
{ found = true;
break;
}
if( found) // Found?
aPtr->display(); // Yes -> display.
// To continue
}
■POINTER ARITHMETIC
Examples for arithmetic with pointers
To step through an array of classes
POINTER ARITHMETIC
■355
In C++ you can perform arithmetic operations and comparisons with pointers, provided
they make sense. This primarily means that the pointer must always point to the ele-
ments of an array. The following examples show some of your options with pointer arith-
metic:
Example: float v[6], *pv = v; // pv points to v[0]
int i = 3;
䊐Moving a Pointer in an Array
As you already know, the addition pv + i results in a pointer to the array element
v[i]. You can use a statement such as pv = pv + i; to store the pointer in the vari-
able pv. This moves the pointer pv i objects, that is, pv now points to v[i].
You can also use the operators ++,--,and += or -= with pointer variables. Some
examples are shown opposite. Please note that the indirection operator, *, and the oper-
ators ++ and -- have the same precedence. Operators and operands thus are grouped
from right to left:
Example: *pv++ is equivalent to *(pv++)
The ++ operator increments the pointer and not the variable referenced by the pointer.
Operations of this type are not possible using the pointer vsince vis a constant.
䊐Subtracting Pointers
An addition performed with two pointers does not return anything useful and is there-
fore invalid. However, it does make sense to perform a subtraction with two pointers,
resulting in an int value that represents the number of array elements between the
pointers. You can use this technique to compute the index of an array element refer-
enced by a pointer. To do so, you simply subtract the starting address of the array. For
example, if pv points to the array element v[3], you can use the following statement
Example: int index = pv - v;
to assign a value of 3to the variable index.
䊐Comparing Pointers
Finally, comparisons can be performed with two pointers of the same type.
Example: for( pv = v + 5; pv >= v; --pv)
cout << setw(10) << *pv;
This loop outputs the numbers contained in vin reverse order. In the example on the
opposite page, the pointer aPtr walks through the first cnt elements of the array
accountTab, as long as aPtr < accountTab + cnt.
356 ■
CHAPTER 17 ARRAYS AND POINTERS
// reverse.cpp
// Defines and calls the function reverse().
// reverse() copies a C string into another C string
// and reverses the order of characters.
// -----------------------------------------------------
#include <iostream>
using namespace std;
#include <string.h> // Header-File for Cstrings,
// here for strlen().
void reverse( char str[], char umstr[]); // Prototype
int main() // Read a word and
{ // output in reversed order.
const int CNT = 81;
char word[CNT], revword[CNT];
cout << "Enter a word: ";
cin.width(CNT); // maximal CNT-1 characters
cin >> word;
reverse( word, revword); // Call
cout << "\nThe \"reversed\" word: " << revword
<< endl ;
return 0;
}
void reverse( char s1[], char s2[]) // Copies the
{ // reversed C string s1 to s2
int j = 0;
for( int i = strlen(s1)-1; i >= 0; i--, j++)
s2[j] = s1[i];
s2[j] = '\0'; // Terminating character
}
■ARRAYS AS ARGUMENTS
Sample program
Sample output:
Enter a word: REGAL
The "reversed" word: LAGER
ARRAYS AS ARGUMENTS
■357
If an array name is passed as an argument when calling a function, the function actually
receives the address of the first array element. The called function can then perform read
or write operations for any element in the array.
䊐Declaring Parameters
If the argument is an array, there are two equivalent methods of declaring parameters.
This point is illustrated by the example using strlen() to return the length of a C
string. For example, calling strlen("REGAL") returns a value of 5.
1. You can declare the parameter as an array.
Example: int strlen( char str[]) // Compute length of
{ int i; // str without '\0'.
for( i = 0; str[i] != '\0'; ++i)
;
return (i);
}
2. You can declare the parameter as a pointer.
Example: int strlen( char *str)
{ /* as above */ }
In both cases the parameter str is a pointer that stores the starting address of the
array. Array notation is preferable if you intend to use an index to access the elements of
an array. Calling strlen("REGAL"); leads to the following situation:
As you can see, the length of a C string is equal to the index of the element containing
the terminating null character.
The function reverse() on the opposite page copies the characters of a C string to
a second char array in reverse order, first copying the last character in s1, that is, the
character with the index strlen(s1)-1, to s2[0], then the second to last character
s2[1], and so on.
䊐Array Length
A function to which an array is passed initially knows only the starting address of the
array but not its length. In the case of C strings, the length is derived implicitly from the
position of the terminating null character. In most other cases the length must be sup-
plied explicitly.
Example: void sort( Account aTab[], int len )
{ /* To sort array aTab of length len */}
str[0] str[1] str[2] str[3] str[4] str[5]
'R' 'E' 'G' 'A' 'L' '\0'
358 ■
CHAPTER 17 ARRAYS AND POINTERS
void strcpy( char s1[], char s2[]) // Copies s2 to s1
{
int i; // Index
for( i = 0; s2[i] != '\0'; ++i) // Copy.
s1[i] = s2[i];
s1[i] = '\0'; // Append terminating
} // character.
void strcpy( char *s1, char *s2) // Copies s2 to s1
{
for( ; *s2 != '\0'; ++s1, ++s2) // Copy
*s1 = *s2;
*s1 = '\0'; // Append terminating
} // character.
void strcpy( char *s1, char *s2) // Copy s2 to s1.
{
while( (*s1++ = *s2++) != '\0' ) // Copy and append
; // terminating
} // character.
■POINTER VERSIONS OF FUNCTIONS
䊐Function strcpy
The standard function strcpy()copies C strings.
Example: char dest[30], source[] = "A string";
strcpy( dest, source);
Here the string source is copied to dest “from left to right” just like an assignment.
The following function strcpy() is somewhat simpler than the standard function
since it has no return value.
Index Version of strcpy()
Pointer version 1 of strcpy()
Pointer version 2 of strcpy()
POINTER VERSIONS OF FUNCTIONS
■359
䊐Using Pointers Instead of Indices
As we have already seen, a parameter for an array argument is always a pointer to the
first array element. When declaring parameters for a given type T:
T name[] is always equivalent to T *name.
So far, in previous sample functions, the pointer has been used like a fixed base
address for the array, with an index being used to access the individual array elements.
However, it is possible to use pointers instead of indices.
Example: A new version of the standard function strlen():
int strlen( char *str) // Computes length
{ // of str without '\0'.
char* p = str;
for( p = str; *p != '\0'; ++p) // Search
; // for \0
return (p - str);
}
In this case, the difference between two pointers results in the string length.
䊐The Sample Functions Opposite
The first version of the function strcpy() “string copy” opposite uses an index,
whereas the second does not. Both versions produce the same results: the string s2 is
copied to s1. When you call the function, you must ensure that the char array refer-
enced by s1 is large enough.
As the parameters s1 and s2 are pointer variables, they can be shifted. The second
“pointer version” of strcpy(), which is also shown opposite, uses this feature, although
the function interface remains unchanged.
Generally, pointer versions are preferable to index versions as they are quicker. In an
expression such as s1[i] the values of the variables s1 and iare read and added to
compute the address of the current object, whereas s1 in the pointer version already
contains the required address.
䊐Multidimensional Arrays as Parameters
In a parameter declaration for multidimensional arrays, you need to state every dimension
with the exception of the first. Thus, a parameter declaration for a two-dimensional array
will always contain the number of columns.
Example: long func( int num[][10] ); // ok.
long func( int *num[10] ); // also ok.
360 ■
CHAPTER 17 ARRAYS AND POINTERS
// accountFct.cpp
// Defines and calls a function, which outputs
// a list of overdrawn accounts.
// --------------------------------------------------
#include "account.h" // Definition of class Account.
Account accountTab[] = // Table with Account-objects.
{ Account("Twain, Mark", 1234567, -3434.30),
Account("Crusoe, Robinson", 200000, 0.00),
Account("Temple, Shirley", 543001, +777.70),
Account("Valentin, Carl", 543002, -1111.10),
};
int cnt = sizeof(accountTab) / sizeof(Account);
// Prototype:
int displayOverdraw( const Account *aTab, int cnt,
double limit);
int main()
{
double limit = 0.0;
cout << "Output the overdrawn accounts!\n"
<< "These are the accounts, which fell below \n"
<< "the limit, ex. -1000.00.\n" << endl;
cout << "What is the limit? ";
cin >> limit;
cout << "Listing the overdrawn accounts:\n" << endl;
if( displayOverdraw( accountTab, cnt, limit) == 0)
cout << "\nNo account found!"
<< endl;
return 0;
}
int displayOverdraw( const Account *aTab, int cnt,
double limit)
{ int count = 0;
const Account* aPtr;
for( aPtr = aTab; aPtr < aTab + cnt; ++aPtr)
if( aPtr->getState() < limit ) // Below the limit?
{
aPtr->display(); // Yes -> display.
++count;
}
return count;
}
■READ-ONLY POINTERS
Sample program
READ-ONLY POINTERS
■361
䊐Pointers to const Objects
You can use a normal pointer for both read and write access to an object. However, just
like the definition used for a reference, you can also define a read-only pointer, that is, a
pointer that can be used for read operations only. In fact, a read-only pointer is obliga-
tory if you need to point to a constant object.
䊐Declaration
You use the keyword const to define a read-only pointer.
Example: const int a = 5, b = 10, *p = &a;
This statement defines the constants aand b, and a pointer pto a constant object of
type int. The referenced object *p can be read but not modified.
Example: cout << *p; // To read is ok.
*p = 1; // Error!
The pointer itself is not a constant, so it can be modified:
Example: p = &b; // ok!
The referenced object also does not need to be a constant. In other words, a read-only
pointer can also point to a non-constant object.
Example: Account depo("Twain, Mark", 1234, 4321.90);
const Account* ptr = &depo; // ok!
ptr->display(); // ok!
prt->setState( 7777.70); // Error!
But ptr can only be used for read access to the non-constant object depo.
䊐Read-Only Pointers as Parameters
Read-only pointers are most commonly found in parameter lists. This guarantees that
arguments cannot be modified.
Example: int strlen( const char *s);
In this example, the parameter sis a read-only pointer. This allows you to pass constant
C strings to the standard function strlen(). You cannot remove the “write protection”
by assigning the read-only pointer sto a normal pointer.
Example: char *temp = s; // Error!
You need to declare a read-only pointer if a constant object may be passed as an argu-
ment.
362 ■
CHAPTER 17 ARRAYS AND POINTERS
// search1.cpp
// A filter to output all lines containing a given
// pattern. The function strstr() is called.
// Call: search1 [ < text.dat ]
// ----------------------------------------------------
#include <iostream>
using namespace std;
#define MAXL 200 // Maximum length of line
namespace MyScope
{ // Self-defined version of function strstr():
char *strstr( const char *str, const char *patt);
}
char line[500], // For a line of text.
patt[] = "is"; // The search pattern.
int main()
{ int lineNr = 0; // As long as a line is left over:
while( cin.getline( line, MAXL))
{
++lineNr;
if( MyScope::strstr( line, patt) != NULL)
{ // If the pattern is found:
cout.width(3);
cout << lineNr << ": " // Output the line
<< line << endl; // number and the line
}
}
return 0;
}
// strstr.cpp
// A self-defined version of the function strstr()
// ---------------------------------------------------
#include <string.h> // For strlen() and strncmp()
namespace MyScope
{
char *strstr( const char *s1, const char *s2)
{ // To search for the string s2 within s1.
int len = strlen( s2);
for( ; *s1 != '\0'; ++s1)
if( strncmp( s1, s2, len) == 0) // s2 found?
return (char *)s1; // Yes -> return pointer
// to this position, or
return NULL; // else the NULL pointer.
}
}
■RETURNING POINTERS
Sample program
RETURNING POINTERS
■363
A function can return a pointer to an object. This makes sense for a function that
searches for a particular object, for example. Such a function will return either a pointer
to the required object or a NULL pointer if the object cannot be found.
The standard C library functions often use pointers as return values. For example, the
functions strcpy(),strcat(), and strstr() each return a pointer to the first char-
acter in a C string.
䊐The Functions strcpy() and strcat()
In contrast to the example on the page entitled “Pointer versions of functions,” the stan-
dard function strcpy()has a return value. The function returns its first argument, that
is, a pointer to the target string and leads to the following:
Prototype: char* strcpy( char* s1, const char* s2);
The second parameter is a read-only pointer, since the source string is read-only.
The standard function strcat() concatenates two C strings, adding the C string
passed as the second argument to the first argument. When you call this function, make
sure that the char array for the first string is large enough to store both strings. The
return value is the first argument. The following example shows one possible implemen-
tation.
Example: char *strcat( char *s1, const char *s2 )
{
char *p = s1 + strlen(s1); // End of s1
strcpy(p, s2);
return s1;
}
䊐Notes on the Sample Program
The program on the opposite page shows a self-defined version of the standard function
strstr(). This version was placed in the MyScope namespace to distinguish it from
the standard function.
The function strstr() searches for a given character sequence within a string. The
standard function strncmp() is used to compare two strings. This function returns zero
if the first ncharacters are identical.
The program uses the strstr() function to display all the lines in the text contain-
ing the letters “is" with line numbers. The exercises for this chapter contain a program
called search.cpp where you can supply a search pattern.
364 ■
CHAPTER 17 ARRAYS AND POINTERS
accPtr[0] "Novack,..", 1234,
"Davis, ..", 2345,
accPtr[1]
accPtr[2]
accPtr[3]
accPtr[4]
Array
accPtr Account objects
// The function displayError() outputs an error message
// to a corresponding error number.
// --------------------------------------------------
#include <iostream>
using namespace std;
void displayError ( int errorNr)
{
static char* errorMsg[] = {
"Invalid error number",
"Error 1: Too much data ",
"Error 2: Not enough memory ",
"Error 3: No data available " };
if( errorNr < 1 || errorNr > 3)
errorNr = 0;
cerr << errorMsg[errorNr] << endl;
}
A string literal, such as “Error..." is a char pointer to the first character in the string. Thus, such a
pointer can be used to initialize another char pointer.
Due to its static declaration, the array is generated only once and remains valid until the program
ends.
✓NOTE
■ARRAYS OF POINTERS
Pointers in the array accPtr
Sample function with pointers to char
ARRAYS OF POINTERS
■365
Pointers offer various possibilities for simple and efficient handling of large amounts of
data. For example, when you are sorting objects it makes sense to define pointers to those
objects and simply place the pointers in order, instead of rearranging the actual order of
the objects in memory.
䊐Defining Arrays of Pointers
Whenever you need a large number of pointers, you can define an array whose elements
are pointers. An array of this type is referred to as a pointer array.
Example: Account* accPtr[5];
The array accPtr contains five Account pointers accPtr[0],accPtr[1], ... ,
accPtr[4]. The individual pointers in the array can now be assigned object addresses.
Any pointers not currently in use should have the value NULL.
Example: Account save("Novack, Kim", 1111, 9999.90);
Account depo("Davis, Sammy", 2222, 1000.);
accPtr[0] = &save;
accPtr[1] = &depo;
for( int i=2; i<5; ++i) accPtr[i] = NULL;
䊐Initialization
As usual, an initialization list is used to initialize the array. In the case of a pointer array,
the list contains either valid addresses or the value NULL.
Example: Account* accPtr[5] = { &depo, &save, NULL};
The value NULL is automatically assigned to any objects for which the list does not con-
tain a value. This produces the same result as in the previous example.
䊐Usage
The individual objects addressed by the pointers in an array do not need to occupy a
contiguous memory space. Normally these objects will be created and possibly destroyed
dynamically at runtime (this will be discussed in detail in a later chapter). This allows for
extremely flexible object handling. The order is defined only by the pointers.
Example: for( int i=0; i<5; ++i)
if( accPtr[i] != NULL)
accPtr[i]->display(); // To output
The function displayError() opposite displays the error message for a correspon-
ding error number, using an array of char pointers to the error messages.
366 ■
CHAPTER 17 ARRAYS AND POINTERS
"C:\...\HELLO.EXE"
"Vivi"
"Jeany"
argv[0]
argv
argv[1]
argv[2]
NULL
// hello.cpp
// Demonstrates the command line arguments.
// Call: hello name1 name2
// ----------------------------------------------------
#include <iostream>
using namespace std;
int main( int argc, char *argv[])
{
if( argc != 3 )
{
cerr << "Use: hello name1 name2" << endl;
return 1;
}
cout << "Hello " << argv[1] << '!' << endl;
cout << "Best wishes \n"
<< "\tyours " << argv[2] << endl;
return 0;
}
■COMMAND LINE ARGUMENTS
Sample program
Example of calling the program:
hello Jeany Vivi
Screen output
Hello Jeany!
Best wishes
Yours Vivi
Array argv in memory
COMMAND LINE ARGUMENTS
■367
䊐Arguments for a Program
When you launch a program, you can use the command line to supply additional charac-
ter sequences other than the program name. These command line arguments are typically
used to govern how a program is executed or to supply the data a program will work with.
Example: copy file1 file2
In this case, the program copy is launched with the arguments file1 and file2. The
individual arguments are separated by spaces. Characters used for redirecting input and
output ( >or <) and a following word are evaluated by the operating system and not
passed to the program. If an argument contains space or redirection characters, you must
place it in double quotes.
䊐Parameters of the Function main()
So far we have only used the function main() without parameters. However, if you
intend to process command line arguments, you must define parameters for main().
Example: int main(int argc, char * argv[] )
{ . . . // Function block }
argc contains the number of arguments passed via the command line. The program
name is one of these, so argc will have a value of at least 1.
The parameter argv is an array of char pointers:
argv[0] points to the program name (and path)
argv[1] points to the first real argument, that is, the word after the pro-
gram name
argv[2] points to the second argument
.....
argv[argc-1] points to the last argument
argv[argc] is the NULL pointer
The parameters are traditionally named argc and argv although any other name could
be used.
Various operating systems, for example WINDOWS 98/00/NT and UNIX, allow you
to declare a third parameter for main(). This parameter is an array with pointers to
environment strings. The exercises for this chapter contain a program that displays the
program environment.
exercises
368 ■
CHAPTER 17 ARRAYS AND POINTERS
// strcmp() compares two C strings lexicographically.
// Return value: < 0, if str1 < str2
// = 0, if str1 == str2
// > 0, if str1 > str2 .
// ----------------------------------------------------
int strcmp( const char str1[], const char str2[])
{
int i;
for( i=0; str1[i] == str2[i] && str1[i] != '\0'; ++i)
;
return (str1[i] - str2[i]);
}
Original array:
After the first loop:
After the second loop:
smallest element
second smallest element
100 50 30
30
30 40 100 70 50
50 100 70 40
70 40
■EXERCISES
For exercise 3
Index version of the standard function strcmp()
Notes on exercise 4
The selection sort algorithm
Method
First find the smallest element in the array and exchange it with the first
element.
This procedure is repeated while i > 0 for the remainder of an array
containing array elements with an initial index of i.
Example
EXERCISES
■369
Exercise 1
Given an array vwith the following definition:
int v[] = { 10, 20, 30, 40 }, i, *pv;
What screen output is caused by the following statements?
a. for( pv = v; pv <= v + 3; pv++ )
cout << " *pv = " << *pv;
b. for( pv = v, i = 1; i <= 3; i++ )
cout << " pv[i] = " << pv[i];
c. for( pv = v, i = 0; pv+i <= &v[3]; pv++,i++)
cout << " *(pv + i) = " << *(pv + i);
d. for( pv = v + 3; pv >= v; --pv )
cout << " v[" << (pv - v) << "] = "
<< v[pv - v];
Exercise 2
Write a program that uses the cin method get() to read a line character by
character and stores it in a char array.The line is then output in reverse order.
Use a pointer, not an index, to address the array elements.
Exercise 3
The standard function strcmp() performs a lexicographical comparison of two
C strings.The opposite page contains an index version of strcmp().The return
value is the difference between two character codes.
Write a pointer version of the function strcmp(). Call this function
str_cmp() to distinguish it from the standard function.
To test the function, use a loop to read two lines of text and output the
results of the comparison.The loop should terminate when both strings are
empty.
Exercise 4
Define and test the function selectionSort() that sorts an array of int
values in ascending order.The principle of the selection sort algorithm is shown
opposite.
Arguments: An int array and its length
Return values: None
Develop both an index version and a pointer version.Test the functions with
random numbers between -10000 and +10000.
370 ■
CHAPTER 17 ARRAYS AND POINTERS
. . .
COMSPEC=C:\COMMAND.COM
PATH=C:\WINDOWS;C:\WINDOWS\COMMAND;C:\DOS;D:\TOOLS;
PROMPT=$p$g
TEMP=C:\TEMP
. . .
Blood-
pressure
Age
<120 >= 160120–129
20–29
30–39
40–49
25
19
6
34
27
15
26
24
35
12
11
36
8
4
18
130–139 140–149
Notes on exercise 5
Sample environment strings for DOS/Windows
Frequency table for exercise 7
EXERCISES
■371
Exercise 5
a. Write a program that outputs its own name and all command line argu-
ments, each in a separate line.
b. Now extend the program to output its own environment.The environ-
ment is a memory area containing strings in the format
NAME=String
A third parameter for the function main() allows access to the environment.
This parameter is an array of pointers just like argv.The array elements are
char pointers to the environment strings, the last element being a NULL
pointer.
Exercise 6
A sample filter program called search1, which outputs lines and the relevant
line numbers for lines containing the search pattern "ei", was introduced in this
chapter.
Modify the program to produce a useful tool called search, to which you can
pass any search pattern via the command line.The program should issue an
error message and terminate if the command line does not contain a search
string. Use the standard function strstr().
Sample call:
search Shanghai < news.txt
Exercise 7
The following frequency was observed during an examination of the relationship
between age and blood pressure for 300 males.
Write a function that calculates the sums of the rows and columns in an int
matrix with three rows and five columns. Store the sums of the rows and
columns separately in a one-dimensional row or column array.
Arguments: The matrix, the row array, and the column array.
Return value: The sum of all the matrix elements.
To test the function, output the matrix, as shown in the graphic opposite
along with the computed sums in your main function.
solutions
372 ■
CHAPTER 17 ARRAYS AND POINTERS
■SOLUTIONS
Exercise 1
Screen Output:
a. *pv = 10 *pv = 20 *pv = 30 *pv = 40
b. pv[i] = 20 pv[i] = 30 pv[i] = 40
c. *(pv+i) = 10 *(pv+i) = 30
d. v[3] = 40 v[2] = 30 v[1] = 20 v[0] = 10
Exercise 2
// -------------------------------------------------------
// reverse.cpp
// Exercise on pointer arithmetic:
// Reads a line and outputs the line in reverse order.
// -------------------------------------------------------
#include <iostream>
using namespace std;
#define MAXLEN 80
int main()
{
char line[MAXLEN], *p;
cout << "Enter a line of text: " << endl;
// Input a line:
for( p = line;
p < line+MAXLEN && cin.get(*p) && *p != '\n';
++p )
;
// Output the line in reverse order:
while( --p >= line)
cout << *p;
cout << endl;
return 0;
}
SOLUTIONS
■373
Exercise 3
// -------------------------------------------------------
// str_cmp.cpp
// Define and test the pointer version str_cmp()
// of the standard function strcmp().
// -------------------------------------------------------
#include <iostream>
using namespace std;
#define MAXLEN 100 // Maximum length of C strings
// Prototype:
int str_cmp( const char* str1, const char* str2);
int main() // Test str_cmp()
{
char text1[MAXLEN], text2[MAXLEN];
cout << "Testing the function str_cmp()" << endl;
while( true)
{
cout << "Enter two lines of text!\n"
"End with two empty lines.\n" << endl;
cout << "1. line: ";
cin.sync(); cin.clear(); cin.get(text1,MAXLEN);
cout << "2. line: ";
cin.sync(); cin.clear(); cin.get(text2,MAXLEN);
if( text1[0] == '\0' && text2[0] == '\0')
break; // Both lines empty.
int cmp = str_cmp( text1, text2);
if( cmp < 0)
cout << "The 1st string is smaller!\n";
else if( cmp == 0)
cout << "Both strings are equal!\n";
else
cout << "The 1st string is greater!\n";
cout << endl;
}
return 0;
}
// ------------------------------------------------------
// Function str_cmp()
// Pointer version of the standard function strcmp().
// ------------------------------------------------------
int str_cmp( const char* str1, const char* str2)
{
for( ; *str1 == *str2 && *str1 != '\0'; ++str1, ++str2)
;
return (*str1 - *str2);
}
374 ■
CHAPTER 17 ARRAYS AND POINTERS
Exercise 4
// -------------------------------------------------------
// selSort.cpp
// Implement the selection sort algorithm
// for int-arrays.
// -------------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstdlib> // For srand(), rand()
#include <ctime> // For time()
using namespace std;
// Prototype:
void selectionSort( int arr[], int len);
const int len = 200;
int intArr[len]; // int-array
int main()
{
cout << "\n *** Selection Sort Algorithm ***\n"
<< endl;
// To initialize an int-array with random numbers:
srand( (unsigned int)time(NULL)); // Initialize the
// random number generator.
for( int n=0; n < len; ++n)
intArr[n] = (rand() % 20000)-10000;
// To sort the numbers
selectionSort( intArr, len);
// To output the numbers
cout << "The sorted numbers:" << endl;
for( int i = 0; i < len; ++i)
cout << setw(8) << intArr[i];
cout << endl;
return 0;
}
inline void swap( int& a, int& b)
{
int temp = a; a = b; b = temp;
}
SOLUTIONS
■375
// Index version:
/*
void selectionSort( int arr[], int len)
{
register int j, mini; // Indices
for( int i = 0; i < len-1; ++i)
{
mini = i; // Search for minimum
for( j = i+1; j < len; ++j) // starting with index i.
if( arr[mini] > arr[j])
mini = j;
swap( arr[i], arr[mini]); // Swap.
}
}
*/
// Pointer version:
void selectionSort( int *arr, int len)
{
register int *p, *minp; // Pointer to array elements,
int *last = arr + len-1; // pointer to the last element
for( ; arr < last; ++arr)
{
minp = arr; // Search for minimum
for( p = arr+1; p <= last; ++p) // starting with arr
if( *minp > *p)
minp = p;
swap( *arr, *minp); // Swap.
}
}
376 ■
CHAPTER 17 ARRAYS AND POINTERS
Exercise 5
// -------------------------------------------------------
// args.cpp
// The program outputs the program name including the path,
// command line arguments and the environment.
// -------------------------------------------------------
#include <iostream>
using namespace std;
int main( int argc, char *argv[], char *env[])
{
cout << "Program: " << argv[0] << endl;
cout << "\nCommand line arguments:" << endl;
int i;
for( i = 1; i < argc; ++i) // Arguments
cout << argv[i] << endl;
cout << "Type <Return> to go on";
cin.get();
cout << "\nEnvironment strings:" << endl;
for( i = 0; env[i] != NULL; ++i) // Environment
cout << env[i] << endl;
return 0;
}
Exercise 6
// -------------------------------------------------------
// search.cpp
// A filter that outputs all lines containing a certain
// pattern. The standard function strstr() is called.
//
// Call: search pattern [ < text.dat ]
//
// If no file name is passed the input is read from the
// keyboard. In this case end input with <Ctrl> + <Z>.
// ------------------------------------------------------
#include <iostream>
#include <cstring> // Standard functions for C strings
using namespace std;
#define MAXL 200 // Maximum length of line
char line[500]; // For a line of text.
SOLUTIONS
■377
int main( int argc, char *argv[])
{
if( argc != 2)
{
cerr << "Call: search pattern [ < text.dat ]"
<< endl;
return 1;
}
int lineNr = 0;
// As long as a line exists:
while( cin.getline( line, MAXL))
{
++lineNr;
if( strstr( line, argv[1]) != NULL)
{ // If the pattern was found:
cout.width(3);
cout << lineNr << ": " // Output the line
<< line << endl; // number and the line
}
}
return 0;
}
Exercise 7
// -------------------------------------------------------
// matrix.cpp
// To compute the sums of rows and columns in a matrix.
// ------------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
// Define and initiate a two-dimensional array:
int matrix[3][5] = { { 25, 34, 26, 12, 8 },
{ 19, 27, 24, 11, 4 },
{ 6, 15, 35, 36, 18 } };
int rowsum[3]; // For the sums of the rows
int colsum[5]; // For the sums of the columns
// Prototype of function matrixsum():
int matrixsum( int arr2D[][5], int vlen,
int rsum[], int csum[]);
378 ■
CHAPTER 17 ARRAYS AND POINTERS
int main()
{
cout << "Testing the function matrixsum().\n"
<< endl;
// Compute sums:
int totalsum =
matrixsum( matrix, 3, rowsum, colsum);
// Output matrix and sums:
cout << "The matrix with the sums "
<< "of rows and columns:\n"
<< endl;
int i,j;
for( i = 0 ; i < 3 ; ++i) // Output rows of the
{ // matrix with row sums.
for( j = 0 ; j < 5 ; ++j)
cout << setw(8) << matrix[i][j];
cout << " | " << setw(8) << rowsum[i] << endl;
}
cout << " -------------------------------------------"
<< endl;
for( j = 0 ; j < 5 ; ++j )
cout << setw(8) << colsum[j];
cout << " | " << setw(8) << totalsum << endl;
return 0;
}
// --------------------------------------------------------
int matrixsum( int v[][5], int len,
int rsum[], int csum[])
{ int ro, co; // Row and column index
for( ro = 0 ; ro < len ; ++ro) // To compute row sums
{
rsum[ro] = 0;
for( co = 0 ; co < 5 ; ++co)
rsum[ro] += v[ro][co];
}
for(co = 0 ; co < 5 ; ++co) // Compute column sums
{
csum[co] = 0;
for( ro = 0 ; ro < len ; ++ro)
csum[co] += v[ro][co];
}
return (rsum[0] + rsum[1] + rsum[2]); // Total sum =
} // sum of row sums.
379
Fundamentals of File
Input and Output
This chapter describes sequential file access using file streams. File
streams provide simple and portable file handling techniques.
chapter
18
380 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
Main Memory External Memory
Write
Read
File
File Buffer
■FILES
File operations
FILES
■381
When a program is terminated, the program data stored in main memory is lost. To store
data permanently, you need to write that data to a file on an external storage medium.
䊐File Operations
Single characters or character strings can be written to text files just like they can be out-
put on screen. However, it is common practice to store records in files. A record contains
data that forms a logical unit, such as the human resource information for a person. A
write operation stores a record in a file, that is, the existing record in the file is updated or
a new record is added. When you read a record, this record is taken from the file and
copied to the data structure of a program.
Objects can be put into permanent storage using similar techniques. However, this
normally involves more than just storing an object’s data. You also need to ensure that
the object can be correctly reconstructed when it is read, and this in turn involves stor-
ing type information and references to other objects.
External mass storage media, such as hard disks, are normally block-oriented—that is,
data is transferred in blocks whose size is a multiple of 512 bytes. Efficient and easy file
management thus implies putting the data you need to store into temporary storage in
main memory, in a so-called file buffer.
䊐File Positions
From the viewpoint of a C++ program, a file is simply a long byte array. The structure of
the file, using records for example, is entirely the programmer’s responsibility, allowing
for a maximum degree of flexibility.
Every character in a file occupies a byte position. The first byte occupies position 0,
the second byte position 1, and so on. The current file position is the position of the byte
that will be read or written next. Each byte that is transferred automatically increases the
current file position by 1.
In the case of sequential access, the data is read or written byte by byte in a fixed order.
The first read operation starts at the beginning of the file. If you need access to some
piece of information in a file, you must read the file content from start to finish. Write
operations can create a new file, overwrite an existing file, or append new data to an
existing file.
Easy access to given data in a file implies being able to set the current file position as
required. This technique is known as random file access and will be discussed in one of the
following chapters.
382 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
ios
istream ostream
iostream
ofstream
ifstream
fstream
■FILE STREAMS
Stream classes for file access
FILE STREAMS
■383
C++ provides various standard classes for file management. These so-called file stream
classes allow for easy file handling. As a programmer you will not need to concern your-
self with file buffer management or system specifics.
Since the file stream classes have been standardized, you can use them to develop
portable C++ programs. One program can thus process files on a Windows NT or UNIX
platform. You simply need to recompile the program for each platform you use.
䊐The File Stream Classes in the iostream Library
The class hierarchy on the opposite page shows that the file stream classes contain the
stream classes, with which you are already familiar, as base classes:
■the ifstream class derives from the istream class and allows file reading
■the ofstream class derives from the ostream stream class and supports writing
to files
■the fstream class derives from the iostream stream class. As you would
expect, it supports both read and write operations for files.
The file stream classes are declared in the fstream header file. An object that
belongs to a file stream class is known as a file stream.
䊐Functionality
The file stream classes inherit the functionality of their base classes. Thus, the methods,
operators, and manipulators you have already used for cin and cout are also available
here. Thus every file stream has:
■methods for non-formatted writing and reading of single characters and/or data
blocks
■the operators << or >> for formatted reading and writing from or to files
■methods and manipulators for formatting character sequences
■methods for state queries.
File handling methods, particularly methods for opening and closing files, round off the
package.
384 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// showfile.cpp
// Reads a text file and outputs it in pages,
// i.e. 20 lines per page.
// Call: showfile filename
// ----------------------------------------------------
#include <iostream>
#include <fstream>
using namespace std;
int main( int argc, char *argv[])
{
if( argc != 2 ) // File declared?
{
cerr << "Use: showfile filename" << endl;
return 1;
}
ifstream file( argv[1]); // Create a file stream
// and open for reading.
if( !file ) // Get status.
{
cerr << "An error occurred when opening the file "
<< argv[1] << endl;
return 2;
}
char line[80];
int cnt = 0;
while( file.getline( line, 80)) // Copy the file
{ // to standard
cout << line << endl; // output.
if( ++cnt == 20)
{
cnt = 0;
cout << "\n\t ---- <return> to continue ---- "
<< endl;
cin.sync(); cin.get();
}
}
if( !file.eof() ) // End-of-file occurred?
{
cerr << "Error reading the file "
<< argv[1] << endl;
return 3;
}
return 0;
}
■CREATING FILE STREAMS
Sample program
CREATING FILE STREAMS
■385
䊐Opening a File
You need to open a file before you can manipulate it. To do so, you can
■state the file name, which can also contain a path
■define a so-called file access mode.
If the path is not explicitly stated, the file must be in the current directory. The file
access mode specifically defines whether read and/or write access to the file is permitted.
Any files still open when a program terminates are automatically closed.
䊐File Stream Definition
You can open a file when you create a file stream—you simply state the file name to do
so. In this case default values are used for the file access mode.
Example: ifstream myfile("test.fle");
The file name test.fle is passed to the constructor of the ifstream class, which
opens the file for reading. Since the path was not stated, the file must be in the current
directory. When a file is opened, the current file position is the beginning of the file.
If you create a file stream for write-only access, the file you state need not exist. In
this case a new file is created.
Example: ofstream yourfile("new.fle");
This statement creates a new file called new.fle and opens the file for writing. But be
careful! If the file already exists, it will be truncated to a length of zero bytes, or in other
words deleted.
You can create a file stream which does not reference a specific file and use the
open() method to open a file later.
Example: ofstream yourfile;
yourfile.open("new.fle");
This example has the same effect as the previous example. More specifically, open()
uses the same default values for file access when opening a file as the default constructor
for the class.
It rarely makes sense to use fixed file names. In the case of the sample program on the
opposite page, you state the file name in the command line when you launch the pro-
gram. If no file name is supplied, the program issues an error message and terminates.
Using interactive user input is another possible way to define a file name.
386 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
ios::in
ios::out
ios::app
ios::trunc
ios::ate
ios::binary
Flag Effects
Opens an existing file for input.
Opens a file for output at the end-of-file.
Perform input and output in binary mode.
An existing file is truncated to zero length.
Open and seek to end immediately after opening.
Without this flag, the starting position after opening is
always at the beginning of the file.
Opens a file for output. This flag implies
ios::trunc if it is not combined with one of the
flags ios::in or ios::app or ios::ate.
ifstream
ofstream
fstream
ios::in
ios::out | ios::trunc
ios::in | ios::out
Class Flags
1. These flags are defined in the baseclass ios, which is common to all stream classes, and
are of the ios::openmode type.
2. By default a file is opened as a text file in so-called text mode. When you read from or
write to a text file, control characters to indicate newlines or the end-of-file are inter-
preted separately and adapted to the current platform (so-called “cooked mode”).When
a file is opened in binary mode, the file contents are left unchanged (the so called “raw
mode”).
✓NOTE
■OPEN MODES
Flags for the open mode of a file
Default settings when opening a file
The constructor and the method open() of all stream classes use the following default
values:
OPEN MODES
■387
To open a file in any but the default mode, you must supply both the file name and the
open mode. This is necessary, for example, to open an existing file for write access with-
out deleting the file.
䊐Open Mode Flags
In addition to the file name, you can pass a second argument for the open mode to the
constructors and the open() method. The open mode is determined by using flags. A
flag represents a single bit in a computer word. If the flag is raised, the bit in question will
contain the value 1, with 0 representing all other cases.
You can use the bit operator, |, to combine various flags. Either the flag ios::in or
ios::out must be stated in all cases. If the flag ios::in is raised, the file must already
exist. If the flag ios::in is not used, the file is created, if it does not already exist.
Example: fstream addresses("Address.fle", ios::out | ios::app);
This opens a file for writing at end-of-file. The file is created, if it does not already exist.
The file will automatically grow after every write operation.
You can use the default mode for the fstream class, that is, ios::in |ios::out,
to open an existing file for reading and writing. This so-called update mode is used for
updating the information in a file and is often seen in conjunction with random file
access.
䊐Error Handling
Errors can occur when opening a file. A user may not have the required access privileges,
or the file you want to read may not exist. The state flag failbit of the ios base class
is raised in this case. The flag can either be queried directly using the fail() method,
or indirectly by querying the status of a file stream in an if condition.
Example: if( !myfile) // or: if( myfile.fail())
The fail bit is also set if a read or write error occurs. If a read operation fails, the end
of the current file may have been reached. To distinguish this normal behavior from a
read error, you can use the eof() method (eof = end-of-file) to query the eof bit:
Example: if( myfile.eof()) // At end-of-file?
The eof bit is set if you try to carry on reading at the end of a file. The sample program
on the previous page illustrates the potential issues.
388 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// fcopy1.cpp : Copies files.
// Call: fcopy1 source [ destination ]
// ----------------------------------------------------
#include <iostream>
#include <fstream>
using namespace std;
inline void openerror( const char *file)
{
cerr << "Error on opening the file " << file << endl;
exit(1); // Ends program closing
} // all opened files.
void copy( istream& is, ostream& os); // Prototype
int main(int argc, char *argv[])
{
if( argc < 2 || argc > 3)
{ cerr << "Call: fcopy1 source [ destination ]"
<< endl;
return 1; // or: exit(1);
}
ifstream infile(argv[1]); // Open 1st file
if( !infile.is_open())
openerror( argv[1]);
if( argc == 2) // Just one sourcefile.
copy( infile, cout);
else // Source and destination
{
ofstream outfile(argv[2]); // Open 2nd file
if( !outfile.is_open() )
openerror( argv[2]);
copy( infile, outfile);
outfile.close(); // Unnecessary.
}
infile.close(); // Unnecessary.
return 0;
}
void copy( istream& is, ostream& os) // Copy it to os.
{
char c;
while( is.get(c) )
os.put(c); // or: os << c ;
}
■CLOSING FILES
Sample program
CLOSING FILES
■389
䊐Motivation
After you have completed file manipulation, the file should always be closed for the fol-
lowing reasons:
■data may be lost, if for some reason the program is not terminated correctly
■there is a limit to the number of files that a program can open simultaneously.
A program that terminates correctly will automatically close any open files before exit-
ing. A file stream destructor will also close a file referenced by a stream. However, if the
file is no longer in use before this point, you should close the file explicitly.
䊐Methods close() and is_open()
Each of the file stream classes contains a definition of a void type method called
close(), which is used to close the file belonging to the stream.
Example: myfile.close();
However, the file stream continues to exist. It is therefore possible to use the stream
to open and manipulate another file.
If you are not sure whether a file stream is currently accessing a file, you can always
perform a test using the is_open() method . In the case of the myfile file stream, the
test is as follows:
Example: if( myfile.is_open() )
{ /* . . . */ } // File is open
䊐The exit() Function
Open files are also closed when you call the global function exit(). The actual reason
for using this function is to terminate a program in an orderly manner and return an error
code to the calling process.
Prototype: void exit( int status );
The calling process, to which the status error code is passed for evaluation, will
often be the command interpreter—a Unix shell, for example. Successful program execu-
tion normally produces the error code 0. The statement return n; is thus equivalent
to the statement exit(n); when used in the main() function.
The program on the opposite page copies a file stated in the command line. If the user
forgets to state a second (target) file, the source file is copied to standard output. In this
case, the source file will need to be a text file.
390 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// Pizza_W.cpp
// Demonstrating output of records block by block.
// ---------------------------------------------------
#include <iostream>
#include <fstream>
using namespace std;
char header[] =
" * * * P I Z Z A P R O N T O * * *\n\n";
// Record structure:
struct Pizza { char name[32]; float price; };
const int MAXCNT = 10;
Pizza pizzaMenu[MAXCNT] =
{
{ "Pepperoni", 9.90F }, { "White Pizza", 15.90F },
{ "Ham Pizza", 12.50F }, { "Calzone", 14.90F } };
int cnt = 4;
char pizzaFile[256] = "pizza.fle";
int main() // To write records.
{
cout << header << endl;
// To write data into the file:
int exitCode = 0;
ofstream outFile( pizzaFile, ios::out|ios::binary );
if( !outFile)
{
cerr << "Error opening the file!" << endl;
exitCode = 1;
}
else
{
for( int i = 0; i < cnt; ++i)
if( !outFile.write( (char*)&pizzaMenu[i],
sizeof(Pizza)) )
{ cerr << "Error writing!" << endl;
exitCode = 2;
}
}
if( exitCode == 0)
cout << "\nData has been added to file "
<< pizzaFile << "\n" << endl;
return exitCode;
}
■READING AND WRITING BLOCKS
Sample program
READING AND WRITING BLOCKS
■391
The file stream classes can use all the public operations defined in their base classes.
This means you can write formatted or unformatted data to a file or read that data from
the file block by block or character by character.
䊐Formatted and Unformatted Input and Output
The previous sample programs illustrated how to use the methods get(),getline(),
and put() to read or write data from or to text files. Formatted input and output of
numerical values, for example, requires the >> and << operators and appropriate manip-
ulators or formatting methods.
Example: double price = 12.34;
ofstream textFile("Test.txt");
textFile << "Price: " << price << "Dollar" << endl;
The file Test.txt will contain a line of text, such as "Price ... " that exactly
matches the screen output.
Converting binary data to legible text is not practicable if you are dealing with large
amounts of data. It makes sense to write the data for a series of measurements to a binary
file in the order in which they occur in the program. To do so, you simply open the file
in binary mode and write the data to the file, or read it from the file, block by block.
䊐Transferring Data Blocks
The ostream method write() transfers given number of bytes from main memory to a
file.
Prototype: ostream& write( const char *buf, int n);
Since write() returns a reference to the stream, you can check to ensure that the write
operation was successful.
Example: if( ! fileStream.write("An example ", 2) )
cerr << "Error in writing!" << endl;
A warning is issued if an error occurs while writing the characters "An". You can use the
read() method to read data blocks from the file. The method transfers a data block
from a file to a program buffer.
Prototype: istream& read( char *buf, int n);
The methods read() and write() are often used for files with fixed length records.
The block that needs to be transferred can contain one or more records. The buffer in
main memory is either a structure variable or an array with elements belonging to the
structure type. You need to cast the address of this memory area to (char *) as shown
in the example opposite.
392 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// Class Account with methods read() and write()
// ---------------------------------------------------
class Account
{
private:
string name; // Account holder
unsigned long nr; // Account number
double balance; // Balance of account
public:
. . . // Constructors, destructor,
// access methods, ...
ostream& Account::write(ostream& os) const;
istream& Account::read(istream& is)
};
// write() outputs an account into the given stream os.
// Returns: The given stream.
ostream& Account::write(ostream& os) const
{
os << name << '\0'; // To write a string
os.write((char*)&nr, sizeof(nr) );
os.write((char*)&balance, sizeof(balance) );
return os;
}
// read() is the opposite function of write().
// read() inputs an account from the stream is
// and writes it into the members of the current object
istream& Account::read(istream& is)
{
getline( is, name, '\0'); // Read a string
is.read( (char*)&nr, sizeof(nr) );
is.read( (char*)&balance, sizeof(balance));
return is;
}
■OBJECT PERSISTENCE
Class Account
Implementing methods read() and write()
OBJECT PERSISTENCE
■393
䊐Storing Objects
Objects are created during program runtime and cleaned up before the program termi-
nates. To avoid this volatility, you can make an object persistent, that is, you can store
the object in a file. However, you must ensure that the object can be reconstructed, as it
was, when read. This means dealing with the following issues:
■Objects can contain other objects. You will generally not know how to store a
member object.
■Objects can contain references to other objects. However, it does not make sense
to store pointer values in a file, as the memory addresses will change each time
you re-launch the program.
For example, the class Account on the opposite page contains the member object
name, which is a string type. As string type objects are used to handle variable
length strings, the object just contains a reference to the string. It therefore makes no
sense to save the memory content of size sizeof(name) occupied by the object name
in a file. Instead, you should write the string itself to a file.
One possible solution to this issue is to store the data to allow them to be passed to a
constructor for the class when read. Another solution involves providing methods to
allow the objects to write their own data members to files or read them from files. This
technique is normally preferable since the class can now handle data storage itself, allow-
ing it to write internal status data while simultaneously preventing external access to
that data.
䊐Storing Account Class Objects
The opposite page shows the Account class, with which you are already familiar. File
input and output methods have been added to the class. A file stream that references a
file opened in binary mode is passed as an argument to the methods read() and
write(). The return value is the stream in both cases, so the status can be queried
when the function is called.
Example: if( ! anAccount.write( outFile) )
cerr << "Error in writing!" << endl;
When you read an account, you can simultaneously create an empty object that the
read() method can access.
Example: if( ! anAccount.read( inFile) )
cerr << "Error in reading!" << endl;
The member object name is saved as a C string, that is, as a string terminated by the null
character, '\0'. The << operator and the function getline() are available for this
task.
exercises
394 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
fcopy file1 file2
A file, file1, is copied to file2. If file2 already exists, it is overwritten.
fcopy file1
A file, file1, is copied to standard output, that is, to the screen if
standard output has not been redirected.
fcopy
For calls without arguments, the source and destination files are entered
in a user dialog.
If is is a file stream that references a file opened for reading, the
following call
Example: char buf[1024];
is.read(buf, 1024);
transfers the next 1024 bytes from file to the buffer buf. Provided that no
error occurs, no less than 1024 bytes will be copied unless end-of-file is
reached. In this case the fail and eof bits are set.The last block of bytes
to be read also has to be written to the destination file.The method
gcount() returns the number of bytes transferred by the last read
operation.
Example: int nread = is.gcount(); // Number of bytes
// in last read op.
■EXERCISES
For exercise 1
Possible calls to the program fcopy:
More details on the istream class method read()
EXERCISES
■395
Exercise 1
The sample program fcopy1, which copies a file to the screen or to a second
file, was introduced in this chapter.Write a program named fcopy to enhance
fcopy1 as follows:
■If the program is launched without any arguments, it does not issue an
error message and terminate but requests user input for the names of the
source and target files. If an empty string is given as the name of the tar-
get file, that is, the Return key is pressed, the source file is displayed on
screen.
■If the command line or the user dialog contains valid source and target
file names, a binary copy operation is performed.
■Copy the data block by block with the read() and write() methods.
The default block size is 1024 bytes.
■The copy() function returns false if an error occurs while copying and
true in all other cases.
Also refer to the notes on the opposite page.
Exercise 2
a. Modify the sample program Pizza_w.cpp in this chapter to allow the
user to add new pizza records to the four standard pizzas and store
these records on file.
b. Then write a program called Pizza_r.cpp, which displays the pizza
menu, that is, outputs the contents of the pizza file.
Exercise 3
Test the methods read() and write() in the Account class.To do so, write a
program called Account_rw.cpp that
■initializes an array with account objects and stores the array in a file
■reads the contents of the file to a second array and displays the accounts
in that array to allow you to check them.
Use binary mode for read or write access to the file.
396 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
New data members:
string filename; // File name
bool dirty; // true, if data is not
// stored yet.
New methods:
const string& getFilename() const;
bool setFilename( const string& fn);
bool isDirty() const;
bool load(); // Read data from the file
bool save(); // Save data.
bool saveAs(); // Save data as ...
* * * * * Telephone List * * * * *
S = Show all entries
F = Find a telephone number
A = Append an entry
D = Delete an entry
-----------------------------------------
O = Open a file
W = Save in the file
U = Save as ...
-----------------------------------------
Q = Quit the program
Your choice:
For Exercise 4
New members of class TelList
Extended menu of the application program
EXERCISES
■397
Exercise 4
The program TelList, which was written as an exercise for Chapter 16, needs
to be modified to allow telephone lists to be saved in a file.
To allow this, first add the data members and methods detailed on the
opposite page to TelList.The string filename is used to store the name of
the file in use.The dirty flag is raised to indicate that the phone list has been
changed but not saved.You will need to modify the existing methods append()
and erase()to provide this functionality.
The strings in the phone list must be saved as C strings in a binary file,
allowing for entries that contain several lines.
Add the following items to the application program menu:
O = Open a file
Read a phone list previously stored in a file.
W = Save
Save the current phone list in a file.
U = Save as . . .
Save the current phone list in a new file.
Choosing one of these menu items calls one of the following methods as
applicable: load(), save() or saveAs().These methods return true for a
successful action and false otherwise.The user must be able to supply a file
name for the save() method, as the list may not have been read from a file
previously.
If the phone list has been modified but not saved, the user should be
prompted to save the current phone list before opening another file or
terminating the program.
solutions
398 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// fcopy.cpp
// Copy files
// Call: fcopy [ source [ destination ] ]
// ----------------------------------------------------
#include <iostream>
#include <fstream>
using namespace std;
char usage[] = "Call: fcopy [source [destination]}";
inline void openerror( const char *file)
{
cerr << "Error opening the file " << file << endl;
exit(1);
}
bool copy( istream& is, ostream& os), // Prototype,
ok = true; // ok flag.
int main(int argc, char *argv[])
{
char source[256] = "", dest[256] = "";
switch( argc )
{
case 1: // No file declared
// ==> input file name.
cout << "Copying source file to "
"destination file!\n"
"Source file: ";
cin.getline( source, 256);
if( strlen(source) == 0)
{ cerr << "No source file declared!" << endl;
return 1;
}
cin.sync(); // No previous input
cout << "Destination file: ";
cin.getline( dest, 256);
break;
case 2: // One file is declared.
strcpy( source, argv[1]);
break;
case 3: // Source and destination files are declared.
strcpy( source, argv[1]);
strcpy( dest, argv[2]);
break;
SOLUTIONS
■399
default: // Invalid call to the program.
cerr << usage << endl;
return 2; // or: exit(2);
}
if( strlen(dest) == 0) // Only source file?
{ // yes ==> output to cout.
ifstream infile(source);
if( !infile )
openerror( source);
ok = copy( infile, cout);
// The file is closed by the ifstream destructor.
}
else // Copy source to destination
{ // file in binary mode.
ifstream infile( source, ios::in | ios::binary);
if( !infile )
openerror( source);
else
{
ofstream outfile( dest, ios::out | ios::binary);
if( !outfile)
openerror( dest);
ok = copy( infile, outfile);
if( ok)
cerr << "File " << source << " to file "
<< dest <<" copied!"<< endl;
}
}
if(!ok)
{ cerr << "Error while copying!" << endl;
return 3;
}
return 0;
}
bool copy( istream& is, ostream& os) // To copy
{ // is to os.
const int BufSize = 1024;
char buf[BufSize];
do
{
is.read( buf, BufSize);
if( is.gcount() > 0)
os.write(buf, is.gcount());
}
while( !is.eof() && !is.fail() && !os.fail() );
if( !is.eof() ) return false;
else return true;
}
400 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
Exercise 2
// ----------------------------------------------------
// Pizza.h
// Header file for Pizza_W.cpp and Pizza_R.cpp.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <fstream>
using namespace std;
// Structure of a record:
struct Pizza { char name[32]; float price; };
#define MAXCNT 20 // Maximum number of pizzas
#define FILENAME "pizza.fle"
inline void header()
{
cout << " * * * P I Z Z A P R O N T O * * *\n\n"
<< endl;
}
// ----------------------------------------------------
// Pizza_w.cpp
// Demonstrating blockwise writing of records.
// ----------------------------------------------------
#include "Pizza.h"
Pizza pizzaMenu[MAXCNT] =
{
{ "Pepperoni", 9.90F }, { "White Pizza", 15.90F },
{ "Ham Pizza", 12.50F }, { "Calzone", 14.90F } };
int cnt = 4;
char pizzaFile[256] = FILENAME;
int main() // Write records.
{
int i;
header();
cout << "\nOur standard offer:\n" << endl;
cout << fixed << setprecision(2);
for( i = 0; i < cnt; ++i)
cout << setw(20) << pizzaMenu[i].name
<< setw(10) << pizzaMenu[i].price << endl;
cout << "\n-----------------------------------------\n"
<< endl;
SOLUTIONS
■401
// Input more pizzas via keyboard:
while( cnt < MAXCNT)
{
cin.sync(); cin.clear();
cout << "What pizza should be added "
<< "to the menu?\n\n" << "Name: ";
cin.getline( pizzaMenu[cnt].name, 32);
if( pizzaMenu[cnt].name[0] == '\0')
break;
cout << "Price: ";
cin >> pizzaMenu[cnt].price;
if( !cin)
cerr << "Invalid input!" << endl;
else
++cnt;
if( cnt < MAXCNT)
cout << "\n... and the next pizza!\n"
<< "Stop with <Return>.\n";
}
// Add data to the file:
int exitCode = 0;
ofstream outFile( pizzaFile, ios::out | ios::binary);
if( !outFile)
{
cerr << "Error opening the file!" << endl;
exitCode = 1;
}
else
{
for( int i = 0; i < cnt; ++i)
if( !outFile.write( (char*)&pizzaMenu[i],
sizeof(Pizza)) )
{
cerr << "Error writing to file!"
<< endl;
exitCode = 2;
}
}
if( exitCode == 0)
cout << "\nData added to file " << pizzaFile
<< ".\n" << endl;
return exitCode;
}
402 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// ----------------------------------------------------
// Pizza_r.cpp
// Demonstrating block by block reading of records.
// ---------------------------------------------------
#include "Pizza.h"
char pizzaFile[256] = FILENAME;
int main() // Read and display records.
{
header();
ifstream inFile( pizzaFile, ios::in | ios::binary);
if( !inFile)
{
cerr << "Pizza file does not exist!" << endl;
return 1;
}
Pizza onePizza;
int cnt = 0;
cout << "\n-------------------------------------------"
<< "\nThe available pizzas:\n" << endl;
cout << fixed << setprecision(2);
while( true)
if( !inFile.read( (char*)&onePizza, sizeof(Pizza)) )
break;
else
{
cout << setw(20) << onePizza.name
<< setw(10) << onePizza.price << endl;
++cnt;
}
cout << "\n------------------------------------------\n"
<< endl;
if( !inFile.eof())
{
cerr << "Error reading file!" << endl;
return 2;
}
else
cerr << "These are " << cnt << " pizzas!\n" << endl;
return 0;
}
SOLUTIONS
■403
Exercise 3
// -------------------------------------------------------
// Account_rw.cpp
// Writes an array with objects of class Account to
// a file and feed the array into another array.
// -------------------------------------------------------
#include "Account.h" // Definition of the class Account
#include <iostream>
#include <fstream>
using namespace std;
Account AccTab1[3] =
{
Account("Lucky, Luke", 707070, -1200.99),
Account("Mickey, Mouse", 123000, 2500.0),
Account("Snoopy, Dog\n" // String can contain more
"Cell #: 01771234567", 543001) // than one line.
};
Account AccTab2[3]; // Calls to default constructor
int cnt = 3;
char file[] = "account.fle";
int main()
{
int i = 0;
// --- Write accounts to file ---
ofstream outFile( file, ios::out | ios::binary );
if( ! outFile)
{
cerr << "Error opening file " << file
<< endl;
return 1;
}
for( i = 0; i < cnt; ++i)
if( !AccTab1[i].write(outFile) )
{
cerr << "Error writing to file " << file
<< endl;
return 2;
}
outFile.close();
404 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
// --- Reads accounts from file ---
ifstream inFile( file, ios::out | ios::binary );
if( ! inFile)
{
cerr << "Error opening file " << file
<< endl;
return 3;
}
for( i = 0; i < cnt; ++i)
if( !AccTab2[i].read(inFile) )
{
cerr << "Error reading file " << file
<< endl;
return 4;
}
inFile.close();
// --- Displays the accounts read ---
cout << "The file " << file << " contains the "
<< "following accounts:" << endl;
for( i = 0; i < cnt; ++i)
AccTab2[i].display();
cout << endl;
return 0;
}
Exercise 4
// -------------------------------------------------------
// telList.h
// A class TelList to represent a list
// containing names and telephone numbers.
// The methods load(), save(), and saveAs() serve for
// loading and saving a telephone list.
// --------- ---------------------------------------------
#ifndef _TelList_
#define _TelList_
#include <string>
using namespace std;
#define PSEUDO -1 // Pseudo position
#define MAX 100 // Maximum number of elements
SOLUTIONS
■405
// Type of a list element:
struct Element { string name, telNr; };
class TelList
{
private:
Element v[MAX]; // The array and the actual
int count; // number of elements.
string filename; // File name
bool dirty; // true if data has been changed
// but not yet saved.
public:
TelList() : count(0), filename(""), dirty(false)
{}
int getCount() { return count; }
Element *retrieve( int i )
{
return (i >= 0 && i < count)? &v[i] : NULL;
}
bool append( const Element& el )
{
return append( el.name, el.telNr);
}
bool append( const string& name, const string& telNr);
bool erase( const string& name);
int search( const string& name) const;
void print() const;
int print( const string& name) const;
int getNewEntries();
const string& getFilename() const { return filename; }
bool setFilename( const string& fn)
{ if( fn.empty()
return false;
else { filename = fn; dirty = true; return true; }
}
bool isDirty() const { return dirty; }
bool load();
bool save();
bool saveAs();
};
#endif // _TelList_
// -------------------------------------------------------
// TelList.cpp
// Implements the methods of class TelList.
// -------------------------------------------------------
#include "telList.h" // Definition of class TelList
#include <iostream>
#include <iomanip>
#include <fstream>
using namespace std;
406 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
bool TelList::append( const string& name,
const string& telNr)
{
if( count < MAX // Any space
&& name.length() > 1 // minimum 2 characters
&& search(name) == PSEUDO) // does not exist
{
v[count].name = name;
v[count].telNr = telNr;
++count;
dirty = true;
return true;
}
return false;
}
bool TelList::erase( const string& key )
{
int i = search(key);
if( i != PSEUDO )
{
if( i != count-1) // Copy the last element
v[i] = v[count-1]; // to position i.
--count;
dirty = true;
return true;
}
return false;
}
// --------------------------------------------------
// Methods search(), print(), getNewEntries()
// are unchanged (refer to solutions of chapter 16).
// --------------------------------------------------
// Methods for loading and saving the telephone list.
bool TelList::load()
{
cout << "\n--- Load the telephone list "
<< "from a file. ---" << "\nFile: ";
string file; // Input file name.
cin.sync(); cin.clear(); // No previous input
getline( cin, file);
if( file.empty())
{
cerr << "No filename declared!" << endl;
return false;
}
SOLUTIONS
■407
// Open the file for reading:
ifstream infile( file.c_str(), ios::in | ios::binary);
if( !infile )
{
cerr << "File " << file
<< " could not be opened!" << endl;
return false;
}
int i = 0;
while( i < MAX)
{
getline( infile, v[i].name, '\0');
getline( infile, v[i].telNr, '\0');
if( !infile)
break;
else
++i;
}
if( i == MAX)
cerr << "Max capacity " << MAX
<< " has been reached!" << endl;
else if( !infile.eof())
{
cerr << "Error reading file " << file << endl;
return false;
}
count = i;
filename = file;
dirty = false;
return true;
}
bool TelList::saveAs()
{
cout << "-- Save the telephone list in a file. --"
<< "\nFile: ";
string file; // Input file name.
cin.sync(); cin.clear(); // No previous input
getline( cin, file);
if( !setFilename(file))
{
cerr << "No file name declared!" << endl;
return false;
}
else
return save();
}
408 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
bool TelList::save() // Save the telephone list.
{
if( filename.empty())
return saveAs();
if( !dirty)
return true;
ofstream outfile( filename.c_str(),
ios::out | ios::binary);
if( !outfile )
{
cerr << "File " << filename
<< " could not be opened!" << endl;
return false;
}
int i = 0;
while( i < count)
{
outfile << v[i].name << '\0';
outfile << v[i].telNr << '\0';
if( !outfile)
break;
else
++i;
}
if( i < count)
{
cerr << "Error writing to file " << filename << endl;
return false;
}
dirty = false;
return true;
}
// -------------------------------------------------------
// TelList_.cpp
// Organize a telephone list with class TelList.
// -------------------------------------------------------
#include "telList.h" // Definition of class TelList
#include <iostream>
#include <string>
#include <cctype>
using namespace std;
inline void cls()
{
cout << "\033[2J\n"; // If ANSI control characters are
} // not available, output new-lines.
SOLUTIONS
■409
inline void go_on()
{
cout << "\n\nGo on with return! ";
cin.sync(); cin.clear(); // No previous input
while( cin.get() != '\n')
;
}
int menu(); // Enter a command
char askForSave(); // Prompt user to save.
char header[] =
"\n\n * * * * * Telephone List * * * * *\n\n";
TelList myFriends; // A telephone list
int main()
{
int action = 0; // Command
string name; // Read a name
while( action != 'Q')
{
action = menu();
cls(); cout << header << endl;
switch( action)
{
// ---------------------------------------------------
// case 'S': case 'F': case 'A': case 'D':
// unchanged (refer to the solutions of chapter 16).
// ---------------------------------------------------
case 'O': // To open a file
if(myFriends.isDirty() && askForSave() == 'y')
myFriends.save();
if( myFriends.load())
cout << "Telephone list read from file "
<< myFriends.getFilename() <<"!"
<< endl;
else
cerr << "Telephone list not read!"
<< endl;
go_on();
break;
case 'U': // Save as ...
if( myFriends.saveAs())
cout << "Telephone list has been saved in file: "
<< myFriends.getFilename() << " !" <<endl;
else
cerr << "Telephone list not saved!" << endl;
go_on();
break;
410 ■
CHAPTER 18 FUNDAMENTALS OF FILE INPUT AND OUTPUT
case 'W': // Save
if( myFriends.save())
cout << "Telephone list has been saved in "
<< "the file "
<< myFriends.getFilename() << endl;
else
cerr << "Telephone list not saved!"
<< endl;
go_on();
break;
case 'Q': // Quit
if( myFriends.isDirty() && askForSave() == 'Y')
myFriends.save();
cls();
break;
}
} // End of while
return 0;
}
int menu()
{
static char menuStr[] =
// . . .
"\n -------------------------------------"
"\n O = Open a file"
"\n W = Save "
"\n U = Save as ..."
"\n -------------------------------------"
"\n Q = Quit the program"
"\n\n Your choice: ";
// ---------------------------------------------------
// everything else unchanged (cf. solutions in Chapter 16)
// ---------------------------------------------------
return choice;
}
char askForSave()
{
char c;
cout << "Do you want to save the phone list(y/n)? ";
do
{ cin.get(c);
c = toupper(c);
}while( c != 'Y' && c != 'N');
return c;
}
411
Overloading Operators
Overloading operators allows you to apply existing operators to objects
of class type. For example, you can stipulate the effect of the + operator
for the objects of a particular class.
This chapter describes various uses of overloaded operators.
Arithmetic operators, comparisons, the subscript operator, and the shift
operators for input and output are overloaded to illustrate the
appropriate techniques.
The concept of friend functions, which is introduced in this context, is
particularly important for overloading operators.
chapter
19
412 ■
CHAPTER 19 OVERLOADING OPERATORS
The assignment operator =, the address operator &, and the comma operator,have a predefined
meaning for each built-in type. This meaning can be changed for classes by a definition of your own.
✓NOTE
■GENERALS
Overloadable operators
䊐Rules
An operator is always overloaded in conjunction with a class. The definition scope of an
operator is simply extended—the characteristics of the operator remain unchanged. The
following rules apply:
■You cannot create “new operators”—that is, you can only overload existing oper-
ators.
■You cannot redefine the operators for fundamental types.
■You cannot change the operands of an operator. A binary operator will always be
binary and a unary operator will always be unary.
■The precedence and the order of grouping operators of the same precedence
remains unchanged.
+ - * / %
== != < <= > >=
& | ^ ~ << >>
&& || !
=
op=
() []
& * -> ,
new delete
++ --
Arithmetic operators
Relational operators
Logical operators
Assignment operators
(op is a binary arithmetic
or a binary bitwise operator)
Bitwise operators
Function call, subscript operator
Other operators
Operators Meaning
GENERALS
■413
䊐Overloading
An operator is said to be overloaded if it is defined for multiple types. In other words,
overloading an operator means making the operator significant for a new type.
Most operators are already overloaded for fundamental types. In the case of the
expression:
Example: a / b
the operand type determines the machine code created by the compiler for the division
operator. If both operands are integral types, an integral division is performed; in all
other cases floating-point division occurs. Thus, different actions are performed depend-
ing on the operand types involved.
䊐Operators for Classes
In addition to defining methods, C++ offers the interesting possibility of defining the
functionality of a class by means of operators. Thus, you can overload the +operator
instead of, or in addition to, using the add() method. For the objects xand yin this
class:
x + y is equivalent to x.add(y)
Using the overloaded operators of a class expressions of this type can be as easily defined
as for fundamental types. Expressions using operators are often more intuitive, and thus
easier to understand than expressions containing function calls.
Many operators belonging to the C++ standard library classes are already overloaded.
This applies to the string class, with which you are already familiar.
Example: string str1("Hello "), str2("Eve");
str1 += str2; // Operator +=
if( str2 < "Alexa") ... // Operator <
cout << str1; // Operator <<
str2[2] = 'i'; // Operators [] and =
The tables on the opposite page show those operators that can be overloaded. Some
operators cannot be overloaded, such as the cast operators, the sizeof operator, and
the following four operators:
. :: .* member access and scope resolution operators
?: conditional operator
These operators either have a fixed significance in the classes for which they are defined,
or overloading the operator makes no sense.
414 ■
CHAPTER 19 OVERLOADING OPERATORS
// DayTime.h
// The class DayTime containing operators < and ++ .
// ---------------------------------------------------
#ifndef _DAYTIME_
#define _DAYTIME_
class DayTime
{
private:
short hour, minute, second;
bool overflow;
public:
DayTime( int h = 0, int m = 0, int s = 0);
bool setTime(int hour, int minute, int second = 0);
int getHour() const { return hour; }
int getMinute() const { return minute; }
int getSecond() const { return second; }
int asSeconds() const // Daytime in seconds
{ return (60*60*hour + 60*minute + second); }
bool operator<( const DayTime& t) const // compare
{ // *this and t
return asSeconds() < t.asSeconds();
}
DayTime& operator++() // Increment seconds
{
++second; // and handle overflow.
return *this;
}
void print() const;
};
#endif // _DAYTIME_
#include "DayTime.h"
. . .
DayTime depart1( 11, 11, 11), depart2(12,0,0);
. . .
if( depart1 < depart2 )
cout << "\nThe 1st plane takes off earlier!" << endl;
. . .
■OPERATOR FUNCTIONS (1)
Operators <and ++ for class DayTime
Calling the Operator <
OPERATOR FUNCTIONS (1)
■415
䊐Naming Operator Functions
To overload an operator, you just define an appropriate operator function. The operator
function describes the actions to be performed by the operator. The name of an operator
function must begin with the operator keyword followed by the operator symbol.
Example: operator+
This is the name of the operator function for the + operator.
An operator function can be defined as a global function or as a class method. Gener-
ally, operator functions are defined as methods, especially in the case of unary operators.
However, it can make sense to define an operator function globally. This point will be
illustrated later.
䊐Operator Functions as Methods
If you define the operator function of a binary operator as a method, the left operand will
always be an object of the class in question. The operator function is called for this
object. The second, right operand is passed as an argument to the method. The method
thus has a single parameter.
Example: bool operator<( const DayTime& t) const;
In this case the lesser than operator is overloaded to compare two DayTime objects. It
replaces the method isLess(), which was formerly defined for this class.
The prefix operator ++ has been overloaded in the example on the opposite page to
illustrate overloading unary operators. The corresponding operator function in this class
has no parameters. The function is called if the object ain the expression ++a is an
object of class DayTime.
䊐Calling an Operator Function
The example opposite compares two times of day:
Example: depart1 < depart2
The compiler will attempt to locate an applicable operator function for this expression
and then call the function. The expression is thus equivalent to
depart1.operator<( depart2)
Although somewhat uncommon, you can call an operator function explicitly. The previ-
ous function call is therefore technically correct.
Programs that use operators are easier to encode and read. However, you should be
aware of the fact that an operator function should perform a similar operation to the cor-
responding operator for the fundamental type. Any other use can lead to confusion.
416 ■
CHAPTER 19 OVERLOADING OPERATORS
// Euro1.h : The class Euro containing arithmetic operators.
// --------------------------------------------------------
#ifndef _EURO_H_
#define _EURO_H_
#include <sstream> // The class stringstream
#include <iomanip>
using namespace std;
class Euro
{
private:
long data; // Euros * 100 + Cents
public:
Euro( int euro = 0, int cents = 0)
{
data = 100L * (long)euro + cents;
}
Euro( double x)
{
x *= 100.0; // Rounding,
data = (long)(x>=0.0 ? x+0.5 : x-0.5); //ex. 9.7 -> 10
}
long getWholePart() const { return data/100; }
int getCents() const { return (int)(data%100); }
double asDouble() const { return (double)data/100.0; }
string asString() const; // Euro as string.
void print( ostream& os) const // Output to stream os.
{ os << asString() << " Euro" << endl; }
// ---- Operator functions ----
Euro operator-() const // Negation (unary minus))
{
Euro temp;
temp.data = -data;
return temp;
}
Euro operator+( const Euro& e2) const // Addition.
{
Euro temp;
temp.data = data + e2.data;
return temp;
}
Euro operator-( const Euro& e2) const // Subtraction.
{ /* Analog just as operator + */ }
Euro& operator+=( const Euro& e2) // Add Euros.
{
data += e2.data;
return *this;
}
Euro& operator-=( const Euro& e2); // Subtract euros.
{ /* Just as operator += */ }
};
// Continued on the next double page.
■OPERATOR FUNCTIONS(2)
Class Euro
OPERATOR FUNCTIONS(2)
■417
䊐Notes on the Sample Class Euro
The opposite page shows the Euro class, which represents the new European currency.
The member data stores a given amount of euros as an integer in the format:
(integer part)*100 + Cents.
Thus data/100 returns the number of euros and data%100 the number of cents. This
technique allows for easy implementation of the arithmetic operations needed for the
Euro class.
In addition to a constructor that is passed whole euros and cents as arguments, there is
a constructor that can process a double value of euros and a standard copy constructor.
Example: Euro e1(9,50), e2(20.07), e3(-e1);
䊐Negation, Addition, and Subtraction
The unary operator -does not change its operand. In the previous example, e3 is thus
assigned a value of -9,50 euro, but e1 remains unchanged. The operator function is
thus a const method that creates and returns a temporary object.
The binary operators +and -do not change their operands either. Thus, the operator
functions also create temporary objects and return them with the correct values.
Example: Euro sum = e1 + e2;
The expression e1 + e2 results in e1.operator+(e2). The return value is used to
initialize the new object, sum.
䊐The += and -= Operators
Although the operators +and -were overloaded for the Euro class, this does not auto-
matically mean that the operators += and -= are overloaded. Both are distinct operators
that require separate definitions. Of course, you should overload the operators to ensure
that the statements
Example: sum += e3; and sum = sum + e3;
produce the same results.
The binary operators += and -= change the current object, that is, the left operand. A
temporary object is not required! The expression sum += e3 represents the current
object after modification. Thus, the operator function returns a reference to *this.
418 ■
CHAPTER 19 OVERLOADING OPERATORS
// Continues file Euro1.h
// --------------------------------------------------------
inline string Euro::asString() const // Euro as string
{
stringstream strStream; // Stream for conversion
long temp = data;
if( temp < 0) { strStream << '-'; temp = -temp; }
strStream << temp/100 << ','
<< setfill('0') << setw(2) << temp%100;
return strStream.str();
}
#endif // _EURO_H_
// Euro1_t.cpp
// Tests the operators of class Euro.
// --------------------------------------------------------
#include "Euro1.h" // Definition of the class
#include <iostream>
using namespace std;
int main()
{
cout << "* * * Testing the class Euro * * *\n" << endl;
Euro wholesale( 20,50), retail;
retail = wholesale; // Standard assignment
retail += 9.49; // += (Euro)9.49
cout << "Wholesale price: "; wholesale.print(cout);
cout << "Retail price: "; retail.print(cout);
Euro discount( 2.10); // double-constructor
retail -= discount;
cout << "\nRetail price including discount: ";
retail.print(cout);
wholesale = 34.10;
cout << "\nNew wholesale price: ";
wholesale.print(cout);
Euro profit( retail - wholesale); // Subtraction and
// copy constructor
cout << "\nThe profit: ";
profit.print(cout); // Negative!
return 0;
}
■USING OVERLOADED OPERATORS
File Euro1.h continued
Sample program
USING OVERLOADED OPERATORS
■419
䊐Calling Operator Functions
The following expressions are valid for the operators in the Euro class.
Example: Euro wholesale(15,30), retail,
profit(7,50), discount(1,75);
retail = wholesale + profit;
// Call: wholesale.operator+( profit)
retail -= discount;
// Call: retail.operator-=( discount)
retail += Euro( 1.49);
// Call: retail.operator+=( Euro(1.49))
These expressions contain only Euro type objects, for which operator functions have
been defined. However, you can also add or subtract int or double types. This is made
possible by the Euro constructors, which create Euro objects from int or double
types. This allows a function that expects a Euro value as argument to process int or
double values.
As the program opposite shows, the statement
Example: retail += 9.49;
is valid. The compiler attempts to locate an operator function that is defined for both the
Euro object and the double type for +=. Since there is no operator function with
these characteristics, the compiler converts the double value to Euro and calls the
existing operator function for euros.
䊐Symmetry of Operands
The available constructors also allow you to call the operator functions of +and – with
int or double type arguments.
Example: retail = wholesale + 10; // ok
wholesale = retail - 7.99; // ok
The first statement is equivalent to
retail = wholesale.operator+( Euro(10));
But the following statement is invalid!
Example: retail = 10 + wholesale; // wrong!
Since the operator function was defined as a method, the left operand must be a class
object. Thus, you cannot simply exchange the operands of the operator +. However, if
you want to convert both operands, you will need global definitions for the operator
functions.
420 ■
CHAPTER 19 OVERLOADING OPERATORS
=
()
[]
->
Assignment operator
Function call
Subscript operator
Class member access
Operators Meaning
The function call operator () is used to represent operations for objects like function calls. The
overloaded operator -> enables the use of objects in the same way as pointers.
✓NOTE
// Euro.h
// The class Euro represents a Euro with
// global operator functions implemented for + and -.
// ---------------------------------------------------
#ifndef _EURO_H_
#define _EURO_H_
// ....
class Euro
{
// Without operator functions for + and -.
// Otherwise unchanged, specifically with regard to
// the operator functions implemented for += and -=.
};
// ----------------------------------------------------
// Global operator functions (inline)
// Addition:
inline Euro operator+( const Euro& e1, const Euro& e2)
{
Euro temp(e1);
temp += e2;
return temp;
}
// Subtraction:
inline Euro operator-( const Euro& e1, const Euro& e2)
{
Euro temp(e1); temp -= e2;
return temp;
}
#endif // _EURO_H_
■GLOBAL OPERATOR FUNCTIONS
Operators overloadable by methods only
The operator functions of the following operators have to be methods:
The new Euro class
GLOBAL OPERATOR FUNCTIONS
■421
䊐Operator Functions: Global or Method?
You can define an operator function as a global function instead of a method. The four
operators listed opposite are the only exceptions.
+= -= *= /= %=
These operators always require a so-called l-value as their left operand, that is, they
require an object with an address in memory.
Global operator functions are generally preferable if one of the following situations
applies:
■the operator is binary and both operands are symmetrical, e.g. the arithmetic
operators +or *
■the operator is to be overloaded for another class without changing that class, e.g.
the << operator for the ostream class.
䊐Defining Global Operator Functions
The operands for a global operator function are passed as arguments to that function.
The operator function of a unary operator thus possesses a single parameter, whereas the
operator function of a binary operator has two.
The Euro class has been modified to provide a global definition of the operator func-
tions for the operators +and -.
Example: Euro operator+(const Euro& e1, const Euro& e2);
Both operands are now peers. More specifically, conversion of int or double to Euro
is performed for both operands now. Given a Euro object net, the following expres-
sions are valid and equivalent:
Example: net + 1.20 and 1.20 + net
They cause the following function calls:
operator+( net, 1.20) and
operator+( 1.20, net)
However, a global function cannot access the private members of the class. The func-
tion operator+() shown opposite therefore uses the += operator, whose operator
function is defined as a method.
A global operator function can be declared as a “friend” of the class to allow it access
to the private members of that class.
422 ■
CHAPTER 19 OVERLOADING OPERATORS
// Euro.h
// The class Euro with operator functions
// declared as friend functions.
// ---------------------------------------------------
#ifndef _EURO_H_
#define _EURO_H_
// ....
class Euro
{
private:
long data; // Euros * 100 + Cents
public:
// Constructors and other methods as before.
// Operators -(unary), +=, -= as before.
// Division Euro / double :
Euro operator/( double x) // Division *this/x
{ // = *this * (1/x)
return (*this * (1.0/x));
}
// Global friend functions
friend Euro operator+( const Euro& e1, const Euro& e2);
friend Euro operator-( const Euro& e1, const Euro& e2);
friend Euro operator*( const Euro& e, double x)
{
Euro temp( ((double)e.data/100.0) * x) ;
return temp;
}
friend Euro operator*( double x, const Euro& e)
{
return e * x;
}
};
// Addition:
inline Euro operator+( const Euro& e1, const Euro& e2)
{
Euro temp; temp.data = e1.data + e2.data;
return temp;
}
// Subtraction:
inline Euro operator-( const Euro& e1, const Euro& e2)
{
Euro temp; temp.data = e1.data - e2.data;
return temp;
}
#endif // _EURO_H_
■FRIEND FUNCTIONS
Class Euro with friend functions
FRIEND FUNCTIONS
■423
䊐The Friend Concept
If functions or individual classes are used in conjunction with another class, you may
want to grant them access to the private members of that class. This is made possible
by a friend declaration, which eliminates data encapsulation in certain cases.
Imagine you need to write a global function that accesses the elements of a numerical
array class. If you need to call the access methods of the class each time, and if these
methods perform range checking, the function runtime will increase considerably. How-
ever, special permission to access the private data members of the class can dramatically
improve the function’s response.
䊐Declaring Friend Functions
A class can grant any function a special permit for direct access to its private members.
This is achieved by declaring the function as a friend. The friend keyword must pre-
cede the function prototype in the class definition.
Example: class A
{ // . . .
friend void globFunc( A* objPtr);
friend int B::elFunc( const A& objRef);
};
Here the global function globFunc() and the method elFunc() of class Bare
declared as friend functions of class A. This allows them direct access to the private
members of class A. Since these functions are not methods of class A, the this pointer is
not available to them. To resolve this issue, you will generally pass the object the func-
tion needs to process as an argument.
It is important to note that the class itself determines who its friends are. If this were
not so, data encapsulation could easily be undermined.
䊐Overloading Operators with Friend Functions
The operator functions for +and -in the Euro class are now defined as friend func-
tions, allowing them direct access to the private member data.
In order to compute interest, it is necessary to multiply and divide euros by double
values. Since both the expression Euro*num and num*Euro are possible, friend func-
tions are implemented to perform multiplications. As the example shows, friend func-
tions can also be defined inline in a class.
424 ■
CHAPTER 19 OVERLOADING OPERATORS
// Result.h
// The class Result to represent a measurement
// and the time the measurement was taken.
// ---------------------------------------------------
#ifndef _RESULT_
#define _RESULT_
#include "DayTime.h" // Class DayTime
class Result
{
private:
double val;
DayTime time;
public:
// Constructor and access methods
friend class ControlPoint; // All methods of
}; // ControlPoint are friends.
#include Result.h
class ControlPoint
{
private:
string name; // Name of control point
Result measure[100]; // Table with results
// . . .
public:
// Constructor and the other methods
// . . .
// Compute static values of measurement results
// (average, deviation from mean, ...).
bool statistic(); // Can access the private
// members of measure[i].
};
■FRIEND CLASSES
Class Result
Class ControlPoint
FRIEND CLASSES
■425
䊐Declaring Friend Classes
In addition to declaring individual friend functions, you can also make entire classes
“friends” of another class. All the methods in this “friendly” class automatically become
friend functions in the class containing the friend declaration.
This technique is useful if a class is used in such close conjunction with another class
that all the methods in that class need access to the private members of the other class.
For example, the class ControlPoint uses objects of the Result class. Calcula-
tions with individual measurements are performed repeatedly. In this case, it makes sense
to declare the ControlPoint class as a friend of the Result class.
Example: class Result
{
// . . .
friend class ControlPoint;
};
It is important to note that the ControlPoint class has no influence over the fact that
it is a friend of the Result class. The Result class itself decides who its friends are and
who has access to its private members.
It does not matter whether a friend declaration occurs in the private or public
section of a class. However, you can regard a friend declaration as an extension of the
public interface. For this reason, it is preferable to place a friend declaration in the
public area of a class.
䊐Using Friend Functions and Classes
Using friend functions and friend classes helps you to create efficient programs.
More specifically, you can utilize global friend functions where methods are not suited
to the task in hand. Some common uses are global operator functions declared as friend
functions.
However, extensive use of friend techniques diffuses the concept of data encapsula-
tion. Allowing external functions to manipulate internal data can lead to inconsistency,
especially if a class is modified or extended in a later version. For this reason, you should
take special care when using friend techniques.
426 ■
CHAPTER 19 OVERLOADING OPERATORS
// Array_t.cpp
// A simple class to represent an array
// with range checking.
// --------------------------------------------------
#include <iostream>
#include <cstdlib> // For exit()
using namespace std;
#define MAX 100
class FloatArr
{
private:
float v[MAX]; // The array
public:
float& operator[](int i);
static int MaxIndex(){ return MAX-1; }
};
float& FloatArr::operator[]( int i )
{
if( i < 0 || i >= MAX )
{ cerr << "\nFloatArr: Outside of range!" << endl;
exit(1);
}
return v[i]; // Reference to i-th element.
}
int main()
{
cout << "\n An array with range checking!\n"
<< endl;
FloatArr random; // Create array.
int i; // An index.
// Fill with random euros:
for( i=0; i <= FloatArr::MaxIndex(); ++i)
random[i] =(rand() - RAND_MAX/2) / 100.0F;
cout << "\nEnter indices between 0 and "
<< FloatArr::MaxIndex() << "!"
<< "\n (Quit by entering invalid input)"
<< endl;
while( cout << "\nIndex: " && cin >> i )
cout << i << ". element: " << random[i];
return 0;
}
■OVERLOADING SUBSCRIPT OPERATORS
A class representing arrays
OVERLOADING SUBSCRIPT OPERATORS
■427
䊐Subscript Operator
The subscript operator [] is normally used to access a single array element. It is a binary
operator and thus has two operands. Given an expression such as v[i], the array name
vwill always be the left operand, whereas the index iwill be the right operand.
The subscript operator for arrays implies background pointer arithmetic, for example,
v[i] is equivalent to *(v+i). Thus, the following restrictions apply to non-overloaded
index operators:
■an operand must be a pointer—an array name, for example
■the other operand must be an integral expression.
䊐Usage in Classes
These restrictions do not apply if the index operator is overloaded for a class. You should
note, however, that the operator function is always a class method with a parameter for
the right operand. The following therefore applies:
■the left operand must be a class object
■the right operand can be any valid type
■the result type is not defined.
This allows for considerable flexibility. However, your overloading should always reflect
the normal use of arrays. More specifically, the return value should be a reference to an
object.
Since an index can be of any valid type, the possibilities are unlimited. For example,
you could easily define associative arrays, that is, arrays whose elements are referenced by
strings.
䊐Notes on the Sample Program
Range checking is not performed when you access the elements of a normal array. An
invalid index can thus lead to abnormal termination of an application program. How-
ever, you can address this issue by defining your own array classes, although this may
impact the speed of your programs.
The opposite page shows a simple array class definition for float values. The sub-
script operator [] has been overloaded to return a reference to the i-th array element.
However, when the array is accessed, range checking is performed to ensure that the
index falls within given boundaries. If an invalid index is found, the program issues an
error message and terminates.
The class FloatArr array has a fixed length. As we will see, variable lengths are pos-
sible using dynamic memory allocation.
428 ■
CHAPTER 19 OVERLOADING OPERATORS
// Euro.h : Class Euro to represent an Euro
// ---------------------------------------------------
#ifndef _EURO_H_
#define _EURO_H_
// ....
class Euro
{ // The class is left unchanged.
// The print() method is now superfluous.
};
// ----------------------------------------------------
// Declaration of shift operators:
ostream& operator<<(ostream& os, const Euro& e);
istream& operator>>(istream& is, Euro& e);
#endif // _EURO_H_
// Euro_io.cpp
// Overload the shift operators
// for input/output of Euro type objects.
// ---------------------------------------------------
#include "Euro.h"
#include <iostream>
using namespace std;
// Output to stream os.
ostream& operator<<(ostream& os, const Euro& e)
{
os << e.asString() << " Euro"; return os;
}
// Input from stream is.
istream& operator>>(istream& is, Euro& e)
{
cout << "Euro amount (Format ...x,xx): ";
int euro = 0, cents = 0; char c = 0;
if( !(is >> euro >> c >> cents)) // Input.
return is;
if( (c != ',' && c != '.')
|| cents>=100) // Error?
is.setstate( ios::failbit); // Yes => Set
else // fail bit.
e = Euro( euro, cents); // No => Accept
return is; // value.
}
■OVERLOADING SHIFT-OPERATORS FOR I/O
Declaration of the operator functions
Definition of operator functions
OVERLOADING SHIFT-OPERATORS FOR I/O
■429
When outputting a Euro class object, price, on screen, the following output statement
causes a compiler error:
Example: cout << price;
cout can only send objects to standard output if an output function has been defined for
the type in question—and this, of course, is not the case for user-defined classes.
However, the compiler can process the previous statement if it can locate a suitable
operator function, operator<<(). To allow for the previous statement, you therefore
need to define a corresponding function.
䊐Overloading the << Operator
In the previous example, the left operand of << is the object cout, which belongs to the
ostream class. Since the standard class ostream should not be modified, it is necessary
to define a global operator function with two parameters. The right operand is a Euro
class object. Thus the following prototype applies for the operator function:
Prototype: ostream& operator<<(ostream& os, const Euro& e);
The return value of the operator function is a reference to ostream. This allows for nor-
mal concatenation of operators.
Example: cout << price << endl;
䊐Overloading the >> Operator
The >> operator is overloaded for input to allow for the following statements.
Example: cout << "Enter the price in Euros: "
cin >> price;
The second statement causes the following call:
operator>>( cin, price);
As cin is an object of the standard istream class, the first parameter of the operator
function is declared as a reference to istream. The second parameter is again a refer-
ence to Euro.
The header file Euro.h contains only the declarations of << and >>. To allow these
functions to access the private members of the Euro class, you can add a friend decla-
ration within the class. However, this is not necessary for the current example.
exercises
430 ■
CHAPTER 19 OVERLOADING OPERATORS
The expression obj++ represents a copy of obj before incrementing.
The prefix and postfix decrement operators -- are distinguished in the same
manner.
✓NOTE
Optimized error handling for the Fraction class will be discussed in Chapter
28, “Exception Handling”
✓NOTE
Addition
Subtraction
Multiplication
Division
a
-
b
+=
c
-
d
a*d + b*c
b*d
a
-
b
-=
c
-
d
a*d - b*c
b*d
a
-
b
*=
c
-
d
a * c
b * d
a
-
b
/=
c
-
d
a * d
b * c
Expression Operator Function Call
++obj (Prefix)
obj++ (Postfix)
obj.operator++()
obj.operator++(0)
■EXERCISES
Prefix and postfix increment
To distinguish the postfix increment operator from the prefix increment
operator, the postfix operator function has an additional parameter of type int.
For exercise 2: Calculating with fractions
EXERCISES
■431
Exercise 1
The <and ++ operators for the sample class DayTime were overloaded at the
beginning of this chapter. Now modify the class as follows:
■Overload the relational operators
< > <= >= == and !=
and the shift operators
>> and << for input and output
using global operator functions.You can define these inline in the
header file.
■Then overload both the prefix and postfix versions of the ++ and --
operators.The operator functions are methods of the class.The -- oper-
ator decrements the time by one second.The time is not decremented
after reaching 0:0:0.
■Write a main function that executes all the overloaded operators and dis-
plays their results.
Exercise 2
You are to develop a class that represents fractions and performs typical
arithmetic operations with them.
■Use a header file called fraction.h to define the Fraction class with a
numerator and a denominator of type long.The constructor has two
parameters of type long: the first parameter (numerator) contains the
default value 0, and the second parameter (denominator) contains the
value 1. Declare operator functions as methods for -(unary), ++ and --
(prefix only), +=,-=,*=, and /=.The operator functions of the binary
operators +,-,*,/and the input / output operators <<,>> are to be
declared as friend functions of the Fraction class.
■Implement the constructor for the Fraction class to obtain a positive
value for the denominator at all times. If the denominator assumes a value
of 0, issue an error message and terminate the program.Then write the
operator functions.The formulae for arithmetic operations are shown
opposite.
■Then write a main function that calls all the operators in the Fraction
class as a test application. Output both the operands and the results.
solutions
432 ■
CHAPTER 19 OVERLOADING OPERATORS
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// DayTime.h
// Class DayTime with all relational operators,
// the operators ++ and -- (prefix and postfix),
// such as the operators << and >> for input/output.
// ----------------------------------------------------
#ifndef _DAYTIME_
#define _DAYTIME_
#include <iostream>
#include <iomanip>
using namespace std;
class DayTime
{
private:
short hour, minute, second;
bool overflow, underflow;
void inc() // private function for ++
{
++second;
if( second >= 60) // handle overflow.
second = 0, ++minute;
if( minute >= 60)
minute = 0, ++hour;
if( hour >= 24)
hour = 0, overflow = true;
}
void dec() // private function for --
{
--second;
if( second < 0) // handle underflow.
second = 59, --minute;
if( minute < 0)
minute = 59, --hour;
if( hour < 0)
hour = 0, underflow = true;
}
public:
DayTime( int h = 0, int m = 0, int s = 0)
{
overflow = underflow = false;
if( !setTime( h, m, s))
hour = minute = second = 0;
}
SOLUTIONS
■433
bool setTime(int hour, int minute, int second = 0)
{
if( hour >= 0 && hour < 24
&& minute >= 0 && minute < 60
&& second >= 0 && second < 60 )
{
this->hour = (short)hour;
this->minute = (short)minute;
this->second = (short)second;
return true;
}
else
return false;
}
int getHour() const { return hour; }
int getMinute() const { return minute; };
int getSecond() const { return second; };
int asSeconds() const // daytime in seconds
{
return (60*60*hour + 60*minute + second);
}
DayTime& operator++() // ++Seconds
{
inc();
return *this;
}
DayTime operator++(int) // Seconds++
{
DayTime temp(*this);
inc();
return temp;
}
DayTime& operator--() // --Seconds
{
dec();
return *this;
}
DayTime operator--(int) // Seconds--
{
DayTime temp(*this);
dec();
return temp;
}
};
// --- Relational operators ---
// t1 < t2
inline bool operator<( const DayTime& t1,
const DayTime& t2)
{ return t1.asSeconds() < t2.asSeconds(); }
434 ■
CHAPTER 19 OVERLOADING OPERATORS
// t1 <= t2
inline bool operator<=( const DayTime& t1,
const DayTime& t2)
{ return t1.asSeconds() <= t2.asSeconds(); }
// t1 == t2
inline bool operator==( const DayTime& t1,
const DayTime& t2)
{ return t1.asSeconds() == t2.asSeconds(); }
// t1 != t2
inline bool operator!=( const DayTime& t1,
const DayTime& t2)
{ return !(t1 == t2); }
// t1 > t2
inline bool operator>( const DayTime& t1,
const DayTime& t2)
{ return (t2 < t1); }
// t1 >= t2
inline bool operator>=(const DayTime& t1,const DayTime& t2)
{ return !(t1 < t2); }
// --- Input and Output ---
ostream& operator<<( ostream& os, const DayTime& t)
{
os << setfill('0')
<< setw(2) << t.getHour() << ':'
<< setw(2) << t.getMinute() << ':'
<< setw(2) << t.getSecond() << " Time";
os << setfill(' ');
return os;
}
istream& operator>>( istream& is, DayTime& t)
{
cout << "Enter daytime in hh:mm:ss format: ";
int hr = 0, min = 0, sec = 0;
char c1 = 0, c2 = 0;
if( !(is >> hr >> c1 >> min >> c2 >> sec))
return is;
if( c1 != ':' || c2 != ':' || ! t.setTime(hr,min,sec))
is.setstate( ios::failbit); // Error!
// => Set fail bit.
return is;
}
#endif // _DAYTIME_
SOLUTIONS
■435
// ----------------------------------------------------
// DayTim_t.cpp
// Testing the operators of class DayTime.
// ----------------------------------------------------
#include "DayTime.h" // Definition of the class
#include <iostream>
using namespace std;
int main()
{
DayTime cinema( 20,30);
cout << "\nThe movie starts at " << cinema << endl;
DayTime now;
cout << "What time is it now?" << endl;
if( !(cin >> now) )
cerr << "Invalid input!" << endl;
else
cout << "\nThe time is now" << now << endl;
cout << "\nThe movie has ";
if( cinema < now)
cout << "already begun!\n" << endl;
else
cout << "not yet begun!\n" << endl;
cout << "Now it is " << now++ << endl;
cout << "After 2 seconds: " << ++now << endl;
DayTime depart(16,0);
cout << "Let's go at " << --depart << endl;
if( depart >= now )
cout << "You can ride with us!" << endl;
else
cout << "We don't have room!" << endl;
return 0;
}
436 ■
CHAPTER 19 OVERLOADING OPERATORS
Exercise 2
// ------------------------------------------------------
// Fraction.h
// A numerical class to represent fractions
// ------------------------------------------------------
#ifndef _FRACTION_
#define _FRACTION_
#include <iostream>
#include <cstdlib>
using namespace std;
class Fraction
{
private:
long numerator, denominator;
public:
Fraction(long n = 0, long d = 1);
Fraction operator-() const
{
return Fraction(-numerator, denominator);
}
Fraction& operator+=(const Fraction& a)
{
numerator = a.numerator * denominator
+ numerator * a.denominator;
denominator *= a.denominator;
return *this;
}
Fraction& operator-=(const Fraction& a)
{
*this += (-a);
return *this;
}
Fraction& operator++()
{
numerator += denominator;
return *this;
}
Fraction& operator--()
{
numerator -= denominator;
return *this;
}
SOLUTIONS
■437
friend Fraction operator+(const Fraction&, const Fraction&);
friend Fraction operator-(const Fraction&, const Fraction&);
friend Fraction operator*(const Fraction&, const Fraction&);
friend Fraction operator/(const Fraction&, const Fraction&);
friend ostream& operator<< (ostream& os, const Fraction& a);
friend istream& operator>> (istream& is, Fraction& a);
};
#endif
// -------------------------------------------------------
// Fraction.cpp
// Defines methods and friend functions.
// -------------------------------------------------------
#include "Fraction.h"
// Constructor:
Fraction::Fraction(long n, long d)
{
if(d == 0)
{ cerr << "\nError: Division by zero!\n";
exit(1);
}
if( n < 0 ) n = -n, d = -d;
numerator = n; denominator = d;
}
Fraction operator+(const Fraction& a, const Fraction& b)
{
Fraction temp;
temp.denominator = a.denominator * b.denominator;
temp.numerator = a.numerator*b.denominator
+ b.numerator * a.denominator;
return temp;
}
Fraction operator-(const Fraction& a, const Fraction& b )
{
Fraction temp = a; temp += (-b);
return temp;
}
Fraction operator*(const Fraction& a, const Fraction& b )
{
Fraction temp;
temp.numerator = a.numerator * b.numerator;
temp.denominator = a.denominator * b.denominator;
return temp;
}
438 ■
CHAPTER 19 OVERLOADING OPERATORS
Fraction operator/(const Fraction& a, const Fraction& b )
{
if( b.numerator == 0)
{
cerr << "\nError: Division by zero!\n";
exit(1);
}
// To multiply a by the inverse of b:
Fraction temp;
temp.numerator = a.numerator * b.denominator;
temp.denominator = a.denominator * b.numerator;
if( temp.denominator < 0 )
temp.numerator = -temp.numerator,
temp.denominator = -temp.denominator;
return temp;
}
ostream& operator<<(ostream& os, const Fraction& a)
{
os << a.numerator << "/" << a.denominator;
return os;
}
istream& operator>>(istream& is, Fraction& a)
{
cout << "Enter a fraction:\n"
" Numerator: "; is >> a.numerator;
cout << " Denominator != 0: "; is >> a.denominator;
if( !is) return is;
if( a.denominator == 0)
{
cout << "\nError: The denominator is 0\n"
" New denominator != 0: ";
is >> a.denominator;
if( a.denominator == 0)
{
cerr << "\nError: Division by zero!\n"; exit(1);
}
}
if( a.denominator < 0 )
a.numerator = -a.numerator,
a.denominator= -a.denominator;
return is;
}
SOLUTIONS
■439
// -------------------------------------------------------
// Fract_t.cpp
// Testing the class Fraction.
// Modules: Fract_t.cpp Fraction.cpp
// -------------------------------------------------------
#include "Fraction.h"
int main()
{
Fraction a(1,3), b(4);
cout << "\nSome test results:\n\n";
cout << " a = " << a << endl;
cout << " b = " << b << endl;
cout << " a + b = " << (a + b) << endl;
cout << " a - b = " << (a - b) << endl;
cout << " a * b = " << (a * b) << endl;
cout << " a / b = " << (a / b) << endl;
cout << " --a = " << --a << endl;
cout << " ++a = " << ++a << endl;
a += Fraction(1,2);
cout << " a+= 1/2; a = " << a << endl;
a -= Fraction(1,2);
cout << " a-= 1/2; a = " << a << endl;
cout << "-b = " << -b << endl;
cout << "\nAnd now an input\n";
cin >> a;
cout << "\nYour input: " << a << endl;
return 0;
}
This page intentionally left blank
441
Type Conversion for
Classes
Implicit type conversion occurs in C++ when an expression cannot be
compiled directly but can be compiled after applying a conversion rule.
The programmer can stipulate how the compiler will perform implicit
type conversion for classes by defining conversion constructors and
functions.
Finally, we discuss ambiguity occurring due to type conversion and
how to avoid it.
chapter
20
442 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
Current
Class
Another
Type
Converting
Constructor
Converting-
Function
// The class Euro defined in the last chapter
// contains the following conversion constructors:
Euro::Euro( int ); // int -> Euro
Euro::Euro( double ); // double -> Euro
// The following declarations are now possible:
// Conversion constructors
Euro my(100), // int-> Euro,
your(321.41); // double -> Euro.
my = 987.12; // Implicit conversion:
// double -> Euro
your += 20; // Implicit conversion:
// int -> Euro
your = Euro(999.99); // Explicit conversion
// (constructor style)
my = (Euro)123.45; // Explicit conversion
// (cast style)
your = my; // No conversion
When the copy constructor performs a type conversion, a temporary object is first created and this
object is used in the assignment. The temporary object is cleaned up later.
✓NOTE
■CONVERSION CONSTRUCTORS
Possible conversions
Converting constructors of class Euro
CONVERSION CONSTRUCTORS
■443
䊐Possible Type Conversions
Implicit and explicit type conversion is also performed for classes in C++. As a program-
mer, you decide what kind of conversion is permissible. You can allow type conversion
between different classes or between classes and fundamental types.
Any type conversion involving a class is defined either
■by a conversion constructor or
■by a conversion function.
A conversion constructor performs type conversion by converting any given type to the
type of the current class. A conversion function performs conversion in the opposite
direction, that is, it converts an object of the current class to another type—a standard
type, for example.
䊐Conversion Constructors
A constructor with a single parameter determines how to form an object of the new class
from the argument passed to it. For this reason, a constructor with only one parameter is
referred to as a conversion constructor. The copy constructor is an exception to this rule: it
creates an object of the same class and does not perform type conversion.
Each conversion constructor is placed by the compiler on a list of possible conver-
sions. The standard string class contains a constructor that creates a string object
from a C string, for example.
Example: string::string( const char* );
This allows you to supply a C string as an argument wherever a string object is
required.
䊐Calling a Conversion Constructor
Conversion constructors have already been used in several examples; for example, in the
Euro class. The compiler uses them to perform implicit and explicit type conversion.
Examples: Euro salary(8765.30);
salary += (Euro)897.1; // explicit
salary += 897.1; // implicit
The last statement initially causes a type mismatch. Addition is not defined for a euro
and a double value. The compiler therefore activates the conversion constructor to cre-
ate a temporary Euro type object from the double value. This object is then added to
the value of the salary object.
444 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
// Euro.h : The class Euro represents a euro.
// -----------------------------------------------------
// . . .
class Euro
{
private:
long data; // Euros * 100 + Cents
public:
Euro( int euro = 0, int cents = 0);
Euro( double x);
// For conversion from Euro to double:
operator double() const { return (double)data/100.0; }
// . . . other methods as before.
};
// Euro_t.cpp : Testing conversions of class Euro.
// -----------------------------------------------------
#include "Euro.h" // Definition of the class
#include <iostream>
using namespace std;
int main()
{
cout << " * * * Testing Conversions * * * \n" << endl;
Euro salary( 8888,80);
double x(0.0);
salary += 1000; // implicit int -> Euro
salary += 0.10; // implicit double -> Euro
x = salary; // implicit Euro -> double
x = (double)salary; // explicit Euro -> double
x = salary.operator double(); // also possible!
// Constructor style is also safe for built-in types:
x = double(salary);
int i = salary; // Euro -> double -> int
// Output:
cout << " salary = " << salary << endl; // 9888,90 Euro
cout << " x = " << x << endl; // 9888.9
cout << " i = " << i << endl; // 9888
return 0;
}
■CONVERSION FUNCTIONS
A converting function for the Euro class
Testing conversions
CONVERSION FUNCTIONS
■445
If you need to convert an object of the current class to another type, you must define a
conversion function to do so. This is an operator function that defines how conversion is
performed. Conversion functions are also automatically used by the compiler to perform
implicit and explicit type conversion.
䊐Defining Conversion Functions
A conversion function is always implemented as a method of the current class. Its name
is made up of the operator keyword and the target type to convert to.
Example: operator int(void) const;
The previous statement declares a conversion function where the target type is int. You
may have noticed that the declaration of a conversion function does not contain a return
type. This is because the return type is implicitly defined by the target type in the name
of the conversion function. The target type can contain multiple keywords, such as
unsigned short or const float*.
Thus, conversion functions must be written to construct a target type object from the
current object,*this, and return the target object.
The Euro shown opposite contains a conversion function with a double target type.
In other words, the function converts a Euro type object to a floating-point number.
Example: double x = oneEuro; // implicit
䊐Conversion Function versus Conversion Constructor
The target type of a conversion function can also be a class. In this case, you must decide
whether it is preferable to use a conversion constructor in the target class.
If you do not want to modify the target class—perhaps because it is a standard class—
a conversion function will perform the task well.
䊐Standard Type Conversion
In addition to user-definable type conversions, the compiler also performs standard type
conversions. In the previous example, an int variable is assigned to a euro object by this
method.
Example: int wholePart = oneEuro;
This first converts a Euro object to double and then to int, that is, the cents are trun-
cated.
446 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
// Euro.h : The class Euro represents a euro.
// -----------------------------------------------------
// . . .
class Euro
{
private:
long data; // Euros * 100 + Cents
public:
explicit Euro( int euro = 0, int cents = 0);
explicit Euro( double x);
// Converting Euro to double:
double asDouble() const { return (double)data/100.0;}
// No conversion function operator double(),
// or as previously seen.
};
// Euro_E_t.cpp
// Tests explicit conversion of class Euro.
// ---------------------------------------------------
#include "Euro_Ex.h" // Class definition
#include <iostream>
using namespace std;
int main()
{
Euro salary( 8888.8); // double constructor
double x(0.0);
/* Now impossible:
salary += 1000; // implicit int -> Euro
salary += 0.10; // implicit double -> Euro
salary = 7777.77;
x = salary; // implicit Euro -> double
x = (double)salary; // There is no method
// operator double().
// The following conversions are ok:
salary = Euro( 7777.77); // explicit double -> Euro
salary += Euro(1000.10);
x = salary.asDouble(); // explicit by method
// Euro -> double
int i = salary.asDouble(); // Euro -> double -> int
return 0;
}
■AMBIGUITIES OF TYPE CONVERSIONS
Explicit type conversion for class Euro
Testing explicit conversions
AMBIGUITIES OF TYPE CONVERSIONS
■447
䊐Type Conversion Failure
Defining a conversion function or conversion constructor can prevent you from compil-
ing a program that is otherwise unchanged.
The Euro class contains a conversion constructor that converts a double value to
euros. This means that the following statement is valid for two objects, wholesale and
retail, of the Euro type.
Example: retail = wholesale + 46.9;
If you now additionally implement the conversion function
operator double()
that converts a euro to a double value, the previous statement can no longer be com-
piled. Since both conversion types double -> Euro and Euro -> double are
defined, two possible conversions could be performed:
prov2 + Euro(546.9) // To add euros
and
double(prov2) + 546.9; // To add values
// of type double
However, the compiler can only perform implicit type conversion if the technique is not
ambiguous. If more than one choice is available, the compiler issues an error message.
䊐Avoiding Implicit Type Conversion
You can prevent ambiguities by stating any desired conversions explicitly. This also has
the advantage of highlighting type conversions in your source code. Moreover, undesir-
able type conversion, which can occur when classes are extended at a later date, can be
avoided.
In order to ensure that some kinds of type conversion are only performed explicitly,
you can use the following techniques:
■you can use an explicit declaration for the conversion constructor. As the
example on the opposite page shows, only explicit calls to the constructor are
possible in this case.
Example: wholesale + Euro(46.9) // ok
■implicit type conversions by conversion functions can be prevented by not defin-
ing the function, of course. Instead you can use a method of an appropriate name,
for example asType(). Type conversion can only be performed by calling this
function explicitly.
exercise
448 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
// Fraction.cpp
// . . .
// To simplify fractions:
void Fraction::simplify()
{
// Divide the numerator and denominator by
// the greatest common divisor.
if( numerator == 0)
{
denominator = 1;
return;
}
// Calculating the greatest common divisor
// using an algorithm by Euclid.
long a = (numerator < 0) ? -numerator : numerator,
b = denominator,
help;
while( b != 0)
{
help = a % b; a = b; b = help;
}
// a is the greatest common divisor
numerator /= a;
denominator /= a;
}
■EXERCISE
Method simplify() of class Fraction
EXERCISE
■449
Exercise
Enhance the numerical class Fraction, which you know from the last chapter, to
convert both double values to fractions and fractions to double. In addition,
fractions should be rounded after arithmetic operations.
■First declare the simplify() method for the Fraction class and insert
the definition on the opposite page in your source code.The method
computes the largest common divisor of numerator and denominator.
The numerator and the denominator are then divided by this value.
■Add an appropriate call to the simplify() function to all operator func-
tions (except ++ and --).
■Then add a conversion constructor with a double type parameter to the
class.
Example: Fraction b(0.5); // yields the fraction 1/2
Double values should be converted to fractions with an accuracy of three
decimal places.The following technique should suffice for numbers below
one million. Multiply the double value by 1000 and add 0.5 for rounding.
Assign the result to the numerator. Set the value of the denominator to
1000.Then proceed to simplify the fraction.
■You now have a conversion constructor for long and double types.To
allow for conversion of int values to fractions, you must write your own
conversion constructor for int!
■Now modify the class to allow conversion of a fraction to a double type
number. Define the appropriate conversion function inline.
Use the function main() to test various type conversions. More specifically, use
assignments and arithmetic functions to do so.Also compute the sum of a
fraction and a floating-point number.
Output the operands and the results on screen.
solution
450 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
■SOLUTION
// -------------------------------------------------------
// Fraction.h
// A numerical class to represent fractions.
// The class converts Fraction <--> double
// and simplifies fractions.
// -------------------------------------------------------
#ifndef _FRACTION_
#define _FRACTION_
#include <iostream.h>
#include <stdlib.h>
class Fraction
{
private: long numerator, denominator;
public:
Fraction(long z, long n);
Fraction(double x); // double-constructor
// Default long- and int-constructor:
Fraction(long z=0) : numerator(z), denominator(1) {}
Fraction(int z) : numerator(z), denominator(1) {}
void simplify();
operator double() // Fraction -> double
{
return (double)numerator / (double)denominator;
}
Fraction operator-() const
{ return Fraction(-numerator, denominator); }
Fraction& operator+=(const Fraction& a)
{
numerator = a.numerator * denominator
+ numerator * a.denominator;
denominator *= a.denominator;
simplify();
return *this;
}
Fraction& operator-=(const Fraction& a)
{
*this += (-a); simplify();
return *this;
}
// The rest of the class including methods
// operator++() and operator--()
// and friend declarations are unchanged.
};
#endif
SOLUTION
■451
// --------------------------------------------------------
// Fraction.cpp
// Defines methods and friend functions
// that are not inline.
// --------------------------------------------------------
#include <iostream.h>
#include <stdlib.h>
#include "Fraction.h"
// Constructors:
Fraction::Fraction(long z, long n)
{
// Unchanged! Same as in Chapter 19.
}
Fraction::Fraction( double x)
{
x *= 1000.0;
x += (x>=0.0) ? 0.5 : -0.5; // Round the 4th digit.
numerator = (long)x;
denominator = 1000;
simplify();
}
Fraction operator+(const Fraction& a, const Fraction& b )
{
Fraction temp;
temp.denominator = a.denominator * b.denominator;
temp.numerator = a.numerator*b.denominator
+ b.numerator * a.denominator;
temp.simplify();
return temp;
}
// The functions
// operator-() operator<<() operator>>()
// are left unchanged.
// The functions
// operator*() and operator/()
// are completed by a call to temp.simplify()
// just like the function operator+().
//
// The code of method Fraction::simplify(), as
// specified in the exercise, should be here.
452 ■
CHAPTER 20 TYPE CONVERSION FOR CLASSES
// -------------------------------------------------------
// Fract_t.cpp
// Tests the class Fraction with type conversions.
// -------------------------------------------------------
#include <iostream.h>
#include "Fraction.h"
int main()
{
Fraction a, b(-1,5), c(2.25);
double x = 0.5, y;
a = x; // double -> Fraction
cout << "\nSome test results:\n" << endl;
cout << " a = " << a << endl;
cout << " b = " << b << endl;
cout << " c = " << c << endl;
cout << "\nThe fractions as double values:\n" << endl;
// Fraction -> double:
cout << " a = " << (double)a << endl;
cout << " b = " << (double)b << endl;
cout << " c = " << (double)c << endl;
cout << "\nAnd calculate with:\n" << endl;
cout << " a + b = " << (a + b) << endl;
cout << " a - b = " << (a - b) << endl;
cout << " a * b = " << (a * b) << endl;
cout << " a / b = " << (a / b) << endl;
cin >> a; // Enter a fraction.
cout << "\nYour input: " << a << endl;
a.simplify();
cout << "\nSimplified: " << a << endl;
cout << "\nAs double value: " << (double)a << endl;
cout << "\nEnter a floating point value: "; cin >> x;
cout << "\nThis is in fraction form: "
<< (Fraction)x << endl;
// To calculate the sum b + x :
cout << " b = " << b << endl;
cout << " x = " << x << endl;
// a = b + x; // Error: ambiguous!
a = b + Fraction(x); // ok! To compute fractions.
y = (double)b + x; // ok! To compute doubles.
cout << " b + x as fraction: " << a << endl;
cout << " b + x as double: " << y << endl;
return 0;
}
453
Dynamic Memory
Allocation
This chapter describes how a program can allocate and release memory
dynamically in line with current memory requirements.
Dynamic memory allocation is an important factor in many C++
programs and the following chapters will contain several additional case
studies to help you review the subject.
chapter
21
454 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
Heap
ptr_long
ptr_double
1234567
1.9
// Dynamic objects of type long and double
// ------------------------------------------------------
long *ptr_long;
ptr_long = new long; // No initialization
// of the long object.
*ptr_long = 1234567; // Assign a value
double *ptr_double;
double z = 1.9;
ptr_double = new double(z); // With initialization
++(*ptr_double); // Increment the value
*ptr_double += *ptr_long; // ok to add long value
ptr_long = new double(2.7); // Error: ptr_long not
// pointing to double!
■THE OPERATOR new
Sample calls to new
On the heap
THE OPERATOR new
■455
䊐Dynamic Memory Allocation
When a program is compiled, the size of the data the program will need to handle is
often an unknown factor; in other words there is no way to estimate the memory require-
ments of the program. In cases like this you will need to allocate memory dynamically,
that is, while the program is running.
Dynamically allocated memory can be released to continually optimize memory usage
with respect to current requirements. This in turn provides a high level of flexibility,
allowing a programmer to represent dynamic data structures, such as trees and linked
lists.
Programs can access a large space of free memory known as the heap. Depending on
the operating system (and how the OS is configured), the heap can also occupy large
amounts of unused space on the hard disk by swapping memory to disk.
C++ uses the new and delete operators to allocate and release memory, and this
means that objects of any type can be created and destroyed. Let’s look at the scenario
for fundamental types first.
䊐Calling new for Fundamental Types
The new operator is an operator that expects the type of object to be created as an argu-
ment. In its simplest form, a call to new follows this syntax
Syntax: ptr = new type;
Where ptr is a pointer to type. The new operator creates an object of the specified
type and returns the address of that object. The address is normally assigned to a pointer
variable. If the pointer belongs to a wrong type, the compiler will issue an error message.
Example: long double *pld = new long double;
This statement allocates memory for a long double type object, that is,
sizeof(long double) bytes.
The previous call to new does not define an initial value for the new object, however,
you can supply a value in parentheses to initialize the object.
Example: pld = new long double(10000.99);
Following this statement pld points to a memory address containing a long double
type with a value of 10000.99. The statement
cout << *pld << endl;
will output this value.
456 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
// DynStd.cpp
// The operators new and delete for built-in types.
// The program contains errors!
// ==> Save all data before starting.
// ---------------------------------------------------
#include <iostream>
using namespace std;
int main()
{
cout << "\nTesting dynamic storage allocation! "
<< endl;
// To allocate storage:
double width = 23.78;
double* ptrWidth = &width;
double* ptrLength = new double(32.54);
double* ptrArea = new double;
// To work with ptrWidth, ptrLength, and ptrArea:
*ptrArea = *ptrWidth * *ptrLength;
delete ptrLength; // Error: The object is still
// in use!
cout << "\nWidth : " << *ptrWidth
<< "\nLength : " << *ptrLength
<< "\nArea : " << *ptrArea << endl;
// To free storage:
delete ptrWidth; // Error: The object has not
// been dynamically reserved
delete ptrLength; // ok
delete ptrArea; // ok
delete ptrLength; // Error: Pointer doesn't
// address any object.
ptrLength = new double(19.45); // ok
// To give a name to a dynamic object:
double& length = *ptrLength; // Reference
cout << "\nNew length : " << length
<< "\nCircumference : " << 2 * width * length
<< endl;
return 0; // On terminating the program
} // allocated storage will be freed.
■THE OPERATOR delete
Sample program
THE OPERATOR delete
■457
A program should make careful use of available memory and always release memory that
is no longer needed. Failure to do so can impact the performance of your computer sys-
tem. Memory that is released is available for further calls to new.
䊐Calling delete
Memory that has been allocated by a call to new can be released using the delete oper-
ator. A call to delete follows this syntax
Syntax: delete ptr;
The operand ptr addresses the memory space to be released. But make sure that this
memory space was dynamically allocated by a call to new!
Example: long *pl = new long(2000000);
. . . . // to work with *pl.
delete pl;
If you do not call delete, the dynamically allocated memory space is not released until
the program terminates.
You can pass a NULL pointer to delete when you call the operator. In this case
nothing happens and delete just returns, so you do not need to check for NULL point-
ers when releasing memory.
Adelete expression is always a void type, so you cannot check whether memory
has been successfully released.
As the sample program illustrates, misuse of delete can be disastrous. More specifi-
cally
■do not call delete twice for the same object
■do not use delete to release statically allocated memory.
䊐Error Handling for new
If there is not enough memory available, the so-called new handler is called. The new
handler is a function designed for central error handling. Thus, you do not need to design
your own error handling routines each time you call new.
The new handler is activated by default and throws an exception. Exceptions can be
caught by the program, allowing the error condition to be remedied (refer to Chapter 28,
Exception Handling). Any exception that is not caught will terminate the program,
however, you can install your own new handler.
If you are working with an older compiler, please note that new returns a NULL
pointer if not enough memory is available.
458 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
// DynObj.cpp
// The operators new and delete for classes.
// ---------------------------------------------------
#include "account.h"
#include <iostream>
using namespace std;
Account *clone( const Account* pK); // Create a copy
// dynamically.
int main()
{
cout << "Dynamically created objects.\n" << endl;
// To allocate storage:
Account *ptrA1, *ptrA2, *ptrA3;
ptrA1 = new Account; // With default constructor
ptrA1->display(); // Show default values.
ptrA1->setNr(302010); // Set the other
ptrA1->setName("Tang, Ming"); // values by access
ptrA1->setStand(2345.87); // methods.
ptrA1->display(); // Show new values.
// Use the constructor with three arguments:
ptrA2 = new Account("Xiang, Zhang", 7531357, 999.99);
ptrA2->display(); // Display new account.
ptrA3 = clone( ptrA1); // Pointer to a dyna-
// mically created copy.
cout << "Copy of the first account: " << endl;
ptrA3->display(); // Display the copy.
delete ptrA1; // Release memory
delete ptrA2;
delete ptrA3;
return 0;
}
Account *clone( const Account* pK) // Create a copy
{ // dynamically.
return new Account(*pK);
}
■DYNAMIC STORAGE ALLOCATION FOR CLASSES
Sample program
DYNAMIC STORAGE ALLOCATION FOR CLASSES
■459
The operators new and delete were designed to create and destroy instances of a class
dynamically. In this case, in addition to allocating memory, a suitable constructor must
be called. Before releasing memory, the destructor must be called to perform cleaning up
tasks. However, the operators new and delete ensure that this happens.
䊐Calling new with a Default Constructor
A call to new for a class is not much different from a call for a fundamental type. Unless
explicitly initialized, the default constructor is called for each new object, but you must
make sure that a default constructor exists!
Example: Euro* pEuro = new Euro;
This statement allocates memory for an object of the Euro class. If enough memory is
available, the default constructor for Euro is executed and the address of a new object
returned.
䊐Explicit Initialization
To initialize an object explicitly, you can state its initial values in parentheses when you
call new.
Syntax: Type *ptr = new Type(initializing_list);
The values in the initialization list are passed as arguments to the constructor. If the
compiler is unable to locate a suitable constructor, an error message occurs.
Example: Euro *pE = new Euro( -123,77);
This statement assigns the address of a new Euro class object to the pointer pE. The
object is initialized using the supplied values. The expression *pE thus represents the
entire object.
Example: *pE += 200; // To add 200 euros.
The public members are referred to via the member access operator ->.
Example: cout << pE->getCents() << endl; // 33
䊐Releasing Memory
When an object that was created dynamically is destroyed, the delete operator makes
sure that the object is cleaned up. The destructor is first called, and only then is the
memory space released.
As previously discussed in the section on fundamental types, when you call delete
you must ensure that the pointer is addressing a dynamic object or that you are dealing
with a NULL pointer.
460 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
// DynArr.cpp
// Operators new[] and delete[] for dynamic arrays.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
cout << "Using a dynamic array.\n" << endl;
int size = 0, cnt = 0, step = 10,
i;
float x, *pArr = NULL;
cout << "Enter some numbers!\n"
"End with q or another character " << endl;
while( cin >> x)
{
if( cnt >= size) // Array too small?
{ // => enlarge it.
float *p = new float[size+step];
// Copy the numbers:
for( i = 0; i < size; ++i)
p[i] = pArr[i];
delete [] pArr; // Release old array:
pArr = p; size += step;
}
pArr[cnt++] = x;
}
// Work with the numbers:
if( cnt == 0)
cout << "No invalid input!" << endl;
else
{
float sum = 0.0;
cout << "Your input: " << endl;
for( i = 0; i < cnt; i++) // To output and
{ // add.
cout << setw(10) << pArr[i];
sum += pArr[i];
}
cout << "\nThe average: " << sum/cnt << endl;
}
delete [] pArr; // To free the storage
return 0;
}
■DYNAMIC STORAGE ALLOCATION FOR ARRAYS
Sample program
DYNAMIC STORAGE ALLOCATION FOR ARRAYS
■461
Imagine you are compiling a program that will store an unknown quantity of elements in
an array. Your best option is to let the program create the array dynamically. An array of
this type is known as a dynamic array.
䊐The new[ ] Operator
The new[ ] operator is available for creating dynamic arrays. When you call the opera-
tor, you must supply the type and quantity of the array elements.
Syntax: vekPtr = new Type[cnt];
The pointer vekPtr will then reference the first of a total of cnt array elements.
vekPtr has to be a pointer to Type for this reason. Of course, Type can also be a class.
Example: Account *pk = new Account[256];
This statement allocates memory for 256 Account type objects and uses the default con-
structor to initialize them. Those objects are
pk[0], pk[1], . . . , pk[255],
or in pointer notation:
*pk, *(pk + 1), ....., *(pk + 255).
If the array elements are of a class type, the class must have a default constructor, since you
cannot supply an initialization list when calling new[]. Starting values for the array ele-
ments cannot be assigned until later.
䊐The delete[ ] Operator
It is always a good idea to release the memory space occupied by a dynamic array, if the
array is no longer needed. To do so, simply call the delete[] operator. The braces []
tell the compiler to release the whole array, and not just a single array element.
Example: delete[] pk;
The operand for delete[]—the pointer pk in this case—must reference the place in
memory that was allocated by a call to new[]! The destructor belonging to the current
class is called for each array element. This shows the big difference to delete, which
would merely call the destructor for *pk, i.e. for the first array element.
The program on the opposite page stores numbers in a dynamic array. The size of the
array is adjusted as required. To do so, a newer bigger array is created, the data is copied
to the new array, and the memory occupied by the old array is released.
462 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
first last
1st element 2nd element 3rd element
Info Info Info
first
New last
element
last
1st element 2nd element 3rd element
Info Info
Info
Info
■APPLICATION: LINKED LISTS
A simple linked list
Appending a list element
Deleting a list element
first
Removed
element
New first
element
New second
element
Info Info InfoInfo
APPLICATION: LINKED LISTS
■463
䊐Dynamic Data Structures
Now, let’s implement a linked list as a sample application. A linked list is a dynamic data
structure that allows easy insertion and deletion of data. A data structure defines how data
can be organized in units, stored, and manipulated—as arrays, lists, or trees, for example.
The type of data structure you choose has a far-reaching effect on the amount of
memory you need, the speed of access to the data involved, and the complexity (or sim-
plicity) of the algorithms (data operations) you need.
In contract to a static data structure, whose size is known before a program is
launched, a dynamic data structure can change size while a program is running. One
example of this is an array whose size can be changed during runtime.
䊐Defining a Linked List
Another example is a linked list that is stored in main memory and has the following
characteristics:
■each list element contains a data store for the live data and a pointer to the next
element in the list
■each list element—except the first and last elements—has exactly one predeces-
sor and one successor. The first element in the list has no predecessor and the last
element no successor.
Some elementary operations are defined for linked lists, such as inserting and deleting list
elements, or searching for and retrieving the information stored in a list element.
䊐Advantages
The storage used for the list elements need not be contiguous. The main advantage of
linked lists is:
■memory for the list elements is only allocated when needed
■you only need to move a pointer when inserting or deleting list elements.
When an array element is inserted or deleted, the other array elements have to be moved
to make room or fill up the “gap” in the array. If there is no room left, you need to allo-
cate memory for a new array and copy the data to it before inserting a new element.
464 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
// List.h
// Defines the classes ListEl and List.
// ---------------------------------------------------
#ifndef _LISTE_H_
#define _LISTE_H_
#include "Date.h" // Class Date from Chapter 14
#include <iostream>
#include <iomanip>
using namespace std;
class ListEl // A list element.
{
private:
Date date; // Date
double amount; // Amount of money
ListEl* next; // Pointer to successor
public:
ListEl( Date d = Date(1,1,1), double b = 0.0,
ListEl* p = NULL)
: date(d), amount(b), next(p) {}
// Access methods:
// getDate(), setDate(), getAmount(), setAmount()
ListEl* getNext() const { return next; }
friend class List;
};
// ----------------------------------------------------
// Defining the class List
class List
{
private:
ListEl* first, *last;
public:
List(){ first = last = NULL; } // Constructor
~List(); // Destructor
// Access to the first and last elements:
ListEl* front() const { return first; }
ListEl* back() const { return last; }
// Append a new element at the end of the list:
void pushBack(const Date& d, double b);
// Delete an element at the beginning of the list
void popFront();
};
#endif // _LIST_H_
■REPRESENTING A LINKED LIST
Classes of header file List.h
REPRESENTING A LINKED LIST
■465
䊐Representing List Elements
You can use a recursive data structure to represent a linked list. A recursive data structure
is a data structure containing a pointer to a data structure of the same type. Of course,
the data structure cannot contain itself—that would be impossible—but it does contain a
pointer to itself.
Now let’s look at a linked list used to represent transactions in a bank account. A
transaction is characterized by a date, a sum of money, and the reason for the transac-
tion. Thus, the list element needed to represent a transaction will contain the transac-
tion data in its data store and a pointer to the next element in the list.
The class shown on the opposite page, ListEl, was designed to represent list ele-
ments. To keep things simple, the data store contains only the date and a sum of money.
The public declaration includes a constructor and access methods for the live data.
Later, we will be overloading the << operator in order to output the list.
It is common practice to let the pointer for the last element in the list point to NULL.
This also provides a termination criterion—the next pointer just needs to be queried for
NULL.
䊐Representing a List
To identify a linked list, you just point a pointer at the first element in the list. You can
then use the pointer to the successor of each element to address any element in the list.
A pointer to the last element in the list is useful for appending new elements.
The opposite page shows the class definition for the List class. The private section
comprises two pointers, which reference the first and last list elements respectively. The
constructor has an easy job—it simply points both pointers to NULL, thus creating an
empty list. The destructor has a more complex task: it has to release the memory occupied
by the remaining list elements.
The pushBack() method is used to append a new element to the end of the list. To
do so, memory is allocated dynamically and the new element becomes the successor of
what was previously the last element and the last pointer is updated. In addition, the
method must deal with a special case, where the list is empty.
The popFront() method deletes the first element in the list. This involves turning
the pointer to the first element around to the second element and releasing the memory
occupied by the first element. The special case with an empty list also applies.
exercises
466 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
■EXERCISES
Notes on exercise 1
Effects of the splice() function
Insert
Position
Result
1st Array
2nd Array
735962
91426835
7391 4268355962
EXERCISES
■467
Exercise 1
Write a global function called splice() that “splices” two int arrays together
by first allocating memory for a dynamic array with enough room for both int
arrays, and then copying the elements from both arrays to the new array, as
follows:
■first, the elements of the first array are inserted up to a given position,
■then the second array is inserted,
■then the remainder of the first array is appended.
Arguments: The two int arrays, their length, and the position at which
they are to be spliced.
Return value: A pointer to the new array
Exercise 2
Write a global function called merge() that merges two sorted int arrays by
first allocating memory for a dynamic array with enough room for both int
arrays and then inserting the elements of both arrays into the new array in
sequence.
Arguments: The two int arrays and their length.
Return value: A pointer to the new array
To test the function, modify the program used to sort arrays in Exercise 4 of
Chapter 17.
Exercise 3
Complete and test the implementation of a linked list found in this chapter.
■First define the access methods shown opposite.Then overload the <<
operator for the class ListEl to allow formatted output of the data in
the list elements.You can use the asString() in the date class to do so.
■Then implement the destructor for the List class.The destructor will
release the memory used by all the remaining elements. Make sure that
you read the pointer to the successor of each element before destroying
it!
■Implement the methods pushBack() and popFront() used for append-
ing and deleting list elements.
■Overload the operator << in the List class to output all the data stored
in the list.
■Test the List class by inserting and deleting several list elements and
repeatedly outputting the list.
solutions
468 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// Splice.cpp
// Implements the splice algorithm.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstdlib> // For srand() and rand()
#include <ctime> // and for time().
using namespace std;
// Prototype:
int *splice( int v1[], int len1,
int v2[], int len2, int pos);
int main()
{
cout << "\n * * * Testing the splice function * * *\n"
<< endl;
int i, len1 = 10, len2 = 5;
int *a1 = new int[len1],
*a2 = new int[len2];
// Initialize the random number generator
// with the current time:
srand( (unsigned)time(NULL));
for( i=0; i < len1; ++i) // Initialize the arrays:
a1[i] = rand(); // with positive and
for( i=0; i < len2; ++i)
a2[i] = -rand(); // negative numbers.
// To output the array:
cout << "1. array: " << endl;
for( i = 0; i < len1; ++i)
cout << setw(12) << a1[i];
cout << endl;
cout << "2. array: " << endl;
for( i = 0; i < len2; ++i)
cout << setw(12) << a2[i];
cout << endl;
cout << "\n At what position do you want to insert "
"\n the 2nd array into 1st array?"
"\n Possible positions: 0, 1, ..., " << len1
<< " : ";
int pos; cin >> pos;
SOLUTIONS
■469
int *a3, len3 = len1 + len2;
a3 = splice( a1, len1, a2, len2, pos);
if( a3 == NULL)
cerr << "\n Invalid position!\n" << endl;
else
{
cout << " The new spliced array: " << endl;
for( i = 0; i < len3; ++i)
cout << setw(12) << a3[i];
cout << endl;
}
delete[] a1; delete[] a2; delete[] a3;
return 0;
}
// -------------------------------------------------------
// Function splice() inserts the array v2 into v1
// starting at position pos.
int *splice( int v1[], int len1,
int v2[], int len2, int pos)
{
if( pos < 0 || pos > len1)
return NULL;
int i = 0, i1 = 0, i2 = 0;
int *v = new int[len1+len2];
for( i = 0, i1 = 0; i1 < pos; ++i, ++i1) // 1st part
v[i] = v1[i1];
for( i2 = 0; i2 < len2; ++i, ++i2) // 2nd part
v[i] = v2[i2];
for( ; i1 < len1; ++i, ++i1) // 3rd part
v[i] = v1[i1];
return v;
}
470 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
Exercise 2
// ----------------------------------------------------
// merge.cpp
// Implements the merge algorithm.
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
using namespace std;
// Prototypes:
void selectionSort( int arr[], int len);
int *merge( int v1[], int len1, int v2[], int len2);
int main()
{
cout << "\n * * * The Merge Algorithm * * *\n"
<< endl;
int i, len1 = 10, len2 = 20;
int *a1 = new int[len1],
*a2 = new int[len2];
// Initialize the random number generator
// with the current time:
srand( (unsigned)time(NULL));
for( i=0; i < len1; ++i) // Initialized arrays:
a1[i] = rand();
for( i=0; i < len2; ++i)
a2[i] = rand();
selectionSort( a1, len1); // To sort array a1.
selectionSort( a2, len2); // To sort array a2.
// Output the arrays:
cout << "The sorted arrays:" << endl;
cout << "1st array: " << endl;
for( i = 0; i < len1; ++i)
cout << setw(12) << a1[i];
cout << endl;
cout << "2nd array: " << endl;
for( i = 0; i < len2; ++i)
cout << setw(12) << a2[i];
cout << endl;
int *a3, len3;
a3 = merge( a1, len1, a2, len2);
len3 = len1 + len2;
SOLUTIONS
■471
cout << "The new merged array: " << endl;
for( i = 0; i < len3; ++i)
cout << setw(12) << a3[i];
cout << endl;
delete[] a1; delete[] a2; delete[] a3;
return 0;
}
// ------------------------------------------------------
// Function selectionSort().
inline void swap( int& a, int& b) // To swap.
{ int temp = a; a = b; b = temp; }
void selectionSort( int *arr, int len)
{
register int *p, *minp; // Pointer to array elements,
int *last = arr + len-1; // Pointer to the last element
for( ; arr < last; ++arr)
{
minp = arr; // Search minimum
for( p = arr+1; p <= last; ++p) // starting at
if( *minp > *p) // position arr.
minp = p;
swap( *arr, *minp); // To swap.
}
}
// ------------------------------------------------------
// merge() : Merges two sorted arrays to create
// a new sorted array.
int *merge( int v1[], int len1, int v2[], int len2)
{
int i = 0, i1 = 0, i2 = 0;
int *v = new int[len1+len2]; // New int array.
for( i=0; i1 < len1 && i2 < len2; ++i)
{
if( v1[i1] <= v2[i2])
v[i] = v1[i1++];
else
v[i] = v2[i2++];
}
if( i1 < len1) // Copy the rest of v1 or v2.
while( i1 < len1)
v[i++] = v1[i1++];
else
while( i2 < len2)
v[i++] = v2[i2++];
return v;
}
472 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
Exercise 3
// ----------------------------------------------------
// date.h : Defines the class Date.
// ----------------------------------------------------
// date.cpp
// Implements the methods of class Date,
// which are not inline defined.
// ----------------------------------------------------
//
// These files are left unchanged
// from Chapter 14 (solutions).
//
// ----------------------------------------------------
// List.h
// Defines the classes ListEl and List
// to represent a linked list.
// ----------------------------------------------------
#ifndef _LIST_H_
#define _LIST_H_
#include "Date.h"
#include <iostream>
#include <iomanip>
using namespace std;
class ListEl
{
private:
Date date; // Date
double amount; // Amount of money
ListEl* next; // Pointer to successor
public:
ListEl( Date d = Date(1,1,1),
double b = 0.0,
ListEl* p = NULL)
: date(d), amount(b), next(p) {}
// Access methods
const Date& getDate() const { return date; }
void setDate() // Sets the current date
{
date.setDate();
}
bool setDate( int day, int month, int year)
{
return setDate( day, month, year);
}
SOLUTIONS
■473
double getAmount() const { return amount; }
void setAmount(double a) { amount = a; }
ListEl* getNext() const { return next; }
friend class List;
};
// Output an element
ostream& operator<<( ostream& os, const ListEl& le);
// ------------------------------------------------------
// Defines the List class
class List
{
private:
ListEl* first, *last;
public:
List(){ first = last = NULL; } // Constructor
~List(); // Destructor
// Access first and last elements:
ListEl* front() const { return first; }
ListEl* back() const { return last; }
// Appends a new element at the end of the list:
void pushBack(const Date& d, double b);
// Deletes an element at the beginning of the list.
void popFront();
};
// Outputs the list
ostream& operator<<( ostream& os, const List& le);
#endif // _LIST_H_
// ----------------------------------------------------
// List.cpp
// Implements the methods of class List,
// which are not previously defined inline.
// ----------------------------------------------------
#include "List.h"
// Destructor of the list:
List::~List()
{
ListEl *pEl = first, *next = NULL;
for( ; pEl != NULL; pEl = next)
{
next = pEl->next;
delete pEl;
}
}
474 ■
CHAPTER 21 DYNAMIC MEMORY ALLOCATION
// Appends a new element at the end of the list:
void List::pushBack(const Date& d, double b)
{
ListEl *pEl = new ListEl( d, b);
if( last == NULL) // List empty?
first = last = pEl;
else
last->next = pEl, last = pEl;
}
// Deletes an element from the beginning of the list.
void List::popFront()
{
if( first != NULL) // Not empty?
{
ListEl *pEl = first; // Save the first element.
first = first->next; // Move to the next element.
delete pEl; // Former first element
if( first == NULL) // Empty now?
last = NULL;
}
}
// --- Global functions for output ---
// Outputs an element:
ostream& operator<<( ostream& os, const ListEl& le)
{
os << le.getDate().asString() << " Amount: ";
os << fixed << setprecision(2) << setw(10)
<< le.getAmount();
return os;
}
// Outputs the list:
ostream& operator<<( ostream& os, const List& List)
{
ListEl *pEl = List.front();
if( pEl == NULL)
os << "The list is empty!" << endl;
for( ; pEl != NULL; pEl = pEl->getNext() )
os << *pEl << endl;
return os;
}
SOLUTIONS
■475
// -------------------------------------------------------
// List_t.cpp
// Tests the List class.
// -------------------------------------------------------
#include "List.h"
int main()
{
cout << "\n * * * Testing the class list * * *\n"
<< endl;
List aList; // A list
cout << aList << endl; // List is still empty.
cout << "\nEnter account changes (Date and Amount)"
"\n(Type invalid input to quit, e.g. q):\n";
Date date;
int month, day, year; char c;
double amount;
while( true)
{
cout << "Date format Month-Day-Year : ";
if( !(cin >> month >> c >> day >> c >> year)
|| ! date.setDate( month, day, year) )
break; // Invalid date.
cout << "Account change: ";
if( !(cin >> amount) ) break;
aList.pushBack( date, amount);
}
cout << "\nContent of the list:\n";
cout << aList << endl;
cout << "\nRemoving the first element of the list:\n";
ListEl *ptrEl = ptrEl = aList.front();
if( ptrEl != NULL)
{
cout << "Deleting: " << *ptrEl << endl;
aList.popFront();
}
cout << "\nContent of the list:\n";
cout << aList << endl;
return 0;
}
This page intentionally left blank
477
Dynamic Members
This chapter describes how to implement classes containing pointers to
dynamically allocated memory.These include
■your own copy constructor definition and
■overloading the assignment operator.
A class designed to represent arrays of any given length is used as a
sample application.
chapter
22
478 ■
CHAPTER 22 DYNAMIC MEMBERS
4.1 6.5 8.2 2.7 ...
Object
fArr
arrPtr
max: 10
cnt: 4
// A class representing dynamic arrays of floats.
// ---------------------------------------------------
class FloatArr
{
private:
float* arrPtr; // Dynamic member
int max; // Maximum quantity without
// reallocating new storage.
int cnt; // Number of present elements
public:
// Public methods here
};
■MEMBERS OF VARYING LENGTH
An object of class FloatArr in memory
Data members of class FloatArr
MEMBERS OF VARYING LENGTH
■479
䊐Dynamic Members
You can exploit the potential of dynamic memory allocation to leverage existing classes
and create data members of variable length. Depending on the amount of data an appli-
cation program really has to handle, memory is allocated as required while the applica-
tion is running. In order to do this the class needs a pointer to the dynamically allocated
memory that contains the actual data. Data members of this kind are also known as
dynamic members of a class.
When compiling a program that contains arrays, you will probably not know how
many elements the array will need to store. A class designed to represent arrays should
take this point into consideration and allow for dynamically defined variable length
arrays.
䊐Requirements
In the following section you will be developing a new version of the FloatArr class to
meet these requirements and additionally allow you to manipulate arrays as easy as fun-
damental types. For example, a simple assignment should be possible for two objects v1
and v2 in the new class.
Example: v2 = v1;
The object v2 itself—and not the programmer—will ensure that enough memory is
available to accommodate the array v1.
Just as in the case of fundamental types, it should also be possible to use an existing
object, v2, to initialize a new object, v3.
Example: FloatArr v3(v2);
Here the object v3 ensures that enough memory is available to accommodate the array
elements of v2.
When an object of the FloatArr is declared, the user should be able to define the
initial length of the array. The statement
Example: FloatArr fArr(100);
allocates memory for a maximum of 100 array elements.
The definition of the FloatArr class therefore comprises a member that addresses a
dynamically allocated array. In addition to this, two int variables are required to store
the maximum and current number of array elements.
480 ■
CHAPTER 22 DYNAMIC MEMBERS
// floatArr.h : Dynamic array of floats.
// ---------------------------------------------------
#ifndef _FLOATARR_
#define _FLOATARR_
class FloatArr
{
private:
float* arrPtr; // Dynamic member
int max; // Maximum quantity without
// reallocation of new storage.
int cnt; // Number of array elements
public:
FloatArr( int n = 256 ); // Constructor
FloatArr( int n, float val);
~FloatArr(); // Destructor
int length() const { return cnt; }
float& operator[](int i); // Subscript operator.
float operator[](int i) const;
bool append(float val); // Append value val.
bool remove(int pos); // Delete position pos.
};
#endif // _FLOATARR_
#include "floatArr.h"
#include <iostream>
using namespace std;
int main()
{
FloatArr v(10); // Array v of 10 float values
FloatArr w(20, 1.0F); // To initialize array w of
// 20 float values with 1.0.
v.append( 0.5F);
cout << " Current number of elements in v: "
<< v.length() << endl; // 1
cout << " Current number of elements in w: "
<< w.length() << endl; // 20
return 0;
}
■CLASSES WITH A DYNAMIC MEMBER
First version of class FloatArr
Creating objects with dynamic members
CLASSES WITH A DYNAMIC MEMBER
■481
The next question you need to ask when designing a class to represent arrays is what
methods are necessary and useful. You can enhance FloatArr class step by step by opti-
mizing existing methods or adding new methods.
The first version of the FloatArr class comprises a few basic methods, which are
introduced and discussed in the following section.
䊐Constructors
It should be possible to create an object of the FloatArr class with a given length and
store a float value in the object, if needed. A constructor that expects an int value as
an argument is declared for this purpose.
FloatArr(int n = 256);
The number 256 is the default argument for the length of the array. This provides for a
default constructor that creates an array with 256 empty array elements.
An additional constructor
FloatArr( int n, int val );
allows you to define an array where the given value is stored in each array element. In
this case you need to state the length of the array.
Example: FloatArr arr( 100, 0.0F));
This statement initializes the 100 elements in the array with a value of 0.0.
䊐Additional Methods
The length() method allows you to query the number of elements in the array.
arr.length() returns a value of 100 for the array arr.
You can overload the subscript operator [] to access individual array elements.
Example: arr[i] = 15.0F;
The index imust lie within the range 0to cnt-1.
The append() method can be used to append a value to the array. The number of
elements is then incremented by one.
When you call the remove() method it does exactly the opposite of append()—
deleting the element at the stated position. This reduces the current count by one, pro-
vided a valid position was stated.
482 ■
CHAPTER 22 DYNAMIC MEMBERS
Object
fArr
arrPtr
max: 10
cnt: 10
??????????
Object
fArr
arrPtr
max: 10
cnt: 10
■CREATING AND DESTROYING OBJECTS
Effects of the declaration FloatArr fArr( 10, 1.0F );
First, memory is allocated for the data members:
Then storage is allocated for 10 array elements and the variables max and cnt are set to
10:
Finally, a value of 1.0 is used to initialize the array elements:
1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0
Object
fArr
arrPtr
max: 10
cnt: 10
CREATING AND DESTROYING OBJECTS
■483
The memory for the array elements is not contained in a FloatArr object and must be
allocated dynamically by the constructor. The object itself only occupies the memory
required for the data members arrPtr,max, and cnt. Thus, sizeof(FloatArr) is a
constant value that defaults to 12 bytes for 32 bit computers.
The additional dynamic memory allocation may need to be adjusted to meet new
requirements, for example, if an assignment is made. Finally, the memory has to be
released explicitly when an object is destroyed.
䊐Constructing an Object
The first constructor in the FloatArr class is defined as follows:
FloatArr::FloatArr( int n )
{
max = n; cnt = 0;
arrPtr = new float[max];
}
This allocates memory for narray elements. The current number of array elements is set
to 0.
The second constructor fills the array with the supplied value and is therefore defined
as follows:
FloatArr::FloatArr(int n, float val)
{
max = cnt = n;
arrPtr = new float[max];
for( int i=0; i < cnt; ++i)
arrPtr[i] = val;
}
The opposite page shows how memory is allocated for the object fArr and how this
object is initialized.
䊐Destroying an Object
When an object is destroyed the dynamic memory the object occupies must be released.
Classes with dynamic members will always need a destructor to perform this task.
The FloatArr class contains a dynamic array, so memory can be released by a call to
the delete[] operator.
FloatArr::~FloatArr()
{
delete[] arrPtr;
}
484 ■
CHAPTER 22 DYNAMIC MEMBERS
// FloatArr.cpp:
// Implementing the methods of class FloatArr.
// -----------------------------------------------------
#include "floatArr.h"
#include <iostream>
using namespace std;
// Constructors and destructor as before.
// Subscript operator for objects that are not const:
float& FloatArr::operator[]( int i )
{
if( i < 0 || i >= cnt ) // Range checking
{
cerr << "\n class FloatArr: Out of range! ";
exit(1);
}
return arrPtr[i];
}
float FloatArr::operator[]( int i ) const
{
// Else as before.
}
bool FloatArr::append( float val)
{
if(cnt < max)
{
arrPtr[cnt++] = val; return true;
}
else // Enlarge the array!
return false;
}
bool FloatArr::remove(int pos)
{
if( pos >= 0 && pos < cnt)
{
for( int i = pos; i < cnt-1; ++i)
arrPtr[i] = arrPtr[i+1];
--cnt;
return true;
}
else
return false;
}
■IMPLEMENTING METHODS
New version of class FloatArr
IMPLEMENTING METHODS
■485
䊐Read and Write Access Using the Subscript Operator
The subscript operator can be overloaded to allow easy manipulation of array elements.
Example: FloatArr v(5, 0.0F);
v[2] = 2.2F;
for( int i=0; i < v.length(); ++i)
cout << v[i];
The operator allows both read and write access to the array elements and cannot be used
for constant objects for this reason. However, you will need to support read-only access
to constant objects.
The FloatArr class contains two versions of the operator function operator[]()
for this purpose. The first version returns a reference to the i-th array element and thus
supports write access. The second, read-only version only supports read access to the
array elements and is automatically called by the compiler when accessing constant
objects.
The implementation of these versions is identical. In both cases range checking is
performed for the index. If the index lies within the valid boundaries, an array element—
or simply a value in the case of the read-only version—is returned.
䊐Appending and Deleting in Arrays
The FloatArr class comprises the methods append() and remove() for appending
and deleting array elements.
In the first version, the append() only works if there is at least one empty slot in the
array. In the exercises, append() is used to extend the array as required. This also
applies for a new method, insert(), which you will write as an exercise in this chapter.
When the remove() method is used to delete an element, the elements following
the deleted element move up one place, preserving the original order. The current count
is decremented by one. What was formerly the last element in the array is not deleted
but overwritten when a new element is inserted.
Another technique would be to copy the last element to the position of the element
that needs to be deleted, simply overwriting that element. Of course, this technique is
quicker and preferable for cases where the order of the elements is not significant.
486 ■
CHAPTER 22 DYNAMIC MEMBERS
4.1 6.5 8.2 2.7
Object
a
arrPtr
max: 10
cnt: 4
Object
b
arrPtr
max: 10
cnt: 4
//floatArr.cpp: Implementing the methods.
// ----------------------------------------------------
FloatArr::FloatArr(const FloatArr& src)
{
max = src.max; cnt = src.cnt;
arrPtr = new float[max];
for( int i = 0; i < cnt; i++ )
arrPtr[i] = src.arrPtr[i];
}
■COPY CONSTRUCTOR
Effect of the standard copy constructor
FloatArr b(a); // Creates a copy of a.
A self-defined copy constructor for class FloatArr
COPY CONSTRUCTOR
■487
䊐Initializing with an Object
The next step is to ensure that an existing object can be used to initialize a new object.
Given an array, a, the following statement should be valid:
Example: FloatArr b(a);
The array bshould now be the same length as the array aand the array elements in b
should contain the same values as in a.
The FloatArr class needs a copy constructor to perform this task. The constructor
has a reference to a constant array as a parameter.
Prototype: FloatArr( const FloatArr& );
䊐Standard Copy Constructor
If a class does not contain a copy constructor, the compiler will automatically create a
minimal version, known as the standard copy constructor. This constructor copies the data
members of the object passed to it to corresponding data members of the new object.
A standard copy constructor is normally sufficient for a class. However, simply copy-
ing the data members would serve no useful purpose for objects containing dynamic
members. This would merely copy the pointers, meaning that the pointers of several dif-
ferent objects would reference the same place in memory. The diagram on the opposite
page illustrates this situation for two FloatArr class objects.
This scenario would obviously mean trouble. Imagine releasing memory allocated for
an object dynamically. The pointer for the second object would reference a memory area
that no longer existed!
䊐Proprietary Version of the Copy Constructor
Clearly you will need to write a new copy constructor for classes with dynamic members,
ensuring that the live data and not just the pointers are copied from the dynamically
allocated memory.
The example on the opposite page shows the definition of the copy constructor for
the FloatArr class. Calling new[] creates a new array and the array elements of the
object passed to the method are then copied to that array.
488 ■
CHAPTER 22 DYNAMIC MEMBERS
// FloatArr.h : Dynamic arrays of floats.
// ---------------------------------------------------
class FloatArr
{
private:
// . . . Data members as before
public:
// . . . Methods as before and
FloatArr(const FloatArr& src); // Copy constructor
FloatArr& operator=( const FloatArr&); // Assignment
};
// In file floatArr.cpp
// The operator function implementing "=".
// ---------------------------------------------------
FloatArr& FloatArr::operator=( const FloatArr& src )
{
if( this != &src ) // No self assignments!
{
max = src.max;
cnt = src.cnt;
delete[] arrPtr; // Release memory,
arrPtr = new float[max]; // reallocate and
for( int i=0; i < cnt; i++) // copy elements.
arrPtr[i] = src.arrPtr[i];
}
return *this;
}
#include "FloatArr.h"
int main()
{
FloatArr v; // Default constructor.
FloatArr w(20, 1.0F); // Array w - 20 float values
// with initial value 1.0.
const FloatArr kw(w); // Use copy constructor
// to create an object.
v = w; // Assignment.
}
■ASSIGNMENT
New declarations in class FloatArr
Defining the assignment
Sample calls
ASSIGNMENT
■489
Each class comprises four implicitly defined default methods, which you can replace with
your own definitions:
■the default constructor and the destructor
■the copy constructor and the standard assignment
In contrast to initialization by means of the copy constructor, which takes place when an
object is defined, an assignment always requires an existing object. Multiple assignments,
which modify an object, are possible.
䊐Default Assignment
Given that v1 and v2 are two FloatArr class objects, the following assignment is
valid:
Example: v1 = v2; // Possible, but ok?
Default assignment is performed member by member. The data members of v2 are copied
to the corresponding data members of v1 just like the copy constructor would copy
them. However, this technique is not suitable for classes with dynamic members. This
would simply point the pointers belonging to different objects at the same dynamic allo-
cated memory. In addition, memory previously addressed by a pointer of the target object
will be unreferenced after the assignment.
䊐Overloading the Assignment Operator
In other words, you need to overload the default assignment for classes containing
dynamic members. Generally speaking, if you need to define a copy constructor, you will
also need to define an assignment.
The operator function for the assignment must perform the following tasks:
■release the memory referenced by the dynamic members
■allocate sufficient memory and copy the source object’s data to that memory.
The operator function is implemented as a class method and returns a reference to the
target object allowing multiple assignments. The prototype of the operator function for
the FloatArr class is thus defined as follows:
FloatArr& FloatArr::operator=( const FloatArr& src)
When implementing the operator function you must avoid self assignment, which would
read memory areas that have already been released.
exercises
490 ■
CHAPTER 22 DYNAMIC MEMBERS
// Copy constructor:
List::List(const List&);
// Assignment:
List& List::operator=( const List&);
// Methods to append a float or an
// array of floats:
void append( float val);
void append( const FloatArr& v);
FloatArr& operator+=( float val);
FloatArr& operator+=( const FloatArr& v);
// Methods to insert a float or an
// array of floats:
bool insert( float val, int pos);
bool insert( const FloatArr& v, int pos );
// In any case, more memory space must be allocated
// to the array if the current capacity is
// insufficient.
■EXERCISES
New methods of class List
New methods of class FloatArr
EXERCISES
■491
The width() method in the ostream class returns the current field width, if you call the method
without any arguments.
✓NOTE
Exercise 1
Complete the definition of the List class, which represents a linked list and test
the class.
First, modify your test program to create a copy of a list. Call the default
assignment for the objects in the List class. Note how your program reacts.
A trial run of the program shows that the class is incomplete. Since the class
contains dynamic members, the following tasks must be performed:
■Define a copy constructor for the List class.
■Overload the assignment operator.
Exercise 2
Add the methods shown opposite to the FloatArr class. In contrast to the
existing method
bool append( float val);
the new method must be able to allocate more memory as required. As this
could also be necessary for other methods, write a private auxiliary function for
this purpose
void expand( int newMax );
The method must copy existing data to the newly allocated memory.
Overload the operator += so it can be used instead of calling the function
append().
The insert() method inserts a float value or a FloatArr object at
position pos. Any elements that follow pos must be pushed.
Also overload the shift operator << to output an array using the field width
originally defined to output the array elements.
Now add calls to the new methods to your test program and output the results
after each call.
solutions
492 ■
CHAPTER 22 DYNAMIC MEMBERS
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// List.h
// Definition of classes ListEl and List
// representing a linked list.
// ---------------------------------------------------
#ifndef _LIST_H_
#define _LIST_H_
#include "Date.h"
#include <iostream>
#include <iomanip>
using namespace std;
class ListEl
{
// Unchanged as in Chapter 21
};
// ------------------------------------------------------
// Definition of class List
class List
{
private:
ListEl* first, *last;
public:
// New methods:
List(const List&); // Copy constructor
List& operator=( const List&); // Assignment
// Otherwise unchanged from Chapter 21
};
#endif // _LIST_H_
// ----------------------------------------------------
// List.cpp
// Implements those methods of class List,
// that are not defined inline.
// ----------------------------------------------------
#include "List.h"
// Copy constructor:
List::List(const List& src)
{
// Appends the elements of src to an empty list.
first = last = NULL;
ListEl *pEl = src.first;
for( ; pEl != NULL; pEl = pEl->next )
pushBack( pEl->date, pEl->amount);
}
SOLUTIONS
■493
// Assignment:
List& List::operator=( const List& src)
{
// Release memory for all elements:
ListEl *pEl = first,
*next = NULL;
for( ; pEl != NULL; pEl = next)
{
next = pEl->next;
delete pEl;
}
first = last = NULL;
// Appends the elements of src to an empty list.
pEl = src.first;
for( ; pEl != NULL; pEl = pEl->next )
pushBack( pEl->date, pEl->amount);
return *this;
}
// All other methods unchanged.
// -------------------------------------------------------
// List_t.cpp
// Tests the class List with copy constructor and
// assignment.
// -------------------------------------------------------
#include "List.h"
int main()
{
cout << "\n * * * Testing the class List * * *\n"
<< endl;
List list1; // A list
cout << list1 << endl; // The list is still empty.
Date date( 11,8,1999); // Insert 3 elements.
double amount( +1234.56);
list1.pushBack( date, amount);
date.setDate( 1, 1, 2002);
amount = -1000.99;
list1.pushBack( date, amount);
date.setDate( 2, 29, 2000);
amount = +5000.11;
list1.pushBack( date, amount);
494 ■
CHAPTER 22 DYNAMIC MEMBERS
cout << "\nThree elements have been inserted!"
"\nContent of the list:" << endl;
cout << list1 << endl;
cout << "\nPress return to continue! "; cin.get();
List list2( list1);
cout << "A copy of the 1st list has been created!\n"
"Contents of the copy:\n" << endl;
cout << list2 << endl;
cout << "\nRemove the first element from the list:\n";
ListEl *ptrEl = ptrEl = list1.front();
if( ptrEl != NULL)
{
cout << "To be deleted: " << *ptrEl << endl;
list1.popFront();
}
cout << "\nContent of the list:\n";
cout << list1 << endl;
list1 = list2; // Reassign the copy.
cout << "The copy has been assigned to the 1st list!\n"
"Contents after assignment:\n" << endl;
cout << list1 << endl;
return 0;
}
SOLUTIONS
■495
Exercise 2
// ------------------------------------------------------
// floatArr.h : Dynamic arrays of floating-point numbers.
// ------------------------------------------------------
#ifndef _FLOATARR_
#define _FLOATARR_
#include <iostream>
using namespace std;
class FloatArr
{
private:
float* arrPtr; // Dynamic member
int max; // Maximum quantity without
// reallocating new storage.
int cnt; // Number of present array elements
void expand( int newMax); // Helps enlarge the array
public:
// Constructors , destructor,
// assignment, subscript operator, and method length()
// as before in this chapter.
// Methods to append a floating-point number
// or an array of floating-point numbers:
void append( float val);
void append( const FloatArr& v);
FloatArr& operator+=( float val)
{
append( val); return *this;
}
FloatArr& operator+=( const FloatArr& v)
{
append(v); return *this;
}
// Methods to insert a floating-point number
// or an array of floating-point numbers:
bool insert( float val, int pos);
bool insert( const FloatArr& v, int pos );
bool remove(int pos); // Delete at position pos.
// To output the array
friend ostream& operator<<( ostream& os,
const FloatArr& v);
};
#endif // _FLOATARR_
496 ■
CHAPTER 22 DYNAMIC MEMBERS
// -----------------------------------------------------
// FloatArr.cpp
// Implements the methods of FloatArr.
// -----------------------------------------------------
#include "floatArr.h"
// Constructors, destructor, assignment,
// and subscript operator unchanged.
// --- The new functions ---
// Private auxiliary function to enlarge the array.
void FloatArr::expand( int new)
{
if( newMax == max)
return;
max = newMax;
if( newMax < cnt)
cnt = newMax;
float *temp = new float[newMax];
for( int i = 0; i < cnt; ++i)
temp[i] = arrPtr[i];
delete[] arrPtr;
arrPtr = temp;
}
// Append floating-point number or an array of floats.
void FloatArr::append( float val)
{
if( cnt+1 > max)
expand( cnt+1);
arrPtr[cnt++] = val;
}
void FloatArr::append( const FloatArr& v)
{
if( cnt + v.cnt > max)
expand( cnt + v.cnt);
int count = v.cnt; // Necessary if v == *this
for( int i=0; i < count; ++i)
arrPtr[cnt++] = v.arrPtr[i];
}
SOLUTIONS
■497
// Insert a float or an array of floats
bool FloatArr::insert( float val, int pos)
{
return insert( FloatArr(1,val), pos);
}
bool FloatArr::insert( const FloatArr& v, int pos )
{
if( pos < 0 || pos >= cnt)
return false; // Invalid position
if( max < cnt + v.cnt)
expand(cnt + v.cnt);
int i;
for( i = cnt-1; i >= pos; --i) // Shift up
arrPtr[i+v.cnt] = arrPtr[i]; // starting at pos
for( i = 0; i < v.cnt; ++i) // Fill gap.
arrPtr[i+pos] = v.arrPtr[i];
cnt = cnt + v.cnt;
return true;
}
// To delete
bool FloatArr::remove(int pos)
{
if( pos >= 0 && pos < cnt)
{
for( int i = pos; i < cnt-1; ++i)
arrPtr[i] = arrPtr[i+1];
--cnt;
return true;
}
else
return false;
}
// Output the array
ostream& operator<<( ostream& os, const FloatArr& v)
{
int w = os.width(); // Save field width.
for( float *p = v.arrPtr; p < v.arrPtr + v.cnt; ++p)
{
os.width(w); os << *p;
}
return os;
}
498 ■
CHAPTER 22 DYNAMIC MEMBERS
// -----------------------------------------------------
// FloatV_t.cpp
// Tests the class FloatArr.
// -----------------------------------------------------
#include "FloatArr.h"
#include <iostream>
#include <iomanip>
using namespace std;
int main()
{
FloatArr v(10); // Array v for 10 float values
FloatArr w(15, 1.0F); // Initialize the array w of
// 15 float values with 1.0.
cout << " Current total of elements in v: "
<< v.length() << endl;
cout << " Current total of elements in w: "
<< w.length() << endl;
float x = -5.0F; // Append values.
for( ; x < 6 ; x += 1.7F)
v.append(x);
v += v; // Also possible!
cout << "\nThe array elements after appending:"
<< endl;
cout << setw(5) << v << endl;
const FloatArr cv(v); // Copy constructor
// creates const object.
cout << "\nThe copy of v has been created.\n";
cout << "\nThe array elements of the copy:\n"
<< setw(5) << v << endl;
w.remove(3); // Erase the element at
// position 3.
w.append(10.0F); // Add a new element.
w.append(20.0F); // And once more!
v = w;
cout << "\nAssignment done.\n";
cout << "\nThe elements after assigning: \n"
<< setw(5) << v << endl;
v.insert( cv, 0);
cout << "\nThe elements after inserting "
" the copy at position 0: \n"
<< setw(5) << v << endl;
return 0;
}
499
Inheritance
This chapter describes how derived classes can be constructed from
existing classes by inheritance. Besides defining derived classes, we will
also discuss
■how members are redefined
■how objects are constructed and destroyed, and
■how access control to base classes can be realized.
chapter
23
500 ■
CHAPTER 23 INHERITANCE
■CONCEPT OF INHERITANCE
Is relation
Car
Properties
and capacities
of class
Car
Properties
and capacities
of class
Car
Properties
and capacities
of class
Car
Additional
properties and
capacities of class
PassCar
Additional
properties and
capacities of class
Truck
PassCar Truck
CONCEPT OF INHERITANCE
■501
䊐Base Classes and Derived Classes
Inheritance allows new classes to be constructed on the basis of existing classes. The new
derived class “inherits” the data and methods of the so-called base class. But you can add
more characteristics and functionality to the new class.
A fleet management program used by a car hire company needs to handle all kinds of
vehicles—automobiles, motorcycles, trucks, and so on. All of these vehicles have an
identification number that indicates the vehicle, the manufacturer, and the vehicle sta-
tus, such as “hired,” “repair shop,” and so on. Additionally, operations such as “modify
status” are required for the class.
To differentiate between vehicle types, various classes are derived from the base class
Car, such as PassCar, which is used to represent passenger-carrying vehicles. This class
has additional attributes, such as the number of seats, type, sunroof (yes/no), and various
additional operations.
䊐Is Relationship
An object of the PassCar type is a special object of the Car class. A passenger vehicle
is a special kind of car. In cases like this we can say that the derived class establishes an is
relationship to the base class.
We distinguish between this close relationship and a so-called has relationship. As
already mentioned, a has relationship occurs between two classes when an member of
one class has another class type. An Account object has a string object to represent
the name of the account holder, for example.
䊐Data Abstraction and Reusability
Inheritance has a number of important benefits for software developers:
■data abstraction: General characteristics and abilities can be handled by generic
(base) classes and specializations can be organized in hierarchical relationships by
means of derived classes. This makes it easier to manage complex situations and
relationships.
■re-usability: Classes that you have defined and tested can be reused and adapted to
perform new tasks. The base class implementation need not be known for this
purpose: only the public interfaces are required.
502 ■
CHAPTER 23 INHERITANCE
BBase class B
C
D
B is a direct
base class
B is an indirect
base class
class C : public B
{
private:
// Declaration of additional private
// data members and member functions
public:
// Declaration of additional public
// data members and member functions
};
■DERIVED CLASSES
Defining a derived class
Direct and indirect derivation
DERIVED CLASSES
■503
When you define a derived class, the base class, the additional data members and meth-
ods, and the access control to the base class are defined.
The opposite page shows a schematic definition of a derived class, C. The Cclass
inherits the Bclass, which is defined in the public section following the colon. The
private and public sections contain additional members of the Cclass.
䊐Access to Public Members in the Base Class
Access privileges to the base class Bare designated by the public keyword that pre-
cedes the B. In other words,
■all the public members in base class Bare publicly available in the derived class
C.
This kind of inheritance ports the public interface of the base class to the derived
class where it is extended by additional declarations. Thus, objects of the derived class
can call the public methods of the base class. A public base class, therefore, imple-
ments the is relationship; this is quite common.
There are some less common cases where access to the members of the base class
needs to be restricted or prohibited. Only the methods of class Ccan still access the
public members of B, but not the users of that class. You can use private or pro-
tected derivation to achieve this (these techniques will be discussed later).
䊐Access to Private Members of the Base Class
The private members of the base class are protected in all cases. That is,
■the methods of the derived class cannot access the private members of the base
class.
Imagine the consequences if this were not so: you would be able to hack access to the
base class by simply defining a derived class, thus undermining any protection offered by
data encapsulation.
䊐Direct and Indirect Base Classes
The derived class Ccan itself be a base class for a further class, D. This allows for class
hierarchies. Class Bthen becomes an indirect base class for class D.
In the graphic on the opposite page, the arrow ↑means directly derived from. That is,
class Dis a direct derivation of class Cand an indirect derivation of B.
504 ■
CHAPTER 23 INHERITANCE
// car.h: Definition of baseclass Car and
// of the derived class PassCar
// --------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
class Car // Base class
{
private:
long nr;
string producer;
public: // Constructor:
Car( long n = 0L, const string& prod = "");
// Access methods:
long getNr(void) const { return nr; }
void setNr( long n ) { nr = n; }
const string& getProd() const{ return producer; }
void setProd(const string& p){ producer = p; }
void display( void ) const; // Display a car
};
class PassCar : public Car // Derived class
{
private:
string passCarType;
bool sunRoof;
public: // Constructor:
PassCar( const string& tp, bool sd,
int n = 0 , const string& h = "");
// Access methods:
const string& getType() const{ return passCarType; }
void setType( const string s) { passCarType = s; }
bool getSunRoof() const { return sunRoof; }
void setSunRoof( bool b ) { sunRoof = b; }
void display() const;
};
■MEMBERS OF DERIVED CLASSES
Base class Car and derived class PassCar
MEMBERS OF DERIVED CLASSES
■505
Let’s look at the example on the opposite page to illustrate how derived classes are
defined. The Car class and a derived class PassCar are defined in the example.
䊐Data Members and Methods
The base class Car contains two data members, nr and producer, which are used to
represent an identification number and the name of the manufacturer. The derived class
PassCar inherits these data members. Thus, an object of the PassCar class also con-
tains the data members nr and producer. The object includes a so-called base sub-
object of type Car.
The PassCar class additionally contains the data members passCarType and
sunRoof to represent a passenger vehicle with or without a sunroof. So a PassCar type
object has a total of four data members. For the sake of simplicity, we have omitted fur-
ther data members, such as the number of seats, etc.
The base class Car contains a constructor, access methods, and the method
display(), which is used for screen output. The methods are also inherited by the
derived class PassCar.
In the PassCar class a constructor, additional access methods, and a second output
function also called display() are declared. The derived class thus inherits a method
called display() and declares a method with the same name. The display()
method is said to have been redefined.
Every member function and every data member in a derived class can be redefined.
The member assumes a new meaning for the derived class. The member inherited from
the base class is also available in the derived class and will retain its original meaning.
We will be looking at this point in more detail later.
䊐Public Interface
Since the Car class is a public base class of the PassCar class, all the public mem-
bers of the base class are available in the derived class. For example, you can call the
getNr() method for an object named cabrio in the PassCar class.
Example: cout << "Car number: "<< cabrio.getNr();
The public interface of the derived class thus comprises
■the public members of the base class and
■the public members additionally defined in the derived class.
506 ■
CHAPTER 23 INHERITANCE
class Car
class PassCar : public Car
void PassCar::display( void) const
private:
private:
public:
public:
<< getNr();
<< getProd();
cout << "Type: "<< passCarType;
cout << "Type: "<< passCarTyp
if( sunRoof) cout << "yes";
else cout << " no";
cout << endl;
cout << "\nCar number: "
string passCarType;
long nr;
string producer;
long getNr(void);
bool sunRoof;
cout << "\nProducer: "
}
{
{
{
}
}
. . .
. . .
. . .
ok
ok
not ok
■MEMBER ACCESS
Accessing members of base class Car
MEMBER ACCESS
■507
䊐Access to Additional Members
The methods of derived classes can access any member additionally defined in the
derived class.
Example: const string& getType() const
{ return passCarType; }
The getType() method directly accesses the private data member passCarType in
the PassCar class in this example.
䊐Access to Private Members of the Base Class
However, a private member of the base class is not directly accessible for the methods of
the derived class. The output function display() in the derived class PassCar, for
example, cannot contain the following statement:
Example: cout << "Producer: " << producer;
As producer is a private data member of the base class Car, the compiler would issue
an error message at this point.
Methods belonging to derived classes only have indirect access to the private data
members of the base class. They use access methods in the public declaration of the
base class for this purpose. The opposite page shows a version of the display() method
that calls the get methods in its base class Car.
When you call an access method, you do not need to state the method’s base class.
The base class is identified by the this pointer, which is passed implicitly as an argu-
ment. The call to getProd() on the opposite page is thus equivalent to:
Example: this->getProd();
䊐Name Lookup
The following rules apply when searching for the name of a method:
■the compiler looks for the name of the method called in the derived class first
■if the name cannot be found, the compiler walks one step up the tree and looks
for a public method with that name.
The above example thus calls the getProd() in the base class Car, as the method is
not defined in the PassCar class.
508 ■
CHAPTER 23 INHERITANCE
// Within file Car.cpp
// This version of method PassCar::display() calls
// the method display() of the base class.
// ---------------------------------------------------
void PassCar::display( void) const
{
Car::display(); // Method in base class
cout << "Type: " << passCarType;
cout << "\nSunroof: ";
if(sunRoof)
cout << "yes "<< endl;
else
cout << "no " << endl;
}
■REDEFINING MEMBERS
New version of method display()
REDEFINING MEMBERS
■509
䊐Redefinition
There are two options for the names of data members or methods in derived classes:
1. The name does not occur in the base class →no redefinition.
2. The name already exists in the base class →redefinition.
In the second case, the member of the same name continues to exist unchanged in the
base class. In other words, redefining members in a derived class has no effect on the base
class.
However, the name lookup rules for the compiler lead to the following scenario:
■if a member is redefined in a derived class, it will mask the corresponding mem-
ber in the base class.
This situation is similar to the one seen for local and global variables. A local variable
will mask a previously defined global variable with the same name.
䊐Redefinition and Overloading
Normally, methods are redefined in derived classes. This adopts the methods to the new
features of the class. When a method is redefined, the signature and the return type of
the method can be changed. However, a redefinition does not overload functions since
the derived class has a different scope.
Redefining a method will always mask a method with the same name in the base class.
Of course, you can overload methods within the same class, and this means you can
repeatedly redefine a base class method for a derived class.
䊐Access to the Members in the Base Class
If you redefine a method in a derived class, this does not alter the fact that the base class
method still exists. If the method was declared in the public section of the base class,
you can call it to redefine a method. The range :: operator is used to access the base
class method.
The new version of the display() method opposite illustrates this point. The
display() method defined in the base class is used to output the data members of the
base class. To do so, you must use the range operator with the name of the base class.
Otherwise the display() method in the derived class will call itself and head off into
an indefinite recursion.
510 ■
CHAPTER 23 INHERITANCE
// First version of the constructor of PassCar
// --------------------------------------------------
PassCar::PassCar(const string& tp, bool sr, int n,
const string& hs)
{
setNr(n); // Initial values for data
setProd(hs); // members of the base class.
passCarType = tp; // Initial values for data mem-
sunRoof = sr; // bers of the derived class
}
// Second version of the constructors of PassCar
// ----------------------------------------------------
PassCar::PassCar(const string& tp, bool sr, int n,
const string& hs) : Car( n, hs)
{
passCarType = tp; // Initial values for data mem-
sunRoof = sr; // bers of the derived class
}
// Third version of the constructor of PassCar
// ----------------------------------------------------
PassCar::PassCar(const string& tp, bool sr, int n,
const string& hs)
: Car( n, hs), passCarType( tp ), sunRoof( sr )
{
// There remains nothing to do
}
■CONSTRUCTING AND DESTROYING DERIVED CLASSES
First version of the constructor of PassCar
Second version with base class initializer
Third version with base class and member initializer
CONSTRUCTING AND DESTROYING DERIVED CLASSES
■511
䊐Constructor Calls
The constructor of a derived class is required to create an object of the derived class type.
As the derived class contains all the members of the base class, the base sub-object must
also be created and initialized. The base class constructor is called to perform this task.
Unless otherwise defined, this will be the default constructor.
The order in which the constructors are called is important. The base class construc-
tor is called first, then the derived class constructor. The object is thus constructed from
its core outwards.
The first version of the constructor for PassCar, as shown opposite, sets initial val-
ues by calling the access methods of the base class. An implicit call to the default con-
structor of the base class occurs prior to this, and the base sub-object is initialized with
default values. This process has the same drawbacks as the technique of creating objects
with member objects. A default constructor must be available in the base class and ini-
tialization with incorrect values before assigning live values impacts the response of the
program.
䊐Base Initializer
If the base class contains a constructor with parameters, it makes sense to call this con-
structor. This immediately initializes the data members with correct values. A base initial-
izer for the constructor of the derived class can be defined for this purpose.
The second version of the constructor for PassCar contains a base initializer.
Example: Car( n, hs )
The syntax of the base initializer for base sub-objects is similar to that of the member ini-
tializer for member sub-objects. This means that you can state both the base and the
member initializer in a list separated by commas. The third version of the PassCar con-
structor illustrates this point.
䊐Destroying Objects
When an object is destroyed, the destructor of the derived class is first called, followed by
the destructor of the base class. The reverse order of the constructor calls applies.
You need to define a destructor for a derived class if actions performed by the con-
structor need to be reversed. The base class destructor need not be called explicitly as it
is executed implicitly.
512 ■
CHAPTER 23 INHERITANCE
// car_t.cpp: Testing the base class Car and
// the derived class PassCar.
// -----------------------------------------------------
#include "car.h"
int main()
{
const PassCar beetle("Beetle", false, 3421, "VW");
beetle.display();
cout << "\nAnd the passenger car number again: "
<< beetle.getNr() << endl;
PassCar cabrio("Carrera", true);
cabrio.setNr(1000);
cabrio.setProd("Porsche");
cabrio.display();
cout << "\nOnly data of the base class: ";
cabrio.Car::display();
return 0;
}
■OBJECTS OF DERIVED CLASSES
Sample program
Screen output
---------------------------------------------
Car number: 3421
Producer: VW
Type: Beetle
Sunroof: no
And the passenger car number again: 3421
---------------------------------------------
Car number: 1000
Producer: Porsche
Type: Carrera
Sunroof: yes
Only data of the base class:
---------------------------------------------
Car number: 1000
Producer: Porsche
OBJECTS OF DERIVED CLASSES
■513
䊐Declaring Objects
The program opposite illustrates how objects of derived classes can be used. Two objects,
beetle and cabrio, of the derived class PassCar type are declared. As the PassCar
class does not contain a default constructor, both objects must be initialized. However, it
is sufficient to state a PassCar type with or without a sunroof as default values exist for
all other data members.
The object beetle is declared as const just to show that the get methods and the
display() method can also be called for constant objects since they were declared as
read-only methods.
However, the following call is invalid:
Example: beetle.setNr( 7564 ); // Error
This means you have to correctly define all the initial values for the object when you
declare it.
䊐Calling Redefined Methods
When you call a redefined method, the object type determines what version of the
method will be executed. In the PassCar class the method display() has been rede-
fined. The statement
Example: cabrio.display();
also outputs the additional data members passCarType and sunRoof. However, in
the case of the van object in the Car class, calling
Example: van.display();
will execute the method in the base class.
䊐Calling Methods in the Base Class
You may be wondering if a base class method can be called for an object of a derived
class, if the method has been redefined in the derived class. This is possible using the
scope resolution operator,::.
If you want to display the basic data of the cabrio object, you can use a direct call to
the base class method display() to do so.
Example: cabrio.Car::display();
The name of the method is preceded by the name of the base class and the scope resolu-
tion operator in this case.
514 ■
CHAPTER 23 INHERITANCE
// safe.h : The classes Safe and Castle
// ---------------------------------------------------
#include <iostream>
using namespace std;
class Safe
{
private:
int topSecret;
protected:
int secret;
void setTopSecret( int n) { topSecret = n;}
int getTopSecret() const { return topSecret;}
void setSecret( int n){ secret = n;}
int getSecret() const { return secret;}
public:
int noSecret;
Safe()
{ topSecret = 100; secret = 10; noSecret = 0; }
};
class Castle : public Safe
{
public:
Castle()
{
// topSecret = 10; // Error, because private
setTopSecret(10); // ok, because protected
secret = 1; // ok, because protected
noSecret = 0; // ok, because public
}
void test()
{
// top.Secret = 200; // Error, because private
setTopSecret(200); // ok, because protected
secret = 20; // ok, because protected
noSecret = 2; // ok, because public
}
};
■PROTECTED MEMBERS
Sample classes
PROTECTED MEMBERS
■515
䊐Access to Sheltered Members
The private members of a base class are equally inaccessible for the methods and friend
functions of a derived class.
When you create a class hierarchy you may want require the methods and friend
functions of a derived class to communicate directly with the members of the base class.
This would be particularly necessary if the base class contained members for use as build-
ing blocks for derived classes but not for general purpose use.
For example, a class used to represent a window on screen could contain the dimen-
sions and other characteristics of a general windows. The characteristics need protecting;
however, methods in derived classes will still need direct access.
䊐Protected Members
To allow methods and friend functions access to the sheltered members of a base class,
let’s introduce an additional level of access control between private and public.
This is achieved by means of protected declarations.
A member declared protected is sheltered from external access just like a pri-
vate member. That means, a protected member is inaccessible for base class objects
and any classes derived from the base class. However, in contrast to a private member,
methods and friend functions of derived classes can access the member.
The classes defined opposite, Safe and Castle, show that protected members of
the base class can be accessed directly in a derived class. In contrast to this, protected
members are inaccessible to users of these classes.
Example: Castle treasure;
treasure.topSecret = 1; // Error: private
treasure.secret = 2; // Error: protected
treasure.setTopSecret(5); // Error: protected
treasure.noSecret = 10; // ok
Protected declarations should be used with caution. If you change the declaration of a
protected member, every class derived from this class must be examined to ascertain
whether additional modifications are necessary.
exercises
516 ■
CHAPTER 23 INHERITANCE
Additional data members:
Type
Number of axles int
Load capacity double
Additional methods:
void setAxles( int a );
int getAxles() const;
void setCapacity( double cp );
void getCapacity() const;
void display() const;
■EXERCISES
For exercise 1
Class Truck being derived from class Car
EXERCISES
■517
Exercise 1
The classes Car and PassCar are to modify to allow objects to be created and
destroyed. In addition, the class Truck is to be added to the class hierarchy.
■Change the classes Car and PassCar to make the constructor issue the
following message:
"Creating an object of type ... ."
■Define a destructor for the Car and PassCar classes.The destructor
should issue the following message:
"Destroying an object of type ...."
■Then define the class Truck, which is derived from Car, using the data
members shown opposite, a constructor, a destructor, and the additional
methods shown opposite.
■Implement the constructor for the Truck class—the constructor should
again issue a suitable message. Use the base initializer to initialize the data
members of Car.
■Define a destructor for Truck—the destructor should again issue a suit-
able message for trucks.
■To test your class, create and display a Truck type object in your main
function. If required by the user, enable your program to create and dis-
play objects of the types PassCar and Car.
Observe how the various objects and member objects are created and
destroyed.
Exercise 2
Derive two classes, DepAcc and SavAcc, from the Account class, which was
defined in Chapter 14, in the section titled “Const Objects and Methods.”
Additionally define an overdraft limit and an interest rate for the DepAcc class.
The SavAcc contains the members of the base class and an interest rate.
■For both classes, define constructors to provide default values for all
parameters, add access methods, and add a display() method for
screen output.
■Test the new classes by initializing objects of the DepAcc and SavAcc
types in the object declarations and outputting them.Then modify both a
savings and a deposit account interactively and display the new values.
518 ■
CHAPTER 23 INHERITANCE
Product
PrepackedFood FreshFood
Properties:
Barcode
Name
Methods:
setCode()
getCode()
...
scanner()
printer()
Properties:
Price per piece
Methods:
getPrice()
setPrice()
...
scanner()
printer()
Properties:
Weight
Price per pound
Methods:
setWght()
getWght()
...
scanner()
printer()
Exercise 3
EXERCISES
■519
Exercise 3
A supermarket chain has asked you to develop an automatic checkout system.
All products are identifiable by means of a barcode and the product name.
Groceries are either sold in packages or by weight. Packed goods have fixed
prices.The price of groceries sold by weight is calculated by multiplying the
weight by the current price per kilo.
Develop the classes needed to represent the products first and organize
them hierarchically.The Product class, which contains generic information on all
products (barcode, name, etc.), can be used as a base class.
■The Product class contains two data members of type long used for
storing barcodes and the product name. Define a constructor with
parameters for both data members.Add default values for the para-
meters to provide a default constructor for the class. In addition to the
access methods setCode() and getCode(), also define the methods
scanner() and printer(). For test purposes, these methods will simply
output product data on screen or read the data of a product from the
keyboard.
■The next step involves developing special cases of the Product class.
Define two classes derived from Product, PrepackedFood and Fresh-
Food. In addition to the product data, the PrepackedFood class should
contain the unit price and the FreshFood class should contain a weight
and a price per kilo as data members.
In both classes define a constructor with parameters providing
default-values for all data members. Use both the base and member ini-
tializer.
Define the access methods needed for the new data members.Also
redefine the methods scanner() and printer() to take the new data
members into consideration.
■Test the various classes in a main function that creates two objects each
of the types Product,PrepackedFood and FreshFood. One object of
each type is fully initialized in the object definition. Use the default con-
structor to create the other object.Test the get and set methods and
the scanner() method and display the products on screen.
solutions
520 ■
CHAPTER 23 INHERITANCE
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// Car.h : Defines the base class Car and
// the derived classes PassCar and Truck
// --------------------------------------------------------
#ifndef _CAR_H_
#define _CAR_H_
#include <iostream>
#include <string>
using namespace std;
class Car
{
// See previous definition in this chapter
};
class PassCar : public Car
{
// See previous definition in this chapter
};
class Truck : public Car
{
private:
int axles;
double tons;
public:
Truck( int a, double t, int n, const string& hs);
~Truck();
void setAxles(int l){ axles = l;}
int getAxles() const { return axles; }
void setCapacity( double t) { tons = t;}
double getCapacity() const { return tons; }
void display() const;
};
#endif
SOLUTIONS
■521
// ------------------------------------------------------
// car.cpp
// Implements the methods of Car, PassCar, and Truck
// ------------------------------------------------------
#include "car.h"
// ------------------------------------------------------
// The methods of base class Car:
Car::Car( long n, const string& prod)
{
cout << "Creating an object of type Car." << endl;
nr = n; producer = prod;
}
Car::~Car()
{
cout << "Destroying an object of type Car" << endl;
}
void Car::display() const
{
cout << "\n---------------------------- "
<< "\nCar number: " << nr
<< "\nProducer: " << producer
<< endl;
}
// -------------------------------------------------------
// The methods of the derived class PassCar:
PassCar::PassCar(const string& tp, bool sd, int n,
const string& hs)
: Car( n, hs), PassCarTyp( tp ), sunRoof( sd )
{
cout << "I create an object of type PassCar." << endl;
}
PassCar::~PassCar()
{
cout << "\nDestroying an object of type PassCar"
<< endl;
}
void PassCar::display( void) const
{
Car::display(); // Base class method
cout << "Type: " << passCarType
<< "\nSunroof: ";
if(sunRoof)
cout << "yes "<< endl;
else
cout << "no " << endl;
}
522 ■
CHAPTER 23 INHERITANCE
// ----------------------------------------------------
// The methods of the derived class Truck:
Truck::Truck( int a, double t, int n, const string& hs)
: Car( n, hs), axles(a), tons(t)
{
cout << "Creating an object of type Truck." << endl;
}
Truck::~Truck()
{
cout << "\nDestroying an object of type Truck\n";
}
void Truck::display() const
{
Car::display();
cout << "Axles: " << axles
<< "\nCapacity: " << tons << " long tons\n";
}
// -----------------------------------------------------
// Car_t.cpp : Tests the base class Car and
// the derived classes PassCar and Truck.
// -----------------------------------------------------
#include "car.h"
int main()
{
Truck toy(5, 7.5, 1111, "Volvo");
toy.display();
char c;
cout << "\nDo you want to create an object of type "
<< " PassCar? (y/n) "; cin >> c;
if( c == 'y' || c == 'Y')
{
const PassCar beetle("Beetle", false, 3421, "VW");
beetle.display();
}
cout << "\nDo you want to create an object "
<< " of type car? (y/n) "; cin >> c;
if( c == 'y' || c == 'Y')
{
const Car oldy(3421, "Rolls Royce");
oldy.display();
}
return 0;
}
SOLUTIONS
■523
Exercise 2
// -----------------------------------------------------
// account.h:
// Defines the classes Account, DepAcc, and SavAcc.
// -----------------------------------------------------
#ifndef _ACCOUNT_H
#define _ACCOUNT_H
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class Account
{
private:
string name; unsigned long nr; double balance;
public:
Account(const string& s="X", unsigned long n
= 1111111L, double st = 0.0)
: name(s), nr(n), balance(st)
{ }
const string& getName() const { return name; }
void setName(const string& n) { name = n;}
unsigned long getNr() const { return nr; }
void setNr(unsigned long n) { nr = n; }
double getBalance() const { return balance; }
void setBalance(double st){ balance = st; }
void display()
{ cout << fixed << setprecision(2)
<< "----------------------------------------\n"
<< "Account holder: " << name << endl
<< "Account number: " << nr << endl
<< "Balance of the account:" << balance <<endl;
}
};
class DepAcc : public Account
{
private:
double limit; // Overdraft limit
double interest; // Interest
public:
DepAcc(const string& s = "X",
unsigned long n = 1111111L, double st = 0.0,
double li = 0.0, double ra = 0.0)
: Account(s, n, st), limit(li), interest(ra)
{ }
524 ■
CHAPTER 23 INHERITANCE
// Access methods:
double getLimit() const { return limit; }
void setLimit(double lt){ limit = lt; }
double getInterest() const { return interest; }
void setInterest(double sl){ interest = sl; }
void display()
{
Account::display();
cout << fixed << setprecision(2)
<< "Overdraft limit: " << limit << endl
<< "Interest rate: " << interest << endl
<< "----------------------------------\n"
<< endl << endl;
}
};
class SavAcc: public Account
{
private:
double interest; // compound interest
public:
SavAcc(const string& s = "X",
unsigned long n = 1111111L, double st = 0.0,
double in = 0.0)
: Account(s, n, st), interest(in)
{ }
// Access methods.
double getInterest() const { return interest; }
void setInterest(double in){ interest = in; }
void display()
{
Account::display();
cout << fixed << setprecision(2)
<< "Interest rate: " << interest << endl
<< "----------------------------------\n"
<< endl << endl;
}
};
#endif
SOLUTIONS
■525
// -------------------------------------------------------
// account_t.cpp
// Tests the classes DepAcc and SavAcc
// derived from class Account
// -------------------------------------------------------
#include "account.h"
int main()
{
string s;
double db;
SavAcc mickey("Mickey Mouse", 1234567,
2.40, 3.5);
mickey.display();
cout << "New name: "; getline(cin, s);
cout << "New interest rate: "; cin >> db;
mickey.setName(s);
mickey.setInterest(db);
mickey.display();
DepAcc dag("Donald Duck", 7654321,
-1245.56, 10000, 12.9);
dag.display();
cout << "New limit: "; cin >> db;
dag.setLimit(db);
dag.display();
return 0;
}
526 ■
CHAPTER 23 INHERITANCE
Exercise 3
// ----------------------------------------------------
// product.h : Defines the classes
// Product, PrepackedFood, and FreshFood
// ----------------------------------------------------
#ifndef _PRODUCT_H
#define _PRODUCT_H
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
class Product
{
private:
long bar;
string name;
public:
Product(long b = 0L, const string& s = "")
: bar(b), name(s)
{ }
void setCode(long b) { bar = b; }
long getCode() const { return bar; }
void setName(const string& s){ name = s; }
const string& getName() const { return name; }
void scanner()
{
cout << "\nBarcode: "; cin >> bar;
cout << "Name: "; cin >> name;
cin.sync(); cin.clear();
}
void printer() const
{
cout << "\n-------------------------------"
<< "\nBarcode: " << bar
<< "\nName: " << name
<< endl;
}
};
class PrepackedFood : public Product
{
private:
double pce_price;
SOLUTIONS
■527
public:
PrepackedFood(double p = 0.0,long b = 0L,
const string& s = "")
: Product(b, s), pce_price(p)
{}
void setPrice(double p){ pce_price = p;}
double getPrice()const { return pce_price; }
void scanner()
{ Product::scanner();
cout << "Price per piece: "; cin >> pce_price;
}
void printer() const
{ Product::printer();
cout << fixed << setprecision(2)
<< "Price per piece: " << pce_price << endl;
}
};
class FreshFood : public Product
{
private:
double wght;
double lbs_price;
public:
FreshFood(double g = 0.0, double p = 0.0,
long b = 0L, const string& s = "")
: Product(b, s), wght(g), lbs_price(p) {}
void setWght(double g) { wght = g;}
double getWght()const { return wght; }
void setPrice(double p) { lbs_price = p;}
double getPrice()const { return lbs_price; }
void scanner()
{ Product::scanner();
cout << "Weight(lbs): "; cin >> wght;
cout << "Price/lbs: "; cin >> lbs_price;
cin.sync(); cin.clear();
}
void printer() const
{
Product::printer();
cout << fixed << setprecision(2)
<< "Price per Lbs: " << lbs_price
<< "\nWeight: " << wght
<< "\nTotal: " << lbs_price * wght
<< endl;
}
};
#endif
528 ■
CHAPTER 23 INHERITANCE
// ------------------------------------------------------
// product_t.cpp
// Tests classes Product, PrepackedFood, and FreshFood.
// ------------------------------------------------------
#include "product.h"
int main()
{
Product p1(12345L, "Flour"), p2;
p1.printer(); // Output the first product
p2.setName("Sugar"); // Set the data members
p2.setCode(543221);
p2.printer(); // Output the second product
// Prepacked products:
PrepackedFood pf1(0.49, 23456, "Salt"), pf2;
pf1.printer(); // Output the first
// prepacked product
cout << "\n Input data of a prepacked product: ";
pf2.scanner(); // Input and output
pf2.printer(); // data of 2nd product
FreshFood pu1(1.5, 1.69, 98765, "Grapes"), pu2;
pu1.printer(); // Output first item
// fresh food
cout <<"\n Input data for a prepacked product: ";
pu2.scanner(); // Input and output
pu2.printer(); // data of 2nd product.
cout << "\n-------------------------------"
<< "\n-------------------------------"
<< "\nAgain in detail: \n"
<< fixed << setprecision(2)
<< "\nBarcode: " << pu2.getCode()
<< "\nName: " << pu2.getName()
<< "\nPrice per Lbs: " << pu2.getPrice()
<< "\nWeight: " << pu2.getWght()
<< "\nEnd price: " << pu2.getPrice()
* pu2.getWght()
<< endl;
return 0;
}
529
Type Conversion in Class
Hierarchies
This chapter describes implicit type conversion within class hierarchies,
which occurs in the context of assignments and function calls.
In addition, explicit type casting in class hierarchies is discussed, in
particular, upcasting and downcasting.
chapter
24
530 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
■CONVERTING TO BASE CLASSES
Example for implicit conversion
#include "car.h"
bool compare( Car&, Car&);
int main()
{
PassCar beetle("New Beetle", false, 3421, "VW"),
miata( "Miata", true, 2512, "Mazda");
bool res = compare( beetle, miata);
// ...
}
// ok!
// Implicit conversion
// to base class.
// Car& a = beetle;
// Car& b = miata;
bool compare( Car& a, Car& b)
{
// Here a is the base part of beetle,
// b is the base part of miata.
// If this is inconvenient, an explicit
// type cast to type PassCar has to be performed.
}
CONVERTING TO BASE CLASSES
■531
䊐Implicit Conversion
If a class is derived from another class by public inheritance, the derived class assumes
the characteristics and features of the base class. Objects of the derived class type then
become special objects of the base class, just like an automobile is a special type of vehi-
cle.
You can utilize the is relationship when handling objects. It is possible to assign an
object of a derived class to an object of the base class. This causes an implicit type conver-
sion to a base class type.
The base class thus becomes a generic term for multiple special cases. Given that the
classes PassCar and Truck were derived from the Car class, objects of the PassCar
or Truck type can always be managed like objects of Car type.
䊐Assignments
Implicit type conversion in class hierarchies occurs in assignments to
■base class objects
■pointers or references to the base class.
䊐Function Calls
Additionally, a similar kind of implicit type conversion takes place for the arguments of
function calls.
Given the function compare() with the following prototype
Example: bool compare( Car& , Car& );
and two objects of the derived PassCar class type, beetle and miata, the following
statement is valid
Example: compare( beetle, miata);
The compiler performs implicit type conversion for the arguments beetle and miata,
converting them to the parameter type, that is, to a reference to the base class Car.
Type conversion for arguments used in function calls is similar to the type conversion
that occurs in assignments, as shown in the following section.
532 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
auto bmw
nr: 4325
producer:
"Bayer...."
nr: 4325
sunRoof: true
producer:
"Bayer...."
passCarType:
"520i"
■TYPE CONVERSIONS IN ASSIGNMENTS
䊐Effect of an assignment
Car auto;
PassCar bmw("520i", true, 4325,
"Bayerische Motorenwerke");
auto = bmw;
TYPE CONVERSIONS IN ASSIGNMENTS
■533
䊐Assignment to a Base Class Object
An object belonging to a derived class type can be assigned to an object of a base class.
Example: Car auto;
PassCar bmw("520i", true, 4325,
"Bayerische Motorenwerke");
auto = bmw;
The object bmw, which belongs to the derived class PassCar, contains all the data
members of the base class, Car, i.e. the vehicle id number and the manufacturer. During
an assignment the object bmw is copied to the data members of the object auto step by
step.
This makes the above statement equivalent to:
auto.nr = bmw.nr;
auto.producer = bmw.producer;
The data members additionally defined in the derived class are not copied!
The following statement outputs the copied data members:
Example: auto.display();
The fact that you can assign an object belonging to a derived class to a base class object
assumes that more will always fill less. The object on the right of the assignment operator
will always contain a member object of the type on the left of the operator.
䊐Assignments to Derived Class Objects
This is not the case when you attempt to assign a base class object to an object of a
derived class. The assignment
Example: bmw = auto; // Error!
is therefore invalid, since the values for the additional data members passCarType and
sunRoof are unknown.
An assignment in reverse order is only possible if you have defined an assignment of
this type or a copy constructor with a parameter of the type “reference to base class.”
Both would be able to supply default values for the additional data members of the
derived classes.
534 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
carPtr cabrio
nr
producer
1001
"Triumph"
carType "Spitfire"
sunRoof true
■CONVERTING REFERENCES AND POINTERS
䊐Effect of a pointer assignment
PassCar cabrio("Spitfire", true, 1001, "Triumph");
Car* carPtr = &cabrio;
carPtr = &cabrio;
CONVERTING REFERENCES AND POINTERS
■535
䊐Converting to Base Class Pointers
The is relationship between a derived class and a base class is also apparent when refer-
ences and pointers are used. A pointer of the type “pointer to base class,” or base class
pointer for short, can reference an object of a derived class type.
Example: Car* carPtr = &cabrio;
In this case cabrio is an object of the class PassCar.
The following rule applies for access to the referenced object:
■a base class pointer can only access the public interface of the base class.
The additional members defined in the derived class are therefore inaccessible. To make
this more clear:
Example: carPtr -> display();
calls the display() method in the base class Car. Although carPtr points to an
object of the PassCar class in this case, it is impossible to call any methods additionally
defined in the derived class.
Example: carPtr->setSunRoof(false); // Error
The object *carPtr belongs to the Car class and only represents the generic part of
cabrio. Thus, the following assignment is also invalid
Example: PassCar auto;
auto = *carPtr; // Error!
although carPtr is pointing at an object of the PassCar type in this case!
䊐Conversions in References to Base Classes
A similar situation arises when you are working with references. A reference of the type
“reference to base class” can point to an object of a derived class. The reference will
address only the generic part of the object in this case.
Example: Car& carRef = cabrio; // ok
carRef.display(); // Output base members
carRef.setSunRoof(true); // Error
PassCar auto;
auto = carRef; // Error
Although the reference carRef points to an object of the PassCar type, it is impossi-
ble to assign the PassCar type object auto to this object.
536 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
Car
PassCar
carPtr
static_cast<PassCar*>(carPtr)
Pointer to
Base class
Derived
class
Downcast
Car
PassCar
static_cast<Car*>(PassCarPtr)
PassCarPtr
Pointer to
Base class
Derived
class
Upcast
■EXPLICIT TYPE CONVERSIONS
Downcast
Upcast
EXPLICIT TYPE CONVERSIONS
■537
䊐Upcasts and Downcasts
Type conversions that walk up a class hierarchy, or upcasts, are always possible and safe.
Upcasting is performed implicitly for this reason.
Type conversions that involve walking down the tree, or downcasts, can only be per-
formed explicitly by means of a cast construction. The cast operator (type), which was
available in C, or the static_cast< > operator are available for this task, and are
equivalent in this case.
䊐Explicit Cast Constructions
Given that cabrio is again an object of the derived class PassCar, the following state-
ments
Example: Car* carPtr = &cabrio;
( (PassCar*) carPtr )->display();
first point the base class pointer carPtr to the cabrio object. carPtr is then cast as a
pointer to the derived class. This allows you to access the display() method of the
derived class PassCar via the pointer. Parentheses are necessary in this case as the
member access operator -> has a higher precedence than the cast operator (type).
The operator static_cast< > conforms to the following
Syntax: static_cast<type>(expression)
and converts the expression to the target type type. The previous example is thus equiv-
alent to
Example: static_cast<PassCar*>(carPtr)->display();
No parentheses are required here as the operators static_cast<> and -> are of equal
precedence. They are read from left to right.
After downcasting a pointer or a reference, the entire public interface of the derived
class is accessible.
䊐Downcast Safety Issues
Type conversions from top to bottom need to be performed with great care. Downcasting
is only safe when the object referenced by the base class pointer really is a derived class
type. This also applies to references to base classes.
To allow safe downcasting C++ introduces the concept of dynamic casting. This tech-
nique is available for polymorphic classes and will be introduced in the next chapter.
exercise
538 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
Product
PrepackedFood FreshFood
■EXERCISE
Class hierarchy of products in a supermarket
EXERCISE
■539
Exercise
The class hierarchy representing a supermarket chain’s checkout system
comprises the base class Product and the derived classes PrepackedFood and
FreshFood.Your job is to test various cast techniques for this class (see also
Exercise 3 in Chapter 23).
■Define a global function isLowerCode() that determines which one of
two products has the lower barcode and returns a reference to the prod-
uct with the lower barcode.
■Define an array with three pointers to the base class Product. Dynami-
cally create one object each of the types Product,PrepackedFood, and
FreshFood.The three objects are to be referenced by the array pointers.
Additionally define a pointer to the derived class FreshFood. Initialize the
pointer with the address of a dynamically allocated object of the same
class.
■Now call the method printer() for all four objects.Which version of
printer() is executed?
■Perform downcasting to execute the correct version of printer() in
every case. Display the pointer values before and after downcasting.
■Use the pointer of the derived class FreshFood to call the base class ver-
sion of printer(). Perform an appropriate upcast.
■Test the function isLowerCode()by multiple calls to the function with
various arguments. Output the product with the lower barcode value in
each case.
solution
540 ■
CHAPTER 24 TYPE CONVERSION IN CLASS HIERARCHIES
■SOLUTION
// ----------------------------------------------------
// product.h : Defines the classes
// Product, PrepackedFood, and FreshFood
// ----------------------------------------------------
//
// Unchanged! See the previous chapter's solutions.
// ------------------------------------------------------
// produc_t.cpp
// Tests up and down casts for the classes
// Product, PrepackedFood, and FreshFood.
// ------------------------------------------------------
#include "product.h"
const Product& isLowerCode(const Product& p1,
const Product& p2);
int main()
{
Product* pv[3];
FreshFood* pu;
pv[0] = new Product(12345L, "Flour");
pv[1] = new PrepackedFood(0.49, 23456, "Salt");
pv[2] = new FreshFood(1.5, 1.69, 98765, "Grapes");
pu = new FreshFood(2.5, 2.69, 56789, "Peaches");
cout << "\nA fresh product: ";
pu->printer();
cout << "\nThe generic data of the other products:";
int i;
for(i=0; i < 3; ++i)
pv[i]->printer();
cin.get();
cout << "\nAnd now the downcast: " << endl;
static_cast<PrepackedFood*>(pv[1])->printer();
static_cast<FreshFood*>(pv[2])->printer();
cin.get();
cout << "\nAnd an upcast: " << endl;
static_cast<Product*>(pu)->printer();
SOLUTION
■541
cout << "\nNow compare the barcodes!" << endl;
cout << "\nIs barcode for flour or salt smaller?";
isLowerCode(*pv[0], *pv[1]).printer();
cout << "\nIs barcode for salt or grapes smaller?";
isLowerCode(*pv[1], *pv[2]).printer();
return 0;
}
const Product& isLowerCode(const Product& p1,
const Product& p2)
{
if(p1.getCode() < p2.getCode())
return p1;
else
return p2;
}
This page intentionally left blank
543
Polymorphism
This chapter describes how to develop and manage polymorphic classes.
In addition to defining virtual functions, dynamic downcasting in
polymorphic class hierarchies is introduced.
chapter
25
544 ■
CHAPTER 25 POLYMORPHISM
Classes with virtual methods:
Base class pointer and objects:
Calling virtual methods:
base class with virtual method display().
derived from Base with its own redefinitions
of method display().
When a virtual method is called, the corresponding version of the method is
executed for the object currently referenced.
Base:
Derived1 and Derived2:
Base* basePtr; // Base class pointer
Derived1 angular; // Objects
// Calling
// Derived1::display()
// Calling
// Derived2::display()
Derived2 round;
basePtr = &angular;
basePtr->display();
basePtr = &round;
basePtr->display();
■CONCEPT OF POLYMORPHISM
Example
CONCEPT OF POLYMORPHISM
■545
䊐Issues
If the special features of derived class objects are insignificant, you can simply concern
yourself with the base members. This is the case when dynamic allocated objects are
inserted into a data structure or deleted from that structure.
It makes sense to use pointers or references to the base class in this case—no matter
what type of concrete object you are dealing with. However, you can only access the
common base members of these objects.
However, you should be able to activate the special features of a derived class when
■the object is accessed by a pointer or reference to the base class and
■the concrete object type will not be known until the program is executed.
Given a base class pointer, carPtr, the statement
Example: carPtr->display();
should output all the data members of the object currently being referenced.
䊐Traditional Approach
Traditional programming languages solved this issue by adding a type field both to the
base class and to the derived classes. The type field stored the type of the current class. A
function that manages objects via the base class pointer could query the concrete type in
a switch statement and call the appropriate method.
This solution has a disadvantage; adding derived classes at a later stage also meant
adding a case label and recompiling.
䊐Object-Oriented Approach
The approach adopted by object-oriented languages is polymorphism (Greek for multi-
form). In C++, virtual methods are used to implement polymorphic classes. Calling a vir-
tual method makes the compiler execute a version of the method suitable for the object
in question, when the object is accessed by a pointer or a reference to the base class!
546 ■
CHAPTER 25 POLYMORPHISM
// virtual.cpp : Tests the virtual method display()
// of the classes Car and PassCar.
// ----------------------------------------------------
#include "car.h"
// The Car class with virtual method display():
// class Car
// {
// ...
// virtual void display() const;
// };
int main()
{
Car* pCar[3]; // Three pointers to the base class.
int i = 0; // Index
pCar[0] = new Car( 5634L, "Mercedes");
pCar[1] = new PassCar("Miata",true,3421,"Mazda");
pCar[2] = new Truck( 5, 7.5, 1234, "Ford");
while( true)
{
cout << "\nTo output an object of type "
"Car, PassCar or Truck!"
"\n 1 = Car, 2 = PassCar, 3 = Truck"
"\nYour input (break with 0): ";
cin >> i;
--i;
if( i < 0 || i > 2)
break;
pCar[i]->display();
}
return 0;
}
■VIRTUAL METHODS
Calling the virtual method display()
VIRTUAL METHODS
■547
䊐Declaring Virtual Methods
The virtual keyword is used to declare a virtual method in a base class.
Example: virtual void display() const;
The definition of a virtual method is no different from the definition of any other mem-
ber function.
A virtual method does not need to be redefined in the derived class. The derived class
then inherits the virtual method from the base class.
䊐Redefinition
However, it is common practice for the derived class to define its own version of the vir-
tual method, which is thus modified to suit the special features of the derived class.
Creating a proprietary version of a virtual method means redefining that method. The
redefinition in the derived class must have
1. the same signature and
2. the same return type
as the virtual method in the base class.
The new version of a virtual method is automatically virtual itself. This means you
can omit the virtual keyword in the declaration.
When you redefine a virtual function, be aware of the following:
■if the return type is a pointer or reference to the base class, the new version of the
virtual method can also return a pointer or reference to a derived class (Note:
Not all compilers support this option.)
■constructors cannot have a virtual declaration
■a base class method does not become virtual just because it is declared as virtual
in a derived class.
If you use a different signature or return type of a virtual base class method to define a
method in a derived class, this simply creates a new method with the same name. The
method will not necessarily be virtual!
However, the virtual method in the base class will be masked by the method in the
derived class. In other words, only the non-virtual version of the method is available for
a derived class object.
548 ■
CHAPTER 25 POLYMORPHISM
// v_destr.cpp
// Base class with a virtual destructor.
// -----------------------------------------------------
#include <iostream>
#include <cstring> // For strcpy()
using namespace std;
class Base
{
public:
Base()
{ cout << "Constructor of class Base\n"; }
virtual ~Base()
{ cout << "Destructor of class Base\n"; }
};
class Data
{
private:
char *name;
public:
Data( const char *n)
{ cout << "Constructor of class Data\n";
name = new char[strlen(n)+1]; strcpy(name, n);
}
~Data()
{ cout << "Destructor of class Data for "
<< "object: " << name << endl;
delete [] name;
}
};
class Derived : public Base
{
private:
Data data;
public:
Derived( const char *n) : data(n)
{ cout << "Constructor of class Derived\n"; }
~Derived() // implicit virtual
{ cout << "Destructor of class Derived\n"; }
};
int main()
{
Base *bPtr = new Derived("DEMO");
cout << "\nCall to the virtual Destructor!\n";
delete bPtr;
return 0;
}
■DESTROYING DYNAMICALLY ALLOCATED OBJECTS
Sample program
DESTROYING DYNAMICALLY ALLOCATED OBJECTS
■549
Dynamically created objects in a class hierarchy are normally handled by a base class
pointer. When such an object reaches the end of its lifetime, the memory occupied by
the object must be released by a delete statement.
Example: Car *carPtr;
carPtr = new PassCar("500",false,21,"Geo");
. . .
delete carPtr;
䊐Destructor Calls
When memory is released, the destructor for an object is automatically called. If multiple
constructors were called to create the object, the corresponding destructors are called in
reverse order. What does this mean for objects in derived classes? The destructor of the
derived class is called first and then the destructor of the base class executed.
If you use a base class pointer to manage an object, the appropriate virtual methods of
the derived class are called. However, non-virtual methods will always execute the base
class version.
In the previous example, only the base class destructor for Car was executed. As the
PassCar destructor is not called, neither is the destructor called for the data member
passCarType, which is additionally defined in the derived class. The data member
passCarType is a string, however, and occupies dynamically allocated memory—
this memory will not be released.
If multiple objects are created dynamically in the derived class, a dangerous situation
occurs. More and more unreferenced memory blocks will clutter up the main memory
without you being able to reallocate them—this can seriously impact your program’s
response and even lead to external memory being swapped in.
䊐Virtual Destructors
This issue can be solved simply by declaring virtual destructors. The opposite page shows
how you would define a virtual destructor for the Car class. Just like any other virtual
method, the appropriate version of the destructor will be executed. The destructors from
any direct or indirect base class then follow.
A class used as a base class for other classes should always have a virtual destructor
defined. Even if the base class does not need a destructor itself, it should at least contain
a dummy destructor, that is, a destructor with an empty function body.
550 ■
CHAPTER 25 POLYMORPHISM
VMT pointer
Object of type Car
Address of Car::display()
Address of Car::~Car()
VMT of Car
nr
producer
12345
Audi
VMT pointer
Address of
Object of type PassCar
VMT of PassCar
nr
producer
54321
Geo
passCarType
PassCar::display()
Address of
PassCar::~PassCar()
500
sunRoof true
VMT pointer
Object of type PassCar
nr
producer
98765
VW
passCarType GTI
sunRoof false
■VIRTUAL METHOD TABLE
VMT for the classes Car and PassCar
VIRTUAL METHOD TABLE
■551
䊐Static Binding
When a non-virtual method is called, the address of the function is known at time of
compilation. The address is inserted directly into the machine code. This is also referred
to as static or early binding.
If a virtual method is called via an object’s name, the appropriate version of this
method is also known at time of compilation. So this is also a case of early binding.
䊐Dynamic Binding
However, if a virtual method is called by a pointer or reference, the function that will be
executed when the program is run is unknown at time of compilation. The statement
Example: carPtr->display();
could execute different versions of the display() method, depending on the object
currently referenced by the pointer.
The compiler is therefore forced to create machine code that does not form an
association with a particular function until the program is run. This is referred to as late
or dynamic binding.
䊐VMT
Dynamic binding is supported internally by virtual method tables (or VMT for short). A
VMT is created for each class with at least one virtual method—that is, an array with the
addresses of the virtual methods in the current class.
Each object in a polymorphic class contains a VMT pointer, that is, a hidden pointer
to the VMT of the corresponding class. Dynamic binding causes the virtual function call
to be executed in two steps:
1. The pointer to the VMT in the referenced object is read.
2. The address of the virtual method is read in the VMT.
In comparison with static binding, dynamic binding does have the disadvantage that
VMTs occupy memory. Moreover, program response can be impacted by indirect
addressing of virtual methods.
However, this is a small price to pay for the benefits. Dynamic binding allows you to
enhance compiled source code without having access to the source code. This is particu-
larly important when you consider commercial class libraries, from which a user can
derive his or her own classes and virtual function versions.
552 ■
CHAPTER 25 POLYMORPHISM
// cast_t.cpp
// Dynamic casts in class hierarchies.
// ----------------------------------------------------
#include "car.h"
bool inspect( PassCar* ), // Inspection of
inspect(Truck* ); // car types.
bool separate(Car* ); // Separates cars
// for inspection.
int main()
{
Car* carPtr = new PassCar("520i", true, 3265, "BMW");
Truck* truckPtr = new Truck(8, 7.5, 5437, "Volvo");
// ... to test some casts and ...
separate(carPtr);
separate(truckPtr);
return 0;
}
bool separate( Car* carPtr)
{
PassCar* PassCarPtr = dynamic_cast<PassCar*>(carPtr);
if( PassCarPtr != NULL)
return inspect( PassCarPtr);
Truck* truckPtr = dynamic_cast<Truck*>(carPtr);
if( truckPtr != NULL)
return inspect( truckPtr);
return false;
}
bool inspect(PassCar* PassCarPtr)
{ cout << "\nI inspect a passenger car!" << endl;
cout << "\nHere it is:";
PassCarPtr->display();
return true;
}
bool inspect(Truck* truckPtr)
{ cout << "\nI inspect a truck!" << endl;
cout << "\nHere it is:";
truckPtr->display();
return true;
}
The compiler’s option “Run Time Type Information (RTTI)” must be activated, for example, under
Project/Settings. The GNU compiler activates these options automatically.
✓NOTE
■DYNAMIC CASTS
Using dynamic casts
DYNAMIC CASTS
■553
䊐Safety Issues in Downcasting
Downcasts in class hierarchies are unsafe if you use a C cast or the static cast operator. If
the referenced object does not correspond to the type of the derived class, fatal runtime
errors can occur.
Given that carPtr is a pointer to the base class Car, which is currently pointing to a
PassCar type, the statement
Example: Truck * truckPtr = static_cast<Truck*>(carPtr);
will not cause a compiler error. But the following statement,
truckPtr->setAxles(10); could cause the program to crash.
䊐The dynamic_cast<> Operator
You can use the cast operator dynamic_cast<> to perform safe downcasting in poly-
morphic classes. At runtime the operator checks whether the required conversion is
valid or not.
Syntax: dynamic_cast<type>(expression)
If so, the expression expression is converted to the target type type. The target type
must be a pointer or reference to a polymorphic class or a void pointer. If it is a pointer
type, expression must also be a pointer type. If the target type is a reference,
expression must identify an object in memory.
䊐Examples
Given a pointer carPtr to the base class Car, the statement
Example: Truck* truckPtr = dynamic_cast<Truck*>(carPtr);
performs a downcast to the derived Truck class, provided the pointer carPtr really
identifies a Truck type object. If this is not so, the dynamic_cast<Truck> operator
will return a NULL pointer.
Given that cabrio is a PassCar type object, the following statements
Example: Car& r_car = cabrio;
PassCar& r_passCar=dynamic_cast<PassCar&>(r_car);
perform a dynamic cast to the “reference to PassCar” type. In any other case, that is, if
the reference r_car does not identify a PassCar type object, an exception of the
bad_cast type is thrown.
The dynamic cast can also be used for upcasting. The classes involved do not need to
be polymorphic in this case. However, type checking is not performed at runtime. An
erroneous upcast is recognized and reported by the compiler.
exercises
554 ■
CHAPTER 25 POLYMORPHISM
* * * Car Rental Management * * *
P = Add a passenger car
T = Add a truck
D = Display all cars
Q = Quit
Your choice:
bool insert(const string& tp, bool sr,
long n, const string& prod);
Add a new passenger car:
bool insert(int a, double t, long n,
const string& prod);
Add a new truck:
■EXERCISES
Menu options
Different versions of method insert()
EXERCISES
■555
Exercise 1
Modify the vehicle management program to allow an automobile rental company
to manage its fleet of automobiles. First, define a class called CityCar that
contains an array of pointers to the 100 objects in the Car class.This also allows
you to store pointers to objects of the derived class types PassCar and Truck.
The objects themselves will be created dynamically at runtime.
■Define a class CityCar with an array of pointers to the Car class and an
int variable for the current number of elements in the array.
The constructor will set the current number of array elements to 0.
The destructor must release memory allocated dynamically for the
remaining objects. Make sure that you use a virtual destructor definition
in the base class Car to allow correct releasing of memory for trucks and
passenger vehicles.
Implement two versions of the insert() method using the prototype
shown opposite. Each version will allocate memory to an object of the
appropriate type—that is of the PassCar or Truck class—and use the
arguments passed to it for initialization.The method should return false
if it is impossible to enter another automobile (that is, if the array is full),
and true in all other cases.
The display() method outputs the data of all vehicles on screen.To
perform this task it calls the existing display() method for each object.
■Create a new function called menu() and store this function in a new
source file.The function will display the menu shown opposite, read, and
return the user’s choice.
■Additionally, write two functions, getPassCar() and getTruck(), which
read the data for a car or a truck from the keyboard and write the data
into the appropriate arguments.
■Create an object of the CityCar type in your main function. Insert one
car and one truck.These will be the first vehicles of the company’s fleet.
If a user chooses “Add car” or “Add truck,” your program must read the
data supplied and call the appropriate version of insert().
556 ■
CHAPTER 25 POLYMORPHISM
Please type next article?
0 = No more articles
1 = Fresh food
2 = Prepacked article
?
Another customer (y/n)?
If yes to record
Dialog with the receptionist
In function record()
Loop of main()
EXERCISES
■557
Exercise 2
An automatic checkout system for a supermarket chain needs to be completed.
■Declare the virtual methods scanner() and printer() in the base class
Product.Also define a virtual destructor.
■Write the record() function, which registers and lists products pur-
chased in the store in a program loop.
The function creates an array of 100 pointers to the base class, Product.
The checkout assistant is prompted to state whether a prepacked or
fresh food item is to be scanned next. Memory for each product scanned
is allocated dynamically and referenced by the next pointer in the array.
After scanning all the available items, a sequential list is displayed.The
prices of all the items are added and the total is output at the end.
■Now create an application program to simulate a supermarket checkout.
The checkout assistant is prompted in a loop to state whether to define a
new customer. If so, the record() function is called; if not, the program
terminates.
solutions
558 ■
CHAPTER 25 POLYMORPHISM
■SOLUTIONS
Exercise 1
// ----------------------------------------------------
// car.h : Defines the base class Car and
// the derived classes PassCar and Truck
// --------------------------------------------------------
#ifndef _CAR_H_
#define _CAR_H_
#include <iostream>
#include <string>
using namespace std;
class Car
{
private:
long nr;
string producer;
public:
Car( long n = 0L, const string& prod = "");
virtual ~Car() {} // Virtual destructor.
// Access methods:
long getNr(void) const { return nr; }
void setNr( long n ) { nr = n; }
const string& getProd() const { return producer; }
void setProd(const string& p){ producer = p; }
virtual void display() const; // Display a car
};
// The derived classes PassCar and Truck are unchanged
// (see Chapter 23).
#endif
// ------------------------------------------------------
// car.cpp
// Implements the methods of Car, PassCar, and Truck
// ------------------------------------------------------
// Unchanged (see Chapter 23).
//
SOLUTIONS
■559
// ------------------------------------------------------
// city.h : Defines the CityCar class
// ------------------------------------------------------
#ifndef _CITY_H_
#define _CITY_H_
#include "car.h"
class CityCar
{
private:
Car* vp[100];
int cnt;
public:
CityCar(){ cnt = 0;}
~CityCar();
bool insert(const string& tp, bool sr,
long n, const string& prod);
bool insert(int a, double t,
long n, const string& prod);
void display() const;
};
#endif // _CITY_H
// ------------------------------------------------------
// city.cpp : Methods of class CityCar
// ------------------------------------------------------
#include "city.h"
CityCar::~CityCar()
{
for(int i=0; i < cnt; ++i)
delete vp[i];
}
// Insert a passenger car:
bool CityCar::insert(const string& tp, bool sr,
long n, const string& prod)
{
if( cnt < 100)
{
vp[cnt++] = new PassCar( tp, sr, n, prod);
return true;
}
else
return false;
}
560 ■
CHAPTER 25 POLYMORPHISM
// Insert a truck:
bool CityCar::insert( int a, double t,
long n, const string& prod)
{
if( cnt < 100)
{
vp[cnt++] = new Truck( a, t, n, prod);
return true;
}
else
return false;
}
void CityCar::display() const
{
cin.sync(); cin.clear(); // No previous input
for(int i=0; i < cnt; ++i)
{
vp[i]->display();
if((i+1)%4 == 0) cin.get();
}
}
// --------------------------------------------------
// city_t.cpp : Test the CityCar class
// --------------------------------------------------
#include "city.h"
char menu(void);
void getPassCar(string&, bool&, long&, string&);
void getTruck(int&, double&, long&, string&);
int main()
{
CityCar carExpress;
string tp, prod; bool sr;
int a; long n; double t;
// Two cars are already present:
carExpress.insert(6, 9.5, 54321, "Ford");
carExpress.insert("A-class", true, 54320, "Mercedes");
char choice;
do
{ choice = menu();
switch( choice )
{
case 'Q':
case 'q': cout << "Bye Bye!" << endl;
break;
SOLUTIONS
■561
case 'P':
case 'p': getPassCar(tp, sr, n, prod);
carExpress.insert(tp, sr, n, prod );
break;
case 'T':
case 't': getTruck(a, t, n, prod);
carExpress.insert(a, t, n, prod);
break;
case 'D':
case 'd': carExpress.display();
cin.get();
break;
default: cout << "\a"; // Beep
break;
}
}while( choice != 'Q' && choice != 'q');
return 0;
}
char menu() // Input a command.
{
cout << "\n * * * Car Rental Management * * *\n\n"
char c;
cout << "\n P = Add a passenger car "
<< "\n T = Add a truck "
<< "\n D = Display all cars "
<< "\n Q = Quit the program "
<< "\n\nYour choice: ";
cin >> c;
return c;
}
void getPassCar(string& tp, bool& sr, long& n,string& prod)
{
char c;
cin.sync(); cin.clear();
cout << "\nEnter data for passenger car:" << endl;
cout << "Car type: "; getline(cin, tp);
cout << "Sun roof (y/n): "; cin >> c;
if(c == 'y' || c == 'Y')
sr = true;
else
sr = false;
cout << "Car number: "; cin >> n;
cin.sync();
cout << "Producer: "; getline(cin, prod);
cin.sync(); cin.clear();
}
562 ■
CHAPTER 25 POLYMORPHISM
void getTruck(int& a, double& t, long& n, string& prod)
{
cout << "\nInput data for truck:" << endl;
cout << "Number of axles: "; cin >> a;
cout << "Weight in tons: "; cin >> t;
cout << "Car number: "; cin >> n;
cin.sync();
cout << "Producer: "; getline(cin, prod);
cin.sync();
}
Exercise 2
// ----------------------------------------------------
// product.h : Defining the classes
// Product, PrepackedFood, and FreshFood
// ----------------------------------------------------
// . . .
class Product
{
private:
long bar;
string name;
public:
Product(long b = 0L, const string& s = "")
: bar(b), name(s)
{ }
// Access methods as previously used.
virtual void scanner(); // Virtual now!
virtual void printer() const;
};
// The classes PrepackedFood and FreshFood are unchanged!
// Refer to the solutions in Chapter 23.
SOLUTIONS
■563
// ------------------------------------------------------
// counter.cpp : Simulates a checkout desk
// ------------------------------------------------------
#include "product.h"
void record();
int main()
{
cout << "\nHere is a checkout desk!" << endl;
char c;
while(true)
{
cin.sync();
cout << "\nAnother customer (y/n)? ";
cin >> c;
if(c == 'y' || c == 'Y')
record();
else
break;
}
return 0;
}
// ------------------------------------------------------
// record() : Records the articles bought by a customer
// and the total price.
void record()
{
Product* v[100];
int x, i, cnt = 0;
double sum = 0.0;
for (i = 0; i < 100; i++)
{
cin.sync();
cout << "\nWhat is the next article?" << endl;
cout << " 0 = No more article\n"
<< " 1 = Fresh Food\n"
<< " 2 = Prepacked article\n"
<< "? " ;
cin >> x;
if( x <= 0 || x >= 3)
break;
564 ■
CHAPTER 25 POLYMORPHISM
switch(x)
{
case 2:
v[i] = new PrepackedFood;
v[i]->scanner();
sum += ((PrepackedFood*)v[i])->getPrice();
break;
case 1:
v[i] = new FreshFood;
v[i]->scanner();
sum += ((FreshFood*)v[i])->getPrice()
* ((FreshFood*)v[i])->getWght();
break;
}
}
cnt = i;
for( i=0; i < cnt; i++) // Output
v[i]->printer();
cout << "\n-----------------------------"
<< fixed << setprecision(2)
<< "\nTotal price: " << sum << endl;
}
565
Abstract Classes
This chapter describes how abstract classes can be created by defining
pure virtual methods and how you can use abstract classes as a
polymorphic interface for derived classes.To illustrate this we will be
implementing an inhomogeneous list, that is, a linked list whose elements
can be of various class types.
chapter
26
566 ■
CHAPTER 26 ABSTRACT CLASSES
// Coworker.h: Defining the abstract class Coworker.
// ----------------------------------------------------
#ifndef _COWORKER_H
#define _COWORKER_H
#include <string>
#include <iostream>
using namespace std;
class Coworker
{
private:
string name;
// more information
public:
Coworker( const string& s = ""){ name = s; }
virtual ~Coworker() {} // Destructor
const string& getName() const{ return name; }
void setName( const string& n){ name = n; }
virtual void display() const;
virtual double income() const = 0;
virtual Coworker& operator=(const Coworker&);
};
#endif
The virtual operator function for the assignment will be described in the section on ”Virtual
Assignments.”
✓NOTE
■PURE VIRTUAL METHODS
The base class Coworker
PURE VIRTUAL METHODS
■567
䊐Motivation
Virtual methods are declared in the base class to ensure that they are available in any
derived classes via the common class interface. It may happen that they rarely perform
any useful tasks in the base class. For example, a destructor in a base class does not need
to perform any explicit cleaning-up operations.
In this case, of course, you can define a virtual dummy method whose address is
entered in the VMT of the base class. However, this creates op-code for a function that
should never be called. It makes more sense not to define a function like this. And this is
where C++ steps in and gives you the opportunity of declaring pure virtual methods.
䊐Declaration
When a pure virtual method is declared, the method is identified by adding the expres-
sion = 0.
Example: virtual void demo()=0; // pure virtual
This informs the compiler that there is no definition of the demo() method in the class.
A NULL pointer is then entered in the virtual method table for the pure virtual method.
䊐The Base Class Coworker
The opposite page shows a definition of the Coworker class, which was designed to rep-
resent human resources data for a company. The class is used as a base class for various
employees, blue-collar, white-collar, and freelancers.
To keep things simple the Coworker class has only a name as a data member. How-
ever, it could also contain the address of an employee, or the division where the
employee works.
The Coworker class does not comprise data members to represent an employee’s
salary. It makes more sense to store data like this in derived classes where the hourly
wage and number of hours worked by a blue-collar worker and the monthly salary for a
white-collar worker are also defined. The income() method is therefore not defined for
the base class and can be declared as a pure virtual method.
568 ■
CHAPTER 26 ABSTRACT CLASSES
// coworker.h: Extending the headerfile.
// --------------------------------------------------
class Laborer : public Coworker
{
private:
double wages; // Pay per hour
int hr;
public:
Laborer(const string& s="", double w=0.0, int h=0)
: Coworker(s), wages(w), hr(h){ }
double getWages() const { return wages; }
void setWages( double w ){ wages = w; }
int getHr() const { return hr; }
void setHr(int h ) { hr = h; }
void display() const;
double income() const;
Laborer& operator=(const Coworker&);
Laborer& operator=(const Laborer&);
};
The operator functions for the assignments are discussed in the section ”Virtual Assignments.”
✓NOTE
■ABSTRACT AND CONCRETE CLASSES
The derived class Laborer
ABSTRACT AND CONCRETE CLASSES
■569
䊐Concrete or Abstract?
If a class comprises pure virtual methods, you cannot create objects of this class type.
Example: Coworker worker("Black , Michael");
The compiler will issue an error message here, as the Coworker class contains the
pure virtual method income(). This avoids calling a method for worker that still
needs to be defined.
A class that does not allow you to create any objects is referred to as an abstract class.
In contrast, a class that allows you to create objects is referred to as a concrete class.
䊐Deriving Abstract Classes
When a class is derived from an abstract class, it inherits all the methods the base class
contains, particularly the pure virtual methods. If all of these pure virtual methods are
implemented in the derived class, you can then create an object of the derived class type.
This means you need to implement the income() method in the Laborer class
shown opposite. Since the hourly wage and the number of hours worked are both defined
for blue-collar workers, it is possible to implement that method.
Example: double Laborer::income()
{
return ( wages * hr );
}
A class derived from a concrete class can again contain pure virtual methods, due to
additional definitions in the derived class. In other words, an abstract class can be
derived from a concrete class.
An abstract class does not necessarily need to contain pure virtual functions. If the
class contains a protected constructor, objects of the class type cannot be created.
The constructor can only be called then by methods in derived classes. A constructor of
this type normally acts as base initializer, when an object of a derived class type is cre-
ated.
A class with pure virtual methods is an abstract class.
✓NOTE
A class derived from a class containing pure virtual methods is a concrete class, if it contains a definition
for each pure virtual function.
✓NOTE
570 ■
CHAPTER 26 ABSTRACT CLASSES
// coworker.h: Extending the headerfile.
// --------------------------------------------------
class Employee : public Coworker
{
private:
double salary; // Pay per month
public:
Employee( const string& s="", double sa = 0.0)
: Coworker(s), salary(g){ }
double getSalary() const { return salary; }
void setSalary( double sa){ salary = sa; }
void display() const;
double income()const { return salary; }
Employee& operator=( const Coworker& );
Employee& operator=( const Employee& );
};
// coworker_t.cpp : Using the Coworker classes.
// -------------------------------------------------
#include "coworker.h"
int main()
{
Coworker* felPtr[2];
felPtr[0] = new Laborer("Young, Neil",45., 40);
felPtr[1] = new Employee("Smith, Eve", 3850.0);
for( int i = 0; i < 2; ++i)
{
felPtr[i]->display();
cout << "\nThe income of " << felPtr[i]->getName()
<< " : " << felPtr[i]->income() << endl;
}
delete felPtr[0]; delete felPtr[1];
return 0;
}
■POINTERS AND REFERENCES TO ABSTRACT CLASSES
The derived class Employee
Sample program
POINTERS AND REFERENCES TO ABSTRACT CLASSES
■571
Although you cannot define objects for abstract classes, you can declare pointers and ref-
erences to abstract classes.
Example: Coworker *felPtr, &coRef;
The pointer felPtr is a base class pointer that can address objects belonging to derived
concrete classes. The reference coRef can also address objects of this type.
䊐References to Abstract Base Classes
References to base classes are often used as parameters in functions. The copy construc-
tor in the Coworker class is just one of them.
Example: Coworker( const Coworker& );
The copy constructor expects an object belonging to a derived class, since the base class
is abstract.
The assignment in the Coworker class has a reference as a parameter and returns a
reference to the abstract class.
䊐Pointers to Abstract Base Classes
Pointers to base classes are generally used to address dynamically allocated objects. If the
base class is abstract, you can only allocate memory for objects belonging to derived, con-
crete classes.
Example: Coworker* felPtr;
felPtr = new Laborer("Young, Neil",45.,40);
cout << felPtr->income();
Since the income()method is virtual, a corresponding function found in the derived
class Laborer is executed.
䊐Polymorphic Interface
Defining pure virtual methods also determines interfaces for general operations, although
the interfaces still need to implemented in the derived classes. If a derived class contains
its own definition of a virtual method, this version will also be executed if an object is
referenced by a base class pointer or reference. Abstract classes are therefore also referred
to as polymorphic interfaces to derived classes.
The opposite page shows the definition of the Employee class, which was also
derived from the abstract class Coworker. The operator functions for the assignments
are discussed and implemented in the following section.
572 ■
CHAPTER 26 ABSTRACT CLASSES
// Virtual Assignment in the base class
Coworker& operator=(const Coworker & m)
{
if( this != &m ) // No self assignment
name = m.name;
return *this;
}
Redefining the virtual operator function operator=(), which returns a reference to the derived class,
is not yet supported by all compilers. In this case the return type must be a reference to the base class
Coworker.
✓NOTE
// Redefinition: virtual
Employee& operator=(const Coworker& m)
{
if( this != &m ) // No self assignment
{
Coworker::operator=( m );
salary = 0.0;
}
return *this;
}
// Standard Assignment: not virtual
Employee& operator=(const Employee& a)
{
if( this != &a )
{
Coworker::operator=( a );
salary = a.salary;
}
return *this;
}
■VIRTUAL ASSIGNMENT
Assignment for class Coworker
Assignments for class Employee
VIRTUAL ASSIGNMENT
■573
䊐Virtual Operator Functions
Operator functions implemented as methods can also be virtual. In this case, you can
ensure that the right version of an operator function will be executed when using a
pointer or reference to a base class to address a derived class object.
One example of this is the operator function for an assignment. If the function decla-
ration is not virtual, and if the function is called via a base class pointer, only the base
data of the object is overwritten. Any additional data members of the derived class
remain unchanged.
䊐Using Virtual Assignments
The assignment was declared virtual in the Coworker base class. The derived classes
Laborer and Employee both contain their own versions. Thus, in the following
Example: void cpy(Coworker& a,const Coworker& b)
{ a = b; }
the assignment of the Employee class is executed, if an object of this class type is the
first argument passed to it. If the object is a Laborer type, the assignment of the
Laborer class is performed.
In the case of the cpy()function, you can therefore assign two objects of any class,
including classes derived at a later stage, without having to modify the function itself!
However, you definitely need to define a version of the assignment for each derived class.
䊐Redefining the Standard Assignment
When you define a new version for a virtual method in a derived class, this implies using
the signature of the original method. Since the standard assignment of a derived class has
a signature of its own, it is not virtual. The standard assignment for the Laborer class
has the following prototype:
Example: Laborer& operator=(const Laborer&);
The type const Laborer& is different from the const Coworker& type of the
parameter in the virtual operator function of the base class. The standard assignment
thus masks the virtual assignment in the base class. This gives rise to two issues:
■the virtual operator function for the assignment must be defined for every derived
class
■to ensure that the standard assignment is also available, the standard assignment
must also be redefined in every derived class.
574 ■
CHAPTER 26 ABSTRACT CLASSES
// cell.h: Defining the classes Cell, BaseEl, and DerivedEl
// --------------------------------------------------------
#ifndef _CELL_
#define _CELL_
#include <string>
#include <iostream>
using namespace std;
class Cell
{
private:
Cell* next;
protected:
Cell(Cell* suc = NULL ){ next = suc; }
public:
virtual ~Cell(){ }
// Access methods to be declared here.
virtual void display() const = 0;
};
class BaseEl : public Cell
{
private:
string name;
public:
BaseEl( Cell* suc = NULL, const string& s = "")
: Cell(suc), name(s){}
// Access methods would be declared here.
void display() const;
};
class DerivedEl : public BaseEl
{
private:
string rem;
public:
DerivedEl(Cell* suc = NULL,const string& s="",
const string& b="")
: BaseEl(suc, s), rem(b) { }
// Access methods would be declared here.
void display() const;
};
#endif
■APPLICATION: INHOMOGENEOUS LISTS
The abstract base class Cell and derived classes
APPLICATION: INHOMOGENEOUS LISTS
■575
1st list element
Info Info
Pointer
2nd list element 3rd list element
Info
Pointer Pointer
䊐Terminology
Let’s look into implementing an application that uses an inhomogeneous list. An inho-
mogeneous list is a linear list whose elements can be of different types. If the data you
need to store consists of objects in a class hierarchy, one list element could contain an
object belonging to the base class, whereas another could contain an object of a derived
class.
Due to implicit type conversions in class hierarchies, you can use the base class point-
ers to manage the list elements, that is, you can manage the elements in a linked list.
The following graphic illustrates this scenario:
䊐Representing List Elements
To separate the management of list elements from the information contained in the list,
we have defined an abstract class called Cell as a base class for all list elements. The
class contains a pointer of type Cell* as the data member used to link list elements.
Since Cell type objects are not be created, the constructor in the Cell class has a
protected declaration.
The Cell class does not contain any data that might need to be output. However,
each class derived from Cell contains data that need to be displayed. For this reason,
Cell contains a declaration of the pure virtual method display(), which can be mod-
ified for multiple derivations.
The classes BaseEl and DerivedEl, which are derived from Cell, represent list
elements used for storing information. To keep things simple, the BaseEl class contains
only a name, and the DerivedEl class additionally contains a comment. The public
declaration section contains a constructor and access method declarations. In addition, a
suitable version of the display() method is defined. Both classes are thus concrete
classes.
576 ■
CHAPTER 26 ABSTRACT CLASSES
Info Info
Pointer
New
element:
Info
Pointer Pointer
Info
// List.h: Defining the class InhomList
// ---------------------------------------------
#ifndef _LIST_H
#define _LIST_H
#include "cell.h"
class InhomList
{private:
Cell* first;
protected:
Cell* getPrev(const string& s);
void insertAfter(const string& s,Cell* prev);
void insertAfter(const string& s,
const string& b,Cell* prev);
public: // Constructor, Destructor etc....
void insert(const string& n);
void insert(const string& n, const string& b);
void displayAll() const;
};
#endif
void InhomList::insertAfter(const string& s, Cell* prev)
{
if( prev == NULL ) // Insert at the beginning,
first = new BaseEl( first, s);
else // middle, or end.
{
Cell* p = new BaseEl(prev->getNext(), s);
prev->setNext(p);
}
}
■IMPLEMENTING AN INHOMOGENEOUS LIST
Defining class InhomList
Inserting a list element in the middle
Definition of insertAfter() version
IMPLEMENTING AN INHOMOGENEOUS LIST
■577
䊐The InhomList Class
The inhomogeneous list must allow sorted insertion of list elements. It is no longer suffi-
cient to append elements to the end of the list; instead, the list must allow insertion at
any given point. A pointer to the first element in the list is all you need for this task. You
can then use the pointer to the next list element to access any given list element.
The definition of the InhomList class is shown opposite. A pointer to Cell has
been declared as a data member. The constructor has very little to do. It simply sets the
base class pointer to NULL, thus creating an empty list.
The list will be sorted by name. When inserting a new element into the list, the inser-
tion point—that is the position of the element that will precede the new element—must
first be located. In our example, we first locate the immediate lexicographical predeces-
sor. The getPrev() method, shown opposite, performs the search and returns either
the position of the predecessor or NULL if there is no predecessor. In this case, the new
list element is inserted as the first element in the list.
䊐Inserting a New List Element
After finding the insertion position you can call the insertAfter() method that allo-
cates memory for a new list element and inserts the element into the list. There are two
cases that need to be looked at:
1. If the new element has to be inserted at the start of the list, what was originally
the first element now becomes the second. The new element becomes the first
element. The first pointer thus needs updating.
2. If the new element is inserted at any other position in the list, the first pointer
is not affected. Instead, you have to modify two pointers. The pointer in the pre-
ceding list element must be pointed at the new element and the pointer in the
new element must be pointed at what was formerly the successor of the preceding
element. This situation also applies in cases where the successor was a NULL
pointer, in other words, when the new element is appended to the list.
Since the list contains objects of the BaseEl and DerivedEl types, the
insertAfter() method has been overloaded with two versions. They differ only in
different calls to the new operator.
The insert() method was overloaded for the same reason. Both versions first call
the getPrev() method and the corresponding version of the insertAfter()
method.
exercise
578 ■
CHAPTER 26 ABSTRACT CLASSES
class InhomList
{
private:
Cell* first;
protected:
Cell* getPrev(const string& s);
Cell* getPos(const string& s);
void insertAfter(const string& s, Cell* prev);
void insertAfter(const string& s,const string& b,
Cell* prev);
void erasePos(Cell* pos);
public:
InhomList(){ first = NULL; }
InhomList(const InhomList& src);
~InhomList();
InhomList& operator=( const InhomList& src);
void insert(const string& n);
void insert(const string& n, const string& b);
void erase(const string& n);
void displayAll() const;
};
■EXERCISE
The complete class InhomList
EXERCISE
■579
Exercise
Modify and complete the definition of the class InhomList, which represents an
inhomogeneous list.
■Write the destructor for the InhomList class.The destructor releases
the memory occupied by the remaining list elements.
■Implement the getPrev() method and both versions of the insert()
and insertAfter() methods.The algorithm needed for inserting list ele-
ments was described in the section “Implementing an Inhomogeneous
List.”
■Implement the displayAll() method, which walks through the list
sequentially, outputting each element.
■Test insertion and output of list elements. Check whether the comments
on the objects are output, if present.
■Define the getPos() method, which locates the position of an element
to be deleted. If the element is in the list, its address is returned. Other-
wise a NULL pointer is returned.
■Write the erasePos() method, which deletes a list element at a given
position. Pay attention to whether the element to be deleted is the first
or any other element in the list. Since the destructor for Cell was
declared virtual, only one version of the deletePos() method is neces-
sary.
■Define the erase() method, which deletes a list element with a given
name from the list.
■Test deletion of list elements. Continually display the remaining elements
in the list to be certain.
■Now implement the copy constructor and assignment. Use the insert()
to construct the list, calling the applicable version of the method.You can
call the typeid() operator to ascertain the type of the list element
currently to be inserted.The operator is declared in the header file
typeinfo.
Example: if( typeid(*ptr) == typeid(DerivedEl)) ...
The expression is true if ptr references a DerivedEl type object.
■Then test the copy constructor and the assignment
solution
580 ■
CHAPTER 26 ABSTRACT CLASSES
■SOLUTION
// ------------------------------------------------------
// cell.h
// Defines the classes Cell, BaseEl, and DerivedEl.
// ------------------------------------------------------
#ifndef _CELL_
#define _CELL_
#include <string>
#include <iostream>
using namespace std;
class Cell
{
private:
Cell* next;
protected:
Cell(Cell* suc = NULL ){ next = suc; }
public:
virtual ~Cell(){ }
Cell* getNext() const { return next; }
void setNext(Cell* suc) { next = suc; }
virtual void display() const = 0;
};
class BaseEl : public Cell
{
private:
string name;
public:
BaseEl( Cell* suc = NULL, const string& s = "")
: Cell(suc), name(s){}
// Access methods:
void setName(const string& s){ name = s; }
const string& getName() const { return name; }
void display() const
{
cout << "\n--------------------------------"
<< "\nName: " << name << endl;
}
};
SOLUTION
■581
class DerivedEl : public BaseEl
{
private:
string rem;
public:
DerivedEl(Cell* suc = NULL,
const string& s="", const string& b="")
: BaseEl(suc, s), rem(b){ }
// Access methods:
void setRem(const string& b){ rem = b; }
const string& getRem() const { return rem; }
void display() const
{
BaseEl::display();
cout << "Remark: " << rem << endl;
}
};
#endif
// ------------------------------------------------------
// List.h : Defines the class InhomList
// ------------------------------------------------------
#ifndef _LIST_H_
#define _LIST_H_
#include "cell.h"
class InhomList
{
private:
Cell* first;
protected:
Cell* getPrev(const string& s);
Cell* getPos( const string& s);
void insertAfter(const string& s, Cell* prev);
void insertAfter(const string& s,const string& b,
Cell* prev);
void erasePos(Cell* pos);
public:
InhomList(){ first = NULL; }
InhomList(const InhomList& src);
~InhomList();
InhomList& operator=( const InhomList& src);
void insert(const string& n);
void insert(const string& n, const string& b);
void erase(const string& s);
void displayAll() const;
};
#endif
582 ■
CHAPTER 26 ABSTRACT CLASSES
// ------------------------------------------------------
// List.cpp : The methods of class InhomList
// ------------------------------------------------------
#include "List.h"
#include <typeinfo>
// Copy constructor:
InhomList::InhomList(const InhomList& src)
{
// Append the elements from src to the empty list.
first = NULL;
Cell *pEl = src.first;
for( ; pEl != NULL; pEl = pEl->getNext() )
if(typeid(*pEl) == typeid(DerivedEl))
insert(dynamic_cast<DerivedEl*>(pEl)->getName(),
dynamic_cast<DerivedEl*>(pEl)->getRem());
else
insert(dynamic_cast<BaseEl*>(pEl)->getName());
}
// Assignment:
InhomList& InhomList::operator=(const InhomList& src)
{
// To free storage for all elements:
Cell *pEl = first,
*next = NULL;
while( pEl != NULL )
{
next = pEl->getNext();
delete pEl;
pEl = next;
}
first = NULL; // Empty list
// Copy the elements from src to the empty list.
pEl = src.first;
for( ; pEl != NULL; pEl = pEl->getNext() )
if(typeid(*pEl) == typeid(DerivedEl))
insert(dynamic_cast<DerivedEl*>(pEl)->getName(),
dynamic_cast<DerivedEl*>(pEl)->getRem());
else
insert(dynamic_cast<BaseEl*>(pEl)->getName());
return *this;
}
SOLUTION
■583
// Destructor:
InhomList::~InhomList()
{
Cell *pEl = first,
*next = NULL;
while( pEl != NULL )
{
next = pEl->getNext();
delete pEl;
pEl = next;
}
}
Cell* InhomList::getPrev(const string& n)
{
Cell *pEl = first,
*prev = NULL;
while( pEl != NULL )
{
if( n > dynamic_cast<BaseEl*>(pEl)->getName() )
{
prev = pEl; pEl = pEl->getNext();
}
else
return prev;
}
return prev;
}
Cell* InhomList::getPos( const string& n)
{
Cell *pEl = first;
while( pEl != NULL &&
(n != dynamic_cast<BaseEl*>(pEl)->getName()))
pEl = pEl->getNext();
if( pEl != NULL &&
n == dynamic_cast<BaseEl*>(pEl)->getName())
return pEl;
else
return NULL;
}
void InhomList::insertAfter(const string& s, Cell* prev)
{
if( prev == NULL ) // Insert at the beginning:
first = new BaseEl( first, s);
else // In the middle or at the end:
{ Cell* p = new BaseEl(prev->getNext(), s);
prev->setNext(p);
}
}
584 ■
CHAPTER 26 ABSTRACT CLASSES
void InhomList::insertAfter( const string& s,
const string& b, Cell* prev)
{
if( prev == NULL ) // Insert at the beginning:
first = new DerivedEl( first, s, b);
else // In the middle or at the end:
{
Cell* p = new DerivedEl(prev->getNext(), s, b);
prev->setNext(p);
}
}
void InhomList::insert(const string& n)
{
Cell* pEl = getPrev(n);
insertAfter(n, pEl);
}
void InhomList::insert(const string& n, const string& b)
{
Cell* pEl = getPrev(n);
insertAfter(n, b, pEl);
}
void InhomList::erasePos(Cell* pos)
{
Cell* temp;
if( pos != NULL)
if( pos == first ) // Delete the first element
{
temp = first;
first = first->getNext();
delete temp;
}
else // Delete from the middle or at the end
{ // Get the predecessor
temp = getPrev( dynamic_cast<BaseEl*>(pos)
->getName());
if(temp != NULL) // and bend pointer.
temp->setNext(pos->getNext());
delete pos;
}
}
void InhomList::erase(const string& n)
{
erasePos( getPos(n));
}
SOLUTION
■585
void InhomList::displayAll() const
{
Cell* pEl = first;
while(pEl != NULL)
{
pEl->display();
pEl = pEl->getNext();
}
}
// ------------------------------------------------------
// List_t.cpp : Tests the sorted inhomogeneous list
// ------------------------------------------------------
#include "List.h"
int main()
{
InhomList liste1;
cout << "\nTo test inserting. " << endl;
liste1.insert("Bully, Max");
liste1.insert("Cheers, Rita", "always merry");
liste1.insert("Quick, John", "topfit");
liste1.insert("Banderas, Antonio");
liste1.displayAll(); cin.get();
cout << "\nTo test deleting. " << endl;
liste1.erase("Banderas, Antonio");
liste1.erase("Quick, John");
liste1.erase("Cheers, Rita");
liste1.displayAll(); cin.get();
cout << "\n----------------------------------"
<< "\nGenerate a copy and insert an element. "
<< endl;
InhomList liste2(liste1), // Copy constructor
liste3; // and an empty list.
liste2.insert("Chipper, Peter", "in good temper");
liste3 = liste2; // Assignment
cout << "\nAfter the assignment: " << endl;
liste3.displayAll();
return 0;
}
This page intentionally left blank
587
Multiple Inheritance
This chapter describes how new classes are created by multiple
inheritance and explains their uses. Besides introducing you to creating
and destroying objects in multiply-derived classes, virtual base classes are
depicted to avoid ambiguity in multiple inheritance.
chapter
27
588 ■
CHAPTER 27 MULTIPLE INHERITANCE
class MotorHome : public Car, public Home
{
private:
// Additional private members here
protected:
// Additional protected members here
public:
// Additional public members here
};
Car Home
MotorHome
■MULTIPLY-DERIVED CLASSES
The multiply-derived class MotorHome
Definition scheme for class MotorHome
MULTIPLY-DERIVED CLASSES
■589
A class can contain not just one but several different base classes. In this case the class is
derived from multiple base classes in a process known as multiple inheritance.
䊐The Multiply-Derived Class MotorHome
This class Car is used to represent vehicles and the class Home contains characteristic
values for an apartment, such as floor space, number and type of rooms, and typical oper-
ations, such as building, selling, or renting.
Using these two classes you can then derive the MotorHome class. The opposite page
shows the inheritance and definition schemes for the new class. An object of the
MotorHome class contains both the members of Car and the members of Home. More
specifically, the object contains two base sub-objects of type Car and Home.
䊐Accessibility of Base Classes
Since the MotorHome class has two public base classes, it assumes the public inter-
faces of both classes. A MotorHome type object not only allows access to the additional
public members but to all the public members of the base classes Car and Home.
When defining a multiply-derived class, the accessibility, private,protected, or
public, must be defined separately for each base class. The MotorHome class could
have the public base class Car and the protected base class Home.
Example: class MotorHome:public Car,protected Home
{ . . . };
If the keyword is omitted, the base class will default to private.
Example: class MotorHome : public Car, Home
{ . . . };
This statement defines the public base class Car and the private base class Home.
This makes all the public members in Home private members of the derived class.
In multiple inheritance each public base class establishes an is relationship. This is
similar to simple inheritance. If the MotorHome class inherits two public base classes,
a motor-home is a special kind of motor vehicle and a special kind of home.
590 ■
CHAPTER 27 MULTIPLE INHERITANCE
Car Car
PassCar Van
SUV
class SUV : public PassCar, public Van
{
// Here are additional methods and data members
};
■MULTIPLE INDIRECT BASE CLASSES
The multiple indirect base class Car
Definition scheme of class SUV
MULTIPLE INDIRECT BASE CLASSES
■591
䊐Multiple Identical Base Classes
When multiply-derived classes are defined, a direct base class cannot be inherited more
than once. The following statement
Example: class B : public A, public A // Error
{ . . . };
causes the compiler to issue an error message.
A class can be derived from several classes that have a common base class, however.
This class is then referred to as a multiple indirect base class.
The inheritance graph on the opposite page shows the multiply-derived class SUV,
which was derived from the classes PassCar and Van. Both base classes were them-
selves derived from the Car class. This makes Car a multiple indirect base class of the
SUV class.
䊐Ambiguity
An object of the SUV class thus contains the members of Car twice. Access to members
of the Car class results in ambiguity.
Example: SUV mySUV(. . .);
cout << mySUV.getProd(); // Error
Both the base classes PassCar and Van contain a method called getProd(), which
they both inherited from the Car class. In this case the compiler cannot decide which
method is meant.
Ambiguity in the context of multiple inheritance is also possible when several base
classes contain members with identical names. If both the Home class and the Car class
contain a method called getNr(), the getNr() method cannot be correctly identified
in the following statement.
Example: MotorHome motorHome( . . .);
motorHome.getNr();
To resolve ambiguity of this kind, you can use the scope resolution operator to determine
which base class is meant.
Example: cout << motorHome.Home::getNr();
cout << mySUV.PassCar::getProd();
The getNr() method in the Home class is called first, followed by the getProd()
method inherited by PassCar from the Car class.
592 ■
CHAPTER 27 MULTIPLE INHERITANCE
Car
PassCar Van
SUV
class PassCar : public virtual Car
{
// Here are additional members
// of class PassCar
};
class Van : public virtual Car
{
// Here are additional members
// of class Van
};
■VIRTUAL BASE CLASSES
The virtual base class Car
Definition scheme
VIRTUAL BASE CLASSES
■593
䊐Issues
You will not normally want a class created by multiple inheritance to contain multiple
instances of an indirect base class. Why should a station wagon contain two versions of
the manufacturer’s name or the chassis number for example? So you might be asking
yourself whether you can define multiply-derived classes that will contain only one
instance of an indirect base class.
C++ uses virtual base classes to do this. An object in a multiply-derived class contains
only one instance of the members in a virtual base class. The inheritance graph on the
opposite page uses the SUV class to illustrate this situation.
䊐Declaration
A direct base class is declared virtual when a derived class is defined. You can use the
virtual keyword, which directly precedes the name of the base class.
In the definition scheme shown opposite, the Car class becomes the virtual base class
of PassCar and Van. However, the fact that the base class Car is virtual has no signifi-
cance at this point.
A virtual base class takes effect in cases of multiple inheritance. The following defi-
nition
Example: class SUV
: public PassCar, public Van
{ . . . };
ensures that the SUV class only contains one instance of the virtual base class Car. An
object my of the SUV class gets sufficient memory for only one Car class sub-object.
More specifically, the statement
Example: cout<<"Producer: " << mySUV.getProd();
does not cause ambiguity.
The following items are important with respect to virtual base classes:
■a virtual base class stays virtual even if further derivations are built. Each class
derived from PassCar also has the Car class as a virtual base class.
■you cannot change the declaration of an indirect base class to virtual.
You must therefore decide what classes are to be declared virtual when you design the
class hierarchy. Later modifications will require modifications to the source code of any
derived classes.
594 ■
CHAPTER 27 MULTIPLE INHERITANCE
Base1 Base2 Base3
MultiDerived
class multiDerived : public Base1, public Base2,
public Base3
{
// Here are additional data members and methods
};
■CONSTRUCTOR CALLS
䊐Building an inheritance graph
Class Definition
Inheritance graph
CONSTRUCTOR CALLS
■595
䊐Initialization
When an object is created in a simply-derived class, the sub-objects of the base classes
are created first on all levels of the class hierarchy. The sub-object whose class is nearer
to the top of the inheritance graph is created first.
The order of the constructor calls is “top down” and follows the inheritance graph.
The activation order used for the constructors in simple inheritance has been generalized
for multiple inheritance.
䊐Inheritance Graph
Again, the inheritance graph, also called sub-object lattice, has an important job to do.
When a derived class is defined, the following rules apply:
■In cases of multiple inheritance, base classes are entered into the inheritance
graph from left to right in the order in which they were stated when the class was
defined. The graph opposite illustrates this point.
If the class hierarchy does not contain any virtual base classes, the following applies to
the activation order of the constructors.
■The base class constructors are executed first, top-down and from left to right on
each level.
■Finally, the constructor belonging to the current class, which is at the bottom of
the inheritance graph, is executed.
If we look at the example on the opposite page, this means that the sub-objects of the
base classes Base1,Base2, and Base3 are created in this order. Then the constructor
of MultiDerived is executed.
䊐Base Initializers
The constructor for the class at the bottom end of the inheritance graph uses base initial-
izers to pass the values to the direct and indirect base classes. If the base initializer defini-
tion is missing in a constructor definition, the default constructor of the base class is
automatically executed.
Initial values are thus passed to the base class constructors “bottom up.”
596 ■
CHAPTER 27 MULTIPLE INHERITANCE
class SUV : public PassCar, public Van
{
private:
// . . .
public:
SUV( ... ) : Car( ... )
{
// Initialize additional data members
}
void display() const
{
PassCar::display();
Van::display();
// Output additional data members
}
};
■INITIALIZING VIRTUAL BASE CLASSES
Class SUV
INITIALIZING VIRTUAL BASE CLASSES
■597
䊐Constructor Calls in Virtual Base Classes
When an object is created for a multiply-derived class, the constructors of the base
classes are called first. However, if there is one virtual base class in the class hierarchy,
the virtual base class constructor is executed before a constructor of a non-virtual base
class is called.
The constructors of the virtual base classes are called first, followed by the constructors of non-virtual
base classes in the order defined in the inheritance graph.
✓NOTE
The constructor of a virtual base class is called with the arguments stated for the base initializer of the
last class to be derived, i.e. class at the bottom end of the inheritance graph.
✓NOTE
The constructor of the virtual base class nearest the top of the inheritance graph is
executed first. This does not necessarily mean the top level of the class hierarchy, since a
virtual base class can be derived from a non-virtual base class.
In our example with the multiply-derived class SUV (Sport Utility Vehicle) the con-
structor for the virtual base class Car is called first, followed by the direct base classes
PassCar and Van, and last but not least, the constructor of the SUV class.
䊐Base Initializers
You may be wondering what arguments are used to call the constructor of a virtual base
class. A base initializer of the directly-derived class or any other derivation could be
responsible. The following applies:
The example opposite shows SUV containing a constructor with one base initializer.
Its arguments are passed to the constructor of the virtual base class Car.
For the purpose of initialization, it does not matter whether a class derived directly
from Car contains a base initializer or not. Base initializers for virtual indirect base
classes defined in the constructor of a direct base class are ignored. If the base classes
PassCar and Van also contained base initializers for the virtual base class Car, these
would be ignored too.
If the constructor for the last derived class does not contain a base initializer, the
default constructor is executed for each virtual base class. Whatever happens, a default
constructor must then exist in every virtual base class! Thus, base initializers that happen
to exist in base classes are also ignored.
exercises
598 ■
CHAPTER 27 MULTIPLE INHERITANCE
Data member: Type
cat CATEGORY
Methods:
MotorHome(CATEGORY, long, const string&, int, double );
void setCategory(CATEGORY )
CATEGORY getCategory() const ;
void display() const;
Car Home
MotorHome
■EXERCISES
The multiply-derived class MotorHome
Additional members of class MotorHome
EXERCISES
■599
Exercise 1
The multiply-derived class MotorHome needs to be fully implemented and tested.
■Define an enumeration type CATEGORY that represents the categories
“Luxury,” “First Class,” “Middle Class,” and “Economy”.
■Develop a class called Home with two data members used to store the
number of rooms and the size in square meters. Supply default values for
your constructor definition to create a default constructor. In addition to
access methods, also define the method display(), which outputs the
data members of an apartment.
■Define a class derived from the Car and Home classes called MotorHome,
which is used to represent motorhomes. Inheritance of public base
classes is used.The MotorHome class contains a new data member used to
store one value of the CATEGORY type. In addition to defining a construc-
tor with default values, also define appropriate access methods and a
display() method for output.
Place your definitions of the Home and MotorHome classes in a separate
header file, which includes the existing header file car.h.
■Write a main function that first fully initializes a MotorHome type object
and then outputs the object.
Then create a second instance of the MotorHome type without initial val-
ues and display the object on screen. Call all the set methods in the
MotorHome class and its base classes to set your own values for the
objects.Then output the object once more.
600 ■
CHAPTER 27 MULTIPLE INHERITANCE
Class hierarchy for the multiply-derived class SUV
Car
Data members:
car number
producer
SUV
Data members:
number of seats
PassCar
Data members:
car type
sun roof (y/n)
Van
Data members:
capacity (lbs)
EXERCISES
■601
Exercise 2
Now fully define the SUV class for testing virtual base classes.
■Change the definition of the PassCar class in the car.h header file to
make Car a virtual base class of the PassCar class.
■Then define the Van class using the Car class as a virtual base class.The
new class should contain an additional data member used to represent
the payload of the van in kilograms.A maximum of 750 kg applies to vans.
The constructor should use default values to initialize the data members
with defaults, thus providing a default constructor for the class.A maxi-
mum value of 750 applies for the payload. In addition to the access
method display(), you still need to define methods for screen output.
■Create the class SUV, which is derived from PassCar and Van, to repre-
sent a station wagon. Store the number of seats available in the station
wagon as a data member.
The constructor in the SUV class should use the base initializer to set all
the values of a station wagon using default values for every data member.
Additionally, define access methods and a display() to define output.
■Use a main function to test the SUV class; the function should create sta-
tion wagons with and without default values and display them on screen.
solutions
602 ■
CHAPTER 27 MULTIPLE INHERITANCE
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// car.h : Definition of base class Car and
// derived classes PassCar and Truck
// ------------------------------------------------------
// car.cpp
// Implementing methods of Car, PassCar, and Truck
// ------------------------------------------------------
//
// These files have been left unchanged
// from Chapters 23 and 25.
//
// ------------------------------------------------------
// motorHome.h : Definition of the class Home and the
// multiply-derived class MotorHome
// ------------------------------------------------------
#ifndef _MOTORHOME_H_
#define _MOTORHOME_H_
#include "car.h"
#include <iomanip>
#include <iostream>
using namespace std;
enum CATEGORY {LUXURY, FIRSTCLASS, SECONDCLASS, ECONOMY};
class Home
{
private:
int room;
double ft2;
public:
Home(int r = 0, double m2 = 0.0)
{ room = r; ft2 = m2;}
void setRoom(int n){ room = n;}
int getRoom() const { return room; }
void setSquareFeet(double m2){ ft2 = m2;}
double getSquareFeet() const { return ft2; }
void display() const
{
cout << "Number of rooms: " << room
<< "\nSquare feet: "
<< fixed << setprecision(2) << ft2 << endl;
}
};
SOLUTIONS
■603
class MotorHome : public Car, public Home
{
private:
CATEGORY cat;
public:
MotorHome(long n=0L, const string& prod="", int ro=0,
double m2=0.0, CATEGORY k=ECONOMY)
: Car(n, prod), Home(ro, m2), cat(k)
{}
void setCategory(CATEGORY c){cat = c;}
CATEGORY getCategory() const { return cat;}
void display() const
{
cout << "\nMotorHome: ";
Car::display();
Home::display();
cout << "Category: ";
switch(cat)
{
case LUXURY: cout << " Luxury";
break;
case FIRSTCLASS: cout << " First class";
break;
case SECONDCLASS: cout << " Second class";
break;
case ECONOMY: cout << " Economy";
break;
}
cout << endl;
}
};
#endif
// ------------------------------------------------------
// motorHome_t.cpp
// Testing the multiply-derived class MotorHome
// ------------------------------------------------------
#include "motorHome.h"
int main()
{
MotorHome rv(12345L, "Texaco", 2, 40.5, LUXURY);
rv.display();
MotorHome holiday;
holiday.display(); // Default values
cin.get();
604 ■
CHAPTER 27 MULTIPLE INHERITANCE
holiday.setNr(54321);
holiday.setProd("VW");
holiday.setRoom(1);
holiday.setSquareFeet(11.5);
holiday.setCategory(SECONDCLASS);
holiday.display();
return 0;
}
Exercise 2
// ------------------------------------------------------
// car.h : Definition of base class Car and
// the derived classes PassCar and Truck
// ------------------------------------------------------
// car.cpp
// Implementing the methods of Car, PassCar, and Truck
// ------------------------------------------------------
//
// These files are carried over from Chapter 23 and 25,
// with the following changes:
//
// Class Car is a virtual base class now
class PassCar : public virtual Car
{
// ...
};
class Truck : public virtual Car
{
// ...
};
// ------------------------------------------------------
// suv.h : Defines the class Van and
// the multiply-derived class SUV
// ------------------------------------------------------
#ifndef _SUV_H
#define _SUV_H
#include "car.h"
class Van : public virtual Car
{
private:
double capacity;
SOLUTIONS
■605
public:
Van(long n=0L, const string& prod="",
double l=0.0)
: Car(n,prod)
{
if(l > 750) l = 750;
capacity = l;
}
void setCapacity(double l)
{
if(l > 750)
capacity= 750;
else
capacity = l;
}
double getCapacity() const { return capacity; }
void display() const
{
cout << "Capacity: "
<< capacity << " kg" << endl;
}
};
class SUV : public PassCar, public Van
{
private:
int cnt; // Number of seats
public:
SUV(const string& tp="without type", bool sb=false,
long n=0L, const string& prod=" none ",
double l=0.0, int z = 1)
: PassCar(tp,sb), Car(n,prod),
Van(n,prod,l), cnt(z)
{ }
void display() const
{
PassCar::display();
Van::display();
cout << "Number of seats: " << cnt << endl;
}
};
#endif
606 ■
CHAPTER 27 MULTIPLE INHERITANCE
// ------------------------------------------------------
// suv_t.cpp : Tests the class SUV
// ------------------------------------------------------
#include "suv.h"
int main()
{
SUV mobil("Bravada", true, 120345, "Oldsmobile",350,6);
mobil.display();
SUV trucky;
trucky.display();
trucky.setNr(543221);
trucky.setProd("Renault");
trucky.setCapacity(1000.);
trucky.display();
return 0;
}
607
Exception Handling
This chapter describes how a C++ program uses error-handling
techniques to resolve error conditions. In addition to throwing and
catching exceptions, we also examine how exception specifications are
declared and exception classes are defined, additionally looking into the
use of standard exception classes.
chapter
28
608 ■
CHAPTER 28 EXCEPTION HANDLING
■TRADITIONAL ERROR HANDLING
Error checking after leaving a function
First calling
function
Second calling
function
Third calling
function
Called function
. . .
{
. . .
if(func()>0)
// every-
// thing
// is ok
else
exit(-1);
. . .
}
. . .
{
. . .
if(func()<=0)
// All errors
// are
// handled
. . .
}
. . .
{
. .
x= func();
if(x == 0)
//1. error
else if
(x == -1)
//2. error
...
}
int func( void)
{
. . .
if(dilemma)
return 0;
. . .
if(catastrophe)
return -1;
. . .
}
TRADITIONAL ERROR HANDLING
■609
䊐Error Conditions
Errors that occur at program runtime can seriously interrupt the normal flow of a pro-
gram. Some common causes of errors are
■division by 0, or values that are too large or small for a type
■no memory available for dynamic allocation
■errors on file access, for example, file not found
■attempt to access an invalid address in main memory
■invalid user input
Anomalies like these lead to incorrect results and may cause a computer to crash. Both of
these cases can have fatal effects on your application.
One of the programmer’s most important tasks is to predict and handle errors. You
can judge a program’s quality by the way it uses error-handling techniques to counteract
any potential error, although this is by no means easy to achieve.
䊐Traditional Error Handling
Traditional structured programming languages use normal syntax to handle errors:
■errors in function calls are indicated by special return values
■global error variables or flags are set when errors occur, and then checked again
later.
If a function uses its return value to indicate errors, the return value must be examined
whenever the function is called, even if no error has occurred.
Example: if( func()> 0 )
// Return value positive => o.k.
else
// Treat errors
Error variables and flags must also be checked after every corresponding action.
In other words, you need to continually check for errors while a program is executing.
If you do happen to forget to check for errors, the consequences may be fatal.
610 ■
CHAPTER 28 EXCEPTION HANDLING
// calc_err.cpp: Defining the function calc(),
// which throws exceptions.
// ----------------------------------------------------
class Error
{
// Infos about the error cause
};
double calc( int a, int b )
{
if ( b < 0 )
throw (string)"Denominator is negative!";
if( b == 0 )
{
Error errorObj;
throw errorObj;
}
return ((double)a/b);
}
■EXCEPTION HANDLING
Using the throw statement
EXCEPTION HANDLING
■611
䊐Exception Handling Concept
C++ introduces a new approach to error handling. Exception handling is based on keeping
the normal functionality of the program separate from error handling. The basic idea is
that errors occurring in one particular part of the program are reported to another part of
the program, known as the calling environment. The calling environment performs central
error handling.
An application program no longer needs to continually check for errors, because in
the case of an error, control is automatically transferred to the calling environment.
When reporting an error, specific information on the error cause can be added. This
information is evaluated by the error-handling routines in the calling environment.
䊐The throw Statement
An exception that occurs is recorded to the calling environment by means of a throw
statement; this is why we also say that an exception has been thrown.
Syntax: throw fault;
The expression fault is an exception object that is thrown to the calling environment. It
can belong to any type except void.
Example: throw "Fire!";
In this example, the exception object is a string that is thrown to the calling environ-
ment.
䊐Exception Classes
Normally, you define your own exception classes to categorize exceptions. In this case
you use the throw statement to throw an object belonging to a specific exception class.
An exception class need not contain data members or methods. However, the type,
which is used by the calling environment to identify the error, is important. Generally,
the exception class will contain members that provide more specific information on the
cause of the error.
In the sample program on the opposite page, the calc() function throws exceptions
in two cases, where the numerator is negative or has a value of 0. In the first case, the
exception thrown is a string. In the second case, the exception is an Error exception
class object. Instead of creating a local exception object errorObj, a temporary object
can be created:
Example: throw Error(); // It is shorter
612 ■
CHAPTER 28 EXCEPTION HANDLING
try
{
// Exceptions thrown by this block will be
// caught by the exception handlers,
// which are defined next.
}
catch( Type1 exc1)
{
// Type1 exceptions are handled here.
}
[catch( Type2 exc2)
{
// Type2 exceptions are handled here.
}
. . . //etc.
]
[catch( ... )
{
// All other exceptions are handled here.
}]
The brackets [...] in a syntax description indicate that the enclosed section is optional.
✓NOTE
■EXCEPTION HANDLERS
Syntax of try and catch blocks
EXCEPTION HANDLERS
■613
䊐How Exception Handling Works
The part of a program that performs central error handling in the calling environment is
referred to as an exception handler. An exception handler catches the exception object
thrown to it and performs error handling. The exception object type determines which
handler will catch it and consequently be executed.
This means that you need to specify two things when implementing exception han-
dling:
■the part of the program that can throw exceptions
■the exception handlers that will process the various exception types.
C++ provides language elements for this task, the keywords try and catch. Each key-
word precedes a code block and thus they are often referred to as try and catch blocks.
Syntactically speaking, each try and catch block is a statement, however.
䊐try and catch Blocks
Atry block contains the program code in which errors can occur and exceptions can be
thrown. Normally, a try block will consist of a group of functions that can produce simi-
lar errors.
Each catch block defines an exception handler, where the exception declaration, which is
enclosed in parentheses, defines the type of exceptions the handler can catch. The
catch blocks immediately follow the try block. A minimum of one catch block is
required.
The exception handlers defined by the catch blocks catch the exceptions thrown
within the try block. If there is no handler defined for a particular exception type, the
program will not simply enter an undefined state but will be orderly terminated by a call
to the standard function terminate().
It is common practice to define specific handlers for certain types of errors and one
generic handler for all other errors. This functionality is provided by a special syntax in
the catch statement with an exception declaration consisting of just three dots.
Syntax: catch( ... )
{ // General handler for
// all other exceptions
}
Since the application program decides what reaction is applicable for certain error condi-
tions, the try and catch blocks are formulated in the application.
614 ■
CHAPTER 28 EXCEPTION HANDLING
// calc_err.cpp: Tests the function calc(),
// which throws exceptions.
// ----------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
double calc( int a, int b );
int main()
{
int x, y;
double res;
bool flag = false;
do
{
try // try block
{
cout << "Enter two positive integers: ";
cin >> x >> y;
res = calc( x, y);
cout << x << "/" << y << " = " << res << endl;
flag = true; // Then to leave the loop.
}
catch( string& s) // 1st catch block
{
cerr << s << endl;
}
catch( Error& ) // 2nd catch block
{
cerr << "Division by 0! " << endl;
}
catch(...) // 3rd catch block
{
cerr << "Unexpected exception! \n";
exit(1);
}
}while( !flag);
// continued ...
return 0;
}
As the Error class contains no data members, the corresponding catch block declares only the type
of exception, and no parameters. This avoids a compiler warning since the parameter is not used.
✓NOTE
■THROWING AND CATCHING EXCEPTIONS
Demonstration program
THROWING AND CATCHING EXCEPTIONS
■615
䊐Backing Out of an Error Condition
When the throw statement is executed, an exception object is thrown. That is, a tem-
porary object of the same type and content as the throw expression is created.
Example: throw "Cyclone!";
This creates a string as an exception object and copies the string "Cyclone!" to it.
Thus, if the throw expression is a class type, the copy constructor is executed.
The exception object is then thrown and the program control leaves the try block.
Any changes to the stack that took place after entering the try block are unwound.
This specifically involves destroying any local, non-static objects. Unwinding the stack
allow you to back out of the normal program flow in an orderly manner.
䊐Searching for Handlers
After leaving the try block, the program control is transferred to an matching handler
in the catch blocks that follow. This search operation is always performed sequentially
beginning with the first catch block and the exception declaration of the handler
determines whether the handler should be executed.
A handler is called when the type in the exception declaration is
■identical to the exception type thrown or
■a base class of the exception type or
■a base class pointer and the exception is a pointer to a derived class.
This is why the general exception handler catch( ... ) always has to be defined last.
Since the first suitable handler will be executed, and any exception thrown will be
caught by the general handler, a handler defined after the general handler would never
be called.
䊐Continuing the Program
After executing a handler, the program continues with the first statement following the
catch blocks, unless the handler throws another exception or terminates the program.
After completing exception handling, the exception object that was thrown is destroyed.
The first two catch blocks handle both exceptions that the calc() function can
throw. In both cases a message is displayed and the program carries on prompting for
input and computing values. If an unexpected exception occurs, a message is again dis-
played, but in this case the program then terminates.
616 ■
CHAPTER 28 EXCEPTION HANDLING
try
{
// Type1 exceptions are thrown here.
try
{
// Type1 and Type2 exceptions are thrown here.
}
catch( Type2 e2)
{
// Type2 exceptions are pre-handled here
throw; // and thrown again.
}
// Other Type1 exceptions
// can be thrown.
}
catch( Type1 e1)
{
// Type1 exceptions are handled here.
}
catch(...)
{
// All remaining exceptions are handled here,
// particularly Type2 exceptions.
}
This scenario assumes that the error classes Type1 and Type2 are not derived one from another. If
class Type2 is derived from class Type1, any Type2 exceptions thrown will be caught by the handler
for the base class Type1.
✓NOTE
■NESTING EXCEPTION HANDLING
Nested try and catch blocks
NESTING EXCEPTION HANDLING
■617
䊐Nested try and catch Blocks
A program will normally contain multiple try blocks with appropriate exception han-
dlers. This allows for various error handling in different parts of the program.
However, a try block can contain additional try blocks. This allows you to use the
handlers in a nested try block for special purpose error handling, leaving the handlers in
the surrounding try block to deal with remaining errors. Handlers in a nested try block
can also pre-handle specific errors and then pass control to the try block wrapper for
final handling.
䊐Re-throwing Exceptions
In the last of these cases an exception thrown by the nested try block has to be passed
to the try block wrapper. This is achieved using a throw statement that does not
expect an exception object.
Example: throw; // in a catch block
This re-throws the pre-handled exception, which can then be processed by the handler
in the surrounding try block. The statement is only valid within a nested catch block
for this reason.
䊐Exception Specifications
The exceptions that a function can throw are features of that function. The application
programmer must have knowledge of both the function prototype and the exceptions the
function can throw to ensure that he or she will be capable of programming correct func-
tion calls and taking appropriate action in case of errors.
The exceptions a function can throw can be stated in a so-called exception specifica-
tion list when you declare a function.
Example: int func(int) throw(BadIndex, OutOfRange);
The list BadIndex,OutOfRange states the exceptions that the function func()can
throw. If the list is empty, that is, if the list contains only the throw() statement, no
exceptions are thrown. If the throw statement is also missing, there is no specific infor-
mation about possible exceptions and any exception can be thrown.
618 ■
CHAPTER 28 EXCEPTION HANDLING
// calc_new.cpp: New version of function calc(),
// which throws exceptions of type
// MathError.
// ----------------------------------------------------
#include <string>
#include <iostream>
using namespace std;
class MathError
{
private:
string message;
public:
MathError( const string& s) : message(s) {}
const string& getMessage() const {return message;}
};
double calc( int a, int b ) throw(MathError);
int main()
{
int x, y; bool flag = false;
do
{
try // try block
{
cout << "Enter two positive integers: ";
cin >> x >> y;
cout << x <<"/"<< y <<" = "<< calc( x, y) << '\n';
flag = true; // To leave the loop.
}
catch( MathError& err) // catch block
{
cerr << err.getMessage() << endl;
}
}while( !flag);
// continued ...
return 0;
}
double calc( int a, int b ) throw (MathError)
{ if ( b < 0 )
throw MathError("Denominator is negative!");
if( b == 0 )
throw MathError("Division by 0!");
return ((double)a/b);
}
■DEFINING YOUR OWN ERROR CLASSES
Exception handling for numeric operations
DEFINING YOUR OWN ERROR CLASSES
■619
䊐Exception Class Members
When an exception is thrown, the exception object type determines which exception
handler will be executed. For this reason, an exception class does not need to have any
members.
However, an exception class can contain data members and methods—just like any
other class. This makes sense, as locally defined non-static objects are destroyed when an
exception has been thrown and the stack is unwound. Thus, the exception handler can
no longer access the previously existing objects.
You can use the data members of error classes to rescue data threatened by an error
condition. For example, you can store data important for exception handling in an
exception object.
䊐The Exception Class MathError
The exception class MathError is defined opposite. The calc() function throws an
exception when a number input by a user is negative or 0. When an exception is thrown,
Example: throw MathError("Division by 0!");
the error message is stored in the exception object. The exception handler can then use
the getMessage() method to evaluate the message.
䊐Exception Hierarchies
New exception classes can be derived from existing exception classes. A base class will
normally represent a general error type and specific errors will be represented by derived
classes.
Thus, the exception class MathError could be defined to represent general errors in
mathematical computations, but it would make sense to define derived exception classes
for special cases, such as “Division by 0” or “Arithmetic overflow.” You could call these
classes DivisionByZero and OverflowError, for example.
Be aware of the following rules for exception handlers in this context:
■given that T is a derived exception class, special errors of this type are handled by
the exception handler
■if T is a base class, the handler will also catch the exception objects thrown by
derived classes, thus providing similar handling for generic and specific errors.
620 ■
CHAPTER 28 EXCEPTION HANDLING
invalid_argument
out_of_range
length_error
domain_error
Invalid argument
Argument value not in its expected range
Length exceeds maximum capacity
Domain error reported by the implementation
range_error
underflow_error
overflow_error
Range error in internal computation
Arithmetic underflow error
Arithmetic overflow error
// Subscript operator of class FloatArr throws
// exceptions with type of standard class out_of_range:
// ---------------------------------------------------
#include <stdexcept>
#include <iostream>
using namespace std;
double& FloatArr::operator[](int i) throw(out_of_range)
{ if( i < 0 || i >= anz )
throw out_of_range("Invalid index!");
else return arrPtr[i];
}
// -------------- Test Program ------------------
int main()
{
try
{
// Uses arrays of type FloatArr
}
catch(out_of_range& err)
{
cerr << err.what() << endl;
}
// The program continues here.
}
■STANDARD EXCEPTION CLASSES
Exception classes derived from logic_error
Exception classes derived from runtime_error
Using standard exception classes
STANDARD EXCEPTION CLASSES
■621
䊐Hierarchy of Standard Exception Classes
The C++ standard library contains various exception classes, which are used in the string
and container libraries, for example. However, the standard exception classes can be
used just like exception classes of your own. Their definitions are to be found in the
header file stdexcept.
The standard exception classes are organized in a hierarchy, the common base class
being the exception class. In addition to a default constructor, a copy constructor, and
an assignment, this class contains a virtual public method, what(), which returns a
message on the error cause as a C string.
䊐Representing Logic Errors and Runtime Errors
The following exception classes are derived from the exception class:
logic_error used to represent logic errors, caused by anomalies in the program’s
logic. These errors can be avoided.
runtime_error used to represent runtime errors, such as under- or overflows occur-
ring in internal computations. These errors are unpredictable.
The opposite page contains on overview of the exception classes derived from the
logic_error and runtime_error classes. For example, the method at() in the
string class throws an out_of_range type exception when an invalid string position
is passed to it. If a string cannot be displayed because of its exceptional length, an excep-
tion of the invalid_argument type is thrown.
An exception of the overflow_error or underflow_error type is thrown if the
value to be displayed is too large or too small for the type in use. The range_error
class shows range errors, which can occur during internal computations.
A constructor with a string as a parameter is defined in every class derived from
exception. This means you can initialize exceptions of these types with error messages.
The what() method returns this error message as a C string.
exercises
622 ■
CHAPTER 28 EXCEPTION HANDLING
Error in reading:
Invalid index: ...
Error in writing:
Invalid index: ...
■EXERCISES
Exercise 1: Error messages of the exception handler
The first exception handler’s message:
The second exception handler’s message:
EXERCISES
■623
Exercise 1
The FloatArr class needs exception handling for cases where an invalid index is
stated when accessing an array member.
■Define the exception class BadIndex for this purpose and store the class
in the header file “floatArr.h”.The exception class must contain a data
member to store the invalid index.The constructor expects an index
that it will copy to the data member.The const access method
getBadIndex() returns the data member.
Both subscript operators should be able to throw BadIndex type excep-
tions.Add an exception specification to the declaration of the subscript
operators.
The methods that expect the position as an argument, such as insert()
and remove(), should also throw exceptions.Add appropriate exception
specifications to the definitions and change the return types from bool to
void.
■Change the definitions of the methods and operator functions to allow a
BadIndex type exception to be thrown when the index passed to the
function is outside the valid range.
■Write a main function where a constant vector is created and initialized
with fixed values. Exception handling is required for the following scenar-
ios.The array elements are displayed and an index is read until an invalid
index value is input.The catch handler should output the information
shown opposite for each invalid index.
Then create a non-constant array.Add further exception handling to be
performed. Elements are appended or inserted within a try block.
Include an invalid element access attempt, which causes the catch han-
dler to output the information shown opposite.Then finally output the
array elements outside the try and catch blocks.
624 ■
CHAPTER 28 EXCEPTION HANDLING
Exercises
For Exercise 2: Error messages of the exception handlers
Messages of the exception handlers
for an exception object of type DivisionByZero:
Error in initializing:
The denominator is 0!
Error in division:
No division by zero!
Error: Denominator is 0!
New denominator != 0: ...
EXERCISES
■625
Exercise 2
Implement exception handling for the Fraction class, which is used to
represent fractions (see Exercise 2, Chapter 19). Dividing by 0 throws an
exception that affects the constructor for the Fraction class and the operator
functions /and >>.
■Define the exception class DivError, which has no data members, within
the Fraction class.The exception class is of the following type
Fraction::DivError
Add an appropriate exception specification to the declarations of the
constructor and the operator functions /and >>.
■Change the definition of the constructor in the Fraction class. If the
value of the denominator is 0, a DivisionByZero type exception should
be thrown.
■Similarly modify the operator functions.
■Now write a main function to test the various exceptions.You will need
to arrange three different try and catch blocks sequentially.
The first try/catch block tests the constructor. Create several fractions,
including some with a numerator value of 0 and one with 0 as its
denominator.The exception handler should issue the error message
shown opposite.
The second try/catch block tests divisions. Use a statement to attempt
to divide by 0.The corresponding exception handler should send the
second error message shown opposite to your standard output.
The third try/catch block reads numerators and denominators of
fractions in dialog. If the value of the denominator is 0, the denominator is
read again. If the value is still 0, the third error message as shown
opposite is output and the program terminates.
solutions
626 ■
CHAPTER 28 EXCEPTION HANDLING
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// floatArr.h : Represents dynamic float arrays.
// Methods throw exceptions for an invalid index.
// ------------------------------------------------------
#ifndef _FLOATARR_
#define _FLOATARR_
#include <iostream>
using namespace std;
class BadIndex
{
private:
int index;
public:
BadIndex(int i){index = i;}
int getBadIndex() const {return index;}
};
class FloatArr
{
private:
float* arrPtr; // Dynamic member
int max; // Maximum number without
// reallocating more memory.
int cnt; // Current number of elements.
void expand( int newSize); // Function to help
// enlarge the array.
public:
FloatArr( int n = 256 ); // Constructors
FloatArr( int n, float val);
FloatArr(const FloatArr& src);
~FloatArr(); // Destructor
FloatArr& operator=( const FloatArr&); // Assignment
int length() const { return cnt; }
// Subscript operators:
float& operator[](int i) throw(BadIndex);
float operator[](int i) const throw(BadIndex);
// Methods to append a float value
// or an array of floats:
void append( float val);
void append( const FloatArr& v);
SOLUTIONS
■627
FloatArr& operator+=( float val)
{
append( val); return *this;
}
FloatArr& operator+=( const FloatArr& v)
{
append(v); return *this;
}
// Methods to insert a float value
// or an array of float values:
void insert( float val, int pos) throw(BadIndex);
void insert(const FloatArr& v,int pos) throw(BadIndex);
void remove(int pos) throw(BadIndex); // Remove
// at pos.
// Output the array
friend ostream& operator<<( ostream& os,
const FloatArr& v)
{
int w = os.width(); // Save field width.
for( float *p = v.arrPtr; p < v.arrPtr + v.cnt; ++p)
{
os.width(w); os << *p;
}
return os;
}
};
#endif // _FLOATARR_
// -----------------------------------------------------
// floatArr.cpp
// Implementing the methods of FloatArr.
// -----------------------------------------------------
#include "floatArr.h"
// --- Constructors ---
FloatArr::FloatArr( int n )
{
max = n; cnt = 0; // Sets max and cnt.
arrPtr = new float[max]; // Allocates memory
}
FloatArr::FloatArr(int n, float val)
{
max = cnt = n;
arrPtr = new float[max];
for( int i=0; i < cnt; ++i)
arrPtr[i] = val;
}
628 ■
CHAPTER 28 EXCEPTION HANDLING
FloatArr::FloatArr(const FloatArr& src)
{
max = src.max;
cnt = src.cnt;
arrPtr = new float[max];
for( int i = 0; i < cnt; i++ )
arrPtr[i] = src.arrPtr[i];
}
// --- Destructor ---
FloatArr::~FloatArr()
{
delete[] arrPtr;
}
// Private functions to help enlarge the array.
void FloatArr::expand( int newSize)
{
if( newSize == max)
return;
max = newSize;
if( newSize < cnt)
cnt = newSize;
float *temp = new float[newSize];
for( int i = 0; i < cnt; ++i)
temp[i] = arrPtr[i];
delete[] arrPtr;
arrPtr = temp;
}
FloatArr& FloatArr::operator=( const FloatArr& src )
{
if( this != &src ) // No self assignment!
{
max = src.max;
cnt = src.cnt;
delete[] arrPtr; // Release memory,
arrPtr = new float[max]; // reallocate,
for( int i=0; i < cnt; i++) // copy elements.
arrPtr[i] = src.arrPtr[i];
}
return *this;
}
SOLUTIONS
■629
float& FloatArr::operator[]( int i ) throw(BadIndex)
{
if( i < 0 || i >= cnt ) throw BadIndex(i);
return arrPtr[i];
}
float FloatArr::operator[]( int i ) const throw(BadIndex)
{
if( i < 0 || i >= cnt ) throw BadIndex(i);
return arrPtr[i];
}
// Append a float value or an array of floats.
void FloatArr::append( float val)
{
if( cnt+1 > max)
expand( cnt+1);
arrPtr[cnt++] = val;
}
void FloatArr::append( const FloatArr& v)
{
if( cnt + v.cnt > max)
expand( cnt + v.cnt);
int count = v.cnt; // Necessary if
// v == *this.
for( int i=0; i < count; ++i)
arrPtr[cnt++] = v.arrPtr[i];
}
// Inserts a float value or an array of floats.
void FloatArr::insert(float val, int pos) throw(BadIndex)
{
insert( FloatArr(1, val), pos);
}
void FloatArr::insert( const FloatArr& v, int pos )
throw( BadIndex )
{
if( pos < 0 || pos > cnt) // Append is also possible.
throw BadIndex(pos);
if( max < cnt + v.cnt)
expand(cnt + v.cnt);
int i;
for( i = cnt-1; i >= pos; --i) // Shift up from
arrPtr[i+v.cnt] = arrPtr[i]; // position pos.
for( i = 0; i < v.cnt; ++i) // Fill the gap.
arrPtr[i+pos] = v.arrPtr[i];
cnt = cnt + v.cnt;
}
630 ■
CHAPTER 28 EXCEPTION HANDLING
// To delete
void FloatArr::remove(int pos) throw(BadIndex)
{
if( pos >= 0 && pos < cnt)
{
for( int i = pos; i < cnt-1; ++i)
arrPtr[i] = arrPtr[i+1];
--cnt;
}
else
throw BadIndex(pos);
}
// -------------------------------------------------------
// arr_h.cpp
// Tests exception handling for float arrays.
// -------------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#include "floatArr.h"
int main()
{
const FloatArr v(10, 9.9f);
bool ok = false;
while( !ok)
{
try
{
cout << "Here is the constant array v: \n";
cout << setw(8) << v <<endl;
int i;
cout << "Index? "; cin >> i;
cout << "\nThe value read: " << v[i] << endl;
ok = true;
}
catch(BadIndex& err)
{
cerr << "Error in reading.\n"
<< "\nInvalid index: "
<< err.getBadIndex() << endl;
}
}
SOLUTIONS
■631
FloatArr w(20); // Array w
try
{
w.insert(1.1F, 0); // To write.
w.insert(2.2F, 1);
// w.insert(3.3F, 3); // Error!
// w[10] = 5.0; // Error!
// w.remove(7); // Error!
}
catch(BadIndex& err)
{
cerr << "\nError in writing! "
<< "\nInvalid index: "
<< err.getBadIndex() << endl;
}
cout << "\nHere is the array: \n";
cout << setw(5) << w << endl;
return 0;
}
Exercise 2
// ------------------------------------------------------
// fraction.h
// A numeric class to represent fractions,
// exception handling is included.
// ------------------------------------------------------
#ifndef _FRACTION_
#define _FRACTION_
#include <iostream>
#include <cstdlib>
using namespace std;
class Fraction
{
private:
long numerator, denominator;
public:
class DivisionByZero
{
// No data members
};
Fraction(long z = 0, long n = 1) throw(DivisionByZero);
Fraction operator-() const
{
return Fraction(-numerator, denominator);
}
632 ■
CHAPTER 28 EXCEPTION HANDLING
Fraction& operator+=(const Fraction& a)
{
numerator = a.numerator * denominator
+ numerator * a.denominator;
denominator *= a.denominator;
return *this;
}
Fraction& operator-=(const Fraction& a)
{
*this += (-a);
return *this;
}
Fraction& operator++()
{
numerator += denominator;
return *this;
}
Fraction& operator--()
{
numerator -= denominator;
return *this;
}
friend Fraction operator+(const Fraction&,
const Fraction&);
friend Fraction operator-(const Fraction&,
const Fraction&);
friend Fraction operator*(const Fraction&,
const Fraction&);
friend Fraction operator/(const Fraction&,
const Fraction&)
throw(Fraction::DivisionByZero);
friend ostream& operator<<(ostream&, const Fraction&);
friend istream& operator>>(istream& is, Fraction& a)
throw(Fraction::DivisionByZero);
};
#endif
SOLUTIONS
■633
// -------------------------------------------------------
// fraction.cpp
// Defines methods and friend functions.
// -------------------------------------------------------
#include <iostream>
#include <cstdlib>
using namespace std;
#include "fraction.h"
// Constructor:
Fraction::Fraction(long z, long n)
throw(Fraction::DivisionByZero)
{
if(n == 0) throw DivisionByZero();
if( n < 0) z = -z, n = -n;
numerator = z; denominator = n;
}
Fraction operator+(const Fraction& a, const Fraction& b)
{
Fraction temp;
temp.denominator = a.denominator * b.denominator;
temp.numerator = a.numerator*b.denominator
+ b.numerator * a.denominator;
return temp;
}
Fraction operator-(const Fraction& a, const Fraction& b )
{
Fraction temp = a; temp += (-b);
return temp;
}
Fraction operator*(const Fraction& a, const Fraction& b )
{
Fraction temp;
temp.numerator = a.numerator * b.numerator;
temp.denominator = a.denominator * b.denominator;
return temp;
}
Fraction operator/(const Fraction& a, const Fraction& b )
throw(Fraction::DivisionByZero)
{
if( b.numerator == 0) throw Fraction::DivisionByZero();
// Multiply a with the inverse of b:
Fraction temp;
temp.numerator = a.numerator * b.denominator;
temp.denominator = a.denominator * b.numerator;
if( temp.denominator < 0 )
temp.numerator = -temp.numerator,
temp.denominator = -temp.denominator;
return temp;
}
634 ■
CHAPTER 28 EXCEPTION HANDLING
ostream& operator<<(ostream& os, const Fraction& a)
{
os << a.numerator << "/" << a.denominator;
return os;
}
istream& operator>>(istream& is, Fraction& a)
throw(Fraction::DivisionByZero)
{
cout << "Enter a fraction:\n"
" Numerator: "; is >> a.numerator;
cout << " Denominator != 0: "; is >> a.denominator;
if( !is) return is;
if( a.denominator == 0)
{
cout << "\nError: The denominator is 0\n"
<< "New Denominator != 0: "; is >> a.denominator;
}
if( a.denominator == 0)
throw Fraction::DivisionByZero();
if( a.denominator < 0 )
a.numerator = -a.numerator,
a.denominator = -a.denominator;
return is;
}
// -------------------------------------------------------
// fract_t.cpp : Testing the class Fraction.
// Modules: fract_t.cpp fraction.cpp
// -------------------------------------------------------
#include <iostream.h>
#include "fraction.h"
int main()
{
try // Tests the exception of the constructor:
{
Fraction c(5,0);
}
catch(Fraction::DivisionByZero& )
{
cout << "\nError on initializing: "
<< "\nThe denominator is 0\n";
}
SOLUTIONS
■635
Fraction a(1,3), b(3);
try
{
cout << "\nSome test output:\n\n";
cout << " a = " << a << endl;
cout << " b = " << b << endl;
cout << " a + b = " << (a + b) << endl;
cout << " a - b = " << (a - b) << endl;
cout << " a * b = " << (a * b) << endl;
b = 0;
cout << " a / b = " << (a / b) << endl; // Error!
}
catch(Fraction::DivisionByZero& )
{
cout << "\nError on dividing: "
<< "\nNo division by zero! 0\n";
}
cout << " --a = " << --a << endl;
cout << " ++a = " << ++a << endl;
a += Fraction(1,2);
cout << " a+= 1/2; a = " << a << endl;
a -= Fraction(1,2);
cout << " a-= 1/2; a = " << a << endl;
cout << "-b = " << -b << endl;
cout << "\nAnd now your input: \n";
try
{
cin >> a;
}
catch(Fraction::DivisionByZero&)
{
cerr << "\nError: The denominator is 0\n";
exit(1);
}
cout << "\nYour input: " << a << endl;
return 0;
}
This page intentionally left blank
637
More about Files
This chapter describes
■random access to files based on file streams
■options for querying file state
■exception handling for files.
We will also illustrate how to make objects in polymorphic classes
persistent, that is, how to save them in files.The applications introduced in
this chapter include simple index files and hash tables.
chapter
29
638 ■
CHAPTER 29 MORE ABOUT FILES
Open mode Effects Must the file
exist?
To open the file for input and
output.
Opens the file for input and
output. If the file already exists,
it will be truncated.
Opens the file for input and
output. If the file does not exist,
it will be created.
Before each writing access, a
seek to end is performed.
yes
no
no
ios::in
| ios::out
ios::in
| ios::out
| ios::trunc
ios::in
| ios::out
| ios::app
1. If the flag ios::binary is additionally set, the file will be opened in binary mode.
2. If the flag ios::ate (“at end”) is additionally set, the current seek position is set to
end-of-file immediately after opening.
✓NOTE
■OPENING A FILE FOR RANDOM ACCESS
Combined open modes for read and write access
OPENING A FILE FOR RANDOM ACCESS
■639
䊐Random File Access
So far we have only looked at sequential file access. If you need access to specific infor-
mation in such a file, you have to walk through the file from top to tail, and new records
are always appended at the end of the file.
Random file access gives you the option of reading and writing information directly at a
pre-defined position. To be able to do this, you need to change the current file position
explicitly, that is, you need to point the get/put pointer to the next byte to be manipu-
lated. After pointing the pointer, you can revert to using the read and write operations
that you are already familiar with.
䊐Open Modes
One prerequisite of random file access is that the position of the records in the file can be
precisely identified. This implies opening the file in binary mode to avoid having to
transfer additional escape characters to the file.
Example: ios::openmode mode = ios::in | ios::out |
ios::app | ios::binary;
fstream fstr("account.fle", mode);
This statement opens the file "Account.fle" in binary mode for reading and append-
ing at end-of-file. The file will be created if it did not previously exist. Random read access
to the file is possible, but for write operations new records will be appended at the end of
the file.
To enable random read and write access to a file, the file can be opened as follows:
Example: ios::openmode mode = ios::in | ios::out |
ios::binary;
fstream fstr("account.fle", mode);
However, this technique can only be used for existing files. If the file does not exist, you
can use the ios::trunc flag to create it.
The section “File State” discusses your error handling options if a file, such as
"account.fle" cannot be found.
640 ■
CHAPTER 29 MORE ABOUT FILES
Access point Positioning flag File
Beginning of the file ios::beg
Current position ios::cur
End of file ios::end
•
•
•
•
•
•
•
■POSITIONING FOR RANDOM ACCESS
The three access points in a file
POSITIONING FOR RANDOM ACCESS
■641
䊐Discovering and Changing the Current Position
The file stream classes comprise methods to discover and change the current position in
a file. The tellp() and tellg() methods return the current position of the put or
get pointers as a long value.
Example: long rpos = myfile.tellg();
This statement queries the current position of the read pointer in the myfile stream.
The current position is always returned as a byte offset relative to the beginning of the
file.
The current file position can be modified using the seekp() or seekg() method.
The position is stated as a byte offset, relative to either the beginning or end of the file,
or relative to the current position in the file.
䊐Positioning Flags
Three ios::seekdir type positioning flags are defined in the ios class for this pur-
pose; these are ios::beg,ios::cur, and ios::end.
Imagine you want to write the object acc to the file "account.fle" at offset pos.
You can use the following statements to do so:
Example: ofstream fstr("account.fle", ios::out |
ios::binary);
fstr.seekp(pos, ios::begin);
acc.write( fstr );
This calls the write() method in the Account class, which allows an object to write
its own data members to a file (see Chapter 18).
If you do not specify a positioning flag, the position will be assumed to be relative to
the beginning of the file. The statement used to position the write pointer in the last
example can therefore be formulated as follows:
Example: fstr.seekp(pos );
The byte offset can also be negative for calls to the methods seekp() and seekg().
However, you cannot position the read/write pointer before the beginning of the file.
In contrast, it is possible to place the pointer at a position after the end of the file and
then perform a write operation, which will create a gap with unknown content in the
file. This only makes sense if all the empty slots in the file are of an equal length, as they
can be overwritten later. This option is often used when programming hash tables.
642 ■
CHAPTER 29 MORE ABOUT FILES
// index.h: Defines the class IndexEntry
// ----------------------------------------------------
#ifndef _INDEX_H
#define _INDEX_H
#include <fstream>
#include <iostream>
using namespace std;
class IndexEntry
{
private:
long key; // Key
long recNr; // Offset
public:
IndexEntry(long k=0L, long n=0L) { key=k; recNr=n;}
// Access methods ... and:
int recordSize() const
{ return sizeof(key) + sizeof(recNr); }
fstream& write( fstream& ind) const;
fstream& read( fstream& ind);
fstream& write_at(fstream& ind, long pos) const;
fstream& read_at( fstream& ind, long pos);
};
#endif
// index.cpp: Implements the methods
// ---------------------------------------------------
#include "index.h"
// . . .
fstream& IndexEntry::write_at( fstream& ind, long pos)
const
{
ind.seekp(pos);
ind.write((char*)&key, sizeof(key) );
ind.write((char*)&recNr, sizeof(recNr) );
return ind;
}
fstream& IndexEntry::read_at(fstream& ind, long pos)
{
ind.seekg(pos);
ind.read((char*)&key, sizeof(key) );
ind.read((char*)&recNr, sizeof(recNr));
return ind;
}
■POSITIONING FOR RANDOM ACCESS (CONTINUED)
Representing an index entry
The read_at() and write_at() methods
POSITIONING FOR RANDOM ACCESS (CONTINUED)
■643
䊐Using Positioning Methods
The following statements are commonly used for random positioning
seekg( 0L); and seekp( 0L, ios::end );
They set the current position to the beginning or end of a file. You should be aware that
the first argument is 0L to indicate that long type is required.
If you need to determine the length of a file, you can point the get pointer to the end
of the file and then query the position of the pointer:
Example: fstr.seekg(0L, ios::end);
unsigned long count = fstr.tellg();
The count variable will then contain the number of bytes occupied by the file.
These positioning methods are useful for files opened in binary mode. However, it
does not make much sense to use them for text files or particularly for devices. In text
mode, conversions of LF <=> CR/LF prevent the methods from working correctly.
䊐Determining Positions in a File
The position of records in a files is easy to compute if all the records in the file are the
same length. Given that size is the length of a record
0L, size, 2*size, ...
are the positions of the first, second, third records, and so on.
If you are working with variable length records, you cannot exactly compute their
positions. To enable random access you therefore need to store the positions of the
records in a separate structure, a so-called index.
The index stores pairs of keys and record positions, so-called index entries in a file. A
key, a social security number, or customer id, for example, must uniquely identify a
record. If the index is sorted, the position that correlates to the required key can be
quickly found using the binary search algorithm.
䊐The IndexEntry Class
The IndexEntry class, used to represent an index entry, is shown opposite. The class
comprises methods for reading and writing an index entry at the current file position or
at any given position. The appropriate file stream is passed as an argument to the
method.
644 ■
CHAPTER 29 MORE ABOUT FILES
// index.h: (continued)
// Adds the definition of an index
// --------------------------------------------------
#include <string>
class IndexFile
{
private:
fstream index;
string name; // Filename of index
public:
IndexFile(const string& s);
~IndexFile() { index.close(); }
void insert( long key, long pos);
long search( long key);
void retrieve(IndexEntry& entry, long pos );
};
// index.cpp: (continued)
// Adds methods of class IndexFile
// ---------------------------------------------------
IndexFile::IndexFile(const string& file)
{
ios::openmode mode = ios::in | ios::out
| ios::binary;
index.open(file.c_str(), mode);
if(!index) // If the file does not exist
{
index.clear();
mode |= ios::trunc;
index.open(file.c_str(), mode);
if(!index)
return;
}
else
name = file;
}
//. . .
■FILE STATE
Representing an index
Constructor of class IndexFile
FILE STATE
■645
䊐State Flags
A file stream can assume various states, for example, when it reaches the end of a file and
cannot continue reading. A file operation can also fail if a file cannot be opened, or if a
block is not transferred correctly.
The ios class uses state flags to define the various states a file can assume. Each state
flag corresponds to a single bit in a status-word, which is represented by the iostate
type in the ios class. The following state flags exist:
■ios::eofbit end of file reached
■ios::failbit last read or write operation failed
■ios::badbit an irrecoverable error occurred
■ios::goodbit the stream is ok, e.g. no other state flag is set.
The “flag” ios::goodbit is an exception to the rule since it is not represented by a
single bit, but by the value 0 if no other flag has been set. In other words a status-word
has the value ios::goodbit if everything is fine!
䊐Discovering and Changing the State
There are multiple methods for discovering and modifying the status of a stream. A
method exists for each state flag; these are eof(),fail(),bad(), and good(). They
return true when the corresponding flag has been raised. This means you can discover
the end of a file with the following statement:
Example: if( fstr.eof() ) ...
The status-word of a stream can be read using the rdstate() method. Individual flags
can then be queried by a simple comparison:
Example: if( myfile.rdstate() == ios::badbit ). . .
The clear() method is available for clearing the status-word. If you call clear()
without any arguments, all the state flags are cleared. An argument of the iostate type
passed to clear() automatically becomes the new status-word for the stream.
䊐The IndexFile Class
The IndexFile class, which uses a file to represent an index, is defined opposite. The
constructor for this class uses the clear() method to reset the fail bit after an invalid
attempt to open a non-existent file. A new file can then be created.
The IndexFile class comprises methods for inserting, seeking, and retrieving index
entries, which we will be implementing later in this chapter.
646 ■
CHAPTER 29 MORE ABOUT FILES
// exceptio.h : Exception classes for file access
// -------------------------------------------------
#ifndef _EXCEPTIO_H
#define _EXCEPTIO_H
#include <string>
#include <iostream>
using namespace std;
class FileError
{
private:
string filename;
public:
FileError( const string& file) : filename(file){ }
string getName() const{ return filename; }
};
class OpenError : public FileError
{
public:
OpenError( const string& file):FileError(file){ }
};
class ReadError : public FileError
{
public:
ReadError( const string& file):FileError(file){ }
};
class WriteError : public FileError
{
public:
WriteError(const string& file):FileError(file){ }
};
#endif
■EXCEPTION HANDLING FOR FILES
Defining your own exception classes
EXCEPTION HANDLING FOR FILES
■647
䊐Implementing Your Own Exception Handling
You can exploit the error tracking options that state flags give you to implement your
own exception handling for files. For example, a method that reads records from a file
can throw an exception when the state flag ios::eof is raised, that is, when the end of
the file is reached.
The opposite page shows typical exception classes organized in a hierarchy that can be
used to represent error conditions on opening, reading from, and writing to a file. In each
case the file name is saved for evaluation by the exception handler.
䊐Standard Exception Handling for Streams
C++ also provides standard exception handling for streams. You can use the excep-
tions() method to specify the flags in the status-word of a stream that will cause
exceptions to be thrown.
The exceptions() method is defined in the ios stream base class. The method
expects one or multiple state flags separated by the |sign. An exception is then thrown
for the flags specified.
Example: ifstream ifstrm("account.fle");
fstrm.exceptions(ios::failbit | ios::badbit);
On accessing the fstrm stream an exception is thrown if either one of the flags
ios::failbit or ios::badbit is raised. The operation that caused the error is then
terminated and the state flags are cleared by a call to clear(rdstate());.
The exception thrown here is of a standard exception class, failure. This type is
defined as a public element in the ios base class and comprises the virtual method
what() that returns a C string containing the cause of the error. The exception handler
will normally send the string to standard error output.
You can call exceptions() without any arguments to discover the state flags in a
status-word of a stream that can cause an exception to be thrown. If a bit is set in the
return value of the exceptions() method, an appropriate exception will be thrown
whenever this error occurs.
Example: iostate except = fstrm.exceptions();
if( except & ios::eofbit) ...
This statement uses a bitwise AND operator to ascertain whether an exception is thrown
when end-of-file is reached.
648 ■
CHAPTER 29 MORE ABOUT FILES
// account.h : Defines the classes
// Account, DepAcc, and SavAcc
// with virtual read and write methods.
// --------------------------------------------------
// . . .
enum TypeId { ACCOUNT, DEP_ACC, SAV_ACC };
class Account
{
private: // Data members: as previously defined.
public: // Constructor, access methods ...
virtual TypeId getTypeId() const { return ACCOUNT;}
virtual ostream& write(ostream& fs) const;
virtual istream& read(istream& fs);
};
class DepAcc : public Account
{ // Data members, constructor, . . .
TypeId getTypeId() const { return DEP_ACC; }
ostream& write(ostream& fs) const;
istream& read(istream& fs);
};
class SavAcc: public Account
{ // Data members, constructor, . . .
TypeId getTypeId() const { return SAV_ACC; }
ostream& write(ostream& fs) const;
istream& read(istream& fs);
};
// account.cpp: Implements the methods.
// ----------------------------------------------------
#include "account.h"
ostream& DepAcc::write(ostream& os) const
{
if(!Account::write(os))
return os;
os.write((char*)&limit, sizeof(limit) );
os.write((char*)&deb, sizeof(deb) );
return os;
}
istream& DepAcc::read(istream& is)
{
if(!Account::read(is))
return is;
is.read((char*)&limit, sizeof(limit) );
is.read((char*)&deb, sizeof(deb));
return is;
}
// . . .
■PERSISTENCE OF POLYMORPHIC OBJECTS
The methods read() and write() of class DepAcc
PERSISTENCE OF POLYMORPHIC OBJECTS
■649
䊐Storing Polymorphic Objects
Imagine you want to make the objects of a polymorphic class hierarchy persistent, that is,
store them in a file. You need to ensure that an object can be reconstructed precisely
when it is read. This gives rise to the fact that objects in polymorphic class hierarchies
contain virtual methods. So it is not simply a case of making the data members of an
object into records and writing them to a file.
1. You must write both the type and the data members of the object to a file.
2. If the objects contain dynamic members, you must save the referenced objects themselves along with
information on the object type.
✓NOTE
To allow the class to assume control over object storage, you need methods that allow
the object to write its own data members to a file. The methods can have a virtual defini-
tion within the class hierarchy. Thus, if pointers are used to reference objects, the appro-
priate read/write operation for each object will be called.
䊐Storing Objects of the Account Hierarchy
The opposite page shows the Account class, with which you should already be familiar.
Virtual file I/O methods have now been added. The implementation of the read() and
write() methods was discussed earlier in Chapter 18, “Fundamentals of File Input and
Output,” and is unchanged.
The derived classes DepAcc and SavAcc also contain definitions of the read() and
write()methods that read only their “own” objects and write them to files. The imple-
mentation first calls the appropriate base class method. If no errors occur, it is simply a
question of transferring the additional data members of the derived class to or from a file.
At present, no type information will be written to file or read from file. This task will
be performed by a special class whose features are used for file management. The follow-
ing section contains more details on this topic.
650 ■
CHAPTER 29 MORE ABOUT FILES
// account.h : (continued)
// Add definition of AccFile class
// --------------------------------------------------
// . . .
#include "exceptio.h"
class AccFile
{
private:
fstream f; // Stream
string name; // Filename
public:
AccFile(const string& s) throw(OpenError);
~AccFile(){ f.close(); }
long append( Account& acc) throw(WriteError);
Account* retrieve( long pos ) throw(ReadError);
};
// account.cpp: (continued)
// Implements methods of class AccFile.
// ---------------------------------------------------
long AccFile::append( Account& acc) throw( WriteError)
{
f.seekp(0L, ios::end); // Seek to end,
long pos = f.tellp(); // save the position.
if( !f )
throw WriteError(name);
TypeId id = acc.getTypeId(); // To write the TypeId
f.write( (char*)&id, sizeof(id));
if(!f)
throw WriteError(name);
else
acc.write(f); // To write an object
// to the file.
if(!f)
throw WriteError(name);
else
return pos;
}
// . . .
■PERSISTENCE OF POLYMORPHIC OBJECTS (CONTINUED)
The append() method
PERSISTENCE OF POLYMORPHIC OBJECTS (CONTINUED)
■651
䊐A Scenario
Imagine you want to save the data for various account types, including current and sav-
ings accounts to a file. Since the objects you need to save belong to different types, you
must save both the data members and the type of object. This is the only way to ensure
that an object will be correctly reconstructed when read.
The methods in the class you are going to define should throw exceptions if errors
occur. The exception type thrown by a method is stated in the exception specification.
䊐The AccFile Class
The AccFile class, which is used for random access to a file with account data, is
defined opposited. The data members are an fstream type file stream and a string used
for storing the file name.
The constructor saves the file name and opens a given file for reading and appending
at end-of-file. If the file cannot be opened, the constructor throws an OpenError class
exception.
The append() method writes an account passed to it as an argument at end-of-file
and returns the position where the account was written into the file.
In order to get the current type of the argument, the virtual method getTypeid() is
called. Depending on this type the append() method will write the appropriate type
field to the file and then call the virtual method write(), allowing the current object
to write itself to the file. If a write error occurs, the method will throw a WriteError
type exception.
The retrieve() method first reads the type identifier and then determines the type
of object it needs to allocate memory for. The data from the file is then transferred to the
dynamically allocated object by a call to the virtual method read(). Here too, an
exception will be thrown if a stream access error occurs.
652 ■
CHAPTER 29 MORE ABOUT FILES
// index.cpp: (continued)
// Implements the methods of class IndexFile.
// ----------------------------------------------------
// . . .
void IndexFile::insert(long k, long n)
throw(ReadError, WriteError)
{
IndexEntry entry;
int size = entry.recordSize(); // Length of an
// index entry.
index.clear();
index.seekg(0, ios::end);
long nr = index.tellg(); // Get file length
// 0, if file is empty.
if(!index) throw ReadError(name);
nr -= size; // Last entry.
bool found = false;
while( nr >= 0 && !found ) // Search for position
{ // to insert
if( !entry.read_at(index, nr))
throw ReadError(name);
if( k < entry.getKey()) // To shift.
{
entry.write_at(index, nr + size);
nr -= size;
}
else
{
found = true;
}
}
entry.setKey(k); entry.setPos(n); // insert
entry.write_at(index, nr + size);
if(!index)
throw WriteError(name);
}
■APPLICATION: INDEX FILES
The insert() method of class IndexFile
APPLICATION: INDEX FILES
■653
Index Primary file
12345 |
It makes sense to organize data sequentially in files if you need to walk through all the
records regularly. This is particularly true for files used to store salary data or phone bills.
However, most applications need to provide quick access to specific data. For exam-
ple, a user would definitely prefer to be able to locate an account quickly by reference to
an account number, rather than searching through a file from top to bottom. Index files
can mean a real performance boost in cases like this.
䊐Index Files
An index file comprises a so-called primary file containing the live data, and an index. The
index consists of pairs of keys and record positions for the primary file. A key stored in the
index will identify a record in the primary file. This situation can be more easily
explained by the following graphic:
The index is sorted by reference to the keys for speed of access, allowing you to perform a
binary search to locate the position of a record.
䊐Inserting into the Index
We can use the IndexFile class definition to represent an index. The insert()
method, which correctly inserts a new record into the sorted index, is defined opposite.
The read pointer is first set to end-of-file for insertions. If the current position is 0,
that is, the file is empty, the entry is inserted at offset 0. In all other cases, any entries
whose keys are greater than the new key are shifted down to make room for the new
entry.
654 ■
CHAPTER 29 MORE ABOUT FILES
// index.h: Defines the class IndexFile
// ---------------------------------------------------
class IndexFileSystem : public AccFile, public IndexFile
{
private:
string name;
public:
IndexFileSystem(const string s)
: AccFile(s + ".prim"), IndexFile(s + ".ind")
{ name = s; }
void insert ( Account& acc );
Account* retrieve( long key );
};
// index.cpp: Implementing the methods.
// ---------------------------------------------------
void IndexFileSystem::insert(Account& acc)
{ // No multiple entries:
if(search(acc.getNr()) == -1)
{
long pos = append(acc); // Insert in primary file
if(pos != -1) // Insert in
IndexFile::insert(acc.getNr(), pos); // index file.
}
}
Account* IndexFileSystem::retrieve(long key )
{ long pos = search(key); // Get the record address:
if( pos == -1 ) // Account number found?
return NULL;
else
{ IndexEntry entry; // Read the index entry:
IndexFile::retrieve(entry, pos);
// Read from primary file:
return( AccFile::retrieve( entry.getPos() ));
}
}
■IMPLEMENTING AN INDEX FILE
Representing the index file
Methods insert() and retrieve() of class IndexFileSystem
IMPLEMENTING AN INDEX FILE
■655
䊐Index File for Account Management
Since an index file consists of a primary file and an index, it makes sense to derive the
class used to represent an index file from the classes of the primary file and the index file.
Let’s now look at a sample index file, used for managing bank accounts.
The IndexFileSystem class, which is derived from the two previously defined
classes AccFile and IndexFile, is defined on the opposite page. The only data mem-
ber is a string for the file name. The constructor expects a file name as an argument and
composes names for the primary file and the index by adding a suitable suffix. Base ini-
tializers are then used to open the corresponding files.
It is not necessary to define a destructor, since files are automatically closed when the
base class destructors are called.
䊐Inserting and Retrieving Records
The insert() method was defined to insert new records. It first calls the search()
method to check whether the account number already exists in the index. If not, a new
record is appended to the end of the primary file using the append() method. Then the
key and the address of the record are inserted into the index.
The IndexFileSystem class also contains the retrieve() method, which is used
to retrieve records from the primary file. The key, key, which is passed to the method, is
used by the search() method to look up the address of the required record in the
index. Then the record is retrieved from the primary file by the AccFile class
retrieve() method.
Only the retrieve() methods for the IndexFile and AccFile classes and the
search() method, which performs a binary search in the index, are needed to com-
plete the index file implementation. It’s your job to implement these three methods as
your next exercise!
Using a sorted file to implement an index has the disadvantage that records need to
be shifted to make room to insert new records. As shifting is time-consuming, an index is
normally represented by a tree, which needs less reorganization.
exercises
656 ■
CHAPTER 29 MORE ABOUT FILES
class IndexFile
{
private:
fstream index;
string name; // Filename of the index
public:
IndexFile(const string s) throw(OpenError);
~IndexFile( ){ index.close(); }
void insert( long key, long pos)
throw(ReadError, WriteError);
long search( long key) throw(ReadError);
void retrieve(IndexEntry& entry, long pos )
throw( ReadError);
};
enum TypeId { ACCOUNT, DEPOSIT, SAVINGS };
class AccFile
{
private:
fstream f;
string name; // Filename of primary file
public:
AccFile(const string s) throw(OpenError);
~AccFile(){ f.close(); }
long append( Account& acc) throw(WriteError);
Account* retrieve( long pos ) throw(ReadError);
};
■EXERCISES
Exercise 1
Class IndexFile
Class AccFile
EXERCISES
■657
Exercise 1
Complete and test the implementation of the IndexFileSystem class.The
methods should throw exceptions of an appropriate FileError type if an error
occurs.
a. Complete the constructor of the IndexFile class in order to throw an
exception of the type OpenError if the file can not be opened.
b. Write the retrieve() method for the IndexFile class.The method
retrieves a record at a given position in the index.
c. Define the search() method, which looks up an index entry for an
account number passed to it as an argument. Base the method on the
binary search algorithm.
Return value: The position of the record found, or -1 if the account
number is not found in the index.
d. Then define the retrieve() method for the AccFile class, which first
evaluates the type field at a given position in the account file, then
dynamically allocates memory for an object of the appropriate type, and
finally reads the data for an account from the file.
e. Write a main() function that uses a try block to create an Index-
FileSystem type object and to insert several accounts of various types
into the index file.The subsequent user dialog reads a key and displays
the corresponding record on screen.Write an exception handler to han-
dle the various errors that could occur.The name of the file and the
cause of the error must be output in any case of error.
658 ■
CHAPTER 29 MORE ABOUT FILES
Hash file
Hash function
Key
4713123434
Hash Files
Hash Files profit from random file access in localizing file records directly. Each
file record must have the same length and it must be identified by a unique key,
the so-called hash key.
The idea behind hashing is to provide a function, the hash function, which is
applied to the hash key of a record and yields the address of the file record. If
the file records are numerated and if the hash key equals the record number, a
simple hash function can be the identical function. However, hash keys, such as
account or insurance numbers, consist of a fixed number of digits that do not
start with 0.
The following example shows a frequently used hash function
Example: Hash(key) = key % b;
This hash function transforms the hash value key into a record number between
0and b-1.The number 0is the first record number and b-1 is the last record
number in the address space of the hash file. For a sufficiently large prime
number b, the function Hash()yields a good distribution of file records in the
address space.
However, the hash function maps the hash key values key,key + b,key + 2*b,
etc. to the same record number. In this case collisions may occur, for example,
when a new record being inserted hashes to an address that already contains a
different record.
To solve this conflict, the new record must be inserted at some other
position.The process of finding another position at which to insert the new
record is called collision resolution.
Linear solution is a common collision resolution technique: Starting at the
occupied position, subsequent positions are checked sequentially until an
available position is found.The new file record is then inserted at this position.
EXERCISES
■659
Exercise 2
A hash file is required for speed of access to customer data.The concept of hash
files is explained on the opposite page.To keep things simple, each record in the
hash file contains only a customer id and a name.The customer id is the key
used by the hash function opposite to compute the address of the record. Use
linear solution as collision resolution technique.
Note: Linear solution provides for adequate access times if the address space is
sufficiently large and not too full. It is also important to distribute the record
numbers yielded by the hash function evenly throughout the available address
space.The hash function opposite will guarantee a good distribution if bis a
sufficiently large prime number.
■Develop the HashEntry class used to represent customer data.You need
to store the customer id as an unsigned long value and the name of
the customer as a char array with a length of 30. Supply default values for
the constructor declaration and additionally declare the read_at() and
write_at() methods that read customer information at a given position
in a stream or write information at that position. Both methods expect
the position and the stream as arguments.
■Define the HashFile class to represent a hash file.The private mem-
bers of the class comprise an fstream type file stream, a string used to
store the file name, an int variable used to store the number b, and the
hash function shown opposite as a method.The public members com-
prise a constructor that expects a file name and a number bas argu-
ments. It opens the corresponding file for read and write access.The
destructor closes the file.
Additionally declare the methods insert() and retrieve() to insert or
retrieve single records. Both methods use a call to the hash function to
compute the appropriate record number in the hash file. If a collision
occurs, the methods perform a sequential search to locate the next free
slot in the address space (mod of address space size), or the desired cus-
tomer data.
■Test the HashFile by writing a main function that creates hash file with a
small address space (e.g. b = 7).Add various customer information
records to the hash file and then retrieve this data. Deliberately provoke
collisions using the customer ids 5, 12, and 19, for example.
solutions
660 ■
CHAPTER 29 MORE ABOUT FILES
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// exceptio.h : Error classes for file processing
// ------------------------------------------------------
// Unchanged (cf. earlier in this chapter).
// ------------------------------------------------------
// account.h :
// Defines the classes
// Account, DepAcc, and SavAcc
// with virtual read and write methods as well as
// the class AccFile to represent account files.
// ------------------------------------------------------
#ifndef _ACCOUNT_H
#define _ACCOUNT_H
#include <fstream>
#include <iostream>
#include <iomanip>
#include <string>
using namespace std;
#include "exceptio.h"
enum TypeId { ACCOUNT, DEP_ACC, SAV_ACC };
class Account
{
private: // Data elements:
string name;
unsigned long nr;
double balance;
public: // Constructor:
Account( const string c_name = "X",
unsigned long c_nr = 1111111L,
double c_balance = 0.0)
: name(c_name), nr(c_nr), balance(c_balance)
{ }
virtual ~Account() {} // Virtual destructor
SOLUTIONS
■661
// Access methods here:
long getNr() const { return nr; }
void setNr(unsigned long n){ nr = n; }
// . . .
// The other methods:
virtual TypeId getTypeId() const { return ACCOUNT; }
virtual ostream& write(ostream& fs) const;
virtual istream& read(istream& fs);
virtual void display() const
{
cout << fixed << setprecision(2)
<< "----------------------------------\n"
<< "Account holder: " << name << endl
<< "Account number: " << nr << endl
<< "Balance of account: " << balance << endl
<< "----------------------------------\n"
<< endl;
}
};
class DepAcc : public Account
{
private:
double limit; // Overdrawn limit
double interest; // Interest rate
public:
DepAcc( const string s = "X",
unsigned long n = 1111111L,
double bal = 0.0,
double li = 0.0,
double ir = 0.0)
: Account(s, n, bal), limit(li), interest(ir)
{ }
// Access methods:
// . . .
// The other methods are implicit virtual:
TypeId getTypeId() const { return DEP_ACC; }
ostream& write(ostream& fs) const;
istream& read(istream& fs);
662 ■
CHAPTER 29 MORE ABOUT FILES
void display() const
{
Account::display();
cout << "Overdrawn limit: " << limit << endl
<< "Competitive interest: " << interest
<< "\n----------------------------------\n"
<< endl;
}
};
class SavAcc: public Account
{
private:
double interest; // Compound interest
public:
// Methods as in class DepAcc.
};
// --------------------------------------------------
// The definition of class AccFile
class AccFile
{
private:
fstream f;
string name; // Filename
public:
AccFile(const string& s) throw(OpenError);
~AccFile(){ f.close(); }
long append( Account& acc) throw(WriteError);
Account* retrieve( long pos) throw(ReadError);
void display() throw( ReadError);
};
#endif
SOLUTIONS
■663
// ----------------------------------------------------
// account.cpp
// Implement methods of the classes
// Account, DepAcc, SavAcc, and AccFile.
// ----------------------------------------------------
#include "account.h"
#include <typeinfo>
ostream& Account::write(ostream& os) const
{
os << name << '\0';
os.write((char*)&nr, sizeof(nr) );
os.write((char*)&balance, sizeof(balance) );
return os;
}
istream& Account::read(istream& is)
{
getline( is, name, '\0');
is.read((char*)&nr, sizeof(nr) );
is.read((char*) &balance, sizeof(balance));
return is;
}
ostream& DepAcc::write(ostream& os) const
{
if(!Account::write(os))
return os;
os.write((char*)&limit, sizeof(limit) );
os.write((char*)&interest, sizeof(interest) );
return os;
}
istream& DepAcc::read(istream& is)
{
if(!Account::read(is))
return is;
is.read((char*)&limit, sizeof(limit) );
is.read((char*)&interest, sizeof(interest));
return is;
}
// ostream& SavAcc::write(ostream& os) const
// istream& SavAcc::read(istream& is)
// as in class DepAcc.
664 ■
CHAPTER 29 MORE ABOUT FILES
// ---- Methods of class AccFile ----
AccFile::AccFile(const string& s) throw( OpenError)
{
ios::openmode mode = ios::in | ios::out | ios::app
| ios::binary;
f.open( s.c_str(), mode);
if(!f)
throw OpenError(s);
else
name = s;
}
void AccFile::display() throw(ReadError)
{
Account acc, *pAcc = NULL;
DepAcc depAcc;
SavAcc savAcc;
TypeId id;
if( !f.seekg(0L))
throw ReadError(name);
cout << "\nThe account file: " << endl;
while( f.read((char*)&id, sizeof(TypeId)) )
{
switch(id)
{
case ACCOUNT: pAcc = &acc;
break;
case DEP_ACC: pAcc = &depAcc;
break;
case SAV_ACC: pAcc = &savAcc;
break;
default: cerr << "Invalid flag in account file"
<< endl;
exit(1);
}
if(!pAcc->read(f))
break;
pAcc->display();
cin.get(); // Go on with return
}
SOLUTIONS
■665
if( !f.eof())
throw ReadError(name);
f.clear();
}
long AccFile::append( Account& acc) throw( WriteError)
{
f.seekp(0L, ios::end); // Seek to end,
long pos = f.tellp(); // save the position.
if( !f )
throw WriteError(name);
TypeId id = acc.getTypeId();
f.write( (char*)&id, sizeof(id)); // Write the TypeId
if(!f)
throw WriteError(name);
else
acc.write(f); // Add an object to the file.
if(!f)
throw WriteError(name);
else
return pos;
}
Account* AccFile::retrieve( long pos) throw(ReadError)
{
f.clear();
f.seekg(pos); // Set the get pointer
if( !f )
throw ReadError(name);
TypeId id;
f.read( (char*)&id, sizeof(id) ); // Get TypeId
if(!f)
throw ReadError(name);
Account* buf;
switch( id )
{
case ACCOUNT: buf = new Account;
break;
case SAV_ACC: buf = new SavAcc;
break;
666 ■
CHAPTER 29 MORE ABOUT FILES
case DEP_ACC: buf = new DepAcc;
break;
}
if( !(buf->read(f))) // Get data
throw ReadError(name);
return buf;
}
// -------------------------------------------------------
// index.h: Contains definitions of classes
// IndexEntry representing an index entry,
// Index representing the index and
// IndexFile representing an index file.
// -------------------------------------------------------
#ifndef _INDEX_H
#define _INDEX_H
#include <fstream>
#include <iostream>
#include <string>
#include "account.h"
using namespace std;
class IndexEntry
{
private:
long key; // Key
long recPos; // Offset
public:
IndexEntry(long k=0L, long n=0L){ key=k; recPos=n; }
void setKey(long k) { key = k; }
long getKey() const { return key; }
void setPos(long p) { recPos = p; }
long getPos() const { return recPos; }
int recordSize() const
{ return sizeof(key) + sizeof(recPos); }
fstream& write( fstream& ind) const;
fstream& read( fstream& ind);
fstream& write_at(fstream& ind, long pos) const;
fstream& read_at( fstream& ind, long pos);
SOLUTIONS
■667
void display() const
{ cout << "Account Nr: " << key
<< " Position: " << recPos << endl;
}
};
class IndexFile
{
private:
fstream index;
string name; // Filename of index
public:
IndexFile( const string& s) throw (OpenError);
~IndexFile() { index.close(); }
void insert( long key, long pos)
throw(ReadError, WriteError);
long search( long key) throw(ReadError);
void retrieve(IndexEntry& entry, long pos )
throw(ReadError);
void display() throw(ReadError);
};
class IndexFileSystem : public AccFile, public IndexFile
{
private:
string name; // Filename without suffix
public:
IndexFileSystem(const string& s)
: AccFile(s + ".prim"), IndexFile(s + ".ind")
{ name = s; }
bool insert( Account& acc);
Account* retrieve( long key);
};
#endif
668 ■
CHAPTER 29 MORE ABOUT FILES
// ------------------------------------------------------
// index.cpp : Methods of the classes
// IndexEntry, Index, and IndexFile
// ------------------------------------------------------
#include "index.h"
fstream& IndexEntry::write_at(fstream& ind, long pos) const
{
ind.seekp(pos);
ind.write((char*)&key, sizeof(key) );
ind.write((char*)&recPos, sizeof(recPos) );
return ind;
}
fstream& IndexEntry::read_at(fstream& ind, long pos)
{
ind.seekg(pos);
ind.read((char*)&key, sizeof(key) );
ind.read((char*)&recPos, sizeof(recPos));
return ind;
}
fstream& IndexEntry::write(fstream& ind) const
{
ind.write((char*)&key, sizeof(key) );
ind.write((char*)&recPos, sizeof(recPos) );
return ind;
}
fstream& IndexEntry::read(fstream& ind)
{
ind.read((char*)&key, sizeof(key) );
ind.read((char*)&recPos, sizeof(recPos));
return ind;
}
// ---------------------------------------------------
// Methods of class IndexFile
IndexFile::IndexFile(const string& file) throw (OpenError)
{
ios::openmode mode = ios::in | ios::out | ios::binary;
// Open file if it already exists:
index.open( file.c_str(), mode);
if(!index) // If the file doesn't exist
{ index.clear();
mode |= ios::trunc;
index.open( file.c_str(), mode);
if(!index)
throw OpenError(name);
}
name = file;
}
SOLUTIONS
■669
void IndexFile::display() throw(ReadError)
{
IndexEntry entry;
index.seekg(0L);
if(!index)
throw ReadError("IndexFile: Setting the get pointer");
cout << endl << "The Index: " << endl;
while( true)
{
if( !entry.read(index))
break;
entry.display();
}
if( !index.eof())
throw ReadError(name);
index.clear();
}
long IndexFile::search(long k) throw(ReadError)
{
IndexEntry entry;
long key;
long mid, begin = 0, end; // Number of file records.
int size = entry.recordSize(); // Length of an index
// entry.
index.clear();
index.seekg(0L, ios::end);
end = index.tellg() / size;
if(!index)
throw ReadError(name);
if( end == 0)
return -1;
end -= 1; // Position of the last entry
while( begin < end )
{
mid = (begin + end +1)/2 ;
entry.read_at(index, mid*size);
if(!index)
throw ReadError(name);
key = entry.getKey();
if( k < key)
end = mid - 1;
else
begin = mid;
}
670 ■
CHAPTER 29 MORE ABOUT FILES
entry.read_at(index, begin * size);
if(!index)
throw ReadError(name);
if( k == entry.getKey() ) // Key found?
return begin * size;
else return -1;
}
void IndexFile::insert(long k, long n)
throw(ReadError, WriteError)
{
IndexEntry entry;
int size = entry.recordSize(); // Length of an index
// entry.
index.clear();
index.seekg(0, ios::end);
long nr = index.tellg(); // Get file length
// 0, if file is empty.
if(!index) throw ReadError(name);
nr -= size; // Last entry.
bool found = false;
while( nr >= 0 && !found ) // Search position
{ // to insert
if(!entry.read_at(index, nr))
throw ReadError(name);
if( k < entry.getKey()) // To shift.
{
entry.write_at(index, nr + size);
nr -= size;
}
else
{
found = true;
}
}
entry.setKey(k); entry.setPos(n); // Insert
entry.write_at(index, nr + size);
if(!index)
throw WriteError(name);
}
SOLUTIONS
■671
void IndexFile::retrieve( IndexEntry& entry, long pos)
throw(ReadError)
{
index.clear();
if(!entry.read_at(index, pos))
throw ReadError(name);
}
// ---------------------------------------------------
// Implementing the methods of class IndexFileSystem.
bool IndexFileSystem::insert( Account& acc)
throw(ReadError, WriteError)
{
if(search(acc.getNr()) == -1) // No multiple entries.
{
long pos = append(acc); // Add to primary file.
IndexFile::insert(acc.getNr(), pos); // Add to Index
return true;
}
else
return false;
}
Account* IndexFileSystem::retrieve(long key )
{
// Get the record address from the index:
long pos = search(key); // Byte offset of
// index entry.
if( pos == -1 ) // Account number doesn't exist.
return NULL;
else // Account number does exist:
{
IndexEntry entry; // To read the index eintry
IndexFile::retrieve( entry, pos);
// Get from primary file:
return( AccFile::retrieve( entry.getPos() ));
}
}
// ------------------------------------------------------
// index_t.cpp : Testing the index file
// ------------------------------------------------------
#include <iostream>
#include <string>
using namespace std;
#include "index.h"
#include "account.h"
672 ■
CHAPTER 29 MORE ABOUT FILES
int main()
{
try
{
IndexFileSystem database("AccountTest");
Account acc1( "Vivi", 490UL, 12340.57);
database.insert( acc1 );
SavAcc acc2( "Ulla", 590UL, 4321.19, 2.5);
database.insert( acc2 );
DepAcc acc3( "Jeany", 390UL, 12340.20, 10000.0, 12.9);
database.insert( acc3 );
database.IndexFile::display();
cin.get();
database.AccFile::display();
unsigned long key;
cout << "Key? "; cin >> key;
if(database.search(key) != -1)
cout << "Key " << key << " found" << endl;
else
cout << "Key " << key << " not found" << endl;
Account* pAcc = database.retrieve(key);
if( pAcc != NULL )
{
pAcc->display();
delete pAcc;
pAcc = NULL;
}
else cout << "Retrieving failed" << endl;
}
catch(OpenError& err)
{
cerr << "Error on opening the file:" << err.getName()
<< endl;
exit(1);
}
catch(WriteError& err)
{
cerr << "Error on writing into the file: "
<< err.getName() << endl;
exit(1);
}
SOLUTIONS
■673
catch(ReadError& err)
{
cerr << "Error on reading from the file: "
<< err.getName() << endl;
exit(1);
}
catch(...)
{
cerr << " Unhandled Exception" << endl;
exit(1);
}
return 0;
}
Exercise 2
// -------------------------------------------------------
// exceptio.h : Error classes for file processing
// -------------------------------------------------------
// As seen previously in this chapter.
// -------------------------------------------------------
// hashFile.h
// Defines the classes
// HashEntry representing a record in a hash file and
// HashFile representing a hash file.
// -------------------------------------------------------
#ifndef _HASH_H_
#define _HASH_H_
#include <fstream>
#include <iostream>
#include <iomanip>
#include <string>
#include <string.h>
using namespace std;
#include "exceptio.h"
class HashEntry
{
private:
unsigned long nr;
char name[30];
674 ■
CHAPTER 29 MORE ABOUT FILES
public:
HashEntry(unsigned long n = 0L, const string& s = "")
{
nr = n;
strncpy(name, s.c_str(), 29); name[30]='\0';
}
long getNr() const { return nr; }
void setNr(unsigned long n){ nr = n; }
string getName() const { return name; }
void setName(const string& s)
{ strncpy(name, s.c_str(), 29); name[30]='\0'; }
int getSize() const
{ return(sizeof(long) + sizeof(name)); }
fstream& write(fstream& fs);
fstream& read(fstream& fs);
fstream& write_at(fstream& fs, unsigned long pos);
fstream& read_at(fstream& fs, unsigned long pos);
virtual void display()
{
cout << fixed << setprecision(2)
<< "----------------------------------\n"
<< "Client number: " << nr << endl
<< "Client: " << name << endl
<< "----------------------------------\n"
<< endl;
cin.get();
}
};
class HashFile
{
private:
fstream f;
string name; // Filename
unsigned long b; // Size of address space
protected:
unsigned long hash_func(unsigned long key)
{ return key%b; }
public:
HashFile(const string s, unsigned long n )
throw(OpenError);
SOLUTIONS
■675
void insert( HashEntry& rec)
throw( ReadError, WriteError );
HashEntry& retrieve( unsigned long key )
throw( ReadError );
void display();
};
#endif
// -------------------------------------------------------
// hashFile.cpp : Methods of classes HashEntry and HashFile
// -------------------------------------------------------
#include "hashFile.h"
fstream& HashEntry::write(fstream& f)
{
f.write((char*)&nr, sizeof(nr) );
f.write( name, sizeof(name) );
return f;
}
fstream& HashEntry::read(fstream& f)
{
f.read((char*)&nr, sizeof(nr) );
f.read( name, sizeof(name));
return f;
}
fstream& HashEntry::write_at(fstream& f, unsigned long pos)
{
f.seekp(pos);
f.write((char*)&nr, sizeof(nr) );
f.write( name, sizeof(name) );
return f;
}
fstream& HashEntry::read_at(fstream& f, unsigned long pos)
{
f.seekg(pos);
f.read((char*)&nr, sizeof(nr) );
f.read( name, sizeof(name));
return f;
}
676 ■
CHAPTER 29 MORE ABOUT FILES
HashFile::HashFile(const string file, unsigned long n)
throw(OpenError)
{
ios::openmode mode = ios::in | ios::out | ios::binary;
f.open(file.c_str(), mode); // Open file if it
// already exists.
if(!f) // If file doesn't exist:
{
f.clear();
mode |= ios::trunc;
f.open(file.c_str(), mode);
if(!f)
throw OpenError(name);
}
name = file;
b = n;
HashEntry rec(0L, "");
f.seekp(0L);
for( unsigned long i=0; i < b; i++) // Initialize
{ // the address space
rec.write(f);
if(!f)
throw WriteError(name);
}
}
void HashFile::insert( HashEntry& rec)
throw( ReadError, WriteError)
{
HashEntry temp;
int size = temp.getSize();
// Hash-Wert:
unsigned long pos = hash_func(rec.getNr());
temp.read_at(f, pos*size); // Read a slot.
if(!f)
throw ReadError(name);
else
{
if(temp.getNr() == 0L) // Slot free?
rec.write_at(f, pos*size); // Yes => Add
// to the file.
else // No => Search for
{ // a free slot.
bool found = false;
unsigned long p = (pos*size + size)%(b*size);
SOLUTIONS
■677
while( !found && p!= pos*size )
{
temp.read_at(f, p);
if(!f)
throw ReadError(name);
else
if(temp.getNr() == 0L) // Free slot
found = true; // found.
else
// Proceed to the next slot:
p = (p + size)%(b*size);
}
if( p == pos*size ) // Address space full.
throw WriteError(name);
if ( found == true ) // Add to file.
rec.write_at(f,p);
}
if(!f)
throw WriteError(name);
}
}
HashEntry& HashFile::retrieve( unsigned long key )
throw(ReadError)
{
static HashEntry temp;
int size = temp.getSize();
unsigned long pos = hash_func(key); // Hash value.
temp.read_at(f, pos*size); // Read a slot.
if(!f) throw ReadError(name);
if(temp.getNr() == key) // Found?
return temp; // Yes => finish
else // No => search
{
unsigned long p = (pos*size + size)%(b*size);
while( p!= pos *size )
{
temp.read_at(f, p);
if(!f)
throw ReadError(name);
678 ■
CHAPTER 29 MORE ABOUT FILES
else
if(temp.getNr() == key) // Record found.
return temp;
else
p = (p + size)%(b*size); // Proceed to the
} // next slot.
temp.setNr(0L); temp.setName(""); // Key doesn't
// exist.
return temp;
}
}
void HashFile::display()
{
HashEntry temp;
f.seekg(0L);
for(unsigned int i = 0; i < b; i++)
{
temp.read(f);
if(!f)
throw ReadError(name);
temp.display();
}
f.clear();
}
// -------------------------------------------------------
// hash_t.cpp : Tests hash files
// -------------------------------------------------------
#include <iostream>
#include <string>
#include "hashFile.h"
using namespace std;
int main()
{
try
{
HashFile hash("Client.fle", 7); // Address space
// of length 7
cout << "\nInsert: " << endl;
HashEntry kde( 3L, "Vivi");
hash.insert( kde );
SOLUTIONS
■679
kde.setNr(10L); kde.setName("Peter");
hash.insert( kde );
kde.setNr(17L); kde.setName("Alexa");
hash.insert( kde );
kde.setNr(21L); kde.setName("Peter");
hash.insert( kde );
kde.setNr(15L); kde.setName("Jeany");
hash.insert( kde );
cout << "\nInsertion complete: " << endl;
hash.display();
unsigned long key;
cout << "Key? "; cin >> key;
HashEntry temp = hash.retrieve(key);
if(temp.getNr() != 0L)
temp.display();
else
cout << "Key " << key
<< " not found" << endl;
}
catch(OpenError& err)
{
cerr << "Error in opening the file:"
<< err.getName() << endl;
exit(1);
}
catch(WriteError& err)
{
cerr << "Error writing to file: "
<< err.getName() << endl;
exit(1);
}
catch(ReadError& err)
{
cerr << "Error reading from file: "
<< err.getName() << endl;
exit(1);
}
return 0;
}
This page intentionally left blank
681
More about Pointers
This chapter describes advanced uses of pointers.These include pointers
to pointers, functions with a variable number of arguments, and pointers
to functions.
An application that defines a class used to represent dynamic
matrices is introduced.
chapter
30
682 ■
CHAPTER 30 MORE ABOUT POINTERS
// accSort.cpp: Sorts an array of pointers to accounts
// according to the account numbers
// ---------------------------------------------------
#include "account.h"
void ptrSwap(Account**, Account** );
void accSort( Account** kptr, int n)
{
Account **temp, **minp, **lastp;
lastp = kptr + n - 1; // Pointer to the last
// pointer in the array.
for( ; kptr < lastp; ++kptr )
{
minp = kptr;
for( temp = kptr + 1; temp <= lastp; ++temp )
{
if( (*temp)->getNr() < (*minp)->getNr() )
minp = temp;
}
ptrSwap( kptr, minp );
}
}
void ptrSwap( Account **p1, Account **p2 )
{
Account *help;
help = *p1; *p1 = *p2; *p2 = help;
}
■POINTER TO POINTERS
The function accSort()
POINTER TO POINTERS
■683
䊐Motivation
Pointer variables are objects that have an address in memory, and this means you can use
pointers to address them. It is thus possible to create pointers to pointers. This is necessary
if
■an array of pointers is to be dynamically allocated, or
■a function expects an array of pointers as an argument.
In both cases you need to declare a pointer variable that can access the first element in
the array. Since each element in the array is a pointer, this pointer variable must be a
pointer to a pointer.
䊐Generating Pointer Arrays Dynamically
Now let’s look into creating a dynamic array of pointers to Account class objects.
Example: Account** ptr = new Account*[400];
The pointer ptr is now pointing at the first pointer in the array with a total of 400
Account* type pointers. The array elements can be addressed as follows:
*ptr and ptr[0] (pointer to index 0)
*(ptr + i) and ptr[i] (pointer to index i)
Access to objects managed by the array is achieved as follows:
**ptr and *ptr[0] (object addressed by pointer at index 0)
**(ptr+i) and *ptr[i] (object addressed by pointer at index i)
䊐Pointer Arrays as Arguments
When you define a function that expects an array of pointers as an argument, you must
define parameters to match.
Example: void accSort( Account **kptr, int len);
You can use the kptr parameter to manipulate a pointer array whose length is stored in
the second parameter, len. After calling
Example: accSort( ptr, 100);
kptr points to the first pointer ptr[0] in the pointer array ptr. Instead of
Account **kptr you can also use the equivalent form Account *kptr[].
The opposite page shows an implementation of the function accSort(). The func-
tion uses the selection sort algorithm (which you have already worked with) for sorting.
In this case it is important not to sort the accounts itself, but to sort the pointers instead.
This saves time-consuming copying.
684 ■
CHAPTER 30 MORE ABOUT POINTERS
Fixed arguments
First varying
argument
Varying arguments
Last varying
argument
va_start(argp,max)
char *buffer
int max
•
•
•
#include <stdarg.h>
int func( char *buffer, int max, ... )
{
va_list argptr; // Declares argument pointer.
long arg3;
. . .
va_start( argptr, max); // Initialization.
arg3 = va_arg( argptr, long ); // Read arguments.
// To use argument arg3.
. . . .
va_end(argptr); // Set argument pointer to NULL.
}
■VARIABLE NUMBER OF ARGUMENTS
Fixed and varying arguments on the stack
Scheme of a function with varying arguments
VARIABLE NUMBER OF ARGUMENTS
■685
C++ allows you to define functions that allow a variable number of arguments. One
example of a function of this type is the standard C function printf(), which requires
at least one argument, a format string. The printf() function uses the conversion
specifiers in the format string to compute the number and type of arguments that follow.
䊐Obligatory and Optional Arguments
Functions with a variable number of arguments always expect a fixed number of obliga-
tory arguments and a variable number of optional arguments. At least one obligatory argu-
ment is required.
As you would expect, you need to define an appropriate parameter for each obligatory
argument when you define a function of this type. The optional arguments are repre-
sented by three dots ... in the parameter list. The function shown opposite, func(),
expects two or more arguments. The prototype is, thus, as follows
Prototype: int func( char *buffer, int max, ...);
To allow functions with a variable number of arguments to be defined, C++ pushes the
last argument onto the stack first. After calling the sample function func(), the stack
looks like the diagram opposite.
The optional arguments are accessed via a pointer, the so-called argument pointer,
which is designated by argptr here. The header files cstdarg or stdarg.h contain
macros, which conform to ANSI standard, to manage the pointer and assure that the
source code will be portable.
䊐Access to Arguments
The following steps are required to read the optional arguments:
1. The va_list type argument pointer argptr must be declared in addition to
other local variables. The type va_list is defined in the header file stdarg.h
as a typeless or char pointer.
2. The macro va_start() is then called to point the argument pointer argptr
to the first optional argument. va_start() expects two arguments: the name of
the argument pointer and the name of the last obligatory parameter.
Example: va_start( argptr, max );
686 ■
CHAPTER 30 MORE ABOUT POINTERS
// input.cpp: The function input() reads characters
// from the keyboard and appends '\0'.
// The input can be corrected with backspace.
// Arguments: 1. Pointer to the input buffer.
// 2. Maximum number of characters to be read
/ 3. Optional arguments: Characters that
// terminate the input.
// This list has to end with CR = '\r'!
// Returns: Character that breaks the input.
// ---------------------------------------------------
#include <stdarg.h>
#include <conio.h> // For getch() and putch()
int input(char *buffer, int max,... )
{
int c, breakc; // Current character, character to
// break with.
int nc = 0; // Number of characters read.
va_list argp; // Pointer to the following arguments.
while(true)
{
*buffer = '\0';
if( ( c = getch()) == 0) // Read a character.
c = getch() + 256; // For special keys:
// Extended code + 256.
va_start(argp, max); // Initialize argp.
do // Compare with break characters:
if( c == (breakc = va_arg(argp,int)) )
return(breakc);
while( breakc != '\r');
va_end( argp);
if( c == '\b' && nc > 0) // Backspace?
{
--nc, --buffer;
putch(c); putch(' '); putch(c);
}
else if( c >= 32 && c <= 255 && nc < max )
{ // Place character into the buffer
++nc, *buffer++ = c; putch(c); // and output.
else if( nc == max) // Is end of buffer reached?
putch('\a'); // Beep.
}
}
■VARIABLE NUMBER OF ARGUMENTS (CONTINUED)
The function input()
VARIABLE NUMBER OF ARGUMENTS (CONTINUED)
■687
3. When the macro va_arg() is called, the optional argument pointed to by
argptr is read from the stack. The arguments of va_arg() are the name of the
argument pointer and the type of the optional argument:
Example: arg3 = va_arg( argptr, long);
Each call to the macro va_arg() sets the argument pointer to the next optional
argument. The result of va_arg() has the type stated in the call. It must be
identical to the type of the corresponding optional argument.
There is no special terminating condition for the last optional argument. A spe-
cific value (such as NULL,⫺1or CR) can be used, or the current number of argu-
ments can be defined by an obligatory argument.
4. After evaluating the arguments the argument pointer is set to NULL by the
va_end() macro:
Example: va_end( argptr);
Optional arguments can also be read more than once. The procedure described above is
repeated beginning at Step 2, that is, with the macro va_start().
䊐Notes on the Example Opposite
The sample function input() on the opposite page uses the getch() function to read
character input from the keyboard and store it in the buffer addressed by the first argu-
ment. The second argument defines the maximum number of characters to be read. All
other arguments are characters that can terminate keyboard input. The last argument
must be a return character ('\r')!
Example: #define ESC 27 // ESC key
#define F1 (256 + 59) // F1 key
input( name, 20, ' ', ESC, F1, '\r');
This call to input() reads up to 20 characters and stores them in the array name. Input
can be terminated by pressing the space, ESC,F1, or return keys. The return value is the
corresponding character code. Non-printable characters are ignored unless stated as
optional arguments.
Special keys, such as the function keys, return a value of 0for the first call to
getch() and the extended code for the second call. For function keys this code is within
the range 59–68. To distinguish extended codes from normal ASCII codes (0–255),
the value 256 is added to the extended code. A table of extended codes is available in
the Appendix.
688 ■
CHAPTER 30 MORE ABOUT POINTERS
// funcptr.cpp: Demonstrates the use of an array
// of pointers to functions.
// --------------------------------------------------
#include <iostream>
#include <cstdlib> // Prototype of atoi()
#include <cctype> // Macros toupper(), tolower()
using namespace std;
void error_message(char *), message(char *),
message_up(char *), message_low(char *);
void (*functab[])(char *) = { error_message, message,
message_up, message_low };
char call[]="Input: 1,2, or 3";
int main()
{
int n = 0;
cout << "Which of the three functions "
<< "do you want call (1,2, or 3)?\n";
cin >> n;
if( n<1 || n>3)
(*functab[0])( call );
else
(*functab[n])("Hello, world\n");
return 0;
}
void error_message( char *s) { cerr << s << endl; }
void message( char *s) { cout << s << endl; }
void message_up( char *s)
{ int c;
for( ; *s != '\0';++s) c = toupper(*s),cout.put(c);
}
void message_low( char *s)
{ int c;
for( ; *s != '\0';++s) c = tolower(*s), cout.put(c);
}
■POINTERS TO FUNCTIONS
A jump table
POINTERS TO FUNCTIONS
■689
䊐Using Pointers to Functions
In C++ the name of a function is a constant pointer to that function. It addresses the
machine code for the function. This is a situation that we have already seen for arrays—
the array name is also a constant pointer to the first array element.
There are many uses for pointers to functions. You can save them in an array to form a
jump table. Individual functions are then accessible via an index.
A pointer to a function can also be passed as an argument to another function. This
makes sense if the function you are calling needs to work with different functions
depending on the current situation.
The standard function qsort() is an example of this. qsort() uses the quick sort
algorithm to sort an array. Depending on the type of the array elements and the sort crite-
ria, the qsort() function will expect as argument another comparison function.
䊐Declaring Pointers to Functions
A pointer to a function is declared as follows:
Syntax: type (* funcptr)( parameter_list );
This defines the variable funcptr, which can store the address of a function. The func-
tion has the type type and the parameter list stated. The first pair of parentheses is also
important for the declaration. The statement type *funcptr(parameter_list);
would declare a function funcptr that returned a pointer.
Now let’s point funcptr to the function compare() and call compare() via the
pointer.
Example: bool compare(double, double); // Prototype
bool (*funcptr)(double, double);
funcptr = compare;
(*funcptr)(9.1, 7.2);
Calling (*funcptr)() is now equivalent to calling compare(). The declaration of
compare() is necessary to let the compiler know that compare is the name of a func-
tion.
In the program shown opposite, functab is an array with four pointers to functions
of the void type, each of which expects a C string as an argument. functab is initial-
ized by the functions stated in its definition and thus functab[0] points to
error_message(),functab[1] to message(), etc. When the program is exe-
cuted, the function with the specified index is called.
690 ■
CHAPTER 30 MORE ABOUT POINTERS
■COMPLEX DECLARATIONS
1st Example:
3.
0. strptr is a
1. pointer to
2. an array with 50 elements of type
3. char.
1. 0. 2.
char (* strptr) [50]
2nd Example:
5.
6. 5. 3. 1. 0. 2. 4.
0. func is a
1. function with return value of type
2. pointer to
3. an array with elements of type
4. pointer to
5. long.
0. funcptr is a
1. pointer to
2. a function with return value of type
3. pointer to
4. an array with elements of type
5. pointer to
6. char.
4. 2. 0. 1.
long * (* func () ) []
3rd Example: char * (* (* funcptr ) () ) []
3.
COMPLEX DECLARATIONS
■691
Operator Significance
Array with elements of type
Function with return value of type
Pointer to
Reference to
[]
()
*
&
䊐Operators and Complex Declarations
In the declaration and definition of a function or a variable the same operators that you
find in expressions are used in addition to the base type and the name. These operators
are:
Acomplex declaration always uses more than one of these operators.
Example: char *strptr[50];
This declares strptr as an array of pointers to char. In a declaration, a combination of
the three operators is permissible, however, the following exceptions apply:
■the elements of an array cannot be functions
■a function cannot return a function or an array (but it can return a pointer to a
function or an array).
Operators have the same precedence in declarations as in expressions. You can use
parentheses to redefine the order of precedence.
䊐Rules
When a complex declaration is evaluated, the following rules are applied:
0. Always start with the identifier being declared.
Then repeat the following steps until all the operators have been resolved:
1. If the parentheses/brackets () or [] are on the right, they are interpreted.
2. If there is nothing or just a right bracket on the right ), the asterisk on the left is
interpreted, if it exists.
At last the base type is interpreted.
This proceeding is demonstrated by the example opposite. The above rules apply to
both the function and each of its arguments.
692 ■
CHAPTER 30 MORE ABOUT POINTERS
1st Example:
typedef DayTime FREETIME;
FREETIME timeArr[100];
2nd Example:
typedef struct { double re, im; } COMPLEX;
COMPLEX z1, z2, *zp;
3rd Example:
typedef enum { Mo, Tu, We, Th, Fr } WORKDAY;
WORKDAY day;
4th Example:
typedef enum { Diamonds, Hearts,
Spades, Clubs } COLOR;
typedef enum { seven, eight, nine, ten ,
jack, queen, king, ace } VALUE;
typedef struct
{
COLOR f;
VALUE w;
} CARD;
typedef CARD[10] HAND;
HAND player1, player2, player3;
■DEFINING TYPENAMES
DEFINING TYPENAMES
■693
䊐The typedef Keyword
C++ allows you to give types a new name using the keyword typedef.
Example: typedef unsigned char BYTE;
This defines the type name BYTE, which can then be used as an abbreviation of the
unsigned char type. The statement
Example: BYTE array[100];
will then define an array array with 100 elements of the unsigned char type. Type
names are normally uppercase, although this is not mandatory.
Examples: typedef int* INTPTR;
typedef enum{ RED, AMBER, GREEN } Lights;
Here, INTPTR identifies the type “pointer to int” and Lights is an enumerated type.
The new type name always assumes the position of a variable name in a typedef
definition. Omitting the typedef prefix will define a variable name but not a new type
name.
Type definitions do not allocate memory and do not create a new type. They simply
introduce a new name for an existing type.
Example: typedef char* (*PTR_TO_FUNC)();
The type name PTR_TO_FUNC is an abbreviation for the type “pointer to a function that
returns a pointer to char.” The declaration
Example: PTR_TO_FUNC search;
is then equivalent to
char* (*search)();
䊐Advantages
The major advantage of using typedef is that it improves the readability of your pro-
grams, especially when complex types are named.
One additional advantage is that you can isolate platform dependent types. When a
program is ported to another platform, you only need to change the platform dependent
type once in the typedef definition.
694 ■
CHAPTER 30 MORE ABOUT POINTERS
// matrix.h: Representing dynamic matrices.
// ---------------------------------------------------
#include <stdexcept>
#include <iostream>
using namespace std;
class Row
{
double *ro; int size;
public:
Row( int s) { size = s; ro = new double[s]; }
~Row(){ delete[]ro; }
double& operator[](int i) throw(out_of_range)
{if(i < 0 || i > size)
throw out_of_range("Column index: Out of Range\n");
else
return ro[i];
}
};
class Matrix
{
Row **mat; // Pointer to array of rows
int lines, cols; // Number of rows and columns
public:
Matrix(int ro , int co )
{ lines = ro; cols = co;
mat = new Row*[lines];
for(int i=0; i < lines; i++)
mat[i] = new Row(cols);
}
~Matrix()
{ for(int i=0; i < lines; i++)
delete mat[i];
delete[] mat;
}
int getLines() const { return lines; }
int getCols() const { return cols; }
Row& operator[](int i) throw(out_of_range)
{ if(i < 0 || i > cols)
throw out_of_range("Row index: Out of Range\n");
else
return *mat[i];
}
};
■APPLICATION: DYNAMIC MATRICES
Class Matrix
APPLICATION: DYNAMIC MATRICES
■695
Now let’s develop an application that uses a class to represent dynamic matrices. Matri-
ces are used for computing vectors needed to move, rotate, or zoom images in graphics
programming, for example.
䊐The Matrix Class
Memory is to be allocated dynamically to a matrix mat runtime. Additionally, it should
be possible to use the index operator to access the elements of the matrix.
Example: m[i][j] // Element in row i, column j
The class will therefore need a dynamic member to reference the matrix. As you already
know, a matrix is a single-dimensional array whose elements are single-dimensional
arrays themselves.
The class Row, which can be used to represent single-dimensional arrays of double
values, is defined opposite. The index operator is overloaded for the Row class to allow
an exception of the out_of_range type to be thrown for invalid indices.
The Matrix class contains a dynamic member, mat, which can address an array of
pointers to Row objects. mat is thus a pointer to a pointer.
䊐Constructor, Destructor, and Subscript Operator
The constructor in the Matrix class creates an array of lines pointers to objects of the
Row type. A loop is then used to allocate memory to the rows dynamically.
In contrast, the destructor releases the memory occupied by the line arrays first, before
releasing the space occupied by the pointer array mat.
The subscript operator in the Matrix class returns the line array ifor a given index
i. When the following expression is evaluated
Example: m[2][3]
the first call is to the subscript operator of the Matrix class, which returns a line array to
index 2. Then the subscript operator of the Row class is called for the line array. It
returns a reference to the double value at index 3.
You will be enhancing the Matrix class in the exercises to this chapter by overload-
ing the copy constructor and the assignment, for example.
exercises
696 ■
CHAPTER 30 MORE ABOUT POINTERS
#include <iostream>
using namespace std;
char* color[] = {"WHITE", "PINK", "BLUE", "GREEN" };
int main()
{
cout << *color[1] << " "
<< *color << " "
<< *(color[3] + 3) << " "
<< color[2] + 1 << " "
<< *( *(color + 1) + 3)
<< endl;
return 0;
}
Since the comparison function compare will be called as a C function, it should
be declared and defined as
extern "C" int compare(....);
Refer to the Appendix “Binding C Functions.”
✓NOTE
■EXERCISES
Listing for exercise 1
For exercise 3
The standard function qsort()
#include <cstdlib>
void qsort( void* array, size_t n, size_t size,
int (*compare)(const void*, const void*));
The function qsort(), ”quick sort,” sorts an array of nelements whose first
element is pointed to by array. The size of each element is specified by size.
The comparison function pointed to by compare is used to sort the content
of the array in ascending order. qsort() calls the function with two arguments
that point to the elements being compared.
You will normally need to define the comparison function yourself.The
function must return an integer less than, equal to, or greater than zero if the
first argument is less than, equal to, or greater than the second.
EXERCISES
■697
You can use the SelectionSort() function defined in Exercise 4 of Chapter 17 as an algorithm for
sorting the int values in this exercise.
Use the standard function qsort(), whose prototype is defined opposite, as quick sort algorithm
for this exercise.
✓NOTE
Exercise 1
What does the program opposite output on screen?
Exercise 2
Write a function min() that computes and returns the minimum of two positive
numbers.The function expects a variable number of unsigned int values as
arguments.The last argument must be the number 0.
Exercise 3
Write a C++ program that compares the speed of the quick sort and selection
sort algorithms
Sort two identical sequences of random numbers of the type int to test the
sort algorithms. Read the maximum number of random numbers from the
keyboard and allocate the needed memory dynamically. Display the time in
seconds required for the sort operation on screen after each sort operation.
Exercise 4
Write additional methods to complete the Matrix class.
■Add a const version of the subscript operator to the Row and Matrix
classes. Use an inline implementation.
■Define a constructor for the Matrix class.The constructor dynamically
allocates a matrix with a given number of rows and columns, and initial-
izes the matrix elements with a given value.Also write a copy construc-
tor.
■Overload the assignment operator =and the compound assignment oper-
ator +=.
Addition is defined for two n⫻nmatrices, Aand B, which have equal num-
bers of rows and columns.The sum Cis a n⫻nmatrix whose elements
are computed by adding elements as follows
C[i,j] = A[i,j] + B[i,j] for i, j = 0, ..., n-1
■Test the Matrix with a suitable main function that calls all the methods.
Display the results of the calculations on screen.
solutions
698 ■
CHAPTER 30 MORE ABOUT POINTERS
■SOLUTIONS
Exercise 1
Screen output: P WHITE E LUE K
Exercise 2
// -------------------------------------------------------
// minivar.cpp
// Defines and tests the function min(), which
// computes and returns the minimum of positive integers.
// The function expects a variable number of arguments
// with unsigned int types.
// The last argument must be 0!
// -------------------------------------------------------
#include <stdarg.h>
unsigned int min( unsigned int first, ... )
{
unsigned int minarg, arg;
va_list argptr; // Pointer to optional arguments
if( first == 0)
return 0;
va_start( argptr, first);
minarg = first;
while( (arg = va_arg(argptr, unsigned int) ) != 0)
if( arg < minarg)
minarg = arg;
va_end (argptr);
return minarg;
}
// ----- A small function main() for testing---------------
#include <iostream>
using namespace std;
int main()
{
cout << "\nThe minimum of : 34 47 19 22 58 "
<< "is: " << min(34, 47, 19, 22, 58, 0)
<< endl;
return 0;
}
SOLUTIONS
■699
Exercise 3
// -- ----------------------------------------------------
// sort_t.cpp
// Compares the performances of sorting algorithms
// quick sort and selection sort
// For this purpose, two identical arrays are dynamically
// generated and initialized with random numbers.
// The times needed for sorting are displayed.
// -------------------------------------------------------
#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>
using namespace std;
void isort(int *v, int lenv);
// For qsort():
extern "C" int intcmp(const void*, const void*);
main()
{
unsigned int i, size;
int *numbers1, *numbers2;
long time1, time2;
cout << "\n The performance of the sorting algorithms"
<< "\n quick sort and selection sort"
<< "\n is being compared.\n\n"
<< "\nHow many numbers are to be sorted? ";
cin >> size;
numbers1 = new int[size];
numbers2 = new int[size];
cout << "\nThere are "
<< size << " random numbers to be generated.\n";
srand((unsigned)time(NULL)); // Initialize the
// random number generator.
for(i = 0 ; i < size ; ++i)
numbers1[i] = numbers2[i] = rand(); // Random numbers
cout << "\nSorting starts! Please wait.\n";
time(&time1); // Length of time
// for quick sort.
qsort(numbers1, size, sizeof(int), intcmp);
time(&time2);
cout << "\nTime taken by the quick sort algorithm: "
<< time2 - time1 << " seconds.\n";
700 ■
CHAPTER 30 MORE ABOUT POINTERS
cout << "\nI am sorting again. Please wait!\n";
time(&time1); // Length of time
isort(numbers2, size); // for selection sort
time(&time2);
cout << "\nTime taken by the insertion sort algorithm: "
<< time2 - time1 << " seconds.\n"
<< "\nOutput sorted numbers? (y/n)\n\n";
char c; cin >> c;
if( c == 'Y' || c == 'y')
for( i = 0 ; i < size ; ++i)
cout << setw(12) << numbers1[i];
cout << endl;
return 0;
}
extern "C" int intcmp( const void *a, const void *b)
{
return (*(int*)a - *(int*)b);
}
// -------------------------------------------------------
// isort() sorts an array of int values
// using the selection sort algorithm.
void isort( int *a, int len) // Sort the array a of
{ // length len in ascending
register int *b, *minp; // order
int *last, help;
last = a + len - 1; // Points to the last element
for( ; a <= last; ++a) // Search for the smallest
{ // element starting at a.
minp = a; // minp points to the "current"
// smallest array element.
for( b = a+1; b <= last; ++b) // Search for the
if( *b < *minp ) // minimum.
minp = b;
help = *a, *a = *minp, *minp = help; // Swap.
}
}
SOLUTIONS
■701
Exercise 4
// -- ----------------------------------------------------
// matrix.h : Represents dynamic matrices
// -------------------------------------------------------
#ifndef _MATRIX_H_
#define _MATRIX_H_
#include <stdexcept>
#include <iostream>
using namespace std;
class Row
{
double *ro;
int size;
public:
Row( int s) { size = s; z = new double[s]; }
~Row(){ delete[]ro; }
double& operator[](int i)
{
if(i < 0 || i > size)
throw out_of_range("Row index: Out of Range\n");
return ro[i];
}
const double& operator[](int i) const
{
if(i < 0 || i > size)
throw out_of_range("Row index: Out of Range\n");
return ro[i];
}
};
class Matrix
{
private:
Row **mat; // Pointer to array of rows
int lines, cols; // Number of rows and columns
public:
Matrix(int ro , int co)
{
lines = ro; cols = co;
mat = new Row*[lines];
for(int i=0; i < lines; i++)
mat[i] = new Row(cols);
}
Matrix:: Matrix( int z, int s, double val);
702 ■
CHAPTER 30 MORE ABOUT POINTERS
Matrix( const Matrix& );
~Matrix()
{ for(int i=0; i < lines; i++)
delete mat[i];
delete[] mat;
}
int getLines() const { return lines; }
int getCols() const { return cols; }
Row& operator[](int i)
{
if(i < 0 || i > cols)
throw out_of_range("Row index: Out of Range\n");
return *mat[i];
}
const Row& operator[](int i) const
{
if(i < 0 || i > cols)
throw out_of_range("Row index: Out of Range\n");
return *mat[i];
}
// Assignments:
Matrix& operator=( const Matrix& );
Matrix& operator+=( const Matrix& );
};
#endif
// ------------------------------------------------------
// matrix.cpp : Defines methods of class Matrix
// ------------------------------------------------------
#include "matrix.h"
Matrix:: Matrix( int ro, int co, double val)
{
lines = ro; cols = co;
mat = new Row*[lines]; // Array of pointers to
// arrays of rows
int i, j;
for(i=0; i < lines; i++) // Arrays of rows:
{
mat[i] = new Row(cols); // Allocate memory
for(j = 0; j < cols; ++j)
(*this)[i][j] = val; // and copy values.
}
}
SOLUTIONS
■703
Matrix:: Matrix( const Matrix& m)
{
lines = m.lines; cols = m.cols; // Rows and columns
mat = new Row*[lines]; // Array of pointers
// to arrays of rows
int i, j;
for(i=0; i < lines; i++) // Arrays of rows:
{
mat[i] = new Row(cols); // To allocate
// storage
for( j = 0; j < cols; ++j)
(*this)[i][j] = m[i][j]; // and copy values.
}
}
Matrix& Matrix::operator=(const Matrix& m)
{
int i, j; // Free "old" storage:
for(i=0; i < lines; i++)
delete mat[i];
delete[] mat;
lines = m.lines; cols = m.cols; // Rows, columns
mat = new Row*[lines]; // Array of pointers
// to arrays of rows
for(i=0; i < lines; ++i) // Array of rows:
{
mat[i] = new Row(cols); // Allocate space
for( j = 0; j < cols; ++j)
(*this)[i][j] = m[i][j]; // and copy values.
}
return *this;
}
Matrix& Matrix::operator+=( const Matrix& m)
{
int i, j;
if( cols == m.cols && lines == m.lines)
for( i=0; i < lines; ++i)
for( j=0; j < cols; ++j)
(*this)[i][j] += m[i][j];
return *this;
}
704 ■
CHAPTER 30 MORE ABOUT POINTERS
// ------------------------------------------------------
// matrix_t.cpp : Tests dynamic matrices
// ------------------------------------------------------
#include "matrix.h"
void display( Matrix& m); // Output a matrix.
int main()
{
Matrix m(4,5);
try
{
int i,j;
for( i=0; i < m.getLines(); i++)
for( j=0; j < m.getCols(); j++)
m[i][j] = (double)i + j/ 100.0;
cout << "Matrix created" << endl;
display(m);
Matrix cop(m);
cout << "Copy generated." << endl;
display(cop);
cop += m;
cout << "Compute the sum:" << endl;
display(cop);
Matrix m1(4, 5, 0.0);
cout << "Initializing a matrix with 0:" << endl;
display(m1);
m = m1;
cout << "Matrix assigned:" << endl;
display(m);
}
catch(out_of_range& err)
{ cerr << err.what() << endl; exit(1); }
return 0;
}
void display( Matrix& m)
{
for(int i=0; i < m.getLines(); i++)
{
for(int j=0; j < m.getCols(); j++)
cout << m[i][j] << " ";
cout << endl;
}
cin.get();
}
705
Manipulating Bits
This chapter describes bitwise operators and how to use bit masks.The
applications included demonstrate calculations with parity bits,
conversion of lowercase and capital letters, and converting binary
numbers. Finally, the definition of bit-fields is introduced.
chapter
31
706 ■
CHAPTER 31 MANIPULATING BITS
Bitwise AND Bitwise inclusive ORResult Result
0 & 0
0 & 1
1 & 0
1 & 1
0 | 0
0 | 1
1 | 0
1 | 1
0
0
0
1
0
1
1
1
0 ˆ 0
0 ˆ 1
1 ˆ 0
1 ˆ 1
~0
~1
0
1
1
0
1
0
a = 5;
b = 12;
c = a & b;
c = a | b;
c = a ˆ b;
c = ~a;
unsigned int a, b, c;
0 0 . . . . . . . 0 0 1 0 1
0 0 . . . . . . . 0 1 1 0 0
0 0 . . . . . . . 0 1 1 0 1
0 0 . . . . . . . 0 1 0 0 1
1 1 . . . . . . . 1 1 0 1 0
0 0 . . . . . . . 0 0 1 0 0
Bit pattern
■BITWISE OPERATORS
“True or False” table for bitwise operators
Examples
BITWISE OPERATORS
■707
䊐Bit Coding Data
In cases where conservative use of memory is imperative, data often need to be bit coded,
a technique to represent information as individual bits. Some examples of bit coded data
can be found in file access rights or the status-word of a stream.
To access bit coded data, you need to be able to read or modify individual bits. C++
has six bitwise operators to perform these tasks:
■Logical bitwise operators
&AND |inclusive OR
^ exclusive OR ∼NOT
■Bitwise shift operators
<< Left shift >> Right shift
Operands for bitwise operators must have integral types. Operands belonging to float
or double types are invalid.
The boolean tables on the opposite page show the effect of the logical bitwise opera-
tors for individual bits. If a bit is set, that is, if it has a value of 1, it will be interpreted as
true. If the bit is not set, and thus has a value of 0, it will be interpreted as false. Exam-
ples for each bitwise operator follow.
The result type of a bitwise operation will be the integral type defined by the operand
type. If, for example, both operands are int types, the result will also be of an int type.
䊐Arithmetic Type Conversions and Precedence
If the operands of a bitwise operator are of different types, normal arithmetic type con-
version will occur. If one operand type is an int and the other a long, the int value
will be converted to long before the operation is performed.
The logical bitwise operators &or |should not be confused with the logical && and
|| operators. The latter do not affect individual bits but interpret the whole value of
their operands as boolean, returning a boolean value. The expression 1 && 2 returns
the value true, whereas 1 & 2 has the value 0.
The precedence of the bitwise NOT operator ∼is high, since ∼is a unary operator. As
you can see from the table of precedence in the appendix, both the binary operators &, ^,
and |have low precedence. However, their precedence is higher than that of the logical
operators && and ||.
708 ■
CHAPTER 31 MANIPULATING BITS
a = 12;
b = a << 3;
b = a >> 2;
unsigned int a, b;
0 0 . . . . 0 0 0 0 1 1 0 0
0 0 . . . . 0 1 1 0 0 0 0 0
0 0 . . . . 0 0 0 0 0 0 1 1
Bit pattern
// getbin_t.cpp: Defines the function getbin(), which
// reads a binary number (ex. 0101... )
// from the standard input, and returns
// a value of type unsigned int.
// --------------------------------------------------
#include <iostream>
using namespace std;
unsigned int getbin()
{
char c;
unsigned int val = 0;
while ( (c = cin.get()) == ' ' || c == '\t' )
; // Ignore leading blanks and tabs
while( c == '0' || c == '1' ) // Read and convert
{ // the binary number
val = (val << 1) | (c - '0');
c = cin.get();
}
return val;
}
■BITWISE SHIFT OPERATORS
Right and left shift
Using shift operators
BITWISE SHIFT OPERATORS
■709
䊐Left and Right Shift
The shift operators << and >> shift the bit pattern of their left operand a certain number
of bit positions. The number of positions is defined by the right operand. The examples
opposite illustrate this point.
In the case of a left shift, 0bits are padded. The bits dropped on the left are lost.
Example: short x = 0xFF00;
x = x << 4; // Result: 0xF000
In the case of a right shift, 0bits are padded from the left if the left operand is an
unsigned type or has a positive value. In all other cases, the compiler determines
whether to pad an expression with 0bits (logical shift) or with the sign bit (arithmetic
shift), although an arithmetic shift normally occurs.
Example: short x = 0xFF00;
x = x >> 4; // Result: 0xFFF0
To ensure portable source code, you should use right shifts for positive values only.
䊐Integral Promotion
Integral promotion is performed for the operands of a shift operator, that is char is
extended to int. The result type in a shift operation is then the same as the type of the
left operand after integral promotion.
The result of a shift operation is unpredictable if the value of the right operand is neg-
ative or larger than the length of the left operand expressed in bits.
Example: char x = 0xFF;
x = x >> 9; // undefined result
䊐Applications
Shift operators allow you to perform efficient multiplication and division with 2n. Shift-
ing a number n places left (or right) is equivalent to a multiplication (or division) by 2n.
Examples: unsigned res,number = 5;
res = number << 3; // 5 * 23= 40
res = number >> 1; // 5 / 21= 2
710 ■
CHAPTER 31 MANIPULATING BITS
Bit pattern
Bit position
higher bits lower bits
n n-1 4 3 2 1 0
1010101 ••••
Current bit patterns: 0 1 0 0 0 0 0 1 'A' = 0x41
| 0 0 1 0 0 0 0 0 MASK = 0x20
0 1 1 0 0 0 0 1 'a' = 0x61
■BIT MASKS
Bit positions
Example
#define MASK 0x20
char c = 'A';
c = c | MASK;
BIT MASKS
■711
䊐Deleting Bits
The bitwise AND operator is normally used to delete specific bits. A so-called mask is used
to determine which bits to delete.
Example: c = c & 0x7F;
In the mask 0x7F the seven least significant bits are set to 1, and all significant bits are
set to 0. This means that all the bits in c, with the exception of the least significant bits,
are deleted. These bits are left unchanged.
The variable ccan be of any integral type. If the variable occupies more than one
byte, the significant bits in the mask, 0x7F, are padded with 0bits when integral promo-
tion is performed.
䊐Setting and Inverting Bits
You can use the bitwise OR operator |to set specific bits. The example on the opposite
page shows how to change the case of a letter. In ASCII code, the only difference
between a lowercase and an uppercase letter is the fifth bit.
Finally, you can use the bitwise exclusive OR operator ^to invert specific bits. Each
0-bit is set to 1and each 1-bit is deleted if the corresponding bit in the mask has a
value of 1.
Example: c = c ^ 0xAA;
The bit pattern for 0xAA is 10101010. Every second bit in the least significant eight
bits of cis therefore inverted.
It is worthy of note that you can perform double inversion using the same mask to
restore the original bit pattern, that is, (x ^ MASK) ^ MASK restores the value x.
The following overview demonstrates the effect of a statement for an integral expres-
sion xand any given mask, MASK:
■x & MASK deletes all bits that have a value of 0 in MASK
■x | MASK sets all bits that have a value of 1 in MASK
■x ^ MASK inverts all bits that have a value of 0 in MASK.
The other bits are left unchanged.
712 ■
CHAPTER 31 MANIPULATING BITS
// parity_t.cpp: Defines the function parity(), which
// computes the parity of an unsigned
// value.
// Returns: 0, if the number of 1-bits is even,
// 1 in all other cases.
// ---------------------------------------------------
inline unsigned int bit0( unsigned int x )
{
return (x & 1);
}
int parity( unsigned int n)
{
unsigned int par = 0;
for( ; n != 0; n >>=1 )
par ^= bit0(n);
return (par);
}
■USING BIT MASKS
Computing the parity of an integer
USING BIT MASKS
■713
䊐Creating Your Own Masks
You can use the bitwise operators to create your own bit masks.
Example: x = x & ∼3;
In the bit pattern of 3, only the bits at positions 0and 1are set. The mask ∼3therefore
contains a whole bunch of 1-bits and only the two least significant bits are 0. The above
expression would thus delete the two least significant bits in x.
The mask ∼3is independent of the word length of the computer and is thus preferable
to the mask 0xFFFC.
The next example shows masks that address exactly one bit in a word. They are cre-
ated by left shifting 1.
Examples: x = x | (1 << 6);
x = x & ∼(1 << 6);
The first expression sets the sixth bit in x. The same bit is then deleted, as only the sixth
bit in the mask ∼(1 << 6) has a value of 0.
Of course, you can also use masks such as (1 << n) where nis a variable containing
the bit position.
Example: int setBit(int x, unsigned int n)
{
if( n < sizeof(int) )
return( x & (1 << n);
}
䊐Bitwise Operators in Compound Assignments
The binary bitwise operators &,|,^,<<, and >> can be used in compound assignments.
Examples: x >>= 1;
x ^= 1;
Both statements are equivalent to
x = x >> 1;
x = x ^ 1
The function parity() shown opposite includes compound assignments with bitwise
operators. Parity bit computation is used to perform error recognition in data communi-
cations.
714 ■
CHAPTER 31 MANIPULATING BITS
Byte
General Flow
Control
Header Error
Control
Virtual Path
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Virtual Channel
Identifier
Payload
Type
CLP
Virtual Path
Identifier
General Flow Control
Virtual Path/Channel Identifier
Payload Type
Controls the data stream
Address of the virtual path/channel
Distinguish between payload and control
data
Mark cells with high priority
Check sum for header
CLP (Cell Loss Priority)
Header Error Control
1
2
3
4
5
struct ATM_Cell
{
unsigned GFC : 4; // General Flow Control
unsigned VPI : 8; // Virtual Path Identifier
unsigned VCI : 16; // Virtual Channel Identifier
unsigned PT : 3; // Payload Type
unsigned CLP : 1 // Cell Loss Priority
unsigned HEC : 8; // Header Error Control
char payload[48]; // Payload
};
■BIT-FIELDS
Header of ATM cells
Representing an ATM cell
BIT-FIELDS
■715
C++ lets you divide a computer word into bit-fields and give the bit-fields a name. Bit-
fields offer major advantages; their uncluttered structure makes them preferable and less
error prone than using masks and bitwise operators to manipulate individual bits.
䊐Defining Bit-Fields
Bit-fields are defined as data members of a class. Each bit-field is of unsigned int type
with an optional name and width. The width is defined as the number of bits the bit-field
occupies in a computer word and is separated from the bit-field name by a colon.
Example: struct { unsigned bit0_4 : 5;
unsigned : 10;
unsigned bit15 : 1; } word;
The member word.bit0_4 designates the 5least significant bits in a computer word
and can store values in the range 0to 31. The second data member does not have a
name and is used to create a gap of 10 bits. The member word.bit15 contains the
value at bit position 15.
You cannot reference nameless bit-fields. They are used to align the subsequent bit-
fields at specific bit positions.
The width of a bit-field cannot be greater than that of the computer word. The width
0has a special significance; the subsequent bit-field is positioned on the next word
boundary, that is, it begins with the next computer word. If a bit-field will not fit in a
computer word, the following bit-field is also positioned on the next word boundary.
There are several special cases you need to consider when dealing with bit-fields:
■you cannot use the address operator for bit-fields. You cannot create arrays of bit-
fields. Neither restriction applies to a class containing bit-field members, how-
ever.
■the order bit-fields are positioned in depends on the machine being used. Some
computer architectures position bit-fields in reverse order. This is true of DEC
Alpha workstations, for example.
䊐The Sample Program Opposite
The opposite page shows a class designed to represent ATM cells. Cells are used for data
transportation in ATM (Asynchronous Transfer Mode) networks. Each cell comprises a 5
byte header with addresses and a checksum for error checking and 48 byte data section
or payload. The header shown here is used to connect a computer to the network in the
User Network Interface.
exercises
716 ■
CHAPTER 31 MANIPULATING BITS
****** BITWISE OPERATORS ******
Please enter two integers.
1st Number --> 57
2nd Number --> -3
The bit pattern of 57 = x : 0000 0000 0011 1001
The bit pattern of -3 = y : 1111 1111 1111 1101
The bit pattern of x & y : 0000 0000 0011 1001
The bit pattern of x | y : 1111 1111 1111 1101
The bit pattern of x ^ y : 1111 1111 1100 0100
How many bit positions is x to be shifted?
Count --> 4
The bit pattern of x << 4 : 0000 0011 1001 0000
The bit pattern of x >> 4 : 0000 0000 0000 0011
Repeat (y/n)?
■EXERCISES
Sample screen output for exercise 1
EXERCISES
■717
Exercise 1
a. Write the function putBits() that outputs the bit pattern of a number
as an unsigned int type. Only the 16 least significant bits are to be
output no matter what the size of the computer word.The number is
passed as an argument to the function, which has no return value.
b. Write a tutorial to demonstrate the effect of bitwise operators. First
read two decimal integers from the keyboard and store them in the vari-
ables xand y.Then use the function putBits() to output the bit pat-
terns of x,x&y,x | y,x ^ y, and ∼x.
To demonstrate the shift operators, shift the value of xa given number of
bit positions right and left. Read the number of bit positions from key-
board input. Use the value 1 in case of invalid input.
The opposite page shows sample output from the program.
Exercise 2
Your task is to encrypt data to prevent spying during data communications.The
sender uses a filter to encrypt the data in question, and the receiver uses the
same filter to decrypt the transmission.
a. Define the function swapBits() that swaps two bits in an int value.The
int value and the positions of the bits to be swapped are passed as argu-
ments to the function.The return value is the new int value. If one of
the positions passed to the function is invalid, the int value should be
returned unchanged.
b. Write a filter that swaps the bits at bit positions 5 and 6, 0 and 4, and 1
and 3 in all characters except control characters (defined as ASCII Code
>= 32).
Test the filter by writing the encrypted output to a file and then using the same
filter to output the new file.The output must comprise the original unencrypted
data.
solutions
718 ■
CHAPTER 31 MANIPULATING BITS
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// bits_t.cpp
// Demonstrates bitwise operators.
// ------------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
void putbits( unsigned int n); // Prototype of putbits()
int main() // Learning bitwise operations
{
int x, y, count;
char yn;
do
{
cout << "\n\n ****** BITWISE OPERATIONS ******\n";
cout << "\nPlease enter two integers.\n\n"
<< "1st number --> ";
cin >> x;
cout << "2nd number --> ";
cin >> y;
cout << "\nThe bit pattern of "
<< setw(6) << x << " = x : ";
putbits(x);
cout << "\nThe bit pattern of "
<< setw(6) << y << " = y : ";
putbits(y);
cout << "\nThe bit pattern of x & y : ";
putbits(x&y);
cout << "\nThe bit pattern of x | y : ";
putbits(x|y);
cout << "\nThe bit pattern of x ^ y : ";
putbits(x^y);
cout << "\n\nHow many bit positions"
" is x to be shifted?"
<< "\nNumber --> ";
cin >> count;
SOLUTIONS
■719
if( count < 0 || count > 15)
{
cout << "Invalid input!"
<< " Shifting by one bit position.\n";
count = 1;
}
cout << "\nThe bit pattern of x << "
<< setw(2) << count << " : ";
putbits( x << count);
cout << "\nThe bit pattern of x >> "
<< setw(2) << count << " : ";
putbits( x >> count);
cout << "\nRepeat (y/n)? ";
cin >> yn;
while( (yn | 0x20) != 'y' && yn != 'n')
;
}while( yn == 'y');
return 0;
}
// ------------------------------------------------------
// Output the bit pattern of n (only the 16 lower bits).
void putbits( unsigned int n )
{
int i;
for( i = 15; i >= 0 ; --i)
{
cout << (char)( ((n>>i) & 1) + '0'); // i-th bit
if( i % 4 == 0 && i > 0) // and after 4 bits
cout << ' '; // one blank.
}
}
Exercise 2
// -------------------------------------------------------
// hide_t.cpp: Filter to encrypt data.
// Swap bits in bit positions 5 and 6,
// 0 and 4, 1 and 3 for all characters
// except control characters.
// Modules: hide_t.cpp, swapbits.cpp
//
// Call: hide_t [ < sourcefile ] [ > destfile ]
// -------------------------------------------------------
720 ■
CHAPTER 31 MANIPULATING BITS
#include <iostream>
using namespace std;
int swapbits( int ch, int bitnr1, int bitnr2); // Prototype
int main() // Encrypt data
{
int c;
while( (c = cin.get()) != EOF)
{
if( c >= 32) // Control character?
{
c = swapbits(c, 5, 6); // Swap bits
c = swapbits(c, 0, 4);
c = swapbits(c, 1, 3);
}
cout << c;
}
return 0;
}
// -------------------------------------------------------
// swapbits.cpp: The function swapbits() swaps two bits
// within an integer.
// Arguments: The integer and two bit positions.
// Returns: The new value.
// -------------------------------------------------------
int swapbits( int x, int bitnr1, int bitnr2)
{ // To swap two bits in x.
int newx, mask1, mask2;
int msb = 8 * sizeof(int) - 1; // Highest bit position
if( bitnr1 < 0 || bitnr1 > msb ||
bitnr2 < 0 || bitnr2 > msb)
return x; // Return, if bit position is invalid
mask1 = (1 << bitnr1); // Shift 1 to position bitnr1
mask2 = (1 << bitnr2); // Shift 1 to position bitnr2
newx = x & ~(mask1 | mask2); // Delete both bits
if( x & mask1 ) newx |= mask2; // Swap bits.
if( x & mask2 ) newx |= mask1;
return( newx);
}
721
Templates
Templates allow you to construct both functions and classes based on
types that have not yet been stated.Thus, templates are a powerful tool
for automating program code generation.
This chapter describes how to define and use function and class
templates. In addition, special options, such as default arguments,
specialization, and explicit instantiation are discussed.
chapter
32
722 ■
CHAPTER 32 TEMPLATES
■FUNCTION AND CLASS TEMPLATES
Template and instantiation
Template
Instantiation for
Type long
Type int
Type char
FUNCTION AND CLASS TEMPLATES
■723
䊐Motivation
As a programmer you will often be faced with implementing multiple versions of similar
functions and classes, which are needed for various types.
A class used to represent an array of int values is very similar to a class representing
an array of double values, for example. The implementation varies only in the type of
elements that you need to represent. Operations performed with elements, such as search
and sort algorithms, must be defined separately for each type.
C++ allows you to define templates—parameterized families of related functions or
classes:
■afunction template defines a group of statements for a function using a parameter
instead of a concrete type
■aclass template specifies a class definition using a parameter instead of a concrete
type.
A class template can provide a generic definition that can be used to represent various
types of arrays, for example. During instantiation, that is, when a concrete type is defined,
an individual class is created based on the template definition.
䊐Advantages of Templates
Templates are powerful programming tools.
■A template need only be coded once. Individual functions or classes are automat-
ically generated when needed.
■A template offers a uniform solution for similar problems allowing type-inde-
pendent code to be tested early in the development phase.
■Errors caused by multiple encoding are avoided.
䊐Templates in the Standard Library
The C++ standard library contains numerous class template definitions, such as the
stream classes for input and output, string, and container classes. The classes string,
istream,ostream,iostream, and so on are instantiations for the char type.
The standard library also includes an algorithm library, which comprises many search
and sort algorithms. The various algorithms are implemented as global function tem-
plates and can be used for any set of objects.
724 ■
CHAPTER 32 TEMPLATES
// stack.h : The class template Stack with
// methods push() and pop().
//-----------------------------------------------------
template<class T>
class Stack
{
private:
T* basePtr; // Pointer to array
int tip; // Stack tip
int max; // Maximum number of elements
public:
Stack(int n){ basePtr = new T[n]; max = n; tip = 0;}
Stack( const Stack<T>&);
~Stack(){ delete[] basePtr; }
Stack<T>& operator=( const Stack<T>& );
bool empty(){ return (tip == 0); }
bool push( const T& x);
bool pop(T& x);
};
template<class T>
bool Stack<T>::push( const T& x)
{
if(tip < max - 1) // If there is enough space
{
basePtr[tip++] = x; return true;
}
else return false;
}
template<class T>
bool Stack<T>::pop( T& x)
{
if(tip > 0) // If the stack is not empty
{
x = basePtr{--tip]; return true;
}
else return false;
}
■DEFINING TEMPLATES
Class template Stack
DEFINING TEMPLATES
■725
䊐Defining Function Templates
The definition of a template is always prefixed by
template<class T>
where the parameter Tis a type name used in the definition that follows. Although you
must state the class keyword, Tcan be any given type, such as an int or double.
Example: template <class T>
void exchange(T& x, T&y)
{
T help(x); x = y; y = help;
}
This defines the function template exchange(). The parameter Trepresents the type
of variables, which are to interchange. The name Tis common but not mandatory.
䊐Defining Class Templates
Example: template <class U>
class Demo
{
U elem; . . . // etc.
};
This defines the class template Demo<U>. Both Uand Demo<U> are treated like normal
types in the class definition. You simply need to state the name of the template, Demo,
within the class scope.
The methods of a class template are also parameterized via the future type. Each
method in a class template is thus a function template. If the definition is external to the
class template, function template syntax is used. The method name is prefixed by the
class template type and the scope resolution operator.
The example on the opposite page illustrates this point by defining a stack template.
A stack is managed according to the last-in-first-out principle, lifo-principle for short; the
last element to be “pushed” onto the stack is the first to be removed, or “popped,” from
the stack.
The methods of a class template are normally defined in the same header file. This
ensures that the definition will be visible to the compiler, since it requires the definition
to generate machine code for concrete template arguments.
726 ■
CHAPTER 32 TEMPLATES
// stack_t.cpp: Testing a stack
// ----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#include "stack.h"
typedef Stack<unsigned> USTACK; // Stack for elements
// of type unsigned.
void fill( USTACK& stk );
void clear( USTACK& stk );
int main()
{
USTACK ustk(256); // Create and fill
fill( ustk); // the original stack.
USTACK ostk(ustk); // Copy.
cout << "The copy: " << endl;
clear( ostk); // Output and clear the copy.
cout << "The original: " << endl;
clear( ustk ); // Output, clear the original.
return 0;
}
void fill( USTACK& stk )
{
unsigned x;
cout << "Enter positive integers (quit with 0):\n";
while( cin >> x && x != 0 )
if( !stk.push(x) )
{
cerr << "Stack is full!"; break;
}
}
void clear( USTACK& stk )
{
if(stk.empty())
cerr << "Stack is empty!" << endl;
else
{
unsigned x;
while( stk.pop(x))
cout << setw(8) << x << " ";
cout << endl;
}
}
■TEMPLATE INSTANTIATION
Sample program
TEMPLATE INSTANTIATION
■727
Defining a template creates neither a concrete function nor a class. The machine code
for functions or methods is not generated until instantiation.
䊐Instantiating Template Functions
Atemplate function is instantiated when it is first called. The compiler determines the
parameter type of Tby the function arguments.
Example: short a = 1, b = 7;
exchange( a, b );
The template is first used to generate the exchange() function machine code for the
short type. The template functions can be called after this step.
This allows you to generate an exchange() template function for any type. Given
that xand yare two double variables, the following statement
Example: exchange( x, y );
creates a second template function for the double type.
䊐Instantiation of Template Classes
The instantiation of a template class is performed implicitly when the class is used for the
first time, for example, when an object of the template class is defined.
Example: Stack<int> istack(256); // implicit
This statement first creates the template class Stack<int>, generating the machine
code of all methods for the int type. After this step has been completed, an istack
object of the Stack<int> type can be constructed.
If a further template class, such as Stack<float> is created, the machine code gen-
erated for the methods in this template class will be different from the machine code of
the Stack<int> methods.
In other words, developing templates will not reduce the amount of machine code
required for a program. However, it does spare the programmer’s extra work required to
develop multiple versions of functions and classes.
Templates are double checked for errors by the compiler—once when the template
definition is compiled and again during instantiation. The first check recognizes errors
that are independent of the template parameters. Errors in parameterization cannot be
detected until instantiation if, for example, an operator for the template argument type
has not been defined.
728 ■
CHAPTER 32 TEMPLATES
// stackn.h: Class Template Stack<T, n>
// ---------------------------------------
template <class T, int n>
class Stack
{
private:
T arr[n]; // Array
int tip; // Tip of stack
int max; // Maximum number of elements
public:
Stack(){ max = n; tip = 0; };
bool empty(){ return (tip == 0); }
bool push( const T& x);
bool pop(T& x);
};
template<class T, int n>
bool Stack<T, n>::push( const T& x)
{
if(tip < max - 1)
{
arr[tip++] = x; return true;
}
else return false;
}
template<class T, int n>
bool Stack<T, n>::pop(T& x )
{
if(tip > 0)
{
x = arr{--tip]; return true;
}
else return false;
}
■TEMPLATE PARAMETERS
The Stack template with two template parameters
TEMPLATE PARAMETERS
■729
䊐Multiple Template Parameters
You can also define templates with multiple parameters like the following class template,
Example: template <class U, class V>
class Demo
{ // . . . };
which has two parameters Uand V. A class Demo<U,V> is defined for each pair of U,V
types.
A template parameter need not always be a type name. Normal function parameters
are also permissible, particularly pointers and references.
Example: template<class T, int n>
class Stack{ . . . };
This defines the class template Stack<T, n> that is parameterized with the type Tand
an integer n.
The example on the opposite page uses the parameter nto specify the size of an array
used to represent a stack. The major advantage is that the number of array elements is
already known when an object of the template class is instantiated. Objects can then be
created without allocating dynamic storage.
This simplifies the definition of the stack template. The copy constructor, the assign-
ment operator, and the destructor no longer need to be defined!
䊐Restrictions
Two restrictions apply to template parameters other than type parameters:
■they cannot be modified
■they cannot be floating-point types.
The following expression would thus be invalid in the definition opposite:
Example: ++n; // Error: changing template parameter
Even though double type template parameters are not permissible,
Example: template<class T, double d> // Error!
class Demo { . . . };
pointers and references to floating-point types are:
Example: template<class T, double& ref>
class Demo { . . . };
730 ■
CHAPTER 32 TEMPLATES
// mini_t.cpp: Passing arguments to
// function templates
// ----------------------------------------------
#include <iostream>
using namespace std;
template <class T>
T min( T x, T y)
{
return( (x < y) ? x : y);
}
int main()
{
short x = 10, y = 2;
cout << "x = " << x << " y = " << y << endl;
cout << "The smaller value is: "
<< min(x, y) << endl; // Call is ok.
double z1 = 2.2;
float z2 = 1.1F;
cout << "\nThe smaller value is: "
<< min(z1, z2) << endl; // Not ok!
double z3 = 1.1;
cout << "\nz1 = " << z1
<< " z3 = " << z3 << endl;
cout << "The smaller value is: "
<< min(z1, z3) << endl; // Call is ok.
return 0;
}
■TEMPLATE ARGUMENTS
Sample program
TEMPLATE ARGUMENTS
■731
䊐Passing Arguments
A template is instantiated when a template argument is passed to it. The argument types
must exactly match to the types of the template parameters.
Not even implicit type conversions, such as float to double, are performed. In the
case of the template function min() this means that both arguments must be of the
same type. The following call
Example: float x = 1.1; double y = 7.7;
min ( x , y );
would lead to an error message, since the template function cannot be defined by the
Prototype: void min( float , double );
䊐Restrictions
There are several restrictions for template arguments other than type names:
■if the template parameter is a reference, only a global or static object can be
passed as a template argument
■if the template parameter is a pointer, only the address of an object or a function
with global scope can be stated
■if the template parameter is neither a reference nor a pointer, only constant
expressions can be used as template arguments.
Example: int cnt = 256; // Error:
typedef Stack<short, cnt> ShortStack;
Since only an int constant is permitted as a template argument, this statement provokes
an error.
Strings, such as "Oktoberfest,"are also invalid as template arguments, as their
scope is static and not global.
Example: template<class T,char* s> class Demo{...};
Only globally defined strings can be used for instantiation, for example
char str[] = "Oktoberfest"; // global
Demo<double, str> income; // ok
732 ■
CHAPTER 32 TEMPLATES
template <class T>
T min( T x, T y)
{
return( (x < y) ? x : y)
}
#include <cstring>
const char* min( const char* s1, const char* s2 )
{
return( (strcmp(s1, s2) < 0 ) ? s1: s2 );
}
#include <cstring>
template<>
const char* min( const char* s1, const char* s2 )
{
return( (strcmp(s1, s2) < 0 ) ? s1: s2 );
}
■SPECIALIZATION
Function template min()
Specializing the function template for C strings
䊐ANSI specialization
The ANSI standard does not differ between template functions and “normal” functions.
The definition of a function template and a function with the same name, which can be
generated by the function template, causes the compiler to output an error message (ex.
“duplicate definition ...”).
That is why the ANSI standard provides its own syntax for defining specializations:
SPECIALIZATION
■733
䊐Motivation
A template function can only be instantiated if all the statements contained in the func-
tion can be executed. If you call the template function exchange() with two objects of
the same class, the copy constructor and the assignment must be defined for this class.
More specifically, all operators used in a template function must be defined for the
current argument type. Thus, the function template min(), which determines the lesser
of two arguments, can only be instantiated if the operator < is defined for the argument
type.
Besides non-executable instructions there are other reasons to prevent a function
template being instantiated for a particular type:
■the generic approach defined by the template does not return any useful results
for a given type
■there are more efficient approaches for some types.
The statement
Example: minStr = min(, "VIVIAN", "vivian" );
only returns the lower of the two addresses at which the C strings are stored.
䊐Defining Specialization
In cases like this, it makes sense to specialize the template function definition. To do so,
you use a function with a separate definition to overload the template function. This
technique is demonstrated on the opposite page using the function template min(),
where a specialization has been defined for the char* type, both for older and more
modern compilers that support the current ANSI standard.
If a template function is replaced by a specialization, the appropriate version must be
executed when a call to the function is made. The order the compiler looks up a function
guarantees that if both a function template and a specialization are defined for a specific
type, the specialization will be called.
This also applies to the methods of a class template, which are function templates, of
course. More specifically, a template class can only be created if all the methods in the
appropriate function template can be instantiated without error.
734 ■
CHAPTER 32 TEMPLATES
// quadMat.h: Defines the template QuadMatrix
// to represent quadratic matrices
// -----------------------------------------------------
#include <iostream>
#include <stdexcept>
using namespace std;
template <class T, int cnt = 10>
class QuadMatrix
{
private:
T mat[cnt][cnt];
public:
int dim() const{ return cnt; }
T* operator[](int line) throw(out_of_range)
{
if( line < 0 || line >= cnt)
throw out_of_range("Matrix: Index out of range");
else
return mat[line];
}
const T* operator[](int line) const
throw(out_of_range)
{
if( line < 0 || line >= cnt)
throw out_of_range("Matrix: Index out of range");
else
return mat[line];
}
friend QuadMatrix& operator+(const QuadMatrix&,
const QuadMatrix&);
// etc.
};
■DEFAULT ARGUMENTS OF TEMPLATES
A class template representing quadratic matrices
DEFAULT ARGUMENTS OF TEMPLATES
■735
䊐Setting Defaults
You can define default arguments for template parameters, just as for function parame-
ters. If an argument required to instantiate a template is missing, the default value is then
used.
You can specify default values in the template definition or when you declare a tem-
plate in a module.
䊐The Class Template QuadMatrix<T, n>
The class template defined opposite, QuadMatrix<T, n>, represents quadratic matri-
ces. The subscript operator is overloaded to allow you to access a matrix element
m[i][j] in a given matrix m. If the line index iis outside of the valid range, a standard
out_of_range type exception is thrown.
The default values are chosen to create a matrix mfor int values with 10 rows and 10
columns following this definition:
Example: typedef QuadMatrix < > IntMat;
IntMat m;
You cannot omit the angled brackets since the QuadMatrix type does not exist.
QuadMatrix is merely the name of a template.
The following definition
Example: typedef QuadMatrix<double> DoubleMat;
DoubleMat dm;
defines a matrix dm of double values with 10 rows and 10 columns.
䊐Rules
The same rules apply to the default arguments of templates as to the default arguments of
functions:
■if you declare a default argument for at least one parameter, you must define
default values for all the remaining parameters
■if a template argument for which a default argument was declared is omitted dur-
ing instantiation, all the remaining template arguments must be omitted.
736 ■
CHAPTER 32 TEMPLATES
// expIns_t.cpp: Tests explicit instantiation
// -----------------------------------------------------
#include <iostream>
#include <iomanip>
using namespace std;
#include "quadMat.h"
// Explicit Instantiation:
template class QuadMatrix<long double, 5>;
int main()
{
QuadMatrix<long double, 5> m;
try
{
for(int k=0; k < m.dim(); k++)
{
for( int l = 0; l < m.dim(); l++)
{
m[k][l] = k*l;
cout << setw(2) << m[k][l] << " ";
}
cout << endl;
}
}
catch(out_of_range& err )
{
cerr << err.what() << endl;
}
return 0;
}
■EXPLICIT INSTANTIATION
Sample program for the class template QuadMatrix
EXPLICIT INSTANTIATION
■737
In addition to implicit instantiation of templates, which occurs when a template func-
tion is called, for example, explicit instantiation is also possible. This is important when
you design libraries that contain template functions and classes for application programs.
䊐Syntax
Explicit instantiation can be achieved by the following
Syntax: template declaration;
where declaration contains the name of the template and the template arguments.
Explicit instantiation for the class template Stack would be performed as follows:
Example: template class Stack<long double, 50>;
This declaration creates the template class Stack<long double, 50> with a maxi-
mum of 50 long double type elements.
Function templates can also be instantiated explicitly.
Example: template short min( short x, short y);
This creates a template function for the short type from the function template min().
䊐ANSI Instantiation
The ANSI standard provides an additional technique for the explicit instantiation of
function templates. Template arguments are stated in the angled brackets that follow the
function name, when the function is first called.
Example: min<long>(x, y);
In this case, a template function min() for the long type is generated. This advanced
syntax for function templates is not supported by all C++ compilers, however.
Explicit instantiation of function templates extends their possible usage:
■function templates can be parameterized by types that cannot be derived from the
function arguments—more specifically, function templates can be defined with-
out function parameters
■function templates can be defined with function parameters that are not template
parameters themselves.
exercises
738 ■
CHAPTER 32 TEMPLATES
The left, sorted part originally consists of only one element, the first array
element.
✓NOTE
■EXERCISES
Interpolation search
The elements of a numerical array are assumed to be unique and sorted in
ascending order.
The given value is compared with the array element at the position where the
value is “expected” to be. For example, if the value searched for is two-thirds of
the way from the lowest to the highest subarray element, the probe would be
made two-thirds from the lowest to the highest index of the subarray. If the
required value is lesser than that of the array element found at the expected
position, the search is continued in the left part of the subarray, just like a binary
search. Otherwise the search continues in the right part of the subarray.
The “expected” position exp in an array vcan be calculated as follows: If key
is the required value, begin is the lowest, and end is the highest index of the
corresponding subarray, the following applies:
double temp = (double)(key-vp[begin]);
temp /= (vp[end]-vp[begin]);
temp = temp * (end - begin) + 0.5;
exp = begin + (int)temp;
Insertion sort algorithm
The following technique is used to divide the array into a left, sorted part and a
right, unsorted part:
Each subsequent element in the unsorted part of the array is selected and
taken out from the array.As long as a greater array element can be found
starting from the end of the left subarray, the element is shifted up by one
position. If a smaller array element is found, the selected element is inserted at
the vacant position.
30
30
30
50
50
50
70
70
40
70
20
20
20
70 > 40? yes
to take out
Graphic:
↑
EXERCISES
■739
Exercise 1
■Define a function template interpolSearch() that looks up a given ele-
ment in a sorted, numeric array.The array elements are of the same type
as the template parameter T.
The function template has three parameters—the value searched for of
type T, a pointer to the first array element, and the number of array ele-
ments.
The function template returns the index of the first element in the array
that corresponds to the searched for value, or –1 if the value cannot be
found in the array.
Implement the function template. Use the technique described opposite
as your algorithm for the interpolation search. Store the function tem-
plate definition in the header file search.h.
■Define a function template, insertionSort(), which sorts a numeric
array in ascending order.The array elements are of the type of the tem-
plate parameter T.
The function template has two parameters—a pointer to the first array
element, and the number of array elements.There is no return value.
■Define a function template display() to display a numeric array on
screen.
The function template has two parameters—a pointer to the first array
element and the number of array elements.There is no return value.
■Use the function templates interpolSearch(), insertionSort(), and
display() to define template functions for double and short types.To
do so, define an array of double values and an array with short values.
Write a main function that creates and calls each template function
insertionSort() for the int and double types.Then display the
sorted arrays.
Add a call to each template function search() in your main function.
Call search() passing values that exist and do not exist in the array.
740 ■
CHAPTER 32 TEMPLATES
class FloatArr // Without conversion functions
{
private:
float* arrPtr; // Dynamic member
int max; // Maximum number, without having
// to reallocate storage.
int cnt; // Current number of elements
void expand( int new Size); // Function to help
// enlarge the array.
public:
FloatArr( int n = 256 );
FloatArr( int n, float val);
FloatArr(const FloatArr& src);
~FloatArr();
FloatArr& operator=( const FloatArr& );
int length() const { return cnt; }
float& operator[](int i) throw(BadIndex);
float operator[](int i) const throw(BadIndex);
void append( float val);
void append( const FloatArr& v);
FloatArr& operator+=( float val)
{
append( val); return *this;
}
FloatArr& operator+=( const FloatArr& v)
{
append(v); return *this;
}
void insert( float val, int pos) throw(BadIndex);
void insert( const FloatArr& v, int pos )
throw(BadIndex);
void remove(int pos) throw(BadIndex);
friend ostream& operator<<( ostream& os,
const FloatArr& v);
};
Exercises
The class FloatArr (as defined in Chap. 28, Ex. 1)
EXERCISES
■741
Exercise 2
Define a class template Array<T, n> to represent an array with a maximum of
nelements of type T.Attempting to address an array element with an invalid
index should lead to an exception of the BadIndex (defined previously) error
class being thrown. If there is no space left to insert an element in the array, an
exception of the OutOfRange type should be thrown.
■First define the error class OutOfRange without any data members. Use
the error class BadIndex, which was defined in Exercise 1 of Chapter 28.
■Change the existing FloatArr class into a class template, Array<T, n>.
Use 255 as the default value for the parameter nof the class template.
This allocates memory for the array statically. Now define a default con-
structor and a constructor that initializes a given number of array ele-
ments with a given value.You do not need to define a copy constructor, a
destructor, or an assignment operator.
As access methods define the size() and the length() methods, the
size() method returns the maximum number of array elements, that
is, n; the length() method returns the current number of elements in
the array.
Also define methods for inserting and deleting elements like those
defined for the FloatArr class.The methods have a void return type and
can throw exceptions of type BadIndex and/or OutOfRange.
Additionally overload the index and shift operators <<.The subscript
operator throws BadIndex type exceptions.
■Test the class template Array<T,n> for double and int types first.
Define arrays of the appropriate types, then insert and delete elements in
the arrays. Output all the elements of the array.
■Modify the test program by adding an array for objects of a class type.
Use the DayTime class from Exercise 1, Chapter 19 for this purpose.
Test the array template by defining an array with 5 DayTime class objects
and inserting a few objects.Then display all the objects on screen.
solutions
742 ■
CHAPTER 32 TEMPLATES
■SOLUTIONS
Exercise 1
// ------------------------------------------------------
// interpol.cpp : Template function interpolSearch()
// ------------------------------------------------------
#include <iostream>
using namespace std;
template <class T>
long interpolSearch(const T& key, T* vp, int len)
{
int expect, begin = 0, end = len - 1;
double temp;
if( end < 0 // Array is empty or
|| key > vp[end] // or key is out of
|| key < vp[begin] ) // range
return -1;
while( begin <= end )
{
if(key > vp[end] || key < vp[begin] ) // Key is not
return -1; // in range.
temp = (double)(key - vp[begin])
/ (vp[end]-vp[begin]);
temp = temp * (end - begin) +0.5;
expect = begin + (int)temp;
if( vp[expect] == key ) // Key found?
return expect;
if( vp[expect] > key)
end = expect - 1;
else begin = expect+1;
}
return -1;
}
template <class T>
void insertionSort( T* vp, int len)
{
T temp;
for( int i=0; i < len; i++)
{
temp = vp[i]; // Take element out.
int j; // Shift greater elements up:
for( j = i-1; j >= 0 && vp[j] > temp; j--)
vp[j+1] = vp[j];
vp[j+1] = temp; // Insert.
}
}
SOLUTIONS
■743
template <class T>
void display(T* vp, int len)
{
cout << "\n\nThe array: " << endl;
for(int i = 0; i < len; i++)
{
cout << vp[i] << " ";
if( (i+1)%10 == 0)
cout << endl;
}
cout << endl; cin.get();
}
// Two arrays for testing:
short sv[5] = { 7, 9, 2, 4, 1};
double dv[5] = { 5.7, 3.5, 2.1, 9.4, 4.3 };
int main()
{
cout << "\nInstantiation for type short: " << endl;
display(sv, 5);
insertionSort(sv, 5);
cout << "\nAfter sorting: ";
display(sv, 5);
short key;
cout << "\nArray element? "; cin >> key; cin.sync();
int pos = interpolSearch(key, sv, 5);
if( pos != -1)
cout << "\nfound!" << endl, cin.get();
else
cout << "\nnot found!" << endl, cin.get();
// -------------------------------------------------
cout << "\nInstantiation for type double: " << endl;
display(dv, 5);
insertionSort(dv, 5);
cout << "\nAfter sorting: ";
display(dv, 5);
double dkey;
cout << "\nArray element? "; cin >> dkey; cin.sync();
pos = interpolSearch(dkey, dv, 5);
if( pos != -1)
cout << "\nfound!" << endl, cin.get();
else
cout << "\nnot found!" << endl, cin.get();
return 0;
}
744 ■
CHAPTER 32 TEMPLATES
Exercise 2
// ------------------------------------------------------
// array.h
// Use of class templates to represent arrays.
// ------------------------------------------------------
#ifndef _ARRAY_H_
#define _ARRAY_H_
#include <iostream>
#include <iomanip>
using namespace std;
class BadIndex
{
private:
int index;
public:
BadIndex(int i):index(i){}
int getBadIndex() const { return index; }
};
class OutOfRange { /* Without data members*/ };
template <class T, int n = 256>
class Array
{
private:
T arr[n]; // The array
int cnt; // Current number of elements
public:
Array( ){ cnt = 0;}
Array(int n, const T& val );
int length() const { return cnt; }
int size() const { return n; }
T& operator[](int i) throw(BadIndex)
{
if( i < 0 || i >= cnt ) throw BadIndex(i);
return arr[i];
}
const T& operator[](int i) const throw(BadIndex)
{
if( i < 0 || i >= cnt ) throw BadIndex(i);
return arr[i];
}
SOLUTIONS
■745
Array& operator+=( float val) throw(OutOfRange)
{
append( val); return *this;
}
Array& operator+=(const Array& v) throw(OutOfRange)
{
append(v); return *this;
}
void append( T val) throw(OutOfRange);
void append( const Array& v) throw(OutOfRange);
void insert( T val, int pos)
throw(BadIndex, OutOfRange);
void insert( const Array& v, int pos )
throw(BadIndex, OutOfRange);
void remove(int pos) throw(BadIndex);
};
template <class T, int n >
Array<T,n>::Array(int m, const T& val )
{
cnt = m;
for(int i=0; i < cnt; i++ )
arr[i] = val;
}
template <class T, int n >
void Array<T,n>::append( T val) throw(OutOfRange)
{
if( cnt < n)
arr[cnt++] = val;
else
throw OutOfRange();
}
template <class T, int n >
void Array<T,n>::append( const Array<T,n>& v) throw(OutOfRange)
{
if( cnt + v.cnt > n) // Not enough space.
throw OutOfRange();
int count = v.cnt; // Necessary if
// v == *this
for( int i=0; i < count; ++i)
arr[cnt++] = v.arr[i];
}
746 ■
CHAPTER 32 TEMPLATES
template <class T, int n >
void Array<T,n>::insert( T val, int pos)
throw(BadIndex, OutOfRange)
{ insert( Array<T,n>(1,val), pos);
}
template <class T, int n >
void Array<T,n>::insert( const Array<T,n>& v, int pos )
throw(BadIndex, OutOfRange)
{
if( pos < 0 || pos >= cnt)
throw BadIndex(); // Invalid position.
if( n < cnt + v.cnt)
throw OutOfRange();
int i;
for( i = cnt-1; i >= pos; --i) // Shift up
arr[i+v.cnt] = arr[i]; // starting at pos.
for( i = 0; i < v.cnt; ++i) // Fill the gap.
arr[i+pos] = v.arr[i];
cnt = cnt + v.cnt;
}
template <class T, int n >
void Array<T,n>::remove(int pos) throw(BadIndex)
{
if( pos >= 0 && pos < cnt)
{
for( int i = pos; i < cnt-1; ++i)
arr[i] = arr[i+1];
--cnt;
}
else throw BadIndex(pos);
}
template <class T, int n >
ostream& operator<<(ostream& os, const Array<T,n>& v)
{
int w = os.width(); // Save the field width
for( int i = 0; i < v.cnt; ++i)
{
os.width(w); os << v.arr[i];
}
os << endl;
return os;
}
#endif
SOLUTIONS
■747
// ----------------------------------------------------
// DayTime.h
// Class DayTime with relational operators,
// the operators ++ and -- (prefix and postfix),
// and the operators << and >> for I/O.
// ----------------------------------------------------
// The same as in Chapter 19.
// ------------------------------------------------------
// Array_t.cpp
// Testing class templates Array<T,n>.
// ------------------------------------------------------
#include "array.h"
#include "DayTime.h"
#include <cstdlib>
#include <iostream>
#include <iomanip>
using namespace std;
typedef Array<int, 100> IntArr;
typedef Array<double> DoubleArr;
typedef Array<DayTime, 5> DayTimeArr;
int main()
{
try
{
const DoubleArr vd(10, 9.9);
DoubleArr kd;
cout << "\nThis is the constant array of doubles: \n";
cout << setw(8) << vd;
kd = vd;
cout << "\nAn array of doubles after the assignment: "
<< endl;
cout << setw(8) << kd;
kd.remove(3); // Delete the element at
// position 3.
kd.append(10.0); // Add a new element.
kd.append(20.0); // And repeat!
cout << "\nThis is the modified array: "
<< endl;
cout << setw(8) << kd;
748 ■
CHAPTER 32 TEMPLATES
IntArr vi;
int i;
for(i=0; i < 10; i++)
vi.append(rand()/100);
cout << "\nThis is the array of int values: \n";
cout << setw(12) << vi;
vi += vi;
cout << "\nAnd append: \n";
cout << setw(12) << vi;
IntArr ki(vi);
cout << "\nThis is the copy of the array: \n";
cout << setw(12) << ki;
DayTimeArr vt; // Array of DayTime objects.
DayTime temp;
for(i=0; i < 3; i++)
{
if( !(cin >> temp))
break;
vt.append(temp);
}
cout << "\nThe array with objects of type DayTime:\n";
for(i=0; i < 3; i++)
cout << setw(20) << vt[i] << endl;
}
catch(BadIndex& err)
{
cerr << "\nIndex " << err.getBadIndex()
<< " invalid";
exit(1);
}
catch(OutOfRange& )
{
cerr << "\nArray is full!";
exit(2);
}
return 0;
}
749
Containers
This chapter describes standard class templates used to represent
containers for more efficient management of object collections.These
include
■sequences, such as lists and double ended queues
■container adapters, such as stacks, queues, and priority queues
■associative containers, such as sets and maps, and
■bitsets.
Besides discussing how to manage containers, we will also be looking at
sample applications, such as bitmaps for raster images, and routing
techniques.
chapter
33
750 ■
CHAPTER 33 CONTAINERS
■CONTAINER TYPES
Sequences and associative containers
Arrays Stacks Queues
etc.
Sets
...
Maps Bitsets
Sequences Associative
Containers
Containers
CONTAINER TYPES
■751
䊐What is a Container?
Containers are used to store objects of the same type and provide operations with which
these objects can be managed. These operations include object insertion, deletion, and
retrieval. Memory is allocated for containers dynamically at runtime. Containers thus
provide a safe and easy way to manage collections of objects.
The C++ standard library provides various class templates for container management
in the Containers Library. These classes can be categorized as follows:
■sequential containers, or sequences, where the objects are arranged sequentially
and access to an object can either be direct or sequential
■associative containers, where the objects are generally organized and managed in
a tree structure and can be referenced using keys.
䊐Sequences
Sequential containers are distinguished by the operations defined for them, which are
either generic or restricted. Restricted operations, such as appending at the end of a con-
tainer, have constant runtimes. That is, the runtime is proportional to a fixed period of
time and does not depend on the number of objects in the container.
The following are sequential containers:
■arrays, which provide the same operations as C arrays but increase and decrease
in size dynamically, in contrast to C arrays
■queues, which are managed on the FIFO (First In First Out) principle. The first
element to be inserted is also removed first
■stacks, which are managed on the LIFO (Last In First Out) principle. The last
element to be inserted is removed first.
䊐Associative Containers and Bitsets
Associative containers comprise sets, which allow quick access to objects via sortable
keys, and maps, which maintain efficient object/key pairs.
There are also so-called bitsets, which represent bit sequences of a given length and
provide bitwise operators, with which bits can be manipulated.
752 ■
CHAPTER 33 CONTAINERS
■SEQUENCES
Operations for sequences
Class Template Time needed to insert or
remove an object
vector<class T, class Allocator
= allocator<T> >
At the end: constant.
At the beginning or in the
middle: linear.
In all positions: constant.
At the beginning or end:
constant.
In the middle: linear
list<class T, class Allocator
= allocator<T> >
deque<class T, class Allocator
= allocator<T> >
Container adapters
Class Template Insertion Deletion
stack<class T, class Container
= dequeue<T> >
at the end at the end
queue<class T, class Container
= dequeue<T> >
at the end at the
beginning
at the
beginning
priority_queue<class T,
class Container = vector<T>,
Compare=less<T> >
priority
based
Sequences and header files
Container Header File
vector<T, Allocator>
list<T, Allocator>
deque<T, Allocator>
stack<T, Container>
queue<T, Container>
priority_queue<T,
<vector>
<list>
<deque>
<stack>
<queue>
<queue>
Container, Compare >
SEQUENCES
■753
䊐Representing Sequences
The Containers Library defines so-called container classes representing containers. These
are class templates parameterized by the type Tof the objects to be managed.
Three basic class templates are defined for sequences.
■The container class vector<T, Allocator> supports standard array opera-
tions, such as direct access to individual objects via the subscript operator [],
and quick appending and deletion at the end of the container. However, the run-
time for insertion and deletion at the beginning or in the middle of the container
is linear, that is, proportional to the number of objects stored in the container.
■The container class list<T, Allocator> provides functionality that is typi-
cal for double linked lists. This includes quick insertion and deletion at any given
position. General list operations, such as sorting and merging, are also defined.
■The container class deque<T, Allocator> (double ended queue, pronounced
“deck”) provides direct access via a subscript operator, just like a normal array,
but offers optimized insertion and deletion at the beginning and end of the con-
tainer. The same operations in the middle of a container have a linear runtime.
The second template parameter is used for any storage allocation to be performed. The
storage management is represented by a so-called allocator class, which is parameterized
by an object of type T. It enables dynamic memory allocation for objects of type T. The
default value of the template parameter is the standard allocator class allocator<T>
that uses the new and delete operators to allocate and release memory.
䊐Adapter Classes
The basic sequence classes are used to construct so-called adapter classes. An adapter class
expects a sequence as a template argument and stores the sequence in a protected
data member.
The opposite page shows various adapter classes. The priority_queue template
represents priority queues. The relationship between the keys used to manage the priori-
ties is defined in the comparator class, Compare. The default value of the template
parameter is the predefined comparator class, less<T>, which uses the lesser than oper-
ator <for type T.
754 ■
CHAPTER 33 CONTAINERS
// Outputs a list containing integers.
// ---------------------------------------------------
#include <list>
#include <iostream>
using namespace std;
typedef list<int> INTLIST; // int list
int display(const INTLIST& c)
{
int z = 0; // Counter
list<int>::const_iterator pos; // Iterator
for( pos = c.begin(); pos != c.end(); pos++, z++)
cout << *pos << endl;
cout << endl;
return z;
}
// iterat_t.cpp: Outputs an array of accounts.
// ---------------------------------------------------
#include <vector>
#include <iostream>
using namespace std;
#include "account.h"
typedef vector<Account> AccVec; // Account vector
void display(const AccVec& v)
{
AccVec::const_iterator pos; // Iterator
for( pos = v.begin(); pos < v.end(); pos++)
pos->display();
cout << endl;
}
■ITERATORS
Iterating lists
Iterating vectors
ITERATORS
■755
䊐Positioning and Iterating in Containers
Each object in a container occupies the specific position where it was stored. To allow
you to work with the objects in a container, the positions of the objects in the container
must be accessible. There must therefore be at least one mechanism that allows:
■read and/or write access to the object at any given position and
■moving from the position of one object to the position of the next object in the
container.
This situation should be familiar from your experience of working with pointers. Given
that iis the index of an element in an array v,(v+i) is its address, *(v+i) the array
element itself, and (v + (++i)) the address of the next array element.
Iterators were introduced in C++ to provide a uniform model for positioning and iter-
ation in containers. An iterator can thus be regarded as an abstraction of a pointer.
䊐Iterator Types
Two types of iterators are important in this context:
■bidirectional iterators, which can be shifted up by the increment operator ++
and down with the decrement operator --, and use the operators *and -> to
provide write or read access to objects
■random access iterators, which are bidirectional iterators that can additionally
perform random positioning. The subscript operator [] was overloaded for this
purpose, and the operations defined for pointer arithmetic, such as addition/sub-
traction of integers or comparison of iterators, are defined.
The container classes vector<T> and deque<T> have random access iterators and the
container class list<T> has bidirectional iterators.
䊐Iterator Classes
The types iterator and const_iterator are defined in all the above classes to rep-
resent iterators. An iterator belonging to one of these classes can reference constant or
non-constant objects.
The methods begin() and end() are also defined. The begin() method accesses
the first position and end() accesses the position after the last container object.
Containers that belong to adapter classes offer only restricted access to the beginning
or end. You cannot use iterators to walk through them.
756 ■
CHAPTER 33 CONTAINERS
// sortVec.h: The Class Template SortVec representing
// a sorted vector.
//---------------------------------------------------
#include <vector> // For class template vector<T>
#include <functional> // For comparator class less<T>
using namespace std;
template <class T, class Compare = less<T> >
class SortVec : public vector<T>
{
public:
SortVec() { }
SortVec(int n, const T& x = T());
void insert(const T& obj); // in sorted order
int search(const T& obj);
void merge(const SortVec<T>& v);
};
// sortv_t.cpp : Tests the template SortVec.
//---------------------------------------------------
#include "sortVec.h"
typedef SortVec<int> IntSortVec;
int main()
{
IntSortVec v, w; // Default constructor
v.insert(2);
v.insert(7); v.insert(1);
int n = v.search(7);
w.insert(3); w.insert(9);
v.merge(w);
return 0;
}
// The array v then contains the elements: 1 2 3 7 9
■DECLARING SEQUENCES
The derived container class sortVec<T, Compare>
Using the container class sortVec
DECLARING SEQUENCES
■757
䊐Constructors of vector, list, and deque
The container classes vector,list, and deque define three constructors and a copy
constructor with which sequences can be created. Their functionality is similar for the
various classes and is discussed in the following section using the vector class as an
example.
The statement
Example: vector<Account> v;
declares an empty container vfor objects of the Account type. You can then insert
individual objects into the container.
However, you can also declare a container and fill it with a predefined number of
object copies.
Example: Fraction x(1, 1);
vector<Fraction> cont(100, x);
This defines the container cont with 100 Fraction type objects, and fills it with the
object x. If the second argument is not supplied, each of the 100 objects is initialized by
the default constructor.
Finally, you can initialize a container with a part of another container. To do so, you
must state a range of iterators.
Example: vector<double> v(first,last);
The arguments first and last are iterators in an existing container. The new con-
tainer, v, is initialized using objects in the range [first,last): this includes all the
objects between the positions first (including first) and last (excluding last).
䊐Constructors for Adapter Classes
Only a default constructor and the copy constructor are defined for adapter classes.
Given that wait is a predefined queue of the container class queue<double>, the fol-
lowing statement
Example: queue<double> w(wait);
creates a new queue, w, and uses the object wait to initialize it.
The opposite page shows the derived container class sortVec, which is used to rep-
resent sorted, dynamic arrays. The class is parameterized by the type Tof array elements.
The second template parameter is a comparator class, which represents a comparison cri-
terion for sorting.
758 ■
CHAPTER 33 CONTAINERS
Method Effect
void push_back(const T&x); Adds x at the end of the
sequence.
Adds x before the first
element of the sequence.
Inserts x after position pos
and returns the position of
the newly inserted element.
Inserts n copies of x after
position pos and returns the
number of inserted elements.
Inserts all elements from
range [first,last) after
position pos into the
sequence.
void push_front(const T&x);
iterator insert(iterator pos,
const T& x = T() );
size_type insert(iterator pos,
size_type n, const T& x)
void insert(iterator pos,
InputIterator first
InputIterator last)
// Method insert() adds a new object at the end
// of the vector and reorganizes in ascending order.
//---------------------------------------------------
template <class T, class Compare >
void SortVec<T, Compare>::insert(const T& obj)
{
SortVec::iterator pos, temp;
push_back(obj); // Add at the end.
pos = end(); pos--; // Last position
while (pos-- > begin()) // Sort:
{
if( obj < *pos) // Swap:
{ temp = pos; *(++temp) = *pos; *pos = obj; }
else break;
}
}
■INSERTING IN SEQUENCES
Inserting methods
Method insert() of the derived container class SortVec
INSERTING IN SEQUENCES
■759
䊐Insertion Methods
The following methods are defined in the container classes vector,deque, and list
■push_back() insert at end
■insert() insert after a given position.
Additionally, the following method is available in the list and deque classes
■push_front() insert at beginning.
This method is not defined in the vector class.
The insert() method is overloaded in various versions, allowing you to insert a sin-
gle object, multiple copies of an object, or object copies from another container. Given
two containers vand wthe following
Example: w.insert(--w.begin(), v.begin(), v.end());
inserts the objects from container vin front of all the other objects in w. A container can
of course be assigned to another container of the same type. The assignment operator is
overloaded for containers to allow this operation.
䊐Runtime Behavior
The push_back() and push_front() methods are preferable on account of their
constant runtime. Insertion of one object with the insert() method also has a con-
stant runtime in the list class. However, this is linear in the vector and deque
classes, that is, the time increases proportionally to the number of objects in the con-
tainer.
This dissimilar runtime behavior for methods can be ascribed to the implementation
of various container classes. Normally, containers of the list type are represented by
double linked lists in which each element possesses a pointer to the preceding and fol-
lowing element. This allows for extremely quick inserting at a given position.
The container classes vector and deque are represented as arrays. Inserting in the
middle means shifting the objects in the container to make place for the new object.
Therefore the runtime will increase proportionally with the number of objects the con-
tainer holds.
䊐Insertion in Adapter Classes
There is only one insertion method for adapter classes: push(). In stacks and queues,
push() appends an object with a constant runtime. Insertion of objects into priority
queues depends on the priority of the object and the runtime is linear.
760 ■
CHAPTER 33 CONTAINERS
// sortvec.h
// Method search() seeks an object obj in the vector
// using the binary search algorithms:
// The object obj is first compared to the element in
// the middle of the vector. If obj is smaller than the
// "middle" element, it must belong to the left half or
// else to the right half of the vector. We repeat this
// process comparing obj with the "middle" element in
// the section where it is known to be, repeatedly
// halving the size of the interval until the interval
// consists of a single point, which is where obj
// belongs.
// This algorithm has logarithmic time and thus
// is very fast.
// ----------------------------------------------------
template <class T, class Compare >
int SortVec<T, Compare>::search(const T& obj)
{
int first = 0, last = end() - begin() - 1, mid;
while( first < last )
{
mid = (first + last + 1)/ 2;
// Search the left half,
if( obj < (*this)[mid] )
last = mid - 1;
else first = mid; // the right half.
}
if ( obj == (*this)[first] ) // Found?
return first; // Yes.
else return size(); // Not found.
}
■ACCESSING OBJECTS
Method search() of container class sortVec
ACCESSING OBJECTS
■761
䊐The front() and back() Methods
Access to individual objects in the container classes vector,deque, and list can be
performed by the following methods
■front() for access to the first element and
■back() for access to the last element.
Both methods return a reference to the object in question.
Example: double z = v.front();
v.front() = 1.9;
This saves the first object in container vin the variable zand then overwrites the object
by 1.9.
䊐Access via Indices
The subscript operator [] is overloaded in the vector and deque classes to permit the
use of indices for access to the objects in a container. An index is a positive integer of
the type size_type.
Example: v[20] = 11.2; // the 21st object
Given that pos is an iterator that references the first object in a container v, the expres-
sion v[20] is equivalent to pos[20].
When you use the subscript operator, you must ensure that the index does not exceed
the valid range. You can use the access method at() to throw an exception if an index
is out of range.
Example: v.at(20) = 11.2;
The at() method throws an exception of the standard error class out_of_range if an
error occurs.
The subscript operator and the at() method are not defined in the list class.
Before you can manipulate the tenth object in the container, for example, you need to
walk through the container sequentially up to that position.
䊐Access to Objects in Adapter Classes
The method top() is defined for access to the element with the highest priority, or the
element at the top of the stack, in the adapter classes priority_queue and stack.
The queue class comprises the front() method, which is used to access the first
element.
762 ■
CHAPTER 33 CONTAINERS
// sortvec.h: Method merge() merges the argument vector
// with the vector *this.
// ---------------------------------------------------
template <class T, class Compare >
void SortVec<T,Compare>::merge(const SortVec<T,
Compare>& v)
{
SortVec temp; // Temporary vector
SortVec::iterator pos = begin(); // Iterator
int n1 = 0, n2 = 0;
// Copy the smaller object into vector temp:
while(n1 < size() && n2 < v.size())
if( pos[n1] <= v[n2] )
temp.push_back(pos[n1++]);
else
temp.push_back(v[n2++]);
// Append the rest:
while( n1 < size())
temp.push_back(pos[n1++]);
while( n2 < v.size())
temp.push_back(v[n2++]);
*this = temp;
}
■LENGTH AND CAPACITY
Method merge() of container class SortVec
LENGTH AND CAPACITY
■763
The identifying features of a container are
■its length, that is, the number of objects held in the container, and
■the capacity, that is, the maximum number of objects the container can store.
The length of a container changes after every insertion or deletion—the capacity does
not.
䊐Length of a Container
The length of a container is discovered by a call to the size() method. The method
returns an integer of the size_type type.
Example: Fraction x(1, 1);
vector<Fraction> v(100, x);
vector<Fraction>::size_type sz = v.size();
The variable sz contains the value 100 in this case.
The length of an empty container is always 0. You can also use the empty() method
to discover whether a container is empty. The method returns true in this case.
Example: while( !cont.empty() ) ...
The methods size() and empty() are defined for all container classes. You can use
the resize() method to change the length of a container.
Example: cont.resize( n, x);
The length is increased to nprovided n > size() is true, or decreased for n <
size(). If n == size(), nothing happens.
If the length is increased, n – size() copies of the object xare appended to the
container. The second argument, x, can be omitted. In this case, the default constructor
for a type Tobject is called as often as necessary.
䊐Capacity
The capacity of a container can be checked using the max_size() method.
Example: size_type k = cont.max_size();
The return value depends on the amount of memory available and the object size.
Only the size() and empty() methods are defined for adapter classes. You cannot
discover the capacity of an object, nor can you call resize() to change its length.
764 ■
CHAPTER 33 CONTAINERS
// prior_t.cpp : Testing a priority queue
// -------------------------------------------------
#include <queue>
#include <string>
#include <iostream>
using namespace std;
class Parcel
{
private:
unsigned int prio; // Priority
string info;
public:
Parcel(unsigned int p, const string& s)
:prio(p), info(s) {}
// Access methods, ... overloaded operators:
friend bool operator<(const Parcel& x,const Parcel& y)
{ return (x.prio < y.prio); }
friend ostream& operator<<(ostream& os,
const Parcel& x)
{ os << x.prio << " "<< x.info << endl; return os; }
};
int main()
{
priority_queue<Parcel> pq;
pq.push(Parcel(7,"Bob")); // Insert
pq.push(Parcel(1,"Peter"));
pq.push(Parcel(4,"Susan"));
while( !pq.empty() )
{
cout << pq.top() << endl; // Output
pq.pop(); // and delete
}
return 0;
}
// Output: 7 Bob
// 4 Susan
// 1 Peter
■DELETING IN SEQUENCES
A priority queue
DELETING IN SEQUENCES
■765
䊐Deletion Methods
The following methods are available for deleting objects in the container classes vec-
tor,deque, and list:
■pop_back() deletes the last object in the container
■erase() deletes the object at a given position, or deletes all the objects in
a given range
■clear() deletes all the objects in a container.
The following method is additionally defined in the deque and list classes:
■pop_front() deletes the first object in the container.
The method does not have a return value, just like the pop_back() method.
The pop_back() and pop_front() methods are preferable on account of their
constant runtimes. Using the erase() method to delete an object at the beginning or
in the middle of a container also provides a constant runtime in the container class
list. However, the runtime is linear in the vector and deque classes, since objects
must be shifted within the container to fill the gap left by the deletion.
䊐Deleting Ranges of Objects
When you use the erase() method to delete the objects in a given range, the position
of the first element to be deleted and the position after the last object to be deleted are
required as arguments.
Example: cont.erase(cont.begin() + 10, cont.end());
This deletes all the remaining objects in the container, starting at position 11. The
erase() method returns the new position of the object immediately after the range of
objects deleted.
䊐Deletion in Adapter Classes
There is only one method of deletion for adapter classes, namely pop(). Given that
wait is a queue of the queue type, the following statement
Example: wait.pop();
deletes the element at the beginning of the queue. In the case of a stack, pop() deletes
the element at the top of the stack, and for priority queues, the object with the highest
priority. The runtime is constant in all cases.
766 ■
CHAPTER 33 CONTAINERS
// list_t.cpp: Tests list operations
// ----------------------------------------------------
#include <list>
#include <cstdlib>
#include <iostream>
using namespace std;
typedef list<int> INTLIST;
int display( const INTLIST& c);
int main()
{
INTLIST ls, sls;
int i;
for( i = 1; i <= 3; i++)
ls.push_back( rand()%10 ); // ex. 1 7 4
ls.push_back(ls.front()); // 1 7 4 1
ls.reverse(); // 1 4 7 1
ls.sort(); // 1 1 4 7
for( i = 1; i <= 3; i++)
sls.push_back( rand()%10 ); // ex. 0 9 4
// Insert first object of sls before the last in ls:
INTLIST::iterator pos = ls.end();
ls.splice(--pos, sls, sls.begin()); // 1 1 4 0 7
display(sls); // 9 4
ls.sort(); // 0 1 1 4 7
sls.sort(); // 4 9
ls.merge(sls); // 0 1 1 4 4 7 9
ls.unique(); // 0 1 4 7 9
return 0;
}
■LIST OPERATIONS
Sample program
LIST OPERATIONS
■767
The container class list comprises methods for list operations that are not defined in
other container classes. These are
■sorting and inverting lists
■merging two sorted lists
■splicing lists.
䊐Sorting, Inverting, and Splicing Lists
A container of the list type, or list container for short, can be sorted by a call to
sort(). This assumes that the operator <is defined in class T. A call to sort() sorts
the container in ascending order.
You can use the reverse() method to invert a list container, that is, to reverse the
order of the objects in the container. What was originally the first element in the con-
tainer becomes the last, and the second element becomes the second to last, and so on.
The merge() method is used to merge two list containers. Given that ls1 and ls2
are two sorted list containers, the following call
Example: ls1.merge(ls2);
creates the sorted list ls1, whose objects comprise the original objects of ls1 and ls2.
The ls2 container is empty following this operation.
䊐Splice Operations
Splice operations insert the objects from one list container at a given position in another
list container and remove them from the original container. You can transfer either a
whole container or just part of a container.
Example: ls1.splice(pos, ls2);
This inserts the whole of container ls2 in front of position pos in ls1.ls2 is emptied
by this statement. The following statement
Example: ls1.splice(pos1, ls2, pos2);
Inserts the element at position pos2 in ls2 before the element at position pos1 in ls1
and deletes it from ls2. If you want to transfer part of a container, the third and fourth
arguments must contain the starting and end position.
You cannot use a splice operation to insert at a position before begin() or after
end().
768 ■
CHAPTER 33 CONTAINERS
Container Class Representing
set< class T,
class Compare = less<T>,
class Allocator = allocator<T> >
collections of objects with
unique keys
multiset< class T,
class Compare = less<T>,
class Allocator = allocator<T> >
collections of objects with
equivalent keys, i.e.
possibly multiple copies of
the same key value
map< class Key, class T,
class Compare = less<T>,
class Allocator = allocator<T> >
collections of objects/key
pairs where the keys are
unique
multimap< class Key, class T,
class Compare = less<T>,
class Allocator = allocator<T> >
collections of objects/key
pairs with possibly
equivalent keys
Container Class Header File
set< T, Compare, Allocator >
multiset<T, Compare, Allocator >
map< Key, T, Compare, Allocator >
multimap< Key, T, Compare, Allocator >
<set>
<set>
<map>
<map>
■ASSOCIATIVE CONTAINERS
Container classes
Associative containers and header files
ASSOCIATIVE CONTAINERS
■769
Sequences store objects in linear order. Searching for a given object will thus require a
linear runtime. If you have only a few objects to deal with, this will not cause any signifi-
cant delay. However, it is a major disadvantage in large collections of objects.
䊐Representing Sets and Maps
Associative containers with different classes that represent sets and maps allow you opti-
mize runtimes. They manage objects in so-called heaps, that is in trees with a minimum
height. Operations are performed by sortable keys. One of the characteristics of a heap is
that the object with the smallest key is always stored at the top of the heap.
Insertion, deletion, and search operations in sets and maps can be performed with log-
arithmic runtimes. That is, the response is proportional to log(n), where nis the num-
ber of objects in a container. Since a logarithmic function grows very slowly, response
will be phenomenally quick.
䊐Unique and Ambiguous Keys
In a set, each object contains a key. This is why we refer to embedded keys. The relation-
ship between the objects is defined by reference to this key. Besides sets, which have
unique keys, you can also define multisets, where multiple objects can have the same key.
Maps are used to manage key/object pairs. In other words, the key is not embedded in
the object but stored separately from it. The type of the key is Key, and Tis the object
type. The relationship between the objects is again defined by the keys.
Besides maps, which contain only unique keys, you can also define multimaps, where
several objects can exist for a single key.
䊐Associative Container Classes
The opposite page shows various classes used to represent sets, multisets, maps, and mul-
timaps. The template parameter Compare is a comparator class and Allocator is an
allocator class. Both parameters have default values, which we already saw in the context
of sequences.
The methods begin() and end() are defined for access to the positions in all asso-
ciative container classes. They return the position of the first element or the position
after the last element.
770 ■
CHAPTER 33 CONTAINERS
// set_t.cpp: Tests sets and multisets
// ----------------------------------------------------
#include <set>
#include <cstdlib>
#include <ctime>
#include <iostream>
using namespace std;
typedef set<int> IntSet; // Define type and
typedef IntSet::iterator SetIter; // iterator type
typedef multiset<int> IntMultiSet; // Multiset and
typedef IntMultiSet::iterator MultiSetIter; // iterator
int main()
{
IntSet lotto; // Create a set.
SetIter pos; // Bidirectional iterator
srand((unsigned) time(NULL));
while( lotto.size() < 6) // Insert
lotto.insert( rand()%50 );
cout << "These are your lotto numbers: " << endl;
for( pos = lotto.begin(); pos != lotto.end(); pos++)
cout << *pos << " ";
cout << endl << endl;
IntMultiSet ms; // Create a multiset.
MultiSetIter mpos; // Bidirectional iterator
for( int i=0; i < 10; i++) // Insert
ms.insert( rand()%10 );
cout << "And now 10 random numbers "
<< " between 0 and 10: " << endl;
for( mpos = ms.begin(); mpos != ms.end(); mpos++)
cout << *mpos << " ";
cout << endl;
return 0;
}
■SETS AND MULTISETS
Sample sets and multisets
SETS AND MULTISETS
■771
Sets and multisets are used for efficient management of object collections with sortable
keys, that is, insertion, deletion, and search operations can be performed with logarith-
mic runtimes. Keys are always parts of objects, thus, keys are data members whose rela-
tionships to one another must be defined in the corresponding class. A lesser than
relationship is normally defined for this purpose, that is, the operator <will be over-
loaded for the class.
䊐Declaring Sets and Multisets
The container classes set and multiset have two constructors each for creating con-
tainers. You can use the default constructor to create sets and multisets with a length of
0. The second constructor inserts objects from a range of iterators into the new set or
multiset.
Example: typedef set<Account> AccountSet;
AccountSet mySet(first, last);
Here, [first, last) is a range of iterators in an existing container whose objects are
of Account type.
The copy constructor is also defined, and this allows you to use an existing container
of the same type to initialize a new container.
䊐Inserting and Deleting
The insert() method is available for insertions. This allows for insertion of individual
objects or multiple objects from a given range of iterators.
Example: mySet.insert(Account(1234,"May, Tom",100));
In contrast to multisets, a new object is only inserted in a set if it does not already exist
in the container.
You can use the erase() method to delete objects. To do so, you can either specify
the object itself or its position in the container.
Example: mySet.erase(mySet.begin());
This deletes the first element in the AccountSet set.
You can delete all objects in a container with the following statement:
Example: mySet.erase( mySet.begin(), mySet.end() );
For erasing a whole container you can also use the clear() method. Calling empty()
will tell you whether the container is empty. The size() method returns the number of
objects in the container.
772 ■
CHAPTER 33 CONTAINERS
// mulmap_t.cpp: Testing multimaps
// -------------------------------------------
#include <map>
#include <string>
#include <iostream>
using namespace std;
typedef multimap<int, string> MULTI_MAP;
typedef MULTI_MAP::iterator ITERATOR;
int main()
{
MULTI_MAP m; // Create a multimap.
ITERATOR pos; // Iterator.
// To insert:
m.insert(pair<int, string>(7, "Bob") );
m.insert(pair<int, string>(3, "Sarah"));
m.insert(pair<int, string>(1, "Diana"));
m.insert(pair<int, string>(1, "Lisa"));
cout << "This is the multimap: " << endl;
for(pos = m.begin(); pos!= m.end(); pos++)
cout << pos->first << " "
<< pos->second << endl;
cout << endl;
pos = m.find(3); // Search for the pair
// with the given key 3
if( pos != m.end()) // and output the pair
cout << pos->first << " " << pos->second << endl;
int key = 1; // Determine the quantity of
// pairs with key value 1:
cout << "There are " << m.count(key)
<< " pairs with key " << key << endl;
return 0;
}
■MAPS AND MULTIMAPS
Using multimaps
MAPS AND MULTIMAPS
■773
䊐Representing Pairs of Keys/Objects
Maps and multimaps store pairs of sorted keys and objects. The key is used again to iden-
tify the object but is stored separately from the object. The comparison criterion is
applied to the keys.
The C++ Standard Library contains the class template pair<const Key,T> with
two public data members first and second, a default constructor, and a copy con-
structor to represent key/object pairs. The first template parameter, Key, is the key type
and the second is the object type T. The data member first is used to store the keys,
and second stores the associated object.
Given that pos is the position of an object in a map or multimap, you can reference
the key with pos->first, and the associated object itself with pos->second.
䊐Using Maps and Multimaps
The container classes map and multimap contain constructors with the same function-
ality as the set and multiset classes. Thus, you can create a container with a length
of 0, or use the objects in an existing container to initialize a new container. The copy
constructor is also defined.
The methods insert() for insertion, and erase() and clear() for deletion have
the same interfaces as in the container classes set and multiset. The methods
size(), which you can use to discover the length of the container, and empty(),
which ascertains whether the container is empty, are also defined.
The find() method is used to look up key/object pairs and expects a key as an argu-
ment in the map and multimap classes. Its return value is the associated position in the
container. In the case of multimaps where several objects can have the same key, it
returns the first position with that key. If the search fails, the value end() is returned as
a pseudo position.
You can use the count() method to discover the number of key/object pairs with a
given key in the container. The method expects a key as an argument. The method
returns 0or 1for maps, depending on whether a pair exists or not. In the case of multi-
maps, the return value can be greater than 1, of course.
774 ■
CHAPTER 33 CONTAINERS
// bitmap.h : Defines the template Bitmap<N>
// representing raster images.
// -------------------------------------------------
#ifndef _BITMAP_
#define _BITMAP_
#include <bitset>
#include <stdexcept>
using namespace std;
template <int N>
class Bitmap : public bitset<N>
{
private:
int lines, cols; // Number of rows and columns
int ax, ay; // Current cursor position
int ai; // Current index in the bitset
public:
Bitmap(int l, int c);
void move(int x, int y);
void draw(int x, int y);
};
template <int N>
Bitmap<N>::Bitmap(int l, int c)
{
if (l*c <= N)
{
reset(); // Set all bits to 0
lines = l; cols = c; // Rows and columns
ax = 0; ay = 0; ai = 0; // Current position
}
else throw invalid_argument("Invalid argument \n");
}
template <int N>
void Bitmap<N>::move(int x, int y)
{
if( x >= 0 && x < lines && y >= 0 && y < cols)
{ ax = x; ay = y; ai = x * cols + y; }
else throw invalid_argument("Invalid argument\n");
}
// to be continued
■BITSETS
Representing raster images with bitmaps
BITSETS
■775
䊐Declaring Bitsets
A bitset stores a bit sequence of a given length. This allows storage of mass bit coded
data, such as raster images, with minimum memory used.
The container class bitset<N> provides the functionality needed to manage bitsets.
The template parameter Nis the length of bitset, that is the maximum number of bits
stored.
You can use the default constructor to create a bitset with no initial values. However,
you can also use a given bit-pattern to initialize a bitset. The bit-pattern is either defined
as an unsigned long value or as a string.
Example: string s = "10101010";
bitset<1024> b(s);
The string scan contain only the '0' or '1' characters. The last character in the string
will be the first bit value (that is 0 or 1) at bit position 0, the second to last character in
the string is the bit value at position 1, and so on. The remaining bits are padded with 0
up to a length of N. This also applies when an unsigned long value is used for initial-
ization purposes.
䊐Notes on the Sample Program Opposite
The container class Bitmap<N>, which is defined opposite, can be used to represent
simple monochrome raster images. A pixel (picture element) is represented by a bit in a
bitset. If the bit is set, a pixel on screen will be illuminated (white) and otherwise turned
off (black).
The number of pixels that can be represented horizontally and vertically is defined by
the resolution. 480 by 800 is a typical screen resolution and 3300 by 2300 is a typical res-
olution for laser printers (at 300 dpi) for an 8 1⁄2⫻11 page size. The value of Nis the
product of the number of pixels in horizontal and vertical direction.
The container class Bitmap<N> is derived from bitset<N> by public inheri-
tance. Thus, the class comprises a bitset and all the public bitset management methods
it inherits. Additional data members are used to store the number of rows and columns of
pixels, the current cursor position, and the current bitset index. The move()method
moves the cursor to the position with the given coordinates. The draw()method draws
a straight line from the current cursor position to the point with the given coordinates.
776 ■
CHAPTER 33 CONTAINERS
// bitmap.h: In addition the Bresenham algorithm.
// ---------------------------------------------------
template <int N>
void Bitmap<N>::draw(int x, int y)
{
if( x >= 0 && x < lines && y >= 0 && y < cols)
{
int savex = x, savey = y;
if(ax > x) // Draw in ascending x-direction
{ // => possibly swap (ax,ay) and (x,y)
int temp = ax; ax = x; x = temp;
temp = ay; ay = y; y = temp;
}
int dx = x - ax, dy = y - ay;
int xinc = 1, yinc; // Increment
if( dy < 0) // Gradient < 0 ?
{ yinc = -1; dy = -dy;} // Decrement y
else yinc = 1; // or else increment.
int count = dx + dy; // Number of pixels to be set
int d = (dx - dy)/2; // Measurement of deviation
// off the line.
while( count-- > 0)
{
ai = ax * cols + ay; // Index in the bitset
set(ai); // Set the bit
if( d < 0) // Next pixel in
{ ay += yinc; d += dx; } // y-direction
else // or else in
{ ax += xinc; d -= dy; } // x-direction
}
ax = savex; ay = savey; // Current cursor position
ai = ax * cols + ay; // Current index in bitset
}
else throw invalid_argument("Invalid argument\n");
}
#endif
■BITSETS (CONTINUED)
Bresenham algorithm
BITSETS (CONTINUED)
■777
䊐Manipulating Bits
The container class Bitset<N> provides the get() and set() methods for reading
and writing individual bits. These methods expect a bit position as an argument. You can
additionally pass a value of 0or 1to the set method, which writes this value at the bit
position stated. The default value here is 1.
If you call the set() method without any arguments, all the bits in the bitset are set
to 1. In contrast, the reset() method deletes all the bits. Bits can be inverted by a call
to the flip() method. Each 0-bit in the bitset is set then to 1and each 1-bit is set to
0.
Bits at specific coordinates can be referenced by the subscript operator. The index is a
bit position, that is a number between 0and N-1.
As you would expect, bitwise operators can also be used for bit manipulation. The bit
operators &,|, and ^are globally overloaded for bitsets. The operator functions for the
NOT operator ~, the shift operators << and >>, and the operators for compound assign-
ments, &=,|=,^=, are implemented as methods of the container class.
䊐The Bresenham Algorithm
The opposite page shows the draw() method that draws a line from the current cursor
position pixel by pixel to the given coordinates. The Bresenham algorithm used here
applies incremental techniques. Starting at the current cursor position, it sets the neigh-
boring pixel in the x- or y-direction. To do this, you only need to increment or decre-
ment the x- or y-coordinate by 1. This avoids time consuming floating-point arithmetic,
which would be required to solve a linear equation.
To allow drawing to take place along a positive x-axis, the starting and target points
of the straight line can be swapped. The difference between the y-coordinates of the
starting and target points dy = y - ay then determines whether to increment or
decrement by 1 along the y-direction.
Drawing neighboring pixels creates a “staircase” effect, which deviates from the
straight line. The variable d = (dx - dy)/2 represents this deviation. If the value of
dis negative, the line is seen to be growing along y-direction and the next pixel is drawn
along the y-direction. Then the deviation is corrected by adding dx. As soon as d
becomes positive, the next pixel is drawn along the x-direction and the deviation is cor-
rected with dy.
exercise
778 ■
CHAPTER 33 CONTAINERS
9 queues have been created.
The queues will now be filled
using the hot potato algorithm.
Some elements of randomly selected
queues are removed.
Output the queues:
1.queue: 28 88 70 60 6
2.queue: 64 6 54 1
3.queue: 2 88 64 30 66 29 11 74 49 41
4.queue: 17 25
5.queue: 96 97 47 27 71 34 87 58
6.queue: 77 82 54
7.queue: 35 65 23 40 5 83 92
8.queue: 32 23 54
9.queue: 28 55 54 73 28 82 21 99
Router Exits
1. Queue
2. Queue
3. Queue
4. Queue
Entrance
■EXERCISE
Hot potato algorithm
Test output
EXERCISE
■779
Exercise
In data communications between two remote computers messages are transmitted via
multiple subnets. En route to the target computers, so-called routers store these messages
in queues before transmitting them towards the target via the next available line. The
routers assume responsibility for route discovery, generally referring to complex address
tables.
There are various routing techniques, including a simple algorithm that can do with-
out address tables, the so-called Hot Potato Algorithm. The router simply tries to dispose
of incoming messages as quickly as possible by sending each incoming message to the
outgoing line with the shortest queue.
■Define a container class VecQueue<T>, which is parameterized with a message
type Tto represent this scenario. The class comprises an array of queues of type
vector< queue<T> > and a data member used to store the current number of
queues in the array.
The constructor creates the number of empty queues passed to it as an argument
for the array. Additionally, you will need to declare the methods size(),
empty(), push() and pop().
Overload the size() method in two versions: If no argument has been passed
to the method, it returns the current number of messages in all queues. If an argu-
ment iof type int has been passed, the method returns the current number of
messages in the i-th queue. Additionally, overload the empty() and
empty(int i) methods, which return true, if all queues or the i-th queue are
empty.
The push() method uses the hot potato algorithm to append a message passed
to it at the end of the shortest queue.
The pop() and pop(int i) methods are used to simulate the assignment of
messages to lines, that is retrieval and removal of messages from queues, in this
exercise. The method pop() retrieves the message at the top of a randomly
selected queue and deletes it, returning the message. The method pop(int i)
retrieves the message at the top of the i-th queue and deletes it, returning the
message.
■To test your class, declare a container of the type VecQueue<int> in your
main function. A message is represented by a number. Use a loop to insert ran-
dom numbers between 0 and 99 into the container and relay some of them to the
outside lines. Then display the remaining messages on screen, as shown opposite,
by calling the pop(int i) method.
solution
780 ■
CHAPTER 33 CONTAINERS
Solution
// ------------------------------------------------------
// vecQueue.h
// Defining the Class Template VecQueue<T>
// to represent a vector of queues.
// ------------------------------------------------------
#ifndef _VECQUEUE_H
#define _VECQUEUE_H
#include <vector>
#include <queue>
#include <cstdlib> // For srand(), rand()
#include <ctime> // For time()
using namespace std;
template <class T>
class VecQueue
{
private:
vector< queue<T> > v;
size_t sz; // Number of queues
public:
VecQueue(size_t n);
size_t size() const; // Current number of all
// elements.
size_t size(int i) const // Number of elements in
{ return v[i].size(); } // the i-th queue.
bool empty() const { return size() == 0; }
bool empty(int i) const { return size(i) == 0; }
void push(const T& a); // Hot potato algorithm
const T& pop(); // Removes the element at the
// beginning of a randomly
// choosen queue.
const T& pop(int i); // Removes the element at the
}; // beginning of the i-th queue
template <class T>
VecQueue<T>::VecQueue( size_t n) // Constructor
{
if(n > 0)
v.resize(n);
sz = n;
srand(time(NULL));
}
SOLUTION
■781
template <class T> // Current number of all elements
size_t VecQueue<T>::size() const
{
size_t count = 0;
for( int i=0; i < sz; ++i)
count += v[i].size();
return count;
}
template <class T> // To insert the argument into the
void VecQueue<T>::push(const T& a) // shortest queue
{
int small = 0; // To determine the
for(int i = 0; i < sz; i++) // shortest queue.
if( v[i].size() < v[small].size())
small = i;
v[small].push(a); // and insert there.
}
template <class T> // To retrieve and delete
const T& VecQueue<T>::pop() // an element in a randomly
{ // choosen queue.
static T temp;
int i, i0;
i = i0 = rand() % sz;
do
{
if(!v[i].empty()) // If i-th queue is not empty:
{ // To retrieve and delete the
temp = v[i].front(); // element at the beginning.
v[i].pop();
break;
}
i = (i+1) % sz; // Or else: Move to the next queue.
}
while( i != i0);
return temp;
}
template <class T> // To retrieve and delete
const T& VecQueue<T>::pop(int i) // an element in the
{ // i-th queue.
static T temp;
if( i >= 0 && i < sz) // If the index is okay:
{ // To retrieve the element
temp = v[i].front(); // at the beginning and
v[i].pop(); // to delete.
}
return temp;
}
#endif // _VECQUEUE_H
782 ■
CHAPTER 33 CONTAINERS
Solutions (continued)
// -----------------------------------------------------
// hotpot_t.cpp : Simulates the hot potato algorithm
// using a vector of queues.
// -----------------------------------------------------
#include <cstdlib> // For srand(), rand()
#include <ctime> // For time()
#include <iostream>
#include <iomanip>
using namespace std;
#include “vecQueue.h”
int main()
{
const int nQueues = 9;
VecQueue<int> vq(9); // Vector of 9 queues
cout << nQueues << “ queues have been created.”
<< endl;
srand(time(NULL));
cout << “\nThe queues will now be filled “
<< “using the hot potato algorithm.”
<< endl;
int i;
for(i = 0; i < 100; i++) // To insert 100 elements
vq.push(rand()%100);
cout << “\nSome elements of randomly selected “
“queues are removed.”
<< endl;
for(i=0; i < 50; i++) // To remove 50 elements
vq.pop();
cout << “\nTo output the queues:” << endl;
// To retrieve, remove
for( i = 0; i < nQueues; ++i) // and display all
{ // remaining elements.
cout << “\n” << i+1 << “.Queue: “;
while( vq.size(i) > 0 )
{
cout << setw(4) << vq.pop(i);
}
cout << endl;
}
return 0;
}
783
This appendix contains
■binary number representation
■preprocessor directives
■pre-defined standard macros
■binding C functions
■operator precedence table
■ASCII Code table
■screen control characters
appendix
784 ■
APPENDIX
■BINARY NUMBERS
The numbers used by a program can be divided into two groups depending on their type:
■integers of the char,signed char,unsigned char,short,unsigned
short,int,unsigned int,long, unsigned long types and
■floating-point numbers of the float,double, and long double types.
Both integral and floating-point numbers are represented internally as binary numbers,
that is, as sequences of 0 and 1 values. However, the formats for representing integral and
floating-point numbers differ. Thus, the bit-pattern of an integer will be interpreted dif-
ferently from that of a floating-point number by the computer.
Representing Signed and Unsigned Integers
The binary format of integers is basically the same for the char,short, int and long
types and differs only in
■the number of bytes available for each type and
■whether the number is interpreted as signed or unsigned.
The bit-pattern of a positive integer can be represented as a base 2 power series. The sign
bit 0additionally indicates that the number is positive in the case of signed types.
The number 4can be represented by the following power series:
0*20+ 0*21+ 1*22+ 0*23+ 0*24...
The binary representation of the number 4as signed char type value (8 bits) is thus
as follows:
Two’s complement is used to represent a negative number, for example -4:
You can also use two’s complement to compute the absolute value of a negative num-
ber. Two’s complement for -4 yields a value of 4.
Sign bits are not required for unsigned types. The bit can then be used to represent
further positive numbers, doubling the range of positive numbers that can be repre-
sented.
The following table contains the binary formats of signed and unsigned integral 8 bit
values:
If the bit-pattern of a negative number is interpreted as an unsigned number, the value
of the number changes. The bit-pattern 1111 1100 of the number ⫺4 will thus yield
the following unsigned value:
0*20+ 0*21+ 1*22+ 1*23+ 1*24+ 1*25+ 1*26+ 1*27
that is, the decimal number 252.
Representing Floating-point Numbers
To represent a given floating-point number, x, the number is first broken down into a
sign, v, a mantissa, m, and a power, exp, with a base of 2:
x = v * m * 2exp
BINARY NUMBERS
■785
Memory for the values v,m, and exp is normally assigned in IEEE (Institute of Elec-
tronics and Electronical Engineers) format. The type float (32 bit) will thus be organ-
ized as follows:
In this “normalized” form, floating-point numbers are unambiguous. The mantissa, m, has
a value that is greater than or equal to 1 and less than 2, the only exception being x ==
0, where the mantissa is 0.
Example: -4.5 = -1 * 1.125 * 22
The first digit of the mantissa is always 1 and need not be stored. The power is stored
along with its bias. A bias of 127 applies for float types; thus a power eof a floating-
point number is represented internally as e + 127.
The memory reserved for the mantissa defines the accuracy, and the memory reserved
for the power defines the range of values for the floating-point number.
If platform-dependent ranges, such as the length of the mantissa or the smallest or
largest value that can be represented, are significant in your programs, you can discover
these ranges in the cfloat or climits header files.
You can use an instantiation of the numeric_limits class template for the type in
question to query platform-dependent ranges by method calls.
786 ■
APPENDIX
■PREPROCESSOR DIRECTIVES
The #define Directive
The #define directive is used to define symbolic constants and macros.
Syntax: #define name[(parameterlist)] [SubstituteText]
The preprocessor replaces name or name(parameterlist)with SubstituteText
throughout the whole program. If SubstituteText is not stated, the preprocessor will
delete the symbolic constant or macro throughout the program code (see also Chapter 7,
“Symbolic Constants and Macros”.)
Example: #define BUFSIZ 512 // Symbolic constant
#define CLS cout << "\033[2J" // Macro
#define MAX(a,b) ((a)>(b) ? (a):(b))// Macro
The # Operator
A macro parameter in a substitute text can be preceded by the # operator (or stringizing
token). When the macro is called, the argument is set in quotes, that is, a string constant
is formed using the characters of the current argument.
Example: #define TITLE(s) "**** " #s " *****"
The call
cout << TITLE(Catalog);
causes the preprocessor to expand the following string
"**** " "Catalog" " ****"
which is then concatenated to "**** Catalog ****".
The characters "and \are represented by \" and \\ within an argument.
Example: #define path(logid,subdir)
"\\user\\" #logid "\\bin\\" #cmd
With path(Smith, games)
the string "\user\Smith\bin\games " is produced.
The ## Operator
When a macro is defined, character sequences can be concatenated in the substitute
text. The past token operator, ##, is used to this effect.
When the macro is called, the parameter preceding or following the ## token is
replaced by the appropriate argument. Then the token and any leading or trailing white-
space character is removed.
PREPROCESSOR DIRECTIVES
■787
Example: #define debug(n) cout << "x" #n "=" << x ## n
Calling
debug(1);
will generate the statement
cout << "x1=" << x1;
The arguments of a macro are not parsed for symbolic constants or macros. However, if
the result of a concatenation is a symbolic constant or a macro, text replacement is again
performed.
The #undef Directive
To change the definition of a symbolic constant or a macro at program runtime, you
must first remove the original definition. To do so, the #undef directive is used.
Syntax: #undef name
Do not supply the parameter list for parameterized macros.
You can then use the #define directive to redefine the macro.
Example: #define BUFSIZE 512
.
.
#undef BUFSIZE
#define BUFSIZE 1024
The #include Directive
The #include directive copies a file to a program. The #include directive is replaced
by the content of the file.
Syntax: #include <filename>
#include "filename"
If the file name is surrounded by <and >, the file will only be looked up in the directo-
ries defined by the environment variable (usually INCLUDE).
If the file name is stated in quotes the file will also be looked up in the current direc-
tory first.
The name filename can include a path. In this case the file is only looked up in
the directory stated.
You can also supply the file name as a symbolic constant. The substitute text must be
in quotes or square brackets in this case.
788 ■
APPENDIX
Example: #include <iostream>
#include "project.h"
#if VERSION == 1
#define MYPROJ_H "version1.h"
#else
#define MYPROJ_H "version2.h"
#endif
#include MYPROJ_H
The #if, #elif, #else, and #endif Directives
You can use the #if,#elif, and #endif directives to compile certain parts of a source
file and ignore others. The directives are known as conditional compilation directives for
this reason.
Syntax: #if expression1
[text1]
[#elif expression2
text2]
.
.
[#elif expression(n)
text(n)]
[#else
text(n+1)]
#endif
Each #if directive must be terminated by an #endif directive. Multiple #elif direc-
tives can occur in between. The #else directive can occur once only.
The preprocessor evaluates expression1,expression2,... in sequence. If
an expression that yields “true” is found, that is its value is not 0, the corresponding code
for this expression is processed.
If none of these expressions is true, the #else directive is executed. If this directive is
omitted, no corresponding code is processed.
expression1,expression1,... must be constant expressions of integral types
and cannot contain the cast operator. Some compilers do not allow the use of the
sizeof operator.
The corresponding text is a source text, that is, it comprises preprocessor directives,
C++ statements, or whole C++ programs.
When a corresponding text is processed, the preprocessor may first execute directives
before passing the expanded source code to the compiler for compilation. Code that the
preprocessor has not processed is removed from the source.
PREPROCESSOR DIRECTIVES
■789
The defined Operator
You can use the defined operator to check whether a symbolic constant or macro has
been defined.
Syntax: defined(name)
The operator returns a value other than 0if valid definition for name exists, and the
value 0in all other cases.
A definition created using the #define directive remains valid until it is removed by
an #undef directive. If the substitute text following name in the #define directive is
missing, the definition is still valid.
The defined operator is normally used in #if or #elif directives, for example
Example: #if defined VERSION
...
Using the defined operator allows you to use its return value in a preprocessor expres-
sion.
The #ifdef and #ifndef Directives
You can also perform the same check using the #ifdef and #ifndef directives.
Syntax: #ifdef name
#ifndef name
The #ifdef directive returns a value other than 0if name is defined and otherwise 0.
In contrast, the #ifndef directive ensures that a value has not yet been defined, that
is, that name is undefined, returning 0if name has been defined, and a non-zero value
in any other case.
Example: #if defined(STATE) && defined(COND)
. . .
The #line Directive
The compiler uses line numbers and the name of a source file to display errors discovered
on compilation. You can use the #line directive to change the line numbers and the
file name.
Syntax: #line new_number ["filename"]
At this position a new line count begins by new_number. If filename is stated, it will
become the new file name that the compiler refers to when issuing error messages.
The new file name must be in quotes and new_number must be an integral constant.
Example: #line 170 "genprog1.cpp"
The line number and the file name can also be stated as symbolic constants.
790 ■
APPENDIX
Example: #if VERSION == 1
#define NEWNUMBER 20
#else
#define NEWNUMBER 25
#line NEWNUMBER
The #line directive is often used by program generators compiling code to produce a
C++ program. In the case of error messages, the message can refer to the appropriate line
and file name in the original code.
The current line number and file name can be accessed using the standard macros
__LINE__ and __FILE__.
Example: cout << "Current line number: "
<< __LINE__ << endl
<< "File name: " << __FILE__ << endl;
The #error Directive
The #error directive can be used to show preprocessor errors.
Syntax: #error errortext;
The message errortext is issued and compilation is terminated.
Example:#if defined VERSION
#if VERSION < 3
#error VERSION too old.\n
##error Version 3 or better needed.
#endif
#include "version.h"
If the symbolic constant VERSION is defined and the value is less than 3, the following
error message is output:
VERSION too old.
Version 3 or better needed.
The #pragma Directive
The #pragma directive is compiler dependent and allows you to define your own pre-
processor commands for a specific compiler.
Syntax: #pragma command
Any other compiler that supports the #pragma directive but does not support the com-
mand following #pragma simply ignores the command.
Example: #pragma pack(1)
This directive causes a Microsoft compiler to align the components of a class bytewise,
avoiding gaps. Other options are pack(2) and pack(4).
PREPROCESSOR DIRECTIVES
■791
■PRE-DEFINED STANDARD MACROS
The ANSI standard provides for six pre-defined standard macros. Their names begin and
end with two underscores:
__LINE__ Returns the number of the line containing the __LINE__ macro. The
first line in a source code is line 1. The numbering can be modified
using the #line directive.
__FILE__ Returns the name of the source file that contains the __FILE__
macro. The name can be modified using the #line directive.
__DATE__ Returns the date in the mmm dd yyyy format, where mmm is an
abbreviation for the month, dd is the day of the month, and yyyy is
the year, resulting in Jul 17 2001, for example.
__DATE__ refers to the point in time when the preprocessor started
processing the source. Thus, the macro returns the same result at any
position in the source code.
__TIME__ Returns the time as a string in the format hh:mm:ss, where hh refers
to hours, mm to minutes, and ss to seconds, e.g., 15:23:47.
__TIME__ refers to the point in time when the preprocessor started
processing the source. Thus, the macro returns the same result at any
position in the source code.
__STDC__ Is only defined if the source code contains ANSI keywords only.
__cplusplus Is defined if the source code is compiled using a C++ compiler.
792 ■
APPENDIX
■BINDING C FUNCTIONS
Calling C Functions in C++ Programs
C functions from C libraries can be called in a C++ program. However, function calls are
interpreted differently by the C++ compiler than you would expect from a C compiler.
You must therefore supply additional binding information that is accessible via an
extern "C" declaration.
Example: extern "C" void oldfunc(int size);
This informs the C++ compiler that a C compiler was used to compile the oldfunc()
function.
If you need to declare multiple C functions, you can declare their prototypes after
extern "C" within curved brackets. If you have already declared the functions in a
header file, you can include the header file in an extern "C" block.
Example: extern "C"
{
#include "graphic.h"
}
It is common practise to declare extern "C" code in C header files. You can then
include the C header both in C and in C++ programs.
Example: #if defined _cplusplus
extern "C"
{
#endif
// Prototypes for C functions here
#if defined __cplusplus
}
#endif
The symbolic constant __cplusplus is evaluated to discover whether the current
compiler is a C or C++ compiler. If __cplusplus is defined, that is, a C++ compiler is
active, the extern "C" block is inserted.
Defining C Functions in C++ Programs
It is also possible to define your own C functions for a C++ program. You need to do this
if you call a function that expects a C function as an argument, such as the standard
qsort() and bsearch() functions.
The definition for a C function in a C++ program must be encased in an extern
"C" block. This instructs the compiler to compile the function as a C function.
BINDING C FUNCTIONS
■793
Example: #include <string>
#include <iostream>
#include <cstdlib>
using namespace std;
static char* city[] = { "Paris", "London",
"Barcelona", "Hollywood"}
static char* key = "New York";
extern "C" int scmp(const void*, const void*);
int main()
{ // Sort cities:
qsort( city, 4, sizeof(char*), scmp);
// Find city:
if( bsearch( &key, city, 4,
sizeof(char*),scmp) == NULL)
cout << "City" << (string) key
<< "not found.\n";
}
extern "C"
{
int scmp(const void *s1, const void *s2)
{
return strcmp( *(const char**)s1,
*(const char**)s2 );
}
}
The C function scmp() is passed to the standard functions bsearch() for binary
searching and qsort() for sorting using the quick sort algorithm.
794 ■
APPENDIX
■OPERATORS OVERVIEW
OPERATORS OVERVIEW
■795
■OPERATORS OVERVIEW (CONTINUED)
796 ■
APPENDIX
■OPERATOR PRECEDENCE TABLE
OPERATOR PRECEDENCE TABLE
■797
■ASCII CODE TABLE
798 ■
APPENDIX
■ASCII CODE TABLE (CONTINUED)
ASCII CODE TABLE
■799
■SCREEN CONTROL SEQUENCES
The following escape sequences reflect the ANSI standard for screen control. Replace
the #sign by the appropriate decimal number in all cases.
ESC[#ACursor # lines up
ESC[#BCursor # lines down
ESC[#CCursor # characters right
ESC[#DCursor # characters left
ESC[z,sHor ESC[z;sfPut cursor in line zand column s
ESC[sSave cursor position
ESC[uLoad saved cursor position
ESC[#K# = 0: Delete from cursor position to line end
# = 1:Delete from start of line to cursor position
# = 2: Delete whole line
ESC[2J Clear screen
ESC[#(;#...)opm# = 0: all attributes normal
# = 1: switch double intensity on
# = 4: Underline on (monochrome screens)
# = 5: Blink on
# = 7: Inverse on
# = 3x: Foreground color
# = 4x: Background color
x = 0: black x = 4: blue
x = 1: red x = 5: magenta
x = 2: green x = 6: cyan
x = 3: yellow x = 7:white
ESC[c1;c2pChange key assignments: The key with decimal code c1 will
then return code c2.
To enable these escape sequences, you must first load an appropriate screen device
driver. To do so for Windows 9x, place the following line in your CONFIG.SYS file
DEVICE = C:\Windows\Command\Ansi.sys
Win NT and Win 2000 do not supply the ANSI screen control characters. Correspond-
ing functions based on system calls are offered for download.
800 ■
APPENDIX
■LITERATURE
International Standard ISO/IEC 14882, Programming Languages—C++; published by
American National Standards Institute, New York NY, 1998.
International Standard ISO/IEC 9899:1999(E), Programming Languages—C; published by
ISO Copyright Office, Case postale 56, CH-1211, Geneva 20, 1999.
Stroustrup, Bjarne, The C++ Programming Language, Addison Wesley, 2000.
Josuttis, Nicolai, The C++ Standard Library, Addison Wesley, 1999.
LITERATURE
■801
This page intentionally left blank
803
Note: Italicized page locators indicate figures.
index
Symbols
&, 223, 231, 691
&&, 91
+, 50, 82, 85, 157
++, 85, 355
-, 82
--, 85, 355, 420, 755
*, 82, 233, 255, 355, 691, 755
/, 82
%, 82
->, 255, 755
=, 87
+=, 50, 87
-=, 87, 355
*=, 87, 157
/=, 87
%=, 87
==, 88, 159
!=, 88, 159
<, 88, 159
<=, 88, 159
>, 88, 159
>=, 88, 159
<<, 9, 88, 229, 429
>>, 44, 229, 429
::, 209
?:, 109
[], 691, 755
//, 91
/, 707
+-, 355
‘\n’, 51, 187
(), 691
A
Abstract classes, 565-585
concrete classes versus, 569
deriving, 569
and inhomogeneous lists, 574,
575-577
pointers and references to, 570
pure virtual methods for, 566
virtual assignment in, 572, 573
Abstraction, 245
Access methods, 274, 275
AccFile class, 656
append() method of, 650, 651
Account class, 267, 392, 393
access methods for, 274
with constructors, 266
defining, 648
methods of, 248
read-only methods in, 276
sample class, 272
Account management index file, 655
accPtr array
pointers in, 364
accSort() function, 682, 683
Accuracy, 21, 786
Adapter classes, 753
access to objects in, 761
constructors for, 757
deletion in, 765
insertion in, 759
Addition, 82, 353, 417
Addresses, array, 351
Address operator, 231, 412
Address space, hash file, 658
Algorithm library
within C++ standard library, 723
Alignment
and fill characters, 67
setting, 67
Ambiguity, 591
Ambiguous keys, 769
American National Standards Institute (ANSI),
3
Ampersand character, 223, 231, 691
Analysis, 245
AND operator, 91
Angled brackets
and header files, 47
template arguments stated in, 737
Appending
in arrays, 485
at end-of-file, 639
list elements, 465, 467
append() method, 333, 481, 485, 655
for class AccFile,650
Applications
dynamic matrices, 694, 695
index files, 652, 653
inhomogeneous lists, 574-575
area() function
defining and calling, 176, 177
Argument pointer, 685
Arguments, 43
access to, 685
arrays as, 356
command line, 366, 367
functions without, 45
macros called with, 123
objects as, 235
objects passed as, 282
passing, 178
pointer arrays as, 683
pointers as, 235
template, 730, 731
variable number of, 684, 685, 686
argv array
in memory, 366
Arithmetic operators, 20, 412
binary, 82 83
precedence of, 84
unary, 84, 85
Arithmetic shifts, 709
Arithmetic type conversions
and precedence, 707
Arithmetic types, 20, 21
in assignments, 145
arr array
in memory, 322
Array elements, 323
addressing, 353
arrays as, 331
index for, 323
pointers interrelated with, 352
Arrays
appending/deleting in, 485
as arguments, 356
as array elements, 331
class FloatArr, 427
class representing, 426, 427
defining, 322, 323
dynamic, 461
804 ■
INDEX
dynamic storage allocation for, 460, 461
encapsulating, 333
initializing, 324, 325
length of, 357
member, 332
multidimensional, 330, 331
name and address of, 351
parameters declared as, 357
of pointers, 364
pointers moved in, 355
and pointer variables, 351
sample program, 350,352
as sequential containers, 751
subscript operator for, 427
Arrow operator, 255
Article class, 287, 311
copy constructor of, 310
ASCII code (American Standard Code for Informa-
tion Interchange), 17, 800
Assignment operator, 87, 253, 412
overloading, 489
Assignments, 279, 488, 489
implicit type conversions in, 145, 531
type conversions in, 145, 532, 533
virtual, 572, 573. See also Compound assignments
Associative arrays, 427
Associative container classes, 769
Associative containers, 750, 751, 768, 769
and bitsets, 751
ATM (Asynchronous Transfer Mode) cells
header of, 714, 715
representing, 714
at() method, 165, 761
auto keyword, 205
Automatic lifetime, 199
auto objects, 205
auto specifier, 204
B
back() method
and container classes vector, deque, and list, 761
Backslashes, 29
bad_cast, 553
badbit, 645
Base classes, 383, 501
accessibility of, 589
access to members in, 503, 509
calling methods in, 513
conversions in references to, 535
converting to, 530, 531
multiple indirect, 590, 591
virtual, 592, 593
with virtual destructors, 548, 549
Base class object assignment, 533
Base class pointer conversion, 535
BaseE1 class, 575
defining, 574
Base initializers, 511, 595, 597, 655
Base subobject, 505
begin() method, 755, 769
Bell Laboratories, 3
Bias, 786
Bidirectional iterators, 755
Binary arithmetic operators, 82, 83
Binary bitwise operators, 713
Binary complement, 143
Binary mode
file opened in, 638
Binary operator, 415, 417
and operands, 82
Binary search algorithm, 643
Binary trees, 187
Binding, 551
Bit coded data, 707
Bit-fields, 714
defining, 715
Bitmap container class, 774, 775
Bitmaps
raster images represented with, 774, 775
Bit masks, 710, 711
creating, 713
using, 712
Bit patterns
retaining, 143
Bits
deleting, 711
manipulating, 777
Bitsets, 774, 775, 776, 777
associative containers and, 751
declaring, 775
Bitwise AND operator, 711
Bitwise exclusive OR operator, 711
INDEX
■805
Bitwise operators, 412,706, 707, 751
for bit manipulation, 777
in compound assignments, 713
for creating bit masks, 713
Bitwise OR operator, 711
Bitwise shift operators, 707, 708, 709
Blocks, building, 97
Block scope
object defined with, 199
Boolean constants, 23
Boolean operator precedence, 91
Boolean values, 17
output of, 68, 69, 71
types for, 16
bool type, 17, 23, 91
Braces
and functions without arguments, 45
and variables, 33
Brackets
and parameters for macros, 123
in syntax descriptions, 612
Branches, 125
break statement, 113
sample program containing, 112
Bresenham algorithm, 776, 777
Bubble sort algorithm, 334
Built-in types, 17
C
C
programming language, 3, 49, 51
standard library header files, 48
C++
characteristics of, 2, 3
conventions in, 31
developing/translating programs in, 6, 7
historical perspective on, 3
keywords in, 30
programming language, 3
sample program, 8,18
standard library, 7, 9, 48, 173, 723, 751, 753, 773
calc() function, 618, 619
Calling environment, 611
Capacities, 5, 763
capital() function
defining, 182
Car class, 504
accessing members of, 506
virtual method table for, 550
Case conversion
and macros, 129
Case labels, 111
Casting, 147
Castle class, 514, 515
Cast operator, 147
catch block, 615
nested, 616, 617
syntax of, 612, 613
Catching exceptions, 614
cctype header file, 129
Cell base class
and derived classes, 574
cerr stream, 58, 59
cfloat header file, 21
Character by character string comparison, 159
Character codes, 17, 69
Character constants, 23, 25
examples for, 24
Character manipulation
standard macros for, 128
Characters, 17
output of, 68, 69
reading and writing, 75
testing, 129
types for, 16
Character set, 17
char arrays, 327
CHAR_MAX, 19
CHAR_MIN, 19
char pointers, 351
array of, 367
sample function, 364
char type, 17, 19, 25, 112
char vectors, 327
cin, 429
cin stream, 47, 49, 58, 61
Class arrays
declaring, 329
sample program, 328
Class(es), 5, 245
abstract, 565-585
adapter, 753
806 ■
INDEX
associative container, 769
base, 501
container, 753
defining, 246, 247
derived, 501
dynamic members of, 479, 480
dynamic storage allocation for, 458
example of, 246
exception, 611
friend, 424, 425
and friend functions, 423
and global functions, 51
I/O stream, 59
iterator, 755
multiply-derived, 588, 589
naming, 247
operators for, 413. See also Abstract classes;
Adapter classes; Base classes; Derived classes;
Type conversion for classes
class keyword, 247, 257
Class member access operator, 253
Class-specific constants, 309
Class template, 723
defining, 725
for sequences, 753
clear() method, 70, 645
for deleting objects in container classes, 765
for erasing containers, 771
and maps/multimaps, 773
Client class, 303
climits header file, 19
clog stream, 58, 59
close() method, 389
Closing files, 388, 389
CLS macro, 123
cmath header file, 41
Collision resolution, 658
Collisions, 658
Colons
after labels, 113
Command line arguments, 367
sample program, 366
Comma operator, 412
syntax for, 101
Commas
for separating member initializers, 301
Comments
C++ program with, 10
examples of, 11
Comparative operators, 88, 159, 355
Comparator class, 753
compare() function, 689
Comparisons
results of, 89, 159, 355
Compiler, 7
Complex declarations, 690
operators and, 691
rules for evaluating, 691
complex header file, 48
Compound assignments, 145
bitwise operators in, 713
demonstration of, 86
operators, 87
Compound conditions, 91
Concatenation operators, 50, 157
Concrete classes
abstract classes versus, 569
Conditional expressions, 109
compilation, 790
structogram for, 108
Conditional inclusion, 126, 127
Conditional operator precedence, 109
conio.h header file, 132
Constants, 23, 25
class-specific, 309
const_iterator type, 755
const keyword, 34, 36, 64, 223
const member object declaration, 303
Const objects/methods
accessing, 276, 277
pointers to, 361
Constructor calls, 594, 595
and initialization, 595
sample program, 268
in virtual base classes, 597
Constructors, 251, 465
Account class with, 266
for adapter classes, 757
calling, 269, 299
conversion, 442, 443
copy, 279
declaring, 267
INDEX
■807
Constructors (continued)
default, 269, 279
defining, 266, 267
initializing, 269
with inline definitions, 273
task of, 267
of vector, list, and deque, 757. See also Destructors
Container adapters, 752
Container classes, 753, 768
deleting objects in, 765
Containers, 749-782
description of, 751
length and capacity of, 763
positioning and iterating in, 755
types of, 750, 751
Containers Library, 751, 753
Contiguous memory space, 323
continue statement, 113
Control, 28
Controlling expression, 97
ControlPoint class, 424, 425
Conversion constructors, 442, 443
Conversion functions, 443
conversion constructor versus, 445
defining, 445
“Cooked mode,” 386
Copy constructor, 279
effect of standard, 486
for FloatArr class, 486, 487
proprietary version of, 487
cos() function, 40
Counter
initializing, 99
count() method
and maps/multimaps, 773
count variable, 643
cout, 9, 30, 32
cout stream, 47, 49, 58, 61
Coworker class, 566, 567
assignment for, 572, 573
CPU registers, 205
cstdlib header file, 45
C strings
initializing, 326
specializing function template for, 732
and string class, 327
ctime() function, 167
ctype.h header file, 48
Current file position, 381
currentTime() global function, 284, 285
D
Data
abstraction, 3, 245, 501
bit coding, 707
class-specific, 305
encapsulation, 3, 245, 273
structures, 463
Data blocks
transferring, 391
Data handling
with traditional procedural programming, 5
Data members, 51, 245
and methods, 505
static, 304, 305
Date class methods, 288
Daytime class operators, 414
DayTime sample class, 280
Debuggers, 7
DEC Alpha workstations
and bit-fields, 715
Decimal constant, 23
Decimals
floating-point numbers represented as, 25
Declarations, 41
within header files, 47
Declaring sequences, 756, 757
dec manipulator, 63, 73
Decrement operator, 85
and bidirectional iterators, 755
Default arguments, 182, 183
defining, 182, 183
rules for and setting of, 735
of templates, 734, 735
default constructors, 269, 279, 299, 461
Default destructors, 271
default label, 111
Default settings, for flags, 61
#define directive, 121, 127
enum constants contrasted with, 309
working with, 124, 125
delete operator, 455, 456, 457, 459
808 ■
INDEX
delete[] operator, 461, 483
Deleting
in arrays, 485
list elements, 465, 467
objects in container classes, 765
in sequences, 764, 765
depAcc class
read() and write() methods of, 648, 649
deque container class, 755, 765
constructors of, 757
Derived classes, 501, 505
constructing/destroying, 510, 511
defining, 502
members of, 504
standard assignment of, 573
Derived class object assignment, 533
DerivedE1 class, 575
defining, 574
Derived type, 323
Destructors, 251, 465, 483, 655
calling, 271, 549
declaring, 271
default, 271
defined, 271
with inline definitions, 273
in Matrix class, 695
sample program, 270.See also Constructors
Direct base class, 503
Direct derivation, 502
displayError() function, 365
display() function, 227
display() method, 247, 253, 509
calling, 546, 547
new version of, 508
Division, 82
Dot operators, 253
Double ended queue, 753
Double quotes
and header files, 47
string constant within, 25
double type, 21, 25
do-while loop, 97
syntax for, 103
do-while statement
structogram for, 102
Downcasting, 536, 537
safety issues in, 537, 553
draw() method
and Bitmap container class, 775
and Bresenham algorithm, 777
Dynamically allocated objects
destroying, 548, 549
Dynamic arrays, 461
Dynamic binding, 551
Dynamic casting, 537
dynamic_cast operator, 553
Dynamic casts
using, 552
Dynamic data structures, 463
Dynamic matrices, 694, 695
Dynamic members, 477-498
classes with, 480
description of, 479
objects created with, 480
of varying length, 478
Dynamic memory allocation, 453-475
for containers, 751
Dynamic storage allocation
for arrays, 460, 461
for classes, 458
E
Early binding, 551
Elementary operations, 463
Element functions
for output in fields, 66
else branch, 105, 107
Else-if chains
structogram for, 106
switch statement contrasted with, 111
Embedded keys, 769
Employee class, 570
assignment for, 572, 573
Empty lists, 465, 577
empty() method, 771
and container classes, 763
Empty statements, 99
Empty strings, 25
Encapsulation, 3, 245, 257
of arrays, 333
and static data members, 307
INDEX
■809
end() method, 755
and associative container classes, 769
endl manipulator, 9, 61
Enumeration
definition, 309
sample program, 308
enum keyword, 309
eof bit, 387
Equals sign
and initialization, 33
erase() method, 161, 771
for deleting objects in container classes, 765
errno.h header file, 48
Error classes
defining, 618, 619
Error condition
backing out of, 615
Error handling, 387
and exception hierarchies, 619
for new handler, 457
traditional, 608, 609
Errors
avoiding, 723
common causes of, 609
input, 73
messages, 7, 43
parity bit computation and recognition of, 713
runtime, 267
templates checked for, 727
Escape sequences, 26, 28, 29, 123
decimal values and effects, 28
Euro class, 416, 417, 418
converting constructors of, 442
converting function for, 444
explicit type conversion for, 446
expressions valid for operators in, 419
with friend functions, 422
new, 420
testing conversions of, 444
Exception classes, 611
defining, 646
standard, 620, 621
Exception class members, 619
Exception declaration, 613
Exception handlers, 612, 613
searching for, 615
Exception handling, 3, 607-635
concept behind, 611
description of, 613
for files, 646
implementing own, 647
nesting, 616-617
Exception hierarchies, 619
Exceptions, 165
catching, 614
re-throwing, 617
throwing, 611, 614, 651
exceptions() method, 647
Exception specification list, 617
exchange() template function, 727
Executable file, 7
Exit code, 9
exit() function, 389
exp() function, 40
Explicit cast constructions, 537
Explicit initialization, 329
of objects, 459
Explicit inline methods, 273
Explicit instantiation
of templates, 736, 737
syntax for, 737
Explicit type conversion, 147, 443, 536, 537
for Euro class, 446
testing, 446
explicit keyword, 447
Exponential notation, 25, 65
Expressions, 83
evaluating, 97
with reference type, 228, 229
in switch statement, 111
Extended codes, 687
External functions, 207
External static object, 203
extern storage class, 200, 201, 207
extern “c”, 795
F
failbit state flag, of ios base class, 387
fail() method, 387
false keyword, 23
Fibonacci numbers, 325
Fibonacci quotients, 325
810 ■
INDEX
Fields
input, 71
output, 66
Field width
defining, 63
specifying, 67
File access
mode, 385
stream classes for, 382
File management
and file buffer, 381
File operations, 380, 381
Files, 381
buffers, 381
closing, 388, 389
default settings for opening, 386
determining positions in, 643
error handling when opening, 387
exception handling for, 646
extensions, 7
names, 385
opening/closing, 383, 385, 387, 638
open mode of, 386
positioning for random access, 640, 641, 642, 643.
See also Header files; Records
File scope
object defined with, 199
File state, 644, 645
File stream classes, 382, 383
functionality of, 383
in iostream library, 383
File streams, 383
definition, 385
sample program/creating, 384
Fill-characters
specifying for field, 67
fill() method, 67
Filter programs
using, 131
Filters, 131
find() method, 163
and maps/multimaps, 773
fixed manipulator, 65
Fixed point output, 65
Flags, 60
for open mode of file, 386
open mode, 387
positioning, 641
state, 645, 647
FloatArr class, 740
constructors in, 483
copy constructor for, 486, 487
data members of, 478
new declarations in, 488
new methods of, 490, 491
prototype of operator function for, 489
versions of, 479, 480, 481, 484, 485
Floating-point constants, 25
examples for, 24
Floating-point division, 413
Floating-point numbers, 17, 21, 25
formatted output of, 64
inputting, 73
Floating-point types, 20, 21
conversion of, to integral type, 145
conversion of, to larger floating-point type, 143
conversion of, to smaller type, 145
Floating-point values
types for, 16
float type, 21, 25, 331
for loops
syntax for, 99
Formatting, 61
options, 63
standard settings, 65
Formatting flags, 61
Formatting operator, 63, 67
for statement, 97
sample program, 100
structogram for, 98
Fraction class, 431
simplify() method of, 448
Fractions
calculating with, 430
Friend classes, 424, 425
declaring, 425
using, 425
Friend declaration, 423
Friend functions, 422, 423
declaring, 423
overloading operators with, 423
using, 425
INDEX
■811
friend keyword, 423
front() method, 761
and container classes vector, deque, and list, 761
fstream class, 383, 387
Function blocks, 175
Function call operator, 420
Function calls
defined, 43
implicit type conversions in, 147, 531
sample program, 42
Function prototype, 11, 41
example of, 40
Functions, 171-195
C++ program with, 10
calling and called, 178
conversion of, 443
declaring, 40-41, 175, 177
default arguments defined for, 182, 183
defining, 174
error checking after leaving, 608
external, 207
general form of, 174, 175
hash, 658
inline, 180, 181
libraries, 173
and macros, 125
operator, 414, 415, 416
overloading, 184, 185
and passing by value, 179
pointers to, 688, 689
pointer versions of, 358, 359
recursive, 186, 187
return value of, 176
sample, 205
scheme of, with varying arguments, 684
signatures, 185
significance of, in C++, 172
static, 207
virtual operator, 573
without arguments, 45
without return value, 45
Function templates, 723
ANSI instantiation of, 737
defining, 725
explicit instantiation of, 737
passing arguments to, 730, 731
Fundamental types, 16, 17, 18,20
example with, 303
operators for, 82-90
G
get() function, 75
getch() function, 132, 687
getline() function, 51, 155
getline() method, 75, 391
get() method, 75, 391
getPassword() function, 203, 207
get pointer, 643
getput() function, 187
get/put pointer, 639
getTypeid() method, 651
Global arrays, 325
Global functions, 9, 51
from C++ standard library, 173
methods versus, 283
programming, 175
Global objects, 199
defining, 201
using, 201
Global operator functions, 420, 421
defining, 421
Global variables, 33, 34
goto statement, 113
Graphical user interfaces, 7, 173
H
“Has-A” relationship, 299
Hash files, 658-659
Hash function, 658
Hashing, 325
Hash key, 658
Hash tables, 641
has relationship, 501
Header files, 7, 9, 41, 249
and associative containers, 768
and macros, 125
multiple inclusions of, 126
searching for, 47
and sequences, 752
standard, 48
standard class definitions in, 47
using, 46, 47
812 ■
INDEX
Heap, 454, 455, 769
Hexadecimal constant, 23
Hexadecimals
displaying, 63
outputting, 63
hex manipulator, 63, 73
Hot potato algorithm, 778, 779
I
Identical types, 323
Identifiers, 31
declaring, 41
read-only, 223
IEEE. See Institute of Electrical and Electronic Engi-
neers
#ifdef directive, 127
if-else statement
structogram for, 104
syntax for, 105
#ifndef directive, 127
if statements
variables defined in, 105
ifstream class, 383
Implicit conversion, 531
example for, 530
Implicit inline methods, 273
Implicit instantiation, 737
of template class, 727
Implicit type conversions, 140, 141, 441, 443
in assignments, 144
avoiding, 447
to base class type, 531
in function calls, 147
#include directive, 47
Include files, 7
include folder, 47
income() method, 567, 569
in constant, 309
Increment operator, 85
and bidirectional iterators, 755
Indefinite recursion, 509
Indentation, 11
Index entries, 643
representing, 642
IndexEntry class, 642, 643
Indexes, 165, 323, 643, 653, 655
access via, 761
for array elements, 323
and bit manipulation, 777
invalid, 165
representing, 644
Index file, 653
implementing, 654, 655
IndexFile class, 656
constructor of, 644
defined, 644, 645
insert() method of, 652, 653
IndexFileSystem class
insert() and retrieve() methods of, 654,
655
Index versions
of functions, 358, 359
Indirect base class, 503
Indirect derivation, 502
Indirection operator, 232, 233, 355
Infinite loops, 101
Inheritance, 3, 59, 499-528
data abstraction and reusability, 501
derived classes, 502
is relation, 500, 501
member access, 506-507
protected members, 514, 515
redefining members, 508, 509. See also Multiple
inheritance
Inheritance graph
building, 594, 595
InhomList class
complete, 578
defining, 576, 577
Inhomogeneous lists
application with, 574
implementing, 576
terminology for, 575
init() call, 253
Initialization, 33
and constructor calls, 595
of constructors, 269
explicit, 329
of member objects, 301
of objects, 251, 279, 455
references, 223
for virtual base classes, 596, 597
INDEX
■813
Initialization list, 325, 329
and arrays of pointers, 365
init() method, 247, 267
Inline functions, 125, 180, 181
definition of, 181
global, 273
and macros, 181, 183
inline keyword, 181
Inline methods, 272, 273
Input
errors, 73
fields, 71
formatted, 70
formatted, for numbers, 72
redirecting standard, 130, 131
stream classes for, 58
streams, 9
input() function, 686, 687
insertAfter() method, 577
Insertion methods
in sequences, 758
in vector, deque, and list container classes, 759
Insertion sort algorithm, 738
insert() method, 161, 485, 771
of class IndexFile,652, 653
of class IndexFileSystem,654, 655
and maps/multimaps, 773
of SortVec derived container class, 758
Instances, class, 51, 251
Instantiation
and template definition, 723
of template functions, 733
of templates, 726, 727
Institute of Electrical and Electronic Engineers, 20
Integer promotions, 140, 141
Integers, 17
computing parity of, 712
formatted output of, 62
inputting, 73
types for, 16
Integer types, 21
Integral constants, 23
examples for, 22
Integral numbers
displaying, 63
Integral promotion, 709
Integral types, 18, 19
conversion of, to floating-point type, 143
conversion of, to smaller type, 145
and operands for bitwise operators, 707
Integrated software development environment, 7
internal manipulator, 67
Internal static object, 203
International Organization for Standardization, 3
Interpolation search, 738
INT_MAX, 19
INT_MIN, 19
int type, 19, 23
Invalid indexes, 427
invalid_argument class, 620
I/O (input/output)
formatted/unformatted, 74, 75, 391
overloading shift operators for, 428
redirecting, 130, 131
iomanip header file, 48, 65, 66
ios baseclass
flags defined in, 386
ios::boolalpha flag, 69
ios class, 59
ios::seekdir type positioning flags, 641
iostream class, 59
iostream header file, 9
iostream library, 59
file stream classes in, 383
isLess() method, 282
islower(c) macro, 129
ISO. See International Organization for Standardiza-
tion
is_open() method, 389
is relationship, 500, 535, 589
istream class, 47, 59, 61
Iterating lists, 754
Iterator classes, 755
Iterators, 754
types of, 755
J
Jump table, 688, 689
K
kbhit() function, 132
814 ■
INDEX
Keys
and adapter classes, 753
and associative containers, 751
hash, 658
representing pairs of, 773
and sets and multisets, 771
unique and ambiguous, 769
Keyword, 29
L
Labels
and goto statement, 113
Laborer class, 568
standard assignment for, 573
Layout
and program flow, 107
of source files, 11
Left shift operators, 708, 709
left manipulator, 66
Legibility, 11
Length, of container, 763
length_error(*) class, 620
length() method, 51, 481
Less-than symbols, 9
Libraries
functions in, 173
Lifetime
object, 199
static, 203
LIFO (last-in-first-out) principle, 179, 725, 751
Lights class, 309
limits header file, 48
Linear solution, 658
Line feed, 187
line() function, 11
Linked lists, 462, 463
advantages with, 463
defining, 463
representing, 464
Linker, 7
List class
class definition for, 464, 465
new methods of, 490, 491
list container class, 767
constructors of, 757
methods for deleting objects in, 765
List elements
appending/deleting, 462, 465, 467
inserting in middle of inhomogeneous list, 576
inserting new, 577
representing, 465, 575
List operations sample program, 766
Lists
representing, 465
sorting, inverting, and splicing, 767
Literals, 23
Local objects, 179, 199
Local variables, 33, 34
LOCATE macro, 123
Logarithmic runtimes, 769
Logical bitwise operators, 707
Logical expressions
examples for, 90
Logical operators, 90, 141, 412
Logical shifts, 709
logic_error
exception classes derived from, 620, 621
long double type, 21, 25
long type, 19
Loop body, 97
Loops, 97
l-value, 233, 421
M
Macro definition
visibility for, 125
Macros
calling with arguments, 123
and case conversion, 129
for character manipulation/classification, 128
defining, 121
in different source files, 124
within header files, 47
and inline functions, 181, 183
redefining, 127
sample program, 120
for screen control, 123, 125
Macros with parameters sample program, 122
main() function, 9, 11, 173, 175
parameters of, 367
structure of, 8
MAKE utility, for module management, 173
INDEX
■815
Manipulators, 61
calling, 60
floating-point numbers formatting, 64
and integers formatting, 62
for output in fields, 66
Maps
and associative containers, 751
representing, 769
using, 773
Masks, bit, 710, 711
Mathematical rules
and expressions, 83
Mathematical standard functions, 40
MathError exception class, 619
math.h header file, 190
Matrix, 331
Matrix class, 695
constructor, destructor, and subscript operator for,
695
Member arrays, 332
Member functions, 9, 51, 245
Member initializers, 300, 301
Member objects, 298, 299
constant, 302, 303
initializing, 301
Members, 247
redefining, 508, 509
Member sub-object, 299
Memory
allocating, 249
objects in, 251
releasing, 459
union and usage of, 259
Memory address
for object of class, 255
merge() method
for merging list containers, 767
of SortVec container class, 762
message() function, 227
Methods, 51, 245
calling, 51
of class template, 725
const and non-const versions of, 277, 279
and data members, 505
defining, 248, 249
global functions versus, 283
name lookup for, 507
operator functions as, 415
operators overloadable by, 420
positioning, 643
pure virtual, 566, 567
standard, 278, 279
min() function template, 732
MIN macro, 127
Modifiers
signed and unsigned, 19
Modular programming, 7, 249
Modules, 7, 173, 199
MotorHome multiply-derived class, 588, 589, 598
move() method
and BitmapN container class, 775
Multidimensional arrays
defining, 331
as parameters, 359
sample program, 330
Multimaps, 769
using, 772, 773
Multiple assignments, 87
Multiple indirect base classes, 590, 591
Multiple inheritance, 587-606
constructor calls, 594
initializing virtual base classes, 596
multiple identical base classes, 591
multiple indirect base classes, 590
multiply-derived classes, 588, 589
virtual base classes, 592
Multiple template parameters, 729
Multiply-derived classes, 588, 589
multiset container class, 771
Multisets, 769
declaring, 771
sample, 770
N
Names and naming
arrays, 351
bit-fields, 715
constructors, 267
declaring, 41
file, 385
macros, 121
operator functions, 415
816 ■
INDEX
source file, 7
valid, 31
of variables, 31
namespace keyword, 209
Namespaces
defining, 208, 209
n-dimensional array, 331
Negation, 417
Negative numbers
converting, 142
outputting as decimals, 63
Nested if-else statements, 105
Nested namespaces, 209
Nesting exception handling, 616, 617
Nesting loops, 103
new handler, 457
New-line characters, 11, 51
new operator, 454
calling for fundamental types, 455
calling with default constructor, 459
new[] operator, 461
noboolalpha manipulator, 69
Nongraphic characters, 28
noshowpoint(*), 64
noshowpos(*) manipulator, 60
NOT operator, 91
nouppercase manipulator, 63
NULL, 365, 465, 577
Null character, 25, 26, 327
NULL pointer, 333, 363, 457
Numbers
formatted input of, 72
Number symbol (#), 9, 11
Numerical constants, 23
Numeric operations
exception handling for, 618, 619
numeric_limits, 786
O
Object-oriented programming, 3, 4, 5, 245
Object persistence, 392, 393
Objects, 5, 33
accessing, 281, 760, 761
as arguments, 235
assigning, 253
cleaning up, 271
creating/destroying, 51, 482, 483, 511
creating with dynamic members, 480
declaring, 513
defining, 250, 251
of derived classes, 512
explicit initialization of, 459
initializing, 251, 455
lifetime of, 199
local, 179
member, 298
in memory, 251
passing as arguments, 282
passing by reference, 283
passing by value, 283
pointers to, 254, 255
references returned to, 285
representing pairs of, 773
returning, 284, 285
static, 203
storage classes of, 198
storing, 393
of union WordByte in memory, 258
using, 252.See also Classes; References
Obligatory arguments, 685
Octal constant, 23
Octal numbers
outputting, 63
oct manipulator, 63, 73
OFF constant, 309
ofstream class, 383
ON constant, 309
OOP. See Object-oriented programming
open() method, 386, 387
Open mode flags, 387
Open modes, of file, 386
Operands
and order of evaluation, 91
symmetry of, 419
Operations
file, 380, 381
for sequences, 752
Operator functions, 414, 415, 416
calling, 415, 419
declaration of, 428
defining global, 421
definition of, 428
INDEX
■817
Operator functions (continued)
global or method, 421
as methods, 415
naming, 415
negation, addition, and subtraction, 417
operator keyword, 415, 445
Operators
bitwise, 706, 707
for classes, 413
and complex declarations, 691
dot, 253
indirection, 232
overloadable, 412
overloading, 413
with pointer variables, 355
reference type, 229
in template functions, 733
unary, 233
Operators for fundamental types
binary arithmetic operators, 82-83
increment/decrement operators, 85
logical operators, 90
relational operators, 88, 89
sign operators, 85
unary arithmetic operators, 84
Optional arguments, 685, 687
OR operator, 91
ostream class, 47, 59, 61
out constant, 309
out_of_range(*), 620
Output
redirecting standard, 130, 131
stream classes for, 58
streams, 9
overflow_error(*) class, 620
Overloaded operators
rules for, 412
using, 418, 419
Overloading
assignment operator, 489
functions, 184, 185
operators, 413, 423
and redefinition, 509
shift operators for I/O, 428, 429
subscript operators, 426, 427, 485
P
Parameters, 175
declaring, 357
multidimensional arrays as, 359
pointers as, 234
read-only pointers as, 361
Parentheses
in syntax description, 33
Parity bit computation, 713
parity() function, 713
PassCar
versions of, 510, 511
PassCar class
virtual method table for, 550
PassCar derived class, 504
Passing arguments
to function templates, 730, 731
Passing by reference, 179, 225, 283
Passing by value, 179, 225, 283
Persistence
object, 392, 393
of polymorphic objects, 648,650
Pixels (picture element), 775
Pointer arithmetic, 354, 355
Pointer arrays
generating dynamically, 683
Pointer assignment
effect of, 534
Pointers, 233, 285, 729, 755
to abstract classes, 570, 571
as arguments, 235
array elements interrelated with, 352
arrays of, 364
comparing, 355
to const objects, 361
defining, 230
defining arrays of, 365
to functions, 688, 689
moving in array, 355
NULL, 333
to objects, 254, 255
as parameters, 234
parameters declared as, 357
read-only, 360
returning, 362, 363
818 ■
INDEX
sample program, 350,352
subtracting, 355
typeless, 351
use of, instead of indices, 359
Pointers to pointers, 682, 683
Pointer types, 231
Pointer variables, 231, 235, 683
addressing with, 353
and arrays, 351
Polymorphic interfaces, 571
Polymorphic objects
persistence of, 648,650
storing, 649
Polymorphism, 3, 543-564
concept of, 544, 545
destroying dynamically allocated objects, 548, 549
dynamic casts, 552, 553
virtual methods, 546, 547
virtual method table, 550, 551
pop_back() method
for deleting objects in container classes, 765
popFront() method, 465, 467
pop() method, 765
Positioning flags, 641
Positioning methods, 643
Positive numbers
converting to, 142
Postfix increment, 430
Postfix notation, 85
effects of, 84
Precedence
of arithmetic operators, 84
and arithmetic type conversions, 707
of Boolean operators, 91
for cast operator (type), 147
for comma operator, 101
for indirection operator, 233
operator, 85
for operators with pointer variables, 355
of relational operators, 88, 89
precision() method, 65
Prefixes, 31
Prefix increment, 430
Prefix notation, 85
effects of, 84
Preprocessor, 9
Preprocessor directives, 11
Primary file, within index file, 653, 655
printf() function, 685
Priority queues, 753
testing, 764
priority_queue template, 753
Private data members
accessing, 275
Private members, 245, 247, 249, 503, 507
Procedures, 5
Program scope
object defined with, 199
Projects, 249
Properties, 5
protected constructors, 569
Protected declarations, 515
Protected members, 515
Prototype, 175, 177
public base classes
is relationship established by, 589
Public interface, of class, 247
Public members, 245, 247
access to, in base class, 503
Public methods, 51
Pure virtual methods, 566, 567
pushBack() method, 465, 467
push_back() method, 759
push_front() method, 759
push() method, 759
put() method, 75, 391
Q
qsort() function, 689, 696, 697
QuadMatrix template, 734, 735
Quadratic matrices
class template representing, 734, 735
Queues
as sequential containers, 751
Quick sort algorithm, 187, 689
Quotient
of Fibonacci number, 325
R
rand(), 45
INDEX
■819
Random access iterators, 755
Random file access, 381, 639
positioning for, 640, 641, 642, 643
Random number generator
initializing, 44, 45
Random positioning statements, 643
Random read and write access, 639
Range checking, 427
Range operator, 305
base class method accessed by, 509
range_error class, 620
Raster images
representing with bitmaps, 774, 775
rdstate() method, 645
Readability
and complex expressions, 109
and empty statements, 99
and loop body, 97
and macros, 121
and typedef, 693
Read access
open mode for, 638
read_at() method, 642
Reading
blocks of records, 390
characters, 75
records, 381
read() method
of classes DepAcc and SavAcc,648, 649
implementing, 392, 393
Read-only methods, 277
Read-only pointers, 360
for non-constant objects, 361
Read-only references, 223, 225
Records, 257
inserting and retrieving, 655
position of, in files, 643
reading, 381
reading/writing blocks, 390
Recursive data structures, 465
Recursive functions, 186, 187
Redefined methods
calling, 513
Redefinition, 509
References, 3, 729
to abstract classes, 570, 571
conversions in, to base classes, 535
defining, 222
as parameters, 224
and pointers, 231
read-only, 223, 225
returning, 285
as return value, 226, 227
sample program, 222
Reference type function
calling, 227
register keyword, 205
Registers
CPU, 205
Register variables, 205
sample function with, 204
Relational operators, 50, 412
precedence of, 88, 89
remove() method, 481, 485
replace() method, 163
reset() method
and manipulating bits, 777
Resistant mistakes
program with, 76
resize() method
and container classes, 763
Result class, 303, 424, 425
constructors for, 298, 299
new version of, 302
with static members, 304
with static methods, 306
retrieve() method, 651
of IndexFileSystem class, 654, 655
Return address, 181
return statement, 9, 177
Return values, 41, 285
Reusability, 5, 501
reverse() function, 357
reverse() method, 767
rfind() method, 163
Right shift operators, 708, 709
Round brackets, 33
Routers, 779
820 ■
INDEX
Row class
defining, 694, 695
RTTI. See Run Time Type Information
Runtime behavior
of container classes, 759
runtime_error, 621
Run time errors, 621
avoiding, 43, 267
exception classes derived from, 620, 621
Run Time Type Information, 552
R-values, 233
S
Safe class, 514, 515
SavAcc class
defining, 648, 649
scientific manipulator, 65
Scope, 199
Scope resolution operator, 209, 211, 249
Screen control macros, 123, 125
Scrolling string output, 334
search() method, 655
seekg() method, 641
seekp() method, 641
SelectionSort() function, 697
Semicolon, 9, 103
Sequences
and header files, 752
operations for, 752
representing, 753
Sequential containers (or sequences), 750, 751
Sequential file access, 381
set container class, 771
setfill() manipulator, 66
setfill() method, 67
setf() method, 60, 61, 69
set() method, 777
setprecision() manipulator, 65
Sets
associative containers within, 751
declaring, 771
representing, 769
sample, 770
setTime() method, 282
setw() manipulator, 66
Shape type, 309
Sheltered members
access to, 515
Shift operators, 708
short type, 19
showpos manipulator
calling, 60
Side effects
avoiding, 87
of macros, 125
Sieve of Eratosthenes, 334
signal.h header file, 48
Signatures
constructor, 267, 269
function, 185
signed char type, 19, 142
Signed integers
converting, 142
signed keyword, 19
Signed type
conversion of, to larger integral type, 143
Sign extension, 143
Sign operators, 85
Simple assignments, 87
Single characters
meaning of, 26
Single quotes
character constants within, 25
size() method
and length of container, 763
and maps/multimaps, 773
and number of objects in container, 771
sizeof operator, 21
sort() method
list container sorted by call to, 767
SortVec container class
merge() method of, 762
search() method of, 760
using, 756
Source code, 7
Source files, 7, 249
layout of, 11
name, 7
INDEX
■821
Spaces, 11
Special characters, 28
Special objects, of base class, 531
splice() function, 466, 467
Splice operations, 767
sqrt() function, 40, 53
srand() function, 45
sstream class, 48
Stack class template, 724
explicit instantiation for, 737
with two template parameters, 728
Stack content
after calling function, 178
Stacks, 179
fixed/varying arguments on, 684
and recursive functions, 187
as sequential containers, 751
testing, 726
Standard copy constructor, 487
Standard exception classes
hierarchy of, 621
using, 620
Standard exception handling
for streams, 647
Standard input, 59
Standard methods, 279
sample program, 278
Standard output, 59
Standard settings, 65
Star character, 233
State flags, 645, 647
Statements, 9
Static arrays, 325
Static binding, 551
Static data members, 304, 305
accessing, 306
declaring, 305
definition and initialization, 305
and encapsulation, 307
Static data structures, 463
Static functions, 207
static keyword, 305
static_cast, 537
Static lifetime, 199, 203
Static member functions, 307
Static objects, 203
static storage class, 202, 203, 207
std standard namespace, 9, 209
Storage classes, 199
of functions, 206
Storage class specifiers, 198
strcat() function
and return pointers, 363
strcmp() function, 327
index version of, 368
strcpy() function, 327
pointer versions of, 358
and return pointers, 363
Stream access errors, 651
Stream class
shift operators, 229
streambuf class, 48
Streams, 9
discovering/changing status of, 645
standard, 59
standard exception handling for, 647
String assignments, 155, 157
string class, 153, 251, 413
C strings and, 327
defining, 155
objects of, 51
sample assignments of, 228
sample program, 50,154
String constants, 23, 25
String literal
internal representation of, 24
Strings
characters accessed in, 164
comparing, 158
concatenating, 156, 157
escape sequences used in, 29
initializing, 154, 155
inserting and erasing in, 160, 161
numbers converted to, 288
output of, 68, 69
searching and replacing in, 162, 163
stringstream class, 288
strlen() function, 327, 359
Stroustrup, Bjarne, 3
strstr() function sample program, 362, 363
struct keyword, 257
structs sample program, 256
822 ■
INDEX
Style, 11
Sub-object lattice, 595
Subroutines, 5, 181
Subscript, 323
Subscript operators, 165, 427
and access via indices, 761
bits referenced by, 777
in Matrix class, 695
overloading, 426, 427
read and write access using, 485
Substrings
erasing, 160
replacing, 162
Subtraction, 355, 417
swap()
implementing as method, 282
swap() function, 235
Swapping, 455
switch statement, 111
else-if chains contrasted with, 111
structogram for, 110
Symbolic constants, 121
sync() method, 70
Syntax, 249
brackets in descriptions, 612
for defining variables, 33
errors, 7
T
Tabs, 11
tan() function, 40
tellg() method, 641
TelList class, 332, 333
methods implemented for, 336, 337
tellp() method, 641
Template arguments
restrictions on, 731
Template function definition, 733
Template functions
motivation for, 733
Template parameters
multiple, 729
restrictions on, 729
Templates, 3, 721-748
advantages of, 723
arguments, 730, 731
in C++ standard library, 723
default arguments of, 734, 735
defining, 724, 725
defining with multiple parameters, 729
function and class, 723
instantiating, 726, 727, 736, 737
parameters, 728, 729
specialization, 732-733
terminate() function, 613
Testing characters, 129
Text
mode, 386
and nesting loops, 103
Text editor, 7
this pointer
sample class DayTime,280
using, 281
Throwing exceptions, 614
throw statement, 611
using, 610
timediff() function, 207
time() function, 167
time_t type, 261
tm struct, 260
Tokens, 11
Tone
and nesting loops, 103
top() method, 761
toupper() macro, 129
Traditional procedural programming, 4, 5
Translation unit, 199
true keyword, 23
trunc open mode, 386
Truth table
for logical operators, 90
try block, 615
nested, 616, 617
syntax of, 612, 613
Two-dimensional arrays
initialization list of, 331
parameter declaration for, 359
Type casting, 351
Type conversion for classes, 441-452
ambiguities of type conversions, 446-447
conversion constructors, 442-443
conversion functions, 444-445
INDEX
■823
Type conversions, 43, 146
ambiguities of, 446, 447
in assignments, 145, 532, 533
explicit, 147, 536, 537
failure, 447
implicit, 140, 144, 147, 531
standard, 445
usual arithmetic, 141, 142
typedef keyword, 693
Type hierarchy, 140
Typeless pointers, 351
Typenames
defining, 692
Types, 611
platform dependent, 693
pointer, 231
U
Unary arithmetic operators, 84, 85
Unary operators, 83, 233
underflow_error class, 620
#undef directive, 127
Underscores
and internal names, 31
Unicode, 17
Union, 258, 259
defined, 259
Unique keys, 769
usetf() method, 60, 61
unsigned char type, 19
unsigned keyword, 19
unsigned short, 19
Unsigned types, 143
Unsigned value, 45
Unwinding the stack, 615
Upcasting, 536, 537, 553
User Network Interface, 715
using declaration, 211
using directive, 211
using keyword, 9, 49, 210
Usual arithmetic type conversions, 141, 145
performing, 142
V
va_arg() macro
arguments of, 687
valarray class, 48
Variables
defining, 33
defining in if statements, 105
names of, 31
pointer, 683
sample program, 32
Variable type, 77
Vector, 323
vector container class, 755
constructors of, 757
methods for deleting objects in, 765
Vectors
iterating, 754
Virtual assignments
using, 573
Virtual base classes, 592, 593
constructor calls in, 597
initializing, 596, 597
Virtual destructors
declaring, 549
virtual keyword, 593
Virtual methods, 546, 547
calling, 544, 545
declaring, 547
pure, 566, 567
redefining, 547
Virtual method tables, 550, 551
Virtual operator functions, 573
VMT. See Virtual method tables
void type, 21
for functions, 44, 45
void* type pointer, 351
volatile keyword, 34, 36
W
Warnings, 7
wchar_t type, 17, 19
what() method, 621
824 ■
INDEX
what() virtual method, 647
while statement
structogram for, 96
structogram for break within, 112
syntax for, 97
Whitespace characters, 11
Width
bit-fields, 715
width() method, 67, 491
Wordbyte union
defining/using, 258
Write access
open mode for, 638
write_at() method, 642
WriteError type exception, 651
write() method, 391, 392, 393
of classes DepAcc and SavAcc,648, 649
Write operation, 381
Writing
blocks of records, 390
characters, 75
X
XOR operator, 707
Z
Zero extension, 143
INDEX
■825

Navigation menu