Google C++ Style Guide

Google C++ Style Guide
Benjy Weinberger
Craig Silverstein
Gregory Eitzmann
Mark Mentovai
Tashana Landray
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Table of Contents
Header Files The #define Guard Header File Dependencies 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 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 
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 NULL sizeof Boost 
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 
Formatting Line Length Non-ASCII Characters Spaces vs. Tabs Function Declarations and Definitions Function Calls Conditionals Loops and Switch Statements Pointer and Reference Expressions Boolean Expressions Return Values Variable and Array Initialization Preprocessor Directives Class Format Initializer Lists Namespace Formatting Horizontal Whitespace Vertical Whitespace 
Exceptions to the Rules Existing Non-conformant Code Windows Code 

Overview
Important Note
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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 ___H_.
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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_
Header File Dependencies
▽ Use forward declarations to minimize use of #include in .h files.
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When you include a header file you introduce a dependency that will cause your code to be recompiled whenever the header file changes. If your header file includes other header files, any change to those files will cause any code that includes your header to be recompiled. Therefore, we prefer to minimize includes, particularly includes of header files in other header files.

You can significantly minimize the number of header files you need to include in your own header files by using forward declarations. For example, if your header file uses the File class in ways that do not require access to the declaration of the File class, your header file can just forward declare class File; instead of having to #include "file/base/file.h".

How can we use a class Foo in a header file without access to its definition?

We can declare data members of type Foo* or Foo&.
We can declare (but not define) functions with arguments, and/or return values, of type Foo.
We can declare static data members of type Foo. This is because static data members are defined outside the class definition.
On the other hand, you must include the header file for Foo if your class subclasses Foo or has a data member of type Foo.

Sometimes it makes sense to have pointer (or better, scoped_ptr) members instead of object members. However, this complicates code readability and imposes a performance penalty, so avoid doing this transformation if the only purpose is to minimize includes in header files.

Of course, .cc files typically do require the definitions of the classes they use, and usually have to include several header files.

Inline Functions
▽ Define functions inline only when they are small, say, 10 lines or less.
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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.
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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.
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All of a project's header files should be listed as descentants 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, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

dir2/foo2.h (preferred location — see details below).
C system files.
C++ system files.
Other libraries' .h files.
Your project's .h files.
The preferred ordering reduces hidden dependencies. We want every header file to be compilable on its own. The easiest way to achieve this is to make sure that every one of them is the first .h file #included in some .cc.

dir/foo.cc and dir2/foo2.h are often in the same directory (e.g. base/basictypes_unittest.cc and base/basictypes.h), but can be in different directories too.

Within each section it is nice to order the includes alphabetically.

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
#include

#include
#include

#include "base/basictypes.h"
#include "base/commandlineflags.h"
#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.
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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 spaces in header files can easily cause violations of the C++ One Definition Rule (ODR).


Decision:

Use namespaces according to the policy described below.

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 { UNUSED, EOF, ERROR };          // Commonly used tokens.
bool AtEof() { return pos_ == EOF; }  // 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. Terminate the unnamed namespace as shown, with a comment // 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
// 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, and in functions and methods in .h files.
// OK in .cc files.
// Must be in a function or method in .h files.
namespace fbz = ::foo::bar::baz;


Nested Classes
▽ Although you may use public nested classes when they are part of an interface, consider a namespace to keep declarations out of the global scope.
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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.
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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.
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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;
i = f();      // Bad -- initialization separate from declaration.
int j = g();  // Good -- declaration has initialization.
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);
}
Global Variables
▽ Global variables of class types are forbidden. Global variables of built-in types are allowed, although non-const globals are forbidden in threaded code. Global variables should never be initialized with the return value of a function.
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Unfortunately the order in which constructors, destructors, and initializers for global variables are called is only partially specified and can change from build to build. This can cause bugs that are very difficult to find.

Therefore we forbid global variables of class types (which includes STL string, vector, etc.) because initialization order might matter for their constructor, now or in the future. Built-in types and structs of built-in types without constructors are okay. If you need a global variable of a class type, use the singleton pattern.

For global string constants, use C style strings, not STL strings:

const char kFrogSays[] = "ribbet";
Although we permit global variables in the global scope, please be judicious in your use of them. Most global variables should either be static data members of some class, or, if only needed in one .cc file, defined in an unnamed namespace. (As an alternative to using an unnamed namespace, you can use static linkage to limit the variable's scope.)

Please note that static class member variables count as global variables, and should not be of class types!

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
▽ Do only trivial initialization in a constructor. If at all possible, use an Init() method for non-trivial initialization.
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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: If your object requires non-trivial initialization, consider having an explicit Init() method and/or adding a member flag that indicates whether the object was successfully initialized.

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.
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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 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.
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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:

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.


Copy Constructors
▽ Use copy constructors only when your code needs to copy a class; most do not need to be copied and so should use DISALLOW_COPY_AND_ASSIGN.
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Definition: The copy constructor is used when copying one object into a new one (especially when passing objects by value).

Pros: Copy constructors make it easy to copy objects. STL containers require that all contents be copyable and assignable.

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:

Most classes do not need to be copyable, and should not have a copy constructor or an assignment operator. Unfortunately, the compiler generates these for you, and makes them public, if you do not declare them yourself.

Consider adding dummy declarations for the copy constructor and assignment operator in the class' private: section, without providing definitions. With these dummy routines marked private, a compilation error will be raised if other code attempts to use them. 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);
};
In almost all cases your class should use the DISALLOW_COPY_AND_ASSIGN macro as described above. If your class is one of the rare classes that does need to be copyable, you should document why this is so in the header file for that class, and you should define the copy constructor and assignment operator appropriately. Remember to check for self-assignment in operator=.

You may be tempted to make your class copyable so that you can use it as a value in STL containers. In almost all such cases you should really put pointers to your objects in the STL container. You may also want to consider using std::tr1::shared_ptr.


Structs vs. Classes
▽ Use a struct only for passive objects that carry data; everything else is a class.
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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.
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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 must always 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.
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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).

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.
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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, they 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.
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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 ==.
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, you can't forward declare classes that overload operator&.

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.

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 all data members private, and provide access to them through accessor functions as needed. Typically a variable would be called foo_ and the accessor function foo(). You may also want a mutator function set_foo().
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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.
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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
Constructors
Destructor
Methods, including static methods
Data Members, including static data members
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.
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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.

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, scoped_ptr is great. You should only use std::tr1::shared_ptr under very specific conditions, such as when objects need to be held by STL containers. You should never use auto_ptr.
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"Smart" pointers are objects that act like pointers but have added semantics. When a scoped_ptr is destroyed, for instance, it deletes the object it's pointing to. shared_ptr is the same way, but implements reference-counting so only the last pointer to an object deletes it.

Generally speaking, we prefer that we design code with clear object ownership. The clearest object ownership is obtained by using an object directly as a field or local variable, without using pointers at all. On the other extreme, by their very definition, reference counted pointers are owned by nobody. The problem with this design is that it is easy to create circular references or other strange conditions that cause an object to never be deleted. It is also slow to perform atomic operations every time a value is copied or assigned.

Although they are not recommended, reference counted pointers are sometimes the simplest and most elegant way to solve a problem.

Other C++ Features
Reference Arguments
▽ All parameters passed by reference must be labeled const.
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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).

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 NULL 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 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.

One case when you might want an input parameter to be a const pointer is if you want to emphasize that the argument is not copied, so it must exist for the lifetime of the object; it is usually best to document this in comments as well. STL adapters such as bind2nd and mem_fun do not permit reference parameters, so you must declare functions with pointer parameters in these cases, too.


Function Overloading
▽ Use overloaded functions (including constructors) only in cases where input can be specified in different types that contain the same information. Do not use function overloading to simulate default function parameters.
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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: One reason to minimize function overloading is that overloading can make it hard to tell which function is being called at a particular call site. Another one is that most people are confused by the semantics of inheritance if a deriving class overrides only some of the variants of a function. Moreover, reading client code of a library may become unnecessarily hard because of all the reasons against default function parameters.

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.
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Pros: Often you have a function that uses lots of 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.

Cons: People often figure out how to use an API by looking at existing code that uses it. Default parameters are more difficult to maintain because copy-and-paste from previous code may not reveal all the parameters. Copy-and-pasting of code segments can cause major problems when the default arguments are not appropriate for the new code.

Decision: We require all arguments to be explicitly specified, to force programmers to consider the API and the values they are passing for each argument rather than silently accepting defaults they may not be aware of.

Variable-Length Arrays and alloca()
▽ We do not allow variable-length arrays or alloca().
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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.
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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.
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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 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 results 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 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)
▽ We do not use Run Time Type Information (RTTI).
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Definition: RTTI allows a programmer to query the C++ class of an object at run time.

Pros:

It is useful in some unittests. 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.

In rare circumstances, it is useful even outside of tests.


Cons: A query of type during run-time typically means a design problem. If you need to know the type of an object at runtime, that is often an indication that you should reconsider the design of your class.

Decision:

Do not use RTTI, except in unittests. If you find yourself in need of writing code that behaves differently based on the class of an object, consider one of the 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.

If you think you truly cannot use those ideas, you may use RTTI. But think twice about it. :-) Then think twice again. Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hierarchies with type tags.


Casting
▽ Use C++ casts like static_cast<>(). Do not use other cast formats like int y = (int)x; or int y = int(x);.
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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.
Do not use dynamic_cast except in test code. If you need to know type information at runtime in this way outside of a unittest, you probably have a design flaw.

Streams
▽ Use streams only for logging.
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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.
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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 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
▽ We strongly recommend that you use const whenever it makes sense to do so.
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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.
However, do not go crazy with const. Something like const int * const * const x; is likely overkill, even if it accurately describes how const x is. Focus on what's really useful to know: in this case, const int** x is probably sufficient.

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 this case, because the "don't go crazy" dictum eliminates most of the uses you'd have to be consistent with. 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
▽ 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 , such as int16_t.
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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:

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 the quantity you are representing is really a bit pattern rather than a number. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this.


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.
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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
#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.
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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:

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.
0 and NULL
▽ Use 0 for integers, 0.0 for reals, NULL for pointers, and '\0' for chars.
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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. Bjarne Stroustrup prefers an unadorned 0. We prefer NULL because it looks like a pointer. In fact, some C++ compilers, such as gcc 4.1.0, provide special definitions of NULL which enable them to give useful warnings, particularly in situatio