Google C++ Style Guide

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Google C++ Style Guide
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Google C++ Style Guide
Revision 3.260
Benjy Weinberger
Craig Silverstein
Gregory Eitzmann
Mark Mentovai
Tashana Landray
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Table of Contents
Header
Files
The #define Guard  Forward Declarations  Inline Functions
The -inl.h Files  Function Parameter Ordering
Names and Order of Includes
Scoping Namespaces  Nested Classes
Nonmember, Static Member, and Global Functions  Local Variables
Static and Global Variables
Classes Doing Work in Constructors  Default Constructors
Explicit Constructors  Copy Constructors  Structs vs. Classes
Inheritance  Multiple Inheritance  Interfaces
Operator Overloading  Access Control  Declaration Order
Write Short Functions
Google-
Specific
Magic
Smart Pointers  cpplint
Other C++
Features
Reference Arguments  Function Overloading  Default Arguments
Variable-Length Arrays and alloca()  Friends  Exceptions
Run-Time Type Information (RTTI)  Casting  Streams
Preincrement and Predecrement  Use of const  Integer Types
64-bit Portability  Preprocessor Macros  0 and nullptr/NULL  sizeof
auto  Brace Initialization  Boost  C++11  unique_ptr
Naming General Naming Rules  File Names  Type Names  Variable Names
Constant Names  Function Names  Namespace Names
Enumerator Names  Macro Names  Exceptions to Naming Rules
Comments Comment Style  File Comments  Class Comments
Function Comments  Variable Comments
Implementation Comments  Punctuation, Spelling and Grammar
TODO Comments  Deprecation Comments
Formatting Line Length  Non-ASCII Characters  Spaces vs. Tabs
Function Declarations and Definitions  Function Calls
Braced Initializer Lists  Conditionals  Loops and Switch Statements
Pointer and Reference Expressions  Boolean Expressions
Return Values  Variable and Array Initialization
Preprocessor Directives  Class Format  Constructor Initializer Lists
Namespace Formatting  Horizontal Whitespace
Vertical Whitespace
Exceptions
to the
Rules
Existing Non-conformant Code  Windows Code
Important Note
Displaying Hidden Details in this Guide
This style guide contains many details that are initially hidden from view. They are marked by

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the triangle icon, which you see here on your left. Click it now. You should see "Hooray"

appear below.

Hooray! Now you know you can expand points to get more details. Alternatively, there's an

"expand all" at the top of this document.

Background

C++ is the main development language used by many of Google's open-source projects. As

every C++ programmer knows, the language has many powerful features, but this power

brings with it complexity, which in turn can make code more bug-prone and harder to read

and maintain.

The goal of this guide is to manage this complexity by describing in detail the dos and don'ts

of writing C++ code. These rules exist to keep the code base manageable while still allowing

coders to use C++ language features productively.

Style

, also known as readability, is what we call the conventions that govern our C++ code.

The term Style is a bit of a misnomer, since these conventions cover far more than just source

file formatting.

One way in which we keep the code base manageable is by enforcing

consistency

. It is very

important that any programmer be able to look at another's code and quickly understand it.

Maintaining a uniform style and following conventions means that we can more easily use

"pattern-matching" to infer what various symbols are and what invariants are true about

them. Creating common, required idioms and patterns makes code much easier to

understand. In some cases there might be good arguments for changing certain style rules,

but we nonetheless keep things as they are in order to preserve consistency.

Another issue this guide addresses is that of C++ feature bloat. C++ is a huge language with

many advanced features. In some cases we constrain, or even ban, use of certain features.

We do this to keep code simple and to avoid the various common errors and problems that

these features can cause. This guide lists these features and explains why their use is restricted.

Open-source projects developed by Google conform to the requirements in this guide.

Note that this guide is not a C++ tutorial: we assume that the reader is familiar with the

language.

Header Files

In general, every .cc file should have an associated .h file. There are some common

exceptions, such as unittests and small .cc files containing just a main() function.

Correct use of header files can make a huge difference to the readability, size and

performance of your code.

The following rules will guide you through the various pitfalls of using header files.

The #define Guard

All header files should have #define guards to prevent multiple inclusion. The format of

the symbol name should be <PROJECT>_<PATH>_<FILE>_H_.

To guarantee uniqueness, they should be based on the full path in a project's source tree. For

example, the file foo/src/bar/baz.h in project foo should have the following guard:

#ifndef FOO_BAR_BAZ_H_

#define FOO_BAR_BAZ_H_

...

#endif // FOO_BAR_BAZ_H_

Forward Declarations

You may forward declare ordinary classes in order to avoid unnecessary #includes.

Definition:

A "forward declaration" is a declaration of a class, function, or template without an

associated definition. #include lines can often be replaced with forward declarations of

whatever symbols are actually used by the client code.

Pros:

In our case, Physika_Src/ is the root directory. For example, the #deﬁne

guard for "Physika_Src/Physika_Core/Utilities/global_conﬁg.h" goes as

below:

#ifndef PHYSIKA_CORE_UTILITIES_GLOBAL_CONFIG_H_

#deﬁne PHYSIKA_CORE_UTILITIES_GLOBAL_CONFIG_H_

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Unnecessary #includes force the compiler to open more files and process more input.

They can also force your code to be recompiled more often, due to changes in the

header.

Cons:

It can be difficult to determine the correct form of a forward declaration in the

presence of features like templates, typedefs, default parameters, and using

declarations.

It can be difficult to determine whether a forward declaration or a full #include is

needed for a given piece of code, particularly when implicit conversion operations

are involved. In extreme cases, replacing an #include with a forward declaration can

silently change the meaning of code.

Forward declaring multiple symbols from a header can be more verbose than simply

#includeing the header.

Forward declarations of functions and templates can prevent the header owners from

making otherwise-compatible changes to their APIs; for example, widening a

parameter type, or adding a template parameter with a default value.

Forward declaring symbols from namespace std:: usually yields undefined behavior.

Structuring code to enable forward declarations (e.g. using pointer members instead

of object members) can make the code slower and more complex.

The practical efficiency benefits of forward declarations are unproven.

Decision:

When using a function declared in a header file, always #include that header.

When using a class template, prefer to #include its header file.

When using an ordinary class, relying on a forward declaration is OK, but be wary of

situations where a forward declaration may be insufficient or incorrect; when in doubt,

just #include the appropriate header.

Do not replace data members with pointers just to avoid an #include.

Always #include the file that actually provides the declarations/definitions you need; do

not rely on the symbol being brought in transitively via headers not directly included. One

exception is that myfile.cc may rely on #includes and forward declarations from its

corresponding header file myfile.h.

Inline Functions

Define functions inline only when they are small, say, 10 lines or less.

Definition:

You can declare functions in a way that allows the compiler to expand them inline rather

than calling them through the usual function call mechanism.

Pros:

Inlining a function can generate more efficient object code, as long as the inlined function is

small. Feel free to inline accessors and mutators, and other short, performance-critical

functions.

Cons:

Overuse of inlining can actually make programs slower. Depending on a function's size,

inlining it can cause the code size to increase or decrease. Inlining a very small accessor

function will usually decrease code size while inlining a very large function can dramatically

increase code size. On modern processors smaller code usually runs faster due to better use

of the instruction cache.

Decision:

A decent rule of thumb is to not inline a function if it is more than 10 lines long. Beware of

destructors, which are often longer than they appear because of implicit member- and base-

destructor calls!

Another useful rule of thumb: it's typically not cost effective to inline functions with loops or

switch statements (unless, in the common case, the loop or switch statement is never

executed).

It is important to know that functions are not always inlined even if they are declared as such;

for example, virtual and recursive functions are not normally inlined. Usually recursive

functions should not be inline. The main reason for making a virtual function inline is to place

its definition in the class, either for convenience or to document its behavior, e.g., for

accessors and mutators.

The -inl.h Files

You may use file names with a -inl.h suffix to define complex inline functions when

needed.

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The definition of an inline function needs to be in a header file, so that the compiler has the

definition available for inlining at the call sites. However, implementation code properly

belongs in .cc files, and we do not like to have much actual code in .h files unless there is a

readability or performance advantage.

If an inline function definition is short, with very little, if any, logic in it, you should put the code

in your .h file. For example, accessors and mutators should certainly be inside a class

definition. More complex inline functions may also be put in a .h file for the convenience of

the implementer and callers, though if this makes the .h file too unwieldy you can instead put

that code in a separate -inl.h file. This separates the implementation from the class

definition, while still allowing the implementation to be included where necessary.

Another use of -inl.h files is for definitions of function templates. This can be used to keep

your template definitions easy to read.

Do not forget that a -inl.h file requires a #define guard just like any other header file.

Function Parameter Ordering

When defining a function, parameter order is: inputs, then outputs.

Parameters to C/C++ functions are either input to the function, output from the function, or

both. Input parameters are usually values or const references, while output and input/output

parameters will be non-const pointers. When ordering function parameters, put all input-

only parameters before any output parameters. In particular, do not add new parameters to

the end of the function just because they are new; place new input-only parameters before

the output parameters.

This is not a hard-and-fast rule. Parameters that are both input and output (often

classes/structs) muddy the waters, and, as always, consistency with related functions may

require you to bend the rule.

Names and Order of Includes

Use standard order for readability and to avoid hidden dependencies: C library, C++

library, other libraries' .h, your project's .h.

All of a project's header files should be listed as descendants of the project's source directory

without use of UNIX directory shortcuts . (the current directory) or .. (the parent directory).

For example, google-awesome-project/src/base/logging.h should be included as

#include "base/logging.h"

In dir/foo.cc or dir/foo_test.cc, whose main purpose is to implement or test the stuff in

dir2/foo2.h, order your includes as follows:

1. dir2/foo2.h (preferred location — see details below).

2. C system files.

3. C++ system files.

4. Other libraries' .h files.

5. Your project's .h files.

With the preferred ordering, if dir2/foo2.h omits any necessary includes, the build of

dir/foo.cc or dir/foo_test.cc will break. Thus, this rule ensures that build breaks show

up first for the people working on these files, not for innocent people in other packages.

dir/foo.cc and dir2/foo2.h are often in the same directory (e.g.

base/basictypes_test.cc and base/basictypes.h), but can be in different directories

too.

Within each section the includes should be ordered alphabetically. Note that older code

might not conform to this rule and should be fixed when convenient.

For example, the includes in google-awesome-

project/src/foo/internal/fooserver.cc might look like this:

#include "foo/public/fooserver.h" // Preferred location.

#include <sys/types.h>

#include <unistd.h>

#include <hash_map>

#include <vector>

#include "base/basictypes.h"

#include "base/commandlineflags.h"

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#include "foo/public/bar.h"

Scoping

Namespaces

Unnamed namespaces in .cc files are encouraged. With named namespaces, choose

the name based on the project, and possibly its path. Do not use a

using-directive

Definition:

Namespaces subdivide the global scope into distinct, named scopes, and so are useful for

preventing name collisions in the global scope.

Pros:

Namespaces provide a (hierarchical) axis of naming, in addition to the (also hierarchical)

name axis provided by classes.

For example, if two different projects have a class Foo in the global scope, these symbols may

collide at compile time or at runtime. If each project places their code in a namespace,

project1::Foo and project2::Foo are now distinct symbols that do not collide.

Cons:

Namespaces can be confusing, because they provide an additional (hierarchical) axis of

naming, in addition to the (also hierarchical) name axis provided by classes.

Use of unnamed namespaces in header files can easily cause violations of the C++ One

Definition Rule (ODR).

Decision:

Use namespaces according to the policy described below. Terminate namespaces with

comments as shown in the given examples.

Unnamed Namespaces

Unnamed namespaces are allowed and even encouraged in .cc files, to avoid

runtime naming conflicts:

namespace { // This is in a .cc file.

// The content of a namespace is not indented

enum { kUnused, kEOF, kError }; // Commonly used tokens.

bool AtEof() { return pos_ == kEOF; } // Uses our namespace's EOF.

} // namespace

However, file-scope declarations that are associated with a particular class may be

declared in that class as types, static data members or static member functions rather

than as members of an unnamed namespace.

Do not use unnamed namespaces in .h files.

Named Namespaces

Named namespaces should be used as follows:

Namespaces wrap the entire source file after includes, gflags definitions/declarations,

and forward declarations of classes from other namespaces:

// In the .h file

namespace mynamespace {

// All declarations are within the namespace scope.

// Notice the lack of indentation.

class MyClass {

public:

...

void Foo();

};

} // namespace mynamespace

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// In the .cc file

namespace mynamespace {

// Definition of functions is within scope of the namespace.

void MyClass::Foo() {

...

}

} // namespace mynamespace

The typical .cc file might have more complex detail, including the need to reference

classes in other namespaces.

#include "a.h"

DEFINE_bool(someflag, false, "dummy flag");

class C; // Forward declaration of class C in the global namespace.

namespace a { class A; } // Forward declaration of a::A.

namespace b {

...code for b... // Code goes against the left margin.

} // namespace b

Do not declare anything in namespace std, not even forward declarations of

standard library classes. Declaring entities in namespace std is undefined behavior,

i.e., not portable. To declare entities from the standard library, include the appropriate

header file.

You may not use a

using-directive

to make all names from a namespace available.

// Forbidden -- This pollutes the namespace.

using namespace foo;

You may use a

using-declaration

anywhere in a .cc file, and in functions, methods or

classes in .h files.

// OK in .cc files.

// Must be in a function, method or class in .h files.

using ::foo::bar;

Namespace aliases are allowed anywhere in a .cc file, anywhere inside the named

namespace that wraps an entire .h file, and in functions and methods.

// Shorten access to some commonly used names in .cc files.

namespace fbz = ::foo::bar::baz;

// Shorten access to some commonly used names (in a .h file).

namespace librarian {

// The following alias is available to all files including

// this header (in namespace librarian):

// alias names should therefore be chosen consistently

// within a project.

namespace pd_s = ::pipeline_diagnostics::sidetable;

inline void my_inline_function() {

// namespace alias local to a function (or method).

namespace fbz = ::foo::bar::baz;

...

}

} // namespace librarian

Note that an alias in a .h file is visible to everyone #including that file, so public headers

(those available outside a project) and headers transitively #included by them, should

avoid defining aliases, as part of the general goal of keeping public APIs as small as

possible.

Nested Classes

Although you may use public nested classes when they are part of an interface, consider a

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namespace to keep declarations out of the global scope.

Definition:

A class can define another class within it; this is also called a

member class

class Foo {

private:

// Bar is a member class, nested within Foo.

class Bar {

...

};

Pros:

This is useful when the nested (or member) class is only used by the enclosing class; making it

a member puts it in the enclosing class scope rather than polluting the outer scope with the

class name. Nested classes can be forward declared within the enclosing class and then

defined in the .cc file to avoid including the nested class definition in the enclosing class

declaration, since the nested class definition is usually only relevant to the implementation.

Cons:

Nested classes can be forward-declared only within the definition of the enclosing class.

Thus, any header file manipulating a Foo::Bar* pointer will have to include the full class

declaration for Foo.

Decision:

Do not make nested classes public unless they are actually part of the interface, e.g., a class

that holds a set of options for some method.

Nonmember, Static Member, and Global Functions

Prefer nonmember functions within a namespace or static member functions to global

functions; use completely global functions rarely.

Pros:

Nonmember and static member functions can be useful in some situations. Putting

nonmember functions in a namespace avoids polluting the global namespace.

Cons:

Nonmember and static member functions may make more sense as members of a new class,

especially if they access external resources or have significant dependencies.

Decision:

Sometimes it is useful, or even necessary, to define a function not bound to a class instance.

Such a function can be either a static member or a nonmember function. Nonmember

functions should not depend on external variables, and should nearly always exist in a

namespace. Rather than creating classes only to group static member functions which do

not share static data, use namespaces instead.

Functions defined in the same compilation unit as production classes may introduce

unnecessary coupling and link-time dependencies when directly called from other

compilation units; static member functions are particularly susceptible to this. Consider

extracting a new class, or placing the functions in a namespace possibly in a separate

library.

If you must define a nonmember function and it is only needed in its .cc file, use an unnamed

namespace or static linkage (eg static int Foo() {...}) to limit its scope.

Local Variables

Place a function's variables in the narrowest scope possible, and initialize variables in the

declaration.

C++ allows you to declare variables anywhere in a function. We encourage you to declare

them in as local a scope as possible, and as close to the first use as possible. This makes it

easier for the reader to find the declaration and see what type the variable is and what it was

initialized to. In particular, initialization should be used instead of declaration and

assignment, e.g.

int i;

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i = f(); // Bad -- initialization separate from declaration.

int j = g(); // Good -- declaration has initialization.

vector<int> v;

v.push_back(1); // Prefer initializing using brace initialization.

v.push_back(2);

vector<int> v = {1, 2}; // Good -- v starts initialized.

Note that gcc implements for (int i = 0; i < 10; ++i) correctly (the scope of i is

only the scope of the for loop), so you can then reuse i in another for loop in the same

scope. It also correctly scopes declarations in if and while statements, e.g.

while (const char* p = strchr(str, '/')) str = p + 1;

There is one caveat: if the variable is an object, its constructor is invoked every time it enters

scope and is created, and its destructor is invoked every time it goes out of scope.

// Inefficient implementation:

for (int i = 0; i < 1000000; ++i) {

Foo f; // My ctor and dtor get called 1000000 times each.

f.DoSomething(i);

}

It may be more efficient to declare such a variable used in a loop outside that loop:

Foo f; // My ctor and dtor get called once each.

for (int i = 0; i < 1000000; ++i) {

f.DoSomething(i);

}

Static and Global Variables

Static or global variables of class type are forbidden: they cause hard-to-find bugs due to

indeterminate order of construction and destruction.

Objects with static storage duration, including global variables, static variables, static class

member variables, and function static variables, must be Plain Old Data (POD): only ints,

chars, floats, or pointers, or arrays/structs of POD.

The order in which class constructors and initializers for static variables are called is only

partially specified in C++ and can even change from build to build, which can cause bugs

that are difficult to find. Therefore in addition to banning globals of class type, we do not

allow static POD variables to be initialized with the result of a function, unless that function

(such as getenv(), or getpid()) does not itself depend on any other globals.

Likewise, the order in which destructors are called is defined to be the reverse of the order in

which the constructors were called. Since constructor order is indeterminate, so is destructor

order. For example, at program-end time a static variable might have been destroyed, but

code still running -- perhaps in another thread -- tries to access it and fails. Or the destructor for

a static 'string' variable might be run prior to the destructor for another variable that contains

a reference to that string.

As a result we only allow static variables to contain POD data. This rule completely disallows

vector (use C arrays instead), or string (use const char []).

If you need a static or global variable of a class type, consider initializing a pointer (which will

never be freed), from either your main() function or from pthread_once(). Note that this must

be a raw pointer, not a "smart" pointer, since the smart pointer's destructor will have the order-

of-destructor issue that we are trying to avoid.

Classes

Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This

section lists the main dos and don'ts you should follow when writing a class.

Doing Work in Constructors

Avoid doing complex initialization in constructors (in particular, initialization that can fail

or that requires virtual method calls).

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Definition:

It is possible to perform initialization in the body of the constructor.

Pros:

Convenience in typing. No need to worry about whether the class has been initialized or not.

Cons:

The problems with doing work in constructors are:

There is no easy way for constructors to signal errors, short of using exceptions (which

are forbidden).

If the work fails, we now have an object whose initialization code failed, so it may be

an indeterminate state.

If the work calls virtual functions, these calls will not get dispatched to the subclass

implementations. Future modification to your class can quietly introduce this problem

even if your class is not currently subclassed, causing much confusion.

If someone creates a global variable of this type (which is against the rules, but still),

the constructor code will be called before main(), possibly breaking some implicit

assumptions in the constructor code. For instance, gflags will not yet have been

initialized.

Decision:

Constructors should never call virtual functions or attempt to raise non-fatal failures. If your

object requires non-trivial initialization, consider using a factory function or Init() method.

Default Constructors

You must define a default constructor if your class defines member variables and has no

other constructors. Otherwise the compiler will do it for you, badly.

Definition:

The default constructor is called when we new a class object with no arguments. It is always

called when calling new[] (for arrays).

Pros:

Initializing structures by default, to hold "impossible" values, makes debugging much easier.

Cons:

Extra work for you, the code writer.

Decision:

If your class defines member variables and has no other constructors you must define a

default constructor (one that takes no arguments). It should preferably initialize the object in

such a way that its internal state is consistent and valid.

The reason for this is that if you have no other constructors and do not define a default

constructor, the compiler will generate one for you. This compiler generated constructor may

not initialize your object sensibly.

If your class inherits from an existing class but you add no new member variables, you are not

required to have a default constructor.

Explicit Constructors

Use the C++ keyword explicit for constructors with one argument.

Definition:

Normally, if a constructor takes one argument, it can be used as a conversion. For instance, if

you define Foo::Foo(string name) and then pass a string to a function that expects a

Foo, the constructor will be called to convert the string into a Foo and will pass the Foo to

your function for you. This can be convenient but is also a source of trouble when things get

converted and new objects created without you meaning them to. Declaring a constructor

explicit prevents it from being invoked implicitly as a conversion.

Pros:

Avoids undesirable conversions.

Cons:

None.

Decision:

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We require all single argument constructors to be explicit. Always put explicit in front of

one-argument constructors in the class definition: explicit Foo(string name);

The exception is copy constructors, which, in the rare cases when we allow them, should

probably not be explicit. Classes that are intended to be transparent wrappers around

other classes are also exceptions. Such exceptions should be clearly marked with comments.

Finally, constructors that take only an initializer_list may be non-explicit. This is to permit

construction of your type using the assigment form for brace init lists (i.e. MyType m = {1,

2} ).

Copy Constructors

Provide a copy constructor and assignment operator only when necessary. Otherwise,

disable them with DISALLOW_COPY_AND_ASSIGN.

Definition:

The copy constructor and assignment operator are used to create copies of objects. The

copy constructor is implicitly invoked by the compiler in some situations, e.g. passing objects

by value.

Pros:

Copy constructors make it easy to copy objects. STL containers require that all contents be

copyable and assignable. Copy constructors can be more efficient than CopyFrom()-style

workarounds because they combine construction with copying, the compiler can elide them

in some contexts, and they make it easier to avoid heap allocation.

Cons:

Implicit copying of objects in C++ is a rich source of bugs and of performance problems. It

also reduces readability, as it becomes hard to track which objects are being passed around

by value as opposed to by reference, and therefore where changes to an object are

reflected.

Decision:

Few classes need to be copyable. Most should have neither a copy constructor nor an

assignment operator. In many situations, a pointer or reference will work just as well as a

copied value, with better performance. For example, you can pass function parameters by

reference or pointer instead of by value, and you can store pointers rather than objects in an

STL container.

If your class needs to be copyable, prefer providing a copy method, such as CopyFrom() or

Clone(), rather than a copy constructor, because such methods cannot be invoked

implicitly. If a copy method is insufficient in your situation (e.g. for performance reasons, or

because your class needs to be stored by value in an STL container), provide both a copy

constructor and assignment operator.

If your class does not need a copy constructor or assignment operator, you must explicitly

disable them. To do so, add dummy declarations for the copy constructor and assignment

operator in the private: section of your class, but do not provide any corresponding

definition (so that any attempt to use them results in a link error).

For convenience, a DISALLOW_COPY_AND_ASSIGN macro can be used:

// A macro to disallow the copy constructor and operator= functions

// This should be used in the private: declarations for a class

#define DISALLOW_COPY_AND_ASSIGN(TypeName) \

TypeName(const TypeName&); \

void operator=(const TypeName&)

Then, in class Foo:

class Foo {

public:

Foo(int f);

~Foo();

private:

DISALLOW_COPY_AND_ASSIGN(Foo);

};

Structs vs. Classes

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Use a struct only for passive objects that carry data; everything else is a class.

The struct and class keywords behave almost identically in C++. We add our own

semantic meanings to each keyword, so you should use the appropriate keyword for the

data-type you're defining.

structs should be used for passive objects that carry data, and may have associated

constants, but lack any functionality other than access/setting the data members. The

accessing/setting of fields is done by directly accessing the fields rather than through method

invocations. Methods should not provide behavior but should only be used to set up the data

members, e.g., constructor, destructor, Initialize(), Reset(), Validate().

If more functionality is required, a class is more appropriate. If in doubt, make it a class.

For consistency with STL, you can use struct instead of class for functors and traits.

Note that member variables in structs and classes have different naming rules.

Inheritance

Composition is often more appropriate than inheritance. When using inheritance, make

it public.

Definition:

When a sub-class inherits from a base class, it includes the definitions of all the data and

operations that the parent base class defines. In practice, inheritance is used in two major

ways in C++: implementation inheritance, in which actual code is inherited by the child, and

interface inheritance, in which only method names are inherited.

Pros:

Implementation inheritance reduces code size by re-using the base class code as it

specializes an existing type. Because inheritance is a compile-time declaration, you and the

compiler can understand the operation and detect errors. Interface inheritance can be used

to programmatically enforce that a class expose a particular API. Again, the compiler can

detect errors, in this case, when a class does not define a necessary method of the API.

Cons:

For implementation inheritance, because the code implementing a sub-class is spread

between the base and the sub-class, it can be more difficult to understand an

implementation. The sub-class cannot override functions that are not virtual, so the sub-class

cannot change implementation. The base class may also define some data members, so

that specifies physical layout of the base class.

Decision:

All inheritance should be public. If you want to do private inheritance, you should be

including an instance of the base class as a member instead.

Do not overuse implementation inheritance. Composition is often more appropriate. Try to

restrict use of inheritance to the "is-a" case: Bar subclasses Foo if it can reasonably be said

that Bar "is a kind of" Foo.

Make your destructor virtual if necessary. If your class has virtual methods, its destructor

should be virtual.

Limit the use of protected to those member functions that might need to be accessed from

subclasses. Note that data members should be private.

When redefining an inherited virtual function, explicitly declare it virtual in the declaration

of the derived class. Rationale: If virtual is omitted, the reader has to check all ancestors of

the class in question to determine if the function is virtual or not.

Multiple Inheritance

Only very rarely is multiple implementation inheritance actually useful. We allow multiple

inheritance only when at most one of the base classes has an implementation; all other

base classes must be pure interface classes tagged with the Interface suffix.

Definition:

Multiple inheritance allows a sub-class to have more than one base class. We distinguish

between base classes that are

pure interfaces

and those that have an

implementation

Pros:

Multiple implementation inheritance may let you re-use even more code than single

inheritance (see Inheritance).

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Cons:

Only very rarely is multiple

implementation

inheritance actually useful. When multiple

implementation inheritance seems like the solution, you can usually find a different, more

explicit, and cleaner solution.

Decision:

Multiple inheritance is allowed only when all superclasses, with the possible exception of the

first one, are pure interfaces. In order to ensure that they remain pure interfaces, they must

end with the Interface suffix.

Note: There is an exception to this rule on Windows.

Interfaces

Classes that satisfy certain conditions are allowed, but not required, to end with an

Interface suffix.

Definition:

A class is a pure interface if it meets the following requirements:

It has only public pure virtual ("= 0") methods and static methods (but see below for

destructor).

It may not have non-static data members.

It need not have any constructors defined. If a constructor is provided, it must take no

arguments and it must be protected.

If it is a subclass, it may only be derived from classes that satisfy these conditions and

are tagged with the Interface suffix.

An interface class can never be directly instantiated because of the pure virtual method(s) it

declares. To make sure all implementations of the interface can be destroyed correctly, the

interface must also declare a virtual destructor (in an exception to the first rule, this should not

be pure). See Stroustrup,

The C++ Programming Language

, 3rd edition, section 12.4 for

details.

Pros:

Tagging a class with the Interface suffix lets others know that they must not add

implemented methods or non static data members. This is particularly important in the case

of multiple inheritance. Additionally, the interface concept is already well-understood by

Java programmers.

Cons:

The Interface suffix lengthens the class name, which can make it harder to read and

understand. Also, the interface property may be considered an implementation detail that

shouldn't be exposed to clients.

Decision:

A class may end with Interface only if it meets the above requirements. We do not require

the converse, however: classes that meet the above requirements are not required to end

with Interface.

Operator Overloading

Do not overload operators except in rare, special circumstances.

Definition:

A class can define that operators such as + and / operate on the class as if it were a built-in

type.

Pros:

Can make code appear more intuitive because a class will behave in the same way as built-

in types (such as int). Overloaded operators are more playful names for functions that are

less-colorfully named, such as Equals() or Add(). For some template functions to work

correctly, you may need to define operators.

Cons:

While operator overloading can make code more intuitive, it has several drawbacks:

It can fool our intuition into thinking that expensive operations are cheap, built-in

operations.

It is much harder to find the call sites for overloaded operators. Searching for Equals()

is much easier than searching for relevant invocations of ==.

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Some operators work on pointers too, making it easy to introduce bugs. Foo + 4 may

do one thing, while &Foo + 4 does something totally different. The compiler does not

complain for either of these, making this very hard to debug.

Overloading also has surprising ramifications. For instance, if a class overloads unary

operator&, it cannot safely be forward-declared.

Decision:

In general, do not overload operators. The assignment operator (operator=), in particular, is

insidious and should be avoided. You can define functions like Equals() and CopyFrom() if

you need them. Likewise, avoid the dangerous unary operator& at all costs, if there's any

possibility the class might be forward-declared.

However, there may be rare cases where you need to overload an operator to interoperate

with templates or "standard" C++ classes (such as operator<<(ostream&, const T&) for

logging). These are acceptable if fully justified, but you should try to avoid these whenever

possible. In particular, do not overload operator== or operator< just so that your class can

be used as a key in an STL container; instead, you should create equality and comparison

functor types when declaring the container.

Some of the STL algorithms do require you to overload operator==, and you may do so in

these cases, provided you document why.

See also Copy Constructors and Function Overloading.

Access Control

Make data members private, and provide access to them through accessor functions

as needed (for technical reasons, we allow data members of a test fixture class to be

protected when using Google Test). Typically a variable would be called foo_ and the

accessor function foo(). You may also want a mutator function set_foo(). Exception:

static const data members (typically called kFoo) need not be private.

The definitions of accessors are usually inlined in the header file.

See also Inheritance and Function Names.

Declaration Order

Use the specified order of declarations within a class: public: before private:,

methods before data members (variables), etc.

Your class definition should start with its public: section, followed by its protected: section

and then its private: section. If any of these sections are empty, omit them.

Within each section, the declarations generally should be in the following order:

Typedefs and Enums

Constants (static const data members)

Constructors

Destructor

Methods, including static methods

Data Members (except static const data members)

Friend declarations should always be in the private section, and the

DISALLOW_COPY_AND_ASSIGN macro invocation should be at the end of the private:

section. It should be the last thing in the class. See Copy Constructors.

Method definitions in the corresponding .cc file should be the same as the declaration order,

as much as possible.

Do not put large method definitions inline in the class definition. Usually, only trivial or

performance-critical, and very short, methods may be defined inline. See Inline Functions for

more details.

Write Short Functions

Prefer small and focused functions.

We recognize that long functions are sometimes appropriate, so no hard limit is placed on

functions length. If a function exceeds about 40 lines, think about whether it can be broken

up without harming the structure of the program.

Even if your long function works perfectly now, someone modifying it in a few months may

add new behavior. This could result in bugs that are hard to find. Keeping your functions short

and simple makes it easier for other people to read and modify your code.

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You could find long and complicated functions when working with some code. Do not be

intimidated by modifying existing code: if working with such a function proves to be difficult,

you find that errors are hard to debug, or you want to use a piece of it in several different

contexts, consider breaking up the function into smaller and more manageable pieces.

Google-Specific Magic

There are various tricks and utilities that we use to make C++ code more robust, and various

ways we use C++ that may differ from what you see elsewhere.

Smart Pointers

If you actually need pointer semantics, unique_ptr is great, and scoped_ptr is fine if

you need to support older versions of C++. You should only use shared_ptr with a non-

const referent when it is truly necessary to share ownership of an object (e.g. inside an STL

container). You should never use auto_ptr.

Definition:

"Smart" pointers are objects that act like pointers, but automate management of the

underlying memory.

Pros:

Smart pointers are extremely useful for preventing memory leaks, and are essential for writing

exception-safe code. They also formalize and document the ownership of dynamically

allocated memory.

Cons:

We prefer designs in which objects have single, fixed owners. Smart pointers which enable

sharing or transfer of ownership can act as a tempting alternative to a careful design of

ownership semantics, leading to confusing code and even bugs in which memory is never

deleted. The semantics of smart pointers (especially auto_ptr) can be nonobvious and

confusing. The exception-safety benefits of smart pointers are not decisive, since we do not

allow exceptions.

Decision:

unique_ptr

See below.

scoped_ptr

Prefer unique_ptr unless C++03 compatibility is required.

auto_ptr

Confusing and bug-prone ownership-transfer semantics. Use unique_ptr instead, if

possible.

shared_ptr

Safe with const referents (i.e. shared_ptr<const T>). Reference-counted pointers

with non-const referents can occasionally be the best design, but try to rewrite with

single owners where possible.

cpplint

Use cpplint.py to detect style errors.

cpplint.py is a tool that reads a source file and identifies many style errors. It is not perfect,

and has both false positives and false negatives, but it is still a valuable tool. False positives

can be ignored by putting // NOLINT at the end of the line.

Some projects have instructions on how to run cpplint.py from their project tools. If the

project you are contributing to does not, you can download cpplint.py separately.

Other C++ Features

Reference Arguments

All parameters passed by reference must be labeled const.

Definition:

In C, if a function needs to modify a variable, the parameter must use a pointer, eg int

foo(int *pval). In C++, the function can alternatively declare a reference parameter: int

foo(int &val).

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Pros:

Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some

applications like copy constructors. Makes it clear, unlike with pointers, that a null pointer is

not a possible value.

Cons:

References can be confusing, as they have value syntax but pointer semantics.

Decision:

Within function parameter lists all references must be const:

void Foo(const string &in, string *out);

In fact it is a very strong convention in Google code that input arguments are values or const

references while output arguments are pointers. Input parameters may be const pointers,

but we never allow non-const reference parameters except when required by convention,

e.g., swap().

However, there are some instances where using const T* is preferable to const T& for input

parameters. For example:

You want to pass in a null pointer.

The function saves a pointer or reference to the input.

Remember that most of the time input parameters are going to be specified as const T&.

Using const T* instead communicates to the reader that the input is somehow treated

differently. So if you choose const T* rather than const T&, do so for a concrete reason;

otherwise it will likely confuse readers by making them look for an explanation that doesn't

exist.

Function Overloading

Use overloaded functions (including constructors) only if a reader looking at a call site

can get a good idea of what is happening without having to first figure out exactly which

overload is being called.

Definition:

You may write a function that takes a const string& and overload it with another that

takes const char*.

class MyClass {

public:

void Analyze(const string &text);

void Analyze(const char *text, size_t textlen);

};

Pros:

Overloading can make code more intuitive by allowing an identically-named function to

take different arguments. It may be necessary for templatized code, and it can be

convenient for Visitors.

Cons:

If a function is overloaded by the argument types alone, a reader may have to understand

C++'s complex matching rules in order to tell what's going on. Also many people are

confused by the semantics of inheritance if a derived class overrides only some of the variants

of a function.

Decision:

If you want to overload a function, consider qualifying the name with some information

about the arguments, e.g., AppendString(), AppendInt() rather than just Append().

Default Arguments

We do not allow default function parameters, except in limited situations as explained

below. Simulate them with function overloading instead, if appropriate.

Pros:

Often you have a function that uses default values, but occasionally you want to override the

defaults. Default parameters allow an easy way to do this without having to define many

functions for the rare exceptions. Compared to overloading the function, default arguments

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have a cleaner syntax, with less boilerplate and a clearer distinction between 'required' and

'optional' arguments.

Cons:

Function pointers are confusing in the presence of default arguments, since the function

signature often doesn't match the call signature. Adding a default argument to an existing

function changes its type, which can cause problems with code taking its address. Adding

function overloads avoids these problems. In addition, default parameters may result in

bulkier code since they are replicated at every call-site -- as opposed to overloaded

functions, where "the default" appears only in the function definition.

Decision:

While the cons above are not that onerous, they still outweigh the (small) benefits of default

arguments over function overloading. So except as described below, we require all

arguments to be explicitly specified.

One specific exception is when the function is a static function (or in an unnamed

namespace) in a .cc file. In this case, the cons don't apply since the function's use is so

localized.

Another specific exception is when default arguments are used to simulate variable-length

argument lists.

// Support up to 4 params by using a default empty AlphaNum.

string StrCat(const AlphaNum &a,

const AlphaNum &b = gEmptyAlphaNum,

const AlphaNum &c = gEmptyAlphaNum,

const AlphaNum &d = gEmptyAlphaNum);

Variable-Length Arrays and alloca()

We do not allow variable-length arrays or alloca().

Pros:

Variable-length arrays have natural-looking syntax. Both variable-length arrays and

alloca() are very efficient.

Cons:

Variable-length arrays and alloca are not part of Standard C++. More importantly, they

allocate a data-dependent amount of stack space that can trigger difficult-to-find memory

overwriting bugs: "It ran fine on my machine, but dies mysteriously in production".

Decision:

Use a safe allocator instead, such as scoped_ptr/scoped_array.

Friends

We allow use of friend classes and functions, within reason.

Friends should usually be defined in the same file so that the reader does not have to look in

another file to find uses of the private members of a class. A common use of friend is to

have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo

correctly, without exposing this state to the world. In some cases it may be useful to make a

unittest class a friend of the class it tests.

Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is

better than making a member public when you want to give only one other class access to it.

However, most classes should interact with other classes solely through their public members.

Exceptions

We do not use C++ exceptions.

Pros:

Exceptions allow higher levels of an application to decide how to handle "can't

happen" failures in deeply nested functions, without the obscuring and error-prone

bookkeeping of error codes.

Exceptions are used by most other modern languages. Using them in C++ would make

it more consistent with Python, Java, and the C++ that others are familiar with.

Some third-party C++ libraries use exceptions, and turning them off internally makes it

harder to integrate with those libraries.

Exceptions are the only way for a constructor to fail. We can simulate this with a factory

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function or an Init() method, but these require heap allocation or a new "invalid"

state, respectively.

Exceptions are really handy in testing frameworks.

Cons:

When you add a throw statement to an existing function, you must examine all of its

transitive callers. Either they must make at least the basic exception safety guarantee,

or they must never catch the exception and be happy with the program terminating as

a result. For instance, if f() calls g() calls h(), and h throws an exception that f

catches, g has to be careful or it may not clean up properly.

More generally, exceptions make the control flow of programs difficult to evaluate by

looking at code: functions may return in places you don't expect. This causes

maintainability and debugging difficulties. You can minimize this cost via some rules

on how and where exceptions can be used, but at the cost of more that a developer

needs to know and understand.

Exception safety requires both RAII and different coding practices. Lots of supporting

machinery is needed to make writing correct exception-safe code easy. Further, to

avoid requiring readers to understand the entire call graph, exception-safe code must

isolate logic that writes to persistent state into a "commit" phase. This will have both

benefits and costs (perhaps where you're forced to obfuscate code to isolate the

commit). Allowing exceptions would force us to always pay those costs even when

they're not worth it.

Turning on exceptions adds data to each binary produced, increasing compile time

(probably slightly) and possibly increasing address space pressure.

The availability of exceptions may encourage developers to throw them when they are

not appropriate or recover from them when it's not safe to do so. For example, invalid

user input should not cause exceptions to be thrown. We would need to make the style

guide even longer to document these restrictions!

Decision:

On their face, the benefits of using exceptions outweigh the costs, especially in new projects.

However, for existing code, the introduction of exceptions has implications on all dependent

code. If exceptions can be propagated beyond a new project, it also becomes problematic

to integrate the new project into existing exception-free code. Because most existing C++

code at Google is not prepared to deal with exceptions, it is comparatively difficult to adopt

new code that generates exceptions.

Given that Google's existing code is not exception-tolerant, the costs of using exceptions are

somewhat greater than the costs in a new project. The conversion process would be slow

and error-prone. We don't believe that the available alternatives to exceptions, such as error

codes and assertions, introduce a significant burden.

Our advice against using exceptions is not predicated on philosophical or moral grounds,

but practical ones. Because we'd like to use our open-source projects at Google and it's

difficult to do so if those projects use exceptions, we need to advise against exceptions in

Google open-source projects as well. Things would probably be different if we had to do it

all over again from scratch.

There is an exception to this rule (no pun intended) for Windows code.

Run-Time Type Information (RTTI)

Avoid using Run Time Type Information (RTTI).

Definition:

RTTI allows a programmer to query the C++ class of an object at run time. This is done by use

of typeid or dynamic_cast.

Cons:

Querying the type of an object at run-time frequently means a design problem. Needing to

know the type of an object at runtime is often an indication that the design of your class

hierarchy is flawed.

Undisciplined use of RTTI makes code hard to maintain. It can lead to type-based decision

trees or switch statements scattered throughout the code, all of which must be examined

when making further changes.

Pros:

The standard alternatives to RTTI (described below) require modification or redesign of the

class hierarchy in question. Sometimes such modifications are infeasible or undesirable,

particularly in widely-used or mature code.

RTTI can be useful in some unit tests. For example, it is useful in tests of factory classes where

the test has to verify that a newly created object has the expected dynamic type. It is also

useful in managing the relationship between objects and their mocks.

RTTI is useful when considering multiple abstract objects. Consider

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bool Base::Equal(Base* other) = 0;

bool Derived::Equal(Base* other) {

Derived* that = dynamic_cast<Derived*>(other);

if (that == NULL)

return false;

...

}

Decision:

RTTI has legitimate uses but is prone to abuse, so you must be careful when using it. You may

use it freely in unittests, but avoid it when possible in other code. In particular, think twice

before using RTTI in new code. If you find yourself needing to write code that behaves

differently based on the class of an object, consider one of the following alternatives to

querying the type:

Virtual methods are the preferred way of executing different code paths depending on

a specific subclass type. This puts the work within the object itself.

If the work belongs outside the object and instead in some processing code, consider

a double-dispatch solution, such as the Visitor design pattern. This allows a facility

outside the object itself to determine the type of class using the built-in type system.

When the logic of a program guarantees that a given instance of a base class is in fact an

instance of a particular derived class, then a dynamic_cast may be used freely on the

object. Usually one can use a static_cast as an alternative in such situations.

Decision trees based on type are a strong indication that your code is on the wrong track.

if (typeid(*data) == typeid(D1)) {

...

} else if (typeid(*data) == typeid(D2)) {

...

} else if (typeid(*data) == typeid(D3)) {

...

Code such as this usually breaks when additional subclasses are added to the class

hierarchy. Moreover, when properties of a subclass change, it is difficult to find and modify

all the affected code segments.

Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as

much to workarounds like class hierarchies with type tags. Moreover, workarounds disguise

your true intent.

Casting

Use C++ casts like static_cast<>(). Do not use other cast formats like int y =

(int)x; or int y = int(x);.

Definition:

C++ introduced a different cast system from C that distinguishes the types of cast operations.

Pros:

The problem with C casts is the ambiguity of the operation; sometimes you are doing a

conversion

(e.g., (int)3.5) and sometimes you are doing a

cast

(e.g., (int)"hello");

C++ casts avoid this. Additionally C++ casts are more visible when searching for them.

Cons:

The syntax is nasty.

Decision:

Do not use C-style casts. Instead, use these C++-style casts.

Use static_cast as the equivalent of a C-style cast that does value conversion, or

when you need to explicitly up-cast a pointer from a class to its superclass.

Use const_cast to remove the const qualifier (see const).

Use reinterpret_cast to do unsafe conversions of pointer types to and from integer

and other pointer types. Use this only if you know what you are doing and you

understand the aliasing issues.

See the RTTI section for guidance on the use of dynamic_cast.

Streams

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Use streams only for logging.

Definition:

Streams are a replacement for printf() and scanf().

Pros:

With streams, you do not need to know the type of the object you are printing. You do not

have problems with format strings not matching the argument list. (Though with gcc, you do

not have that problem with printf either.) Streams have automatic constructors and

destructors that open and close the relevant files.

Cons:

Streams make it difficult to do functionality like pread(). Some formatting (particularly the

common format string idiom %.*s) is difficult if not impossible to do efficiently using streams

without using printf-like hacks. Streams do not support operator reordering (the %1s

directive), which is helpful for internationalization.

Decision:

Do not use streams, except where required by a logging interface. Use printf-like routines

instead.

There are various pros and cons to using streams, but in this case, as in many other cases,

consistency trumps the debate. Do not use streams in your code.

Extended Discussion

There has been debate on this issue, so this explains the reasoning in greater depth. Recall the

Only One Way guiding principle: we want to make sure that whenever we do a certain type

of I/O, the code looks the same in all those places. Because of this, we do not want to allow

users to decide between using streams or using printf plus Read/Write/etc. Instead, we

should settle on one or the other. We made an exception for logging because it is a pretty

specialized application, and for historical reasons.

Proponents of streams have argued that streams are the obvious choice of the two, but the

issue is not actually so clear. For every advantage of streams they point out, there is an

equivalent disadvantage. The biggest advantage is that you do not need to know the type

of the object to be printing. This is a fair point. But, there is a downside: you can easily use the

wrong type, and the compiler will not warn you. It is easy to make this kind of mistake without

knowing when using streams.

cout << this; // Prints the address

cout << *this; // Prints the contents

The compiler does not generate an error because << has been overloaded. We discourage

overloading for just this reason.

Some say printf formatting is ugly and hard to read, but streams are often no better.

Consider the following two fragments, both with the same typo. Which is easier to discover?

cerr << "Error connecting to '" << foo->bar()->hostname.first

<< ":" << foo->bar()->hostname.second << ": " << strerror(errno);

fprintf(stderr, "Error connecting to '%s:%u: %s",

foo->bar()->hostname.first, foo->bar()->hostname.second,

strerror(errno));

And so on and so forth for any issue you might bring up. (You could argue, "Things would be

better with the right wrappers," but if it is true for one scheme, is it not also true for the other?

Also, remember the goal is to make the language smaller, not add yet more machinery that

someone has to learn.)

Either path would yield different advantages and disadvantages, and there is not a clearly

superior solution. The simplicity doctrine mandates we settle on one of them though, and the

majority decision was on printf + read/write.

Preincrement and Predecrement

Use prefix form (++i) of the increment and decrement operators with iterators and other

template objects.

Definition:

When a variable is incremented (++i or i++) or decremented (--i or i--) and the value of

the expression is not used, one must decide whether to preincrement (decrement) or

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postincrement (decrement).

Pros:

When the return value is ignored, the "pre" form (++i) is never less efficient than the "post" form

(i++), and is often more efficient. This is because post-increment (or decrement) requires a

copy of i to be made, which is the value of the expression. If i is an iterator or other non-

scalar type, copying i could be expensive. Since the two types of increment behave the

same when the value is ignored, why not just always pre-increment?

Cons:

The tradition developed, in C, of using post-increment when the expression value is not used,

especially in for loops. Some find post-increment easier to read, since the "subject" (i)

precedes the "verb" (++), just like in English.

Decision:

For simple scalar (non-object) values there is no reason to prefer one form and we allow

either. For iterators and other template types, use pre-increment.

Use of const

Use const whenever it makes sense.

Definition:

Declared variables and parameters can be preceded by the keyword const to indicate the

variables are not changed (e.g., const int foo). Class functions can have the const

qualifier to indicate the function does not change the state of the class member variables

(e.g., class Foo { int Bar(char c) const; };).

Pros:

Easier for people to understand how variables are being used. Allows the compiler to do

better type checking, and, conceivably, generate better code. Helps people convince

themselves of program correctness because they know the functions they call are limited in

how they can modify your variables. Helps people know what functions are safe to use

without locks in multi-threaded programs.

Cons:

const is viral: if you pass a const variable to a function, that function must have const in its

prototype (or the variable will need a const_cast). This can be a particular problem when

calling library functions.

Decision:

const variables, data members, methods and arguments add a level of compile-time type

checking; it is better to detect errors as soon as possible. Therefore we strongly recommend

that you use const whenever it makes sense to do so:

If a function does not modify an argument passed by reference or by pointer, that

argument should be const.

Declare methods to be const whenever possible. Accessors should almost always be

const. Other methods should be const if they do not modify any data members, do

not call any non-const methods, and do not return a non-const pointer or non-const

reference to a data member.

Consider making data members const whenever they do not need to be modified

after construction.

The mutable keyword is allowed but is unsafe when used with threads, so thread safety

should be carefully considered first.

Where to put the const

Some people favor the form int const *foo to const int* foo. They argue that this is

more readable because it's more consistent: it keeps the rule that const always follows the

object it's describing. However, this consistency argument doesn't apply in codebases with

few deeply-nested pointer expressions since most const expressions have only one const,

and it applies to the underlying value. In such cases, there's no consistency to maintain.

Putting the const first is arguably more readable, since it follows English in putting the

"adjective" (const) before the "noun" (int).

That said, while we encourage putting const first, we do not require it. But be consistent with

the code around you!

Integer Types

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Of the built-in C++ integer types, the only one used is int. If a program needs a variable

of a different size, use a precise-width integer type from <stdint.h>, such as int16_t. If

your variable represents a value that could ever be greater than or equal to 2^31 (2GiB), use

a 64-bit type such as int64_t. Keep in mind that even if your value won't ever be too large

for an int, it may be used in intermediate calculations which may require a larger type.

When in doubt, choose a larger type.

Definition:

C++ does not specify the sizes of its integer types. Typically people assume that short is 16

bits, int is 32 bits, long is 32 bits and long long is 64 bits.

Pros:

Uniformity of declaration.

Cons:

The sizes of integral types in C++ can vary based on compiler and architecture.

Decision:

<stdint.h> defines types like int16_t, uint32_t, int64_t, etc. You should always use

those in preference to short, unsigned long long and the like, when you need a

guarantee on the size of an integer. Of the C integer types, only int should be used. When

appropriate, you are welcome to use standard types like size_t and ptrdiff_t.

We use int very often, for integers we know are not going to be too big, e.g., loop counters.

Use plain old int for such things. You should assume that an int is at least 32 bits, but don't

assume that it has more than 32 bits. If you need a 64-bit integer type, use int64_t or

uint64_t.

For integers we know can be "big", use int64_t.

You should not use the unsigned integer types such as uint32_t, unless there is a valid

reason such as representing a bit pattern rather than a number, or you need defined

overflow modulo 2^N. In particular, do not use unsigned types to say a number will never be

negative. Instead, use assertions for this.

If your code is a container that returns a size, be sure to use a type that will accommodate

any possible usage of your container. When in doubt, use a larger type rather than a smaller

type.

Use care when converting integer types. Integer conversions and promotions can cause non-

intuitive behavior.

On Unsigned Integers

Some people, including some textbook authors, recommend using unsigned types to

represent numbers that are never negative. This is intended as a form of self-documentation.

However, in C, the advantages of such documentation are outweighed by the real bugs it

can introduce. Consider:

for (unsigned int i = foo.Length()-1; i >= 0; --i) ...

This code will never terminate! Sometimes gcc will notice this bug and warn you, but often it

will not. Equally bad bugs can occur when comparing signed and unsigned variables.

Basically, C's type-promotion scheme causes unsigned types to behave differently than one

might expect.

So, document that a variable is non-negative using assertions. Don't use an unsigned type.

64-bit Portability

Code should be 64-bit and 32-bit friendly. Bear in mind problems of printing,

comparisons, and structure alignment.

printf() specifiers for some types are not cleanly portable between 32-bit and 64-bit

systems. C99 defines some portable format specifiers. Unfortunately, MSVC 7.1 does

not understand some of these specifiers and the standard is missing a few, so we have

to define our own ugly versions in some cases (in the style of the standard include file

inttypes.h):

// printf macros for size_t, in the style of inttypes.h

#ifdef _LP64

#define __PRIS_PREFIX "z"

#else

#define __PRIS_PREFIX

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#endif

// Use these macros after a % in a printf format string

// to get correct 32/64 bit behavior, like this:

// size_t size = records.size();

// printf("%"PRIuS"\n", size);

#define PRIdS __PRIS_PREFIX "d"

#define PRIxS __PRIS_PREFIX "x"

#define PRIuS __PRIS_PREFIX "u"

#define PRIXS __PRIS_PREFIX "X"

#define PRIoS __PRIS_PREFIX "o"

Type DO NOT use DO use Notes

void * (or any

pointer) %lx %p

int64_t %qd, %lld %"PRId64"

uint64_t %qu, %llu,

%llx

%"PRIu64",

%"PRIx64"

size_t %u %"PRIuS",

%"PRIxS"

C99 specifies

%zu

ptrdiff_t %d %"PRIdS" C99 specifies

%zd

Note that the PRI* macros expand to independent strings which are concatenated by

the compiler. Hence if you are using a non-constant format string, you need to insert

the value of the macro into the format, rather than the name. It is still possible, as usual,

to include length specifiers, etc., after the % when using the PRI* macros. So, e.g.

printf("x = %30"PRIuS"\n", x) would expand on 32-bit Linux to printf("x =

%30" "u" "\n", x), which the compiler will treat as printf("x = %30u\n", x).

Remember that sizeof(void *) != sizeof(int). Use intptr_t if you want a

pointer-sized integer.

You may need to be careful with structure alignments, particularly for structures being

stored on disk. Any class/structure with a int64_t/uint64_t member will by default

end up being 8-byte aligned on a 64-bit system. If you have such structures being

shared on disk between 32-bit and 64-bit code, you will need to ensure that they are

packed the same on both architectures. Most compilers offer a way to alter structure

alignment. For gcc, you can use __attribute__((packed)). MSVC offers #pragma

pack() and __declspec(align()).

Use the LL or ULL suffixes as needed to create 64-bit constants. For example:

int64_t my_value = 0x123456789LL;

uint64_t my_mask = 3ULL << 48;

If you really need different code on 32-bit and 64-bit systems, use #ifdef _LP64 to

choose between the code variants. (But please avoid this if possible, and keep any

such changes localized.)

Preprocessor Macros

Be very cautious with macros. Prefer inline functions, enums, and const variables to

macros.

Macros mean that the code you see is not the same as the code the compiler sees. This can

introduce unexpected behavior, especially since macros have global scope.

Luckily, macros are not nearly as necessary in C++ as they are in C. Instead of using a macro

to inline performance-critical code, use an inline function. Instead of using a macro to store a

constant, use a const variable. Instead of using a macro to "abbreviate" a long variable

name, use a reference. Instead of using a macro to conditionally compile code ... well, don't

do that at all (except, of course, for the #define guards to prevent double inclusion of

header files). It makes testing much more difficult.

Macros can do things these other techniques cannot, and you do see them in the codebase,

especially in the lower-level libraries. And some of their special features (like stringifying,

concatenation, and so forth) are not available through the language proper. But before

using a macro, consider carefully whether there's a non-macro way to achieve the same

result.

The following usage pattern will avoid many problems with macros; if you use macros, follow

it whenever possible:

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Don't define macros in a .h file.

#define macros right before you use them, and #undef them right after.

Do not just #undef an existing macro before replacing it with your own; instead, pick a

name that's likely to be unique.

Try not to use macros that expand to unbalanced C++ constructs, or at least

document that behavior well.

Prefer not using ## to generate function/class/variable names.

0 and nullptr/NULL

Use 0 for integers, 0.0 for reals, nullptr (or NULL) for pointers, and '\0' for chars.

Use 0 for integers and 0.0 for reals. This is not controversial.

For pointers (address values), there is a choice between 0 and NULL (and, for C++11,

nullptr). For projects that allow C++11 features, use nullptr. For C++03 projects, we prefer

NULL because it looks like a pointer. In fact, some C++ compilers provide special definitions

of NULL which enable them to give useful warnings, particularly in situations where

sizeof(NULL) is not equal to sizeof(0).

Use '\0' for chars. This is the correct type and also makes code more readable.

sizeof

Prefer sizeof(varname) to sizeof(type).

Use sizeof(varname) when you take the size of a particular variable. sizeof(varname)

will update appropriately if someone changes the variable type either now or later. You may

use sizeof(type) for code unrelated to any particular variable, such as code that

manages an external or internal data format where a variable of an appropriate C++ type is

not convenient.

Struct data;

memset(&data, 0, sizeof(data));

memset(&data, 0, sizeof(Struct));

if (raw_size < sizeof(int)) {

LOG(ERROR) << "compressed record not big enough for count: " << raw_size;

return false;

}

auto

Use auto to avoid type names that are just clutter. Continue to use manifest type

declarations when it helps readability, and never use auto for anything but local

variables.

Definition:

In C++11, a variable whose type is given as auto will be given a type that matches that of the

expression used to initialize it. You can use auto either to initialize a variable by copying, or to

bind a reference.

vector<string> v;

...

auto s1 = v[0]; // Makes a copy of v[0].

const auto& s2 = v[0]; // s2 is a reference to v[0].

Pros:

C++ type names can sometimes be long and cumbersome, especially when they involve

templates or namespaces. In a statement like

sparse_hash_map<string, int>::iterator iter = m.find(val);

the return type is hard to read, and obscures the primary purpose of the statement. Changing

it to

auto iter = m.find(val);

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makes it more readable.

Without auto we are sometimes forced to write a type name twice in the same expression,

adding no value for the reader, as in

diagnostics::ErrorStatus* status = new diagnostics::ErrorStatus("xyz");

Using auto makes it easier to use intermediate variables when appropriate, by reducing the

burden of writing their types explicitly.

Cons:

Sometimes code is clearer when types are manifest, especially when a variable's initialization

depends on things that were declared far away. In an expression like

auto i = x.Lookup(key);

it may not be obvious what i's type is, if x was declared hundreds of lines earlier.

Programmers have to understand the difference between auto and const auto& or they'll

get copies when they didn't mean to.

The interaction between auto and C++11 brace-initialization can be confusing. The

declarations

auto x(3); // Note: parentheses.

auto y{3}; // Note: curly braces.

mean different things — x is an int, while y is an initializer_list. The same applies to

other normally-invisible proxy types.

If an auto variable is used as part of an interface, e.g. as a constant in a header, then a

programmer might change its type while only intending to change its value, leading to a

more radical API change than intended.

Decision:

auto is permitted, for local variables only. Do not use auto for file-scope or namespace-

scope variables, or for class members. Never assign a braced initializer list to an auto-typed

variable.

The auto keyword is also used in an unrelated C++11 feature: it's part of the syntax for a new

kind of function declaration with a trailing return type. Function declarations with trailing

return types are not permitted.

Brace Initialization

You may use brace initialization.

In C++03, aggregate types (arrays and structs with no constructor) could be initialized using

braces.

struct Point { int x; int y; };

Point p = {1, 2};

In C++11, this syntax has been expanded for use with all other datatypes. The brace

initialization form is called

braced-init-list

. Here are a few examples of its use.

// Vector takes lists of elements.

vector<string> v{"foo", "bar"};

// The same, except this form cannot be used if the initializer_list

// constructor is explicit. You may choose to use either form.

vector<string> v = {"foo", "bar"};

// Maps take lists of pairs. Nested braced-init-lists work.

map<int, string> m = {{1, "one"}, {2, "2"}};

// braced-init-lists can be implicitly converted to return types.

vector<int> test_function() {

return {1, 2, 3};

}

// Iterate over a braced-init-list.

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for (int i : {-1, -2, -3}) {}

// Call a function using a braced-init-list.

void test_function2(vector<int> v) {}

test_function2({1, 2, 3});

User data types can also define constructors that take initializer_list, which is

automatically created from

braced-init-list

class MyType {

public:

// initializer_list is a reference to the underlying init list,

// so it can be passed by value.

MyType(initializer_list<int> init_list) {

for (int element : init_list) {}

}

};

MyType m{2, 3, 5, 7};

Finally, brace initialization can also call ordinary constructors of data types that do not have

initializer_list constructors.

double d{1.23};

// Calls ordinary constructor as long as MyOtherType has no

// initializer_list constructor.

class MyOtherType {

public:

explicit MyOtherType(string);

MyOtherType(int, string);

};

MyOtherType m = {1, "b"};

// If the constructor is explicit, you can't use the "= {}" form.

MyOtherType m{"b"};

Never assign a

braced-init-list

to an auto local variable. In the single element case, what this

means can be confusing.

auto d = {1.23}; // d is an initializer_list<double>

auto d = double{1.23}; // Good -- d is a double, not an initializer_list.

Boost

Use only approved libraries from the Boost library collection.

Definition:

The Boost library collection is a popular collection of peer-reviewed, free, open-source C++

libraries.

Pros:

Boost code is generally very high-quality, is widely portable, and fills many important gaps in

the C++ standard library, such as type traits, better binders, and better smart pointers. It also

provides an implementation of the TR1 extension to the standard library.

Cons:

Some Boost libraries encourage coding practices which can hamper readability, such as

metaprogramming and other advanced template techniques, and an excessively

"functional" style of programming.

Decision:

In order to maintain a high level of readability for all contributors who might read and

maintain code, we only allow an approved subset of Boost features. Currently, the following

libraries are permitted:

Call Traits from boost/call_traits.hpp

Compressed Pair from boost/compressed_pair.hpp

The Boost Graph Library (BGL) from boost/graph, except serialization

(adj_list_serialize.hpp) and parallel/distributed algorithms and data structures

(boost/graph/parallel/* and boost/graph/distributed/*).

Property Map from boost/property_map, except parallel/distributed property maps

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(boost/property_map/parallel/*).

The part of Iterator that deals with defining iterators:

boost/iterator/iterator_adaptor.hpp,

boost/iterator/iterator_facade.hpp, and

boost/function_output_iterator.hpp

The part of Polygon that deals with Voronoi diagram construction and doesn't

depend on the rest of Polygon: boost/polygon/voronoi_builder.hpp,

boost/polygon/voronoi_diagram.hpp, and

boost/polygon/voronoi_geometry_type.hpp

Bimap from boost/bimap

We are actively considering adding other Boost features to the list, so this list may be

expanded in the future.

The following libraries are permitted, but their use is discouraged because they've been

superseded by standard libraries in C++11:

Array from boost/array.hpp: use std::array instead.

Pointer Container from boost/ptr_container: use containers of std::unique_ptr

instead.

C++11

Use only approved libraries and language extensions from C++11 (formerly known as

C++0x). Consider portability to other environments before using C++11 features in your

project.

Definition:

C++11 is the latest ISO C++ standard. It contains significant changes both to the language

and libraries.

Pros:

C++11 has become the official standard, and eventually will be supported by most C++

compilers. It standardizes some common C++ extensions that we use already, allows

shorthands for some operations, and has some performance and safety improvements.

Cons:

The C++11 standard is substantially more complex than its predecessor (1,300 pages versus

800 pages), and is unfamiliar to many developers. The long-term effects of some features on

code readability and maintenance are unknown. We cannot predict when its various

features will be implemented uniformly by tools that may be of interest (gcc, icc, clang,

Eclipse, etc.).

As with Boost, some C++11 extensions encourage coding practices that hamper readability

—for example by removing checked redundancy (such as type names) that may be helpful

to readers, or by encouraging template metaprogramming. Other extensions duplicate

functionality available through existing mechanisms, which may lead to confusion and

conversion costs.

Decision:

Use only C++11 libraries and language features that have been approved for use. Currently

only the following C++11 features are approved:

auto (for local variables only).

constexpr (for ensuring constants).

Use of >> with no intervening space to close multiple levels of template arguments, as

in set<list<string>>, where C++03 required a space as in set<list<string> >.

Range-based for loops.

Use of the LL and ULL suffixes on numeric literals to guarantee that their type is at least

64 bits wide.

Variadic macros (but note that use of macros is discouraged).

All of the new STL algorithms in the <algorithm> and <numeric> headers.

Use of local types as template parameters.

nullptr and nullptr_t.

static_assert.

Everything in <array>.

Everything in <tuple>.

Variadic templates.

Alias templates (the new using syntax) may be used. Don't use an alias declaration

where a typedef will work.

unique_ptr, with restrictions (see below).

Brace initialization syntax. See the full section for more detail.

Other features will be approved individually as appropriate. Avoid writing code that is

incompatible with C++11 (even though it works in C++03).

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unique_ptr

Use unique_ptr freely within classes and functions, but do not use it to transfer

ownership outside of a single .cc/.h pair.

Definition:

unique_ptr is a "smart" pointer type which expresses exclusive ownership of the underlying

object. It provides "move semantics", which enables it to be stored in containers and used as

a function parameter or return value, even though it cannot be copied (to preserve the

unique-ownership property).

Pros:

It fully automates memory management of singly-owned pointers, virtually eliminating

the risk of leaking the underlying memory.

It provides a single, universal abstraction for singly-owned pointers, replacing close to

a dozen overlapping partial solutions in common use at Google.

It can be passed into and returned from functions, providing a self-documenting and

compiler-enforced way to transfer ownership between scopes.

Its performance is essentially identical to a plain pointer.

Cons:

It cannot be used in code that requires C++03 compatibility.

Move semantics implicitly rely on rvalue references, a new C++11 feature which is

unfamiliar to many Googlers.

Google code currently uses a completely different set of conventions for ownership

transfer. Mixing unique_ptr with the existing conventions could add complexity and

create confusion.

Best practices for using unique_ptr within Google have not yet been established.

Decision:

Use of unique_ptr as a class member or local variable is encouraged, as is storing

unique_ptr in containers that support it. However, for the time being it is forbidden to use

unique_ptr as a function parameter or return value, except for functions that are local to a

single .cc/.h pair.

Note that the std::move() function, which is often used to pass unique_ptr into function

calls, remains forbidden.

Naming

The most important consistency rules are those that govern naming. The style of a name

immediately informs us what sort of thing the named entity is: a type, a variable, a function, a

constant, a macro, etc., without requiring us to search for the declaration of that entity. The

pattern-matching engine in our brains relies a great deal on these naming rules.

Naming rules are pretty arbitrary, but we feel that consistency is more important than

individual preferences in this area, so regardless of whether you find them sensible or not, the

rules are the rules.

General Naming Rules

Function names, variable names, and filenames should be descriptive; eschew

abbreviation.

Give as descriptive a name as possible, within reason. Do not worry about saving horizontal

space as it is far more important to make your code immediately understandable by a new

reader. Do not use abbreviations that are ambiguous or unfamiliar to readers outside your

project, and do not abbreviate by deleting letters within a word.

int price_count_reader; // No abbreviation.

int num_errors; // "num" is a widespread convention.

int num_dns_connections; // Most people know what "DNS" stands for.

int n; // Meaningless.

int nerr; // Ambiguous abbreviation.

int n_comp_conns; // Ambiguous abbreviation.

int wgc_connections; // Only your group knows what this stands for.

int pc_reader; // Lots of things can be abbreviated "pc".

int cstmr_id; // Deletes internal letters.

File Names

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Filenames should be all lowercase and can include underscores (_) or dashes (-). Follow

the convention that your project uses. If there is no consistent local pattern to follow,

prefer "_".

Examples of acceptable file names:

my_useful_class.cc

my-useful-class.cc

myusefulclass.cc

myusefulclass_test.cc // _unittest and _regtest are deprecated.

C++ files should end in .cc and header files should end in .h.

Do not use filenames that already exist in /usr/include, such as db.h.

In general, make your filenames very specific. For example, use http_server_logs.h rather

than logs.h. A very common case is to have a pair of files called, e.g., foo_bar.h and

foo_bar.cc, defining a class called FooBar.

Inline functions must be in a .h file. If your inline functions are very short, they should go

directly into your .h file. However, if your inline functions include a lot of code, they may go

into a third file that ends in -inl.h. In a class with a lot of inline code, your class could have

three files:

url_table.h // The class declaration.

url_table.cc // The class definition.

url_table-inl.h // Inline functions that include lots of code.

See also the section -inl.h Files

Type Names

Type names start with a capital letter and have a capital letter for each new word, with

no underscores: MyExcitingClass, MyExcitingEnum.

The names of all types — classes, structs, typedefs, and enums — have the same naming

convention. Type names should start with a capital letter and have a capital letter for each

new word. No underscores. For example:

// classes and structs

class UrlTable { ...

class UrlTableTester { ...

struct UrlTableProperties { ...

// typedefs

typedef hash_map<UrlTableProperties *, string> PropertiesMap;

// enums

enum UrlTableErrors { ...

Variable Names

Variable names are all lowercase, with underscores between words. Class member

variables have trailing underscores. For instance: my_exciting_local_variable,

my_exciting_member_variable_.

Common Variable names

For example:

string table_name; // OK - uses underscore.

string tablename; // OK - all lowercase.

string tableName; // Bad - mixed case.

Class Data Members

Data members (also called instance variables or member variables) are lowercase with

optional underscores like regular variable names, but always end with a trailing underscore.

string table_name_; // OK - underscore at end.

string tablename_; // OK.

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Struct Variables

Data members in structs should be named like regular variables without the trailing

underscores that data members in classes have.

struct UrlTableProperties {

string name;

int num_entries;

}

See Structs vs. Classes for a discussion of when to use a struct versus a class.

Global Variables

There are no special requirements for global variables, which should be rare in any case, but

if you use one, consider prefixing it with g_ or some other marker to easily distinguish it from

local variables.

Constant Names

Use a k followed by mixed case: kDaysInAWeek.

All compile-time constants, whether they are declared locally, globally, or as part of a class,

follow a slightly different naming convention from other variables. Use a k followed by words

with uppercase first letters:

const int kDaysInAWeek = 7;

Function Names

Regular functions have mixed case; accessors and mutators match the name of the

variable: MyExcitingFunction(), MyExcitingMethod(),

my_exciting_member_variable(), set_my_exciting_member_variable().

Regular Functions

Functions should start with a capital letter and have a capital letter for each new word. No

underscores.

If your function crashes upon an error, you should append OrDie to the function name. This

only applies to functions which could be used by production code and to errors that are

reasonably likely to occur during normal operation.

AddTableEntry()

DeleteUrl()

OpenFileOrDie()

Accessors and Mutators

Accessors and mutators (get and set functions) should match the name of the variable they

are getting and setting. This shows an excerpt of a class whose instance variable is

num_entries_.

class MyClass {

public:

...

int num_entries() const { return num_entries_; }

void set_num_entries(int num_entries) { num_entries_ = num_entries; }

private:

int num_entries_;

};

You may also use lowercase letters for other very short inlined functions. For example if a

function were so cheap you would not cache the value if you were calling it in a loop, then

lowercase naming would be acceptable.

Namespace Names

Namespace names are all lower-case, and based on project names and possibly their

directory structure: google_awesome_project.

See Namespaces for a discussion of namespaces and how to name them.

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Enumerator Names

Enumerators should be named

either

like constants or like macros: either kEnumName or

ENUM_NAME.

Preferably, the individual enumerators should be named like constants. However, it is also

acceptable to name them like macros. The enumeration name, UrlTableErrors (and

AlternateUrlTableErrors), is a type, and therefore mixed case.

enum UrlTableErrors {

kOK = 0,

kErrorOutOfMemory,

kErrorMalformedInput,

};

enum AlternateUrlTableErrors {

OK = 0,

OUT_OF_MEMORY = 1,

MALFORMED_INPUT = 2,

};

Until January 2009, the style was to name enum values like macros. This caused problems with

name collisions between enum values and macros. Hence, the change to prefer constant-

style naming was put in place. New code should prefer constant-style naming if possible.

However, there is no reason to change old code to use constant-style names, unless the old

names are actually causing a compile-time problem.

Macro Names

You're not really going to define a macro, are you? If you do, they're like this:

MY_MACRO_THAT_SCARES_SMALL_CHILDREN.

Please see the description of macros; in general macros should

not

be used. However, if they

are absolutely needed, then they should be named with all capitals and underscores.

#define ROUND(x) ...

#define PI_ROUNDED 3.0

Exceptions to Naming Rules

If you are naming something that is analogous to an existing C or C++ entity then you

can follow the existing naming convention scheme.

bigopen()

function name, follows form of open()

uint

typedef

bigpos

struct or class, follows form of pos

sparse_hash_map

STL-like entity; follows STL naming conventions

LONGLONG_MAX

a constant, as in INT_MAX

Comments

Though a pain to write, comments are absolutely vital to keeping our code readable. The

following rules describe what you should comment and where. But remember: while

comments are very important, the best code is self-documenting. Giving sensible names to

types and variables is much better than using obscure names that you must then explain

through comments.

When writing your comments, write for your audience: the next contributor who will need to

understand your code. Be generous — the next one may be you!

Comment Style

Use either the // or /* */ syntax, as long as you are consistent.

You can use either the // or the /* */ syntax; however, // is

much

more common. Be

consistent with how you comment and what style you use where.

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File Comments

Start each file with license boilerplate, followed by a description of its contents.

Legal Notice and Author Line

Every file should contain license boilerplate. Choose the appropriate boilerplate for the

license used by the project (for example, Apache 2.0, BSD, LGPL, GPL).

If you make significant changes to a file with an author line, consider deleting the author line.

File Contents

Every file should have a comment at the top describing its contents.

Generally a .h file will describe the classes that are declared in the file with an overview of

what they are for and how they are used. A .cc file should contain more information about

implementation details or discussions of tricky algorithms. If you feel the implementation

details or a discussion of the algorithms would be useful for someone reading the .h, feel free

to put it there instead, but mention in the .cc that the documentation is in the .h file.

Do not duplicate comments in both the .h and the .cc. Duplicated comments diverge.

Class Comments

Every class definition should have an accompanying comment that describes what it is

for and how it should be used.

// Iterates over the contents of a GargantuanTable. Sample usage:

// GargantuanTableIterator* iter = table->NewIterator();

// for (iter->Seek("foo"); !iter->done(); iter->Next()) {

// process(iter->key(), iter->value());

// }

// delete iter;

class GargantuanTableIterator {

...

};

If you have already described a class in detail in the comments at the top of your file feel free

to simply state "See comment at top of file for a complete description", but be sure to have

some sort of comment.

Document the synchronization assumptions the class makes, if any. If an instance of the class

can be accessed by multiple threads, take extra care to document the rules and invariants

surrounding multithreaded use.

Function Comments

Declaration comments describe use of the function; comments at the definition of a

function describe operation.

Function Declarations

Every function declaration should have comments immediately preceding it that describe

what the function does and how to use it. These comments should be descriptive ("Opens the

file") rather than imperative ("Open the file"); the comment describes the function, it does not

tell the function what to do. In general, these comments do not describe how the function

performs its task. Instead, that should be left to comments in the function definition.

Types of things to mention in comments at the function declaration:

What the inputs and outputs are.

For class member functions: whether the object remembers reference arguments

beyond the duration of the method call, and whether it will free them or not.

If the function allocates memory that the caller must free.

Whether any of the arguments can be a null pointer.

If there are any performance implications of how a function is used.

If the function is re-entrant. What are its synchronization assumptions?

Here is an example:

// Returns an iterator for this table. It is the client's

// responsibility to delete the iterator when it is done with it,

// and it must not use the iterator once the GargantuanTable object

// on which the iterator was created has been deleted.

// The iterator is initially positioned at the beginning of the table.

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// This method is equivalent to:

// Iterator* iter = table->NewIterator();

// iter->Seek("");

// return iter;

// If you are going to immediately seek to another place in the

// returned iterator, it will be faster to use NewIterator()

// and avoid the extra seek.

Iterator* GetIterator() const;

However, do not be unnecessarily verbose or state the completely obvious. Notice below

that it is not necessary to say "returns false otherwise" because this is implied.

// Returns true if the table cannot hold any more entries.

bool IsTableFull();

When commenting constructors and destructors, remember that the person reading your

code knows what constructors and destructors are for, so comments that just say something

like "destroys this object" are not useful. Document what constructors do with their arguments

(for example, if they take ownership of pointers), and what cleanup the destructor does. If this

is trivial, just skip the comment. It is quite common for destructors not to have a header

comment.

Function Definitions

If there is anything tricky about how a function does its job, the function definition should

have an explanatory comment. For example, in the definition comment you might describe

any coding tricks you use, give an overview of the steps you go through, or explain why you

chose to implement the function in the way you did rather than using a viable alternative. For

instance, you might mention why it must acquire a lock for the first half of the function but

why it is not needed for the second half.

Note you should

not

just repeat the comments given with the function declaration, in the .h

file or wherever. It's okay to recapitulate briefly what the function does, but the focus of the

comments should be on how it does it.

Variable Comments

In general the actual name of the variable should be descriptive enough to give a good

idea of what the variable is used for. In certain cases, more comments are required.

Class Data Members

Each class data member (also called an instance variable or member variable) should have

a comment describing what it is used for. If the variable can take sentinel values with special

meanings, such as a null pointer or -1, document this. For example:

private:

// Keeps track of the total number of entries in the table.

// Used to ensure we do not go over the limit. -1 means

// that we don't yet know how many entries the table has.

int num_total_entries_;

Global Variables

As with data members, all global variables should have a comment describing what they are

and what they are used for. For example:

// The total number of tests cases that we run through in this regression test.

const int kNumTestCases = 6;

Implementation Comments

In your implementation you should have comments in tricky, non-obvious, interesting, or

important parts of your code.

Class Data Members

Tricky or complicated code blocks should have comments before them. Example:

// Divide result by two, taking into account that x

// contains the carry from the add.

for (int i = 0; i < result->size(); i++) {

x = (x << 8) + (*result)[i];

(*result)[i] = x >> 1;

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x &= 1;

}

Line Comments

Also, lines that are non-obvious should get a comment at the end of the line. These end-of-

line comments should be separated from the code by 2 spaces. Example:

// If we have enough memory, mmap the data portion too.

mmap_budget = max<int64>(0, mmap_budget - index_->length());

if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))

return; // Error already logged.

Note that there are both comments that describe what the code is doing, and comments

that mention that an error has already been logged when the function returns.

If you have several comments on subsequent lines, it can often be more readable to line

them up:

DoSomething(); // Comment here so the comments line up.

DoSomethingElseThatIsLonger(); // Comment here so there are two spaces between

// the code and the comment.

{ // One space before comment when opening a new scope is allowed,

// thus the comment lines up with the following comments and code.

DoSomethingElse(); // Two spaces before line comments normally.

}

DoSomething(); /* For trailing block comments, one space is fine. */

nullptr/NULL, true/false, 1, 2, 3...

When you pass in a null pointer, boolean, or literal integer values to functions, you should

consider adding a comment about what they are, or make your code self-documenting by

using constants. For example, compare:

bool success = CalculateSomething(interesting_value,

10,

false,

NULL); // What are these arguments??

versus:

bool success = CalculateSomething(interesting_value,

10, // Default base value.

false, // Not the first time we're calling this.

NULL); // No callback.

Or alternatively, constants or self-describing variables:

const int kDefaultBaseValue = 10;

const bool kFirstTimeCalling = false;

Callback *null_callback = NULL;

bool success = CalculateSomething(interesting_value,

kDefaultBaseValue,

kFirstTimeCalling,

null_callback);

Don'ts

Note that you should

never

describe the code itself. Assume that the person reading the code

knows C++ better than you do, even though he or she does not know what you are trying to

do:

// Now go through the b array and make sure that if i occurs,

// the next element is i+1.

... // Geez. What a useless comment.

Punctuation, Spelling and Grammar

Pay attention to punctuation, spelling, and grammar; it is easier to read well-written

comments than badly written ones.

Comments should be as readable as narrative text, with proper capitalization and

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punctuation. In many cases, complete sentences are more readable than sentence

fragments. Shorter comments, such as comments at the end of a line of code, can sometimes

be less formal, but you should be consistent with your style.

Although it can be frustrating to have a code reviewer point out that you are using a comma

when you should be using a semicolon, it is very important that source code maintain a high

level of clarity and readability. Proper punctuation, spelling, and grammar help with that

goal.

TODO Comments

Use TODO comments for code that is temporary, a short-term solution, or good-enough

but not perfect.

TODOs should include the string TODO in all caps, followed by the name, e-mail address, or

other identifier of the person who can best provide context about the problem referenced by

the TODO. A colon is optional. The main purpose is to have a consistent TODO format that can

be searched to find the person who can provide more details upon request. A TODO is not a

commitment that the person referenced will fix the problem. Thus when you create a TODO, it

is almost always your name that is given.

// TODO(kl@gmail.com): Use a "*" here for concatenation operator.

// TODO(Zeke) change this to use relations.

If your TODO is of the form "At a future date do something" make sure that you either include a

very specific date ("Fix by November 2005") or a very specific event ("Remove this code when

all clients can handle XML responses.").

Deprecation Comments

Mark deprecated interface points with DEPRECATED comments.

You can mark an interface as deprecated by writing a comment containing the word

DEPRECATED in all caps. The comment goes either before the declaration of the interface or

on the same line as the declaration.

After the word DEPRECATED, write your name, e-mail address, or other identifier in

parentheses.

A deprecation comment must include simple, clear directions for people to fix their callsites.

In C++, you can implement a deprecated function as an inline function that calls the new

interface point.

Marking an interface point DEPRECATED will not magically cause any callsites to change. If

you want people to actually stop using the deprecated facility, you will have to fix the

callsites yourself or recruit a crew to help you.

New code should not contain calls to deprecated interface points. Use the new interface

point instead. If you cannot understand the directions, find the person who created the

deprecation and ask them for help using the new interface point.

Formatting

Coding style and formatting are pretty arbitrary, but a project is much easier to follow if

everyone uses the same style. Individuals may not agree with every aspect of the formatting

rules, and some of the rules may take some getting used to, but it is important that all project

contributors follow the style rules so that they can all read and understand everyone's code

easily.

To help you format code correctly, we've created a settings file for emacs.

Line Length

Each line of text in your code should be at most 80 characters long.

We recognize that this rule is controversial, but so much existing code already adheres to it,

and we feel that consistency is important.

Pros:

Those who favor this rule argue that it is rude to force them to resize their windows and there is

no need for anything longer. Some folks are used to having several code windows side-by-

side, and thus don't have room to widen their windows in any case. People set up their work

environment assuming a particular maximum window width, and 80 columns has been the

traditional standard. Why change it?

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Cons:

Proponents of change argue that a wider line can make code more readable. The 80-

column limit is an hidebound throwback to 1960s mainframes; modern equipment has wide

screens that can easily show longer lines.

Decision:

80 characters is the maximum.

Exception: if a comment line contains an example command or a literal URL longer than 80

characters, that line may be longer than 80 characters for ease of cut and paste.

Exception: an #include statement with a long path may exceed 80 columns. Try to avoid

situations where this becomes necessary.

Exception: you needn't be concerned about header guards that exceed the maximum

length.

Non-ASCII Characters

Non-ASCII characters should be rare, and must use UTF-8 formatting.

You shouldn't hard-code user-facing text in source, even English, so use of non-ASCII

characters should be rare. However, in certain cases it is appropriate to include such words in

your code. For example, if your code parses data files from foreign sources, it may be

appropriate to hard-code the non-ASCII string(s) used in those data files as delimiters. More

commonly, unittest code (which does not need to be localized) might contain non-ASCII

strings. In such cases, you should use UTF-8, since that is an encoding understood by most

tools able to handle more than just ASCII. Hex encoding is also OK, and encouraged where it

enhances readability — for example, "\xEF\xBB\xBF" is the Unicode zero-width no-break

space character, which would be invisible if included in the source as straight UTF-8.

Spaces vs. Tabs

Use only spaces, and indent 2 spaces at a time.

We use spaces for indentation. Do not use tabs in your code. You should set your editor to

emit spaces when you hit the tab key.

Function Declarations and Definitions

Return type on the same line as function name, parameters on the same line if they fit.

Functions look like this:

ReturnType ClassName::FunctionName(Type par_name1, Type par_name2) {

DoSomething();

...

}

If you have too much text to fit on one line:

ReturnType ClassName::ReallyLongFunctionName(Type par_name1, Type par_name2,

Type par_name3) {

DoSomething();

...

}

or if you cannot fit even the first parameter:

ReturnType LongClassName::ReallyReallyReallyLongFunctionName(

Type par_name1, // 4 space indent

Type par_name2,

Type par_name3) {

DoSomething(); // 2 space indent

...

}

Some points to note:

The return type is always on the same line as the function name.

The open parenthesis is always on the same line as the function name.

There is never a space between the function name and the open parenthesis.

There is never a space between the parentheses and the parameters.

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The open curly brace is always at the end of the same line as the last parameter.

The close curly brace is either on the last line by itself or (if other style rules permit) on the

same line as the open curly brace.

There should be a space between the close parenthesis and the open curly brace.

All parameters should be named, with identical names in the declaration and

implementation.

All parameters should be aligned if possible.

Default indentation is 2 spaces.

Wrapped parameters have a 4 space indent.

If some parameters are unused, comment out the variable name in the function definition:

// Always have named parameters in interfaces.

class Shape {

public:

virtual void Rotate(double radians) = 0;

}

// Always have named parameters in the declaration.

class Circle : public Shape {

public:

virtual void Rotate(double radians);

}

// Comment out unused named parameters in definitions.

void Circle::Rotate(double /*radians*/) {}

// Bad - if someone wants to implement later, it's not clear what the

// variable means.

void Circle::Rotate(double) {}

Function Calls

On one line if it fits; otherwise, wrap arguments at the parenthesis.

Function calls have the following format:

bool retval = DoSomething(argument1, argument2, argument3);

If the arguments do not all fit on one line, they should be broken up onto multiple lines, with

each subsequent line aligned with the first argument. Do not add spaces after the open

paren or before the close paren:

bool retval = DoSomething(averyveryveryverylongargument1,

argument2, argument3);

If the function has many arguments, consider having one per line if this makes the code more

readable:

bool retval = DoSomething(argument1,

argument2,

argument3,

argument4);

Arguments may optionally all be placed on subsequent lines, with one line per argument:

if (...) {

...

if (...) {

DoSomething(

argument1, // 4 space indent

argument2,

argument3,

argument4);

}

In particular, this should be done if the function signature is so long that it cannot fit within the

maximum line length.

Braced Initializer Lists

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On one line if it fits, otherwise wrap at the open brace.

Put everything on one line where possible. If everything can't fit on one line, the open

brace should be the last character on its line, and the close brace should be the first

character on its line.

// Examples of braced init list on a single line.

return {foo, bar};

functioncall({foo, bar});

pair<int, int> p{foo, bar};

// When you have to wrap.

MyType m = {

superlongvariablename1,

superlongvariablename2,

{short, interior, list},

{

interiorwrappinglist,

interiorwrappinglist2

}

};

// Wrapping inside a function call.

function({

wrapped, long,

list, here

});

// If the variable names are really long.

function(

{

wrapped,

list

});

Conditionals

Prefer no spaces inside parentheses. The else keyword belongs on a new line.

There are two acceptable formats for a basic conditional statement. One includes spaces

between the parentheses and the condition, and one does not.

The most common form is without spaces. Either is fine, but

be consistent

. If you are modifying

a file, use the format that is already present. If you are writing new code, use the format that

the other files in that directory or project use. If in doubt and you have no personal

preference, do not add the spaces.

if (condition) { // no spaces inside parentheses

... // 2 space indent.

} else if (...) { // The else goes on the same line as the closing brace.

...

} else {

...

}

If you prefer you may add spaces inside the parentheses:

if ( condition ) { // spaces inside parentheses - rare

... // 2 space indent.

} else { // The else goes on the same line as the closing brace.

...

}

Note that in all cases you must have a space between the if and the open parenthesis. You

must also have a space between the close parenthesis and the curly brace, if you're using

one.

if(condition) // Bad - space missing after IF.

if (condition){ // Bad - space missing before {.

if(condition){ // Doubly bad.

if (condition) { // Good - proper space after IF and before {.

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Short conditional statements may be written on one line if this enhances readability. You may

use this only when the line is brief and the statement does not use the else clause.

if (x == kFoo) return new Foo();

if (x == kBar) return new Bar();

This is not allowed when the if statement has an else:

// Not allowed - IF statement on one line when there is an ELSE clause

if (x) DoThis();

else DoThat();

In general, curly braces are not required for single-line statements, but they are allowed if you

like them; conditional or loop statements with complex conditions or statements may be

more readable with curly braces. Some projects require that an if must always always have

an accompanying brace.

if (condition)

DoSomething(); // 2 space indent.

if (condition) {

DoSomething(); // 2 space indent.

}

However, if one part of an if-else statement uses curly braces, the other part must too:

// Not allowed - curly on IF but not ELSE

if (condition) {

foo;

} else

bar;

// Not allowed - curly on ELSE but not IF

if (condition)

foo;

else {

bar;

}

// Curly braces around both IF and ELSE required because

// one of the clauses used braces.

if (condition) {

foo;

} else {

bar;

}

Loops and Switch Statements

Switch statements may use braces for blocks. Annotate non-trivial fall-through between

cases. Empty loop bodies should use {} or continue.

case blocks in switch statements can have curly braces or not, depending on your

preference. If you do include curly braces they should be placed as shown below.

If not conditional on an enumerated value, switch statements should always have a default

case (in the case of an enumerated value, the compiler will warn you if any values are not

handled). If the default case should never execute, simply assert:

switch (var) {

case 0: { // 2 space indent

... // 4 space indent

break;

}

case 1: {

...

break;

}

default: {

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assert(false);

}

Empty loop bodies should use {} or continue, but not a single semicolon.

while (condition) {

// Repeat test until it returns false.

}

for (int i = 0; i < kSomeNumber; ++i) {} // Good - empty body.

while (condition) continue; // Good - continue indicates no logic.

while (condition); // Bad - looks like part of do/while loop.

Pointer and Reference Expressions

No spaces around period or arrow. Pointer operators do not have trailing spaces.

The following are examples of correctly-formatted pointer and reference expressions:

x = *p;

p = &x;

x = r.y;

x = r->y;

Note that:

There are no spaces around the period or arrow when accessing a member.

Pointer operators have no space after the * or &.

When declaring a pointer variable or argument, you may place the asterisk adjacent to

either the type or to the variable name:

// These are fine, space preceding.

char *c;

const string &str;

// These are fine, space following.

char* c; // but remember to do "char* c, *d, *e, ...;"!

const string& str;

char * c; // Bad - spaces on both sides of *

const string & str; // Bad - spaces on both sides of &

You should do this consistently within a single file, so, when modifying an existing file, use the

style in that file.

Boolean Expressions

When you have a boolean expression that is longer than the standard line length, be

consistent in how you break up the lines.

In this example, the logical AND operator is always at the end of the lines:

if (this_one_thing > this_other_thing &&

a_third_thing == a_fourth_thing &&

yet_another && last_one) {

...

}

Note that when the code wraps in this example, both of the && logical AND operators are at

the end of the line. This is more common in Google code, though wrapping all operators at

the beginning of the line is also allowed. Feel free to insert extra parentheses judiciously

because they can be very helpful in increasing readability when used appropriately. Also

note that you should always use the punctuation operators, such as && and ~, rather than the

word operators, such as and and compl.

Return Values

Do not needlessly surround the return expression with parentheses.

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Use parentheses in return expr; only where you would use them in x = expr;.

return result; // No parentheses in the simple case.

return (some_long_condition && // Parentheses ok to make a complex

another_condition); // expression more readable.

return (value); // You wouldn't write var = (value);

return(result); // return is not a function!

Variable and Array Initialization

Your choice of =, (), or {}.

You may choose between =, (), and {}; the following are all correct:

int x = 3;

int x(3);

int x{3};

string name = "Some Name";

string name("Some Name");

string name{"Some Name"};

Be careful when using the {} on a type that takes an initializer_list in one of its

constructors. The {} syntax prefers the initializer_list constructor whenever possible. To

get the non- initializer_list constructor, use ().

vector<int> v(100, 1); // A vector of 100 1s.

vector<int> v{100, 1}; // A vector of 100, 1.

Also, the brace form prevents narrowing of integral types. This can prevent some types of

programming errors.

int pi(3.14); // OK -- pi == 3.

int pi{3.14}; // Compile error: narrowing conversion.

Preprocessor Directives

The hash mark that starts a preprocessor directive should always be at the beginning of

the line.

Even when preprocessor directives are within the body of indented code, the directives

should start at the beginning of the line.

// Good - directives at beginning of line

if (lopsided_score) {

#if DISASTER_PENDING // Correct -- Starts at beginning of line

DropEverything();

# if NOTIFY // OK but not required -- Spaces after #

NotifyClient();

# endif

#endif

BackToNormal();

}

// Bad - indented directives

if (lopsided_score) {

#if DISASTER_PENDING // Wrong! The "#if" should be at beginning of line

DropEverything();

#endif // Wrong! Do not indent "#endif"

BackToNormal();

}

Class Format

Sections in public, protected and private order, each indented one space.

The basic format for a class declaration (lacking the comments, see Class Comments for a

discussion of what comments are needed) is:

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class MyClass : public OtherClass {

public: // Note the 1 space indent!

MyClass(); // Regular 2 space indent.

explicit MyClass(int var);

~MyClass() {}

void SomeFunction();

void SomeFunctionThatDoesNothing() {

}

void set_some_var(int var) { some_var_ = var; }

int some_var() const { return some_var_; }

private:

bool SomeInternalFunction();

int some_var_;

int some_other_var_;

DISALLOW_COPY_AND_ASSIGN(MyClass);

};

Things to note:

Any base class name should be on the same line as the subclass name, subject to the

80-column limit.

The public:, protected:, and private: keywords should be indented one space.

Except for the first instance, these keywords should be preceded by a blank line. This

rule is optional in small classes.

Do not leave a blank line after these keywords.

The public section should be first, followed by the protected and finally the private

section.

See Declaration Order for rules on ordering declarations within each of these sections.

Constructor Initializer Lists

Constructor initializer lists can be all on one line or with subsequent lines indented four

spaces.

There are two acceptable formats for initializer lists:

// When it all fits on one line:

MyClass::MyClass(int var) : some_var_(var), some_other_var_(var + 1) {}

// When it requires multiple lines, indent 4 spaces, putting the colon on

// the first initializer line:

MyClass::MyClass(int var)

: some_var_(var), // 4 space indent

some_other_var_(var + 1) { // lined up

...

DoSomething();

...

}

Namespace Formatting

The contents of namespaces are not indented.

Namespaces do not add an extra level of indentation. For example, use:

namespace {

void foo() { // Correct. No extra indentation within namespace.

...

}

} // namespace

Do not indent within a namespace:

namespace {

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// Wrong. Indented when it should not be.

void foo() {

...

}

} // namespace

When declaring nested namespaces, put each namespace on its own line.

namespace foo {

namespace bar {

Horizontal Whitespace

Use of horizontal whitespace depends on location. Never put trailing whitespace at the

end of a line.

General

void f(bool b) { // Open braces should always have a space before them.

...

int i = 0; // Semicolons usually have no space before them.

int x[] = { 0 }; // Spaces inside braces for braced-init-list are

int x[] = {0}; // optional. If you use them, put them on both sides!

// Spaces around the colon in inheritance and initializer lists.

class Foo : public Bar {

public:

// For inline function implementations, put spaces between the braces

// and the implementation itself.

Foo(int b) : Bar(), baz_(b) {} // No spaces inside empty braces.

void Reset() { baz_ = 0; } // Spaces separating braces from implementation.

...

Adding trailing whitespace can cause extra work for others editing the same file, when they

merge, as can removing existing trailing whitespace. So: Don't introduce trailing whitespace.

Remove it if you're already changing that line, or do it in a separate clean-up operation

(preferably when no-one else is working on the file).

Loops and Conditionals

if (b) { // Space after the keyword in conditions and loops.

} else { // Spaces around else.

}

while (test) {} // There is usually no space inside parentheses.

switch (i) {

for (int i = 0; i < 5; ++i) {

switch ( i ) { // Loops and conditions may have spaces inside

if ( test ) { // parentheses, but this is rare. Be consistent.

for ( int i = 0; i < 5; ++i ) {

for ( ; i < 5 ; ++i) { // For loops always have a space after the

... // semicolon, and may have a space before the

// semicolon.

for (auto x : counts) { // Range-based for loops always have a

... // space before and after the colon.

}

switch (i) {

case 1: // No space before colon in a switch case.

...

case 2: break; // Use a space after a colon if there's code after it.

Operators

x = 0; // Assignment operators always have spaces around

// them.

x = -5; // No spaces separating unary operators and their

++x; // arguments.

if (x && !y)

...

v = w * x + y / z; // Binary operators usually have spaces around them,

v = w*x + y/z; // but it's okay to remove spaces around factors.

v = w * (x + z); // Parentheses should have no spaces inside them.

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link ▽

Templates and Casts

vector<string> x; // No spaces inside the angle

y = static_cast<char*>(x); // brackets (< and >), before

// <, or between >( in a cast.

vector<char *> x; // Spaces between type and pointer are

// okay, but be consistent.

set<list<string>> x; // Permitted in C++11 code.

set<list<string> > x; // C++03 required a space in > >.

set< list<string> > x; // You may optionally use

// symmetric spacing in < <.

Vertical Whitespace

Minimize use of vertical whitespace.

This is more a principle than a rule: don't use blank lines when you don't have to. In particular,

don't put more than one or two blank lines between functions, resist starting functions with a

blank line, don't end functions with a blank line, and be discriminating with your use of blank

lines inside functions.

The basic principle is: The more code that fits on one screen, the easier it is to follow and

understand the control flow of the program. Of course, readability can suffer from code

being too dense as well as too spread out, so use your judgement. But in general, minimize

use of vertical whitespace.

Some rules of thumb to help when blank lines may be useful:

Blank lines at the beginning or end of a function very rarely help readability.

Blank lines inside a chain of if-else blocks may well help readability.

Exceptions to the Rules

The coding conventions described above are mandatory. However, like all good rules, these

sometimes have exceptions, which we discuss here.

Existing Non-conformant Code

You may diverge from the rules when dealing with code that does not conform to this

style guide.

If you find yourself modifying code that was written to specifications other than those

presented by this guide, you may have to diverge from these rules in order to stay consistent

with the local conventions in that code. If you are in doubt about how to do this, ask the

original author or the person currently responsible for the code. Remember that

consistency

includes local consistency, too.

Windows Code

Windows programmers have developed their own set of coding conventions, mainly

derived from the conventions in Windows headers and other Microsoft code. We want to

make it easy for anyone to understand your code, so we have a single set of guidelines for

everyone writing C++ on any platform.

It is worth reiterating a few of the guidelines that you might forget if you are used to the

prevalent Windows style:

Do not use Hungarian notation (for example, naming an integer iNum). Use the

Google naming conventions, including the .cc extension for source files.

Windows defines many of its own synonyms for primitive types, such as DWORD, HANDLE,

etc. It is perfectly acceptable, and encouraged, that you use these types when calling

Windows API functions. Even so, keep as close as you can to the underlying C++ types.

For example, use const TCHAR * instead of LPCTSTR.

When compiling with Microsoft Visual C++, set the compiler to warning level 3 or

higher, and treat all warnings as errors.

Do not use #pragma once; instead use the standard Google include guards. The path

in the include guards should be relative to the top of your project tree.

In fact, do not use any nonstandard extensions, like #pragma and __declspec, unless

you absolutely must. Using __declspec(dllimport) and __declspec(dllexport)

is allowed; however, you must use them through macros such as DLLIMPORT and

DLLEXPORT, so that someone can easily disable the extensions if they share the code.

However, there are just a few rules that we occasionally need to break on Windows:

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Normally we forbid the use of multiple implementation inheritance; however, it is

required when using COM and some ATL/WTL classes. You may use multiple

implementation inheritance to implement COM or ATL/WTL classes and interfaces.

Although you should not use exceptions in your own code, they are used extensively in

the ATL and some STLs, including the one that comes with Visual C++. When using the

ATL, you should define _ATL_NO_EXCEPTIONS to disable exceptions. You should

investigate whether you can also disable exceptions in your STL, but if not, it is OK to

turn on exceptions in the compiler. (Note that this is only to get the STL to compile. You

should still not write exception handling code yourself.)

The usual way of working with precompiled headers is to include a header file at the

top of each source file, typically with a name like StdAfx.h or precompile.h. To

make your code easier to share with other projects, avoid including this file explicitly

(except in precompile.cc), and use the /FI compiler option to include the file

automatically.

Resource headers, which are usually named resource.h and contain only macros,

do not need to conform to these style guidelines.

Parting Words

Use common sense and

BE CONSISTENT

If you are editing code, take a few minutes to look at the code around you and determine its

style. If they use spaces around their if clauses, you should, too. If their comments have little

boxes of stars around them, make your comments have little boxes of stars around them too.

The point of having style guidelines is to have a common vocabulary of coding so people

can concentrate on what you are saying, rather than on how you are saying it. We present

global style rules here so people know the vocabulary. But local style is also important. If

code you add to a file looks drastically different from the existing code around it, the

discontinuity throws readers out of their rhythm when they go to read it. Try to avoid this.

OK, enough writing about writing code; the code itself is much more interesting. Have fun!

Revision 3.260

Benjy Weinberger

Craig Silverstein

Gregory Eitzmann

Mark Mentovai

Tashana Landray

Google C++ Style Guide

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