Google C++ Style Guide

Why do we have this document?

There are a few core goals that we believe this guide should serve. These are the fundamental whys that underlie all of the individual rules. By bringing these ideas to the fore, we hope to ground discussions and make it clearer to our broader community why the rules are in place and why particular decisions have been made. If you understand what goals each rule is serving, it should be clearer to everyone when a rule may be waived (some can be), and what sort of argument or alternative would be necessary to change a rule in the guide.

The goals of the style guide as we currently see them are as follows:

Style rules should pull their weight

The benefit of a style rule must be large enough to justify asking all of our engineers to remember it. The benefit is measured relative to the codebase we would get without the rule, so a rule against a very harmful practice may still have a small benefit if people are unlikely to do it anyway. This principle mostly explains the rules we don’t have, rather than the rules we do: for example, goto contravenes many of the following principles, but is already vanishingly rare, so the Style Guide doesn’t discuss it.

Optimize for the reader, not the writer

Our codebase (and most individual components submitted to it) is expected to continue for quite some time. As a result, more time will be spent reading most of our code than writing it. We explicitly choose to optimize for the experience of our average software engineer reading, maintaining, and debugging code in our codebase rather than ease when writing said code.  "Leave a trace for the reader" is a particularly common sub-point of this principle: When something surprising or unusual is happening in a snippet of code (for example, transfer of pointer ownership), leaving textual hints for the reader at the point of use is valuable (std::unique_ptr demonstrates the ownership transfer unambiguously at the call site).

Be consistent with existing code

Using one style consistently through our codebase lets us focus on other (more important) issues. Consistency also allows for automation: tools that format your code or adjust your #includes only work properly when your code is consistent with the expectations of the tooling. In many cases, rules that are attributed to "Be Consistent" boil down to "Just pick one and stop worrying about it"; the potential value of allowing flexibility on these points is outweighed by the cost of having people argue over them.

Be consistent with the broader C++ community when appropriate

Consistency with the way other organizations use C++ has value for the same reasons as consistency within our code base. If a feature in the C++ standard solves a problem, or if some idiom is widely known and accepted, that‘s an argument for using it. However, sometimes standard features and idioms are flawed, or were just designed without our codebase‘s needs in mind. In those cases (as described below) it‘s appropriate to constrain or ban standard features.  In some cases we prefer a homegrown or third-party library over a library defined in the C++ Standard, either out of perceived superiority or insufficient value to transition the codebase to the standard interface.

Avoid surprising or dangerous constructs

C++ has features that are more surprising or dangerous than one might think at a glance. Some style guide restrictions are in place to prevent falling into these pitfalls. There is a high bar for style guide waivers on such restrictions, because waiving such rules often directly risks compromising program correctness.

Avoid constructs that our average C++ programmer would find tricky or hard to maintain

C++ has features that may not be generally appropriate because of the complexity they introduce to the code. In widely used code, it may be more acceptable to use trickier language constructs, because any benefits of more complex implementation are multiplied widely by usage, and the cost in understanding the complexity does not need to be paid again when working with new portions of the codebase. When in doubt, waivers to rules of this type can be sought by asking  your project leads. This is specifically important for our codebase because code ownership and team membership changes over time: even if everyone that works with some piece of code currently understands it, such understanding is not guaranteed to hold a few years from now.

Be mindful of our scale

With a codebase of 100+ million lines and thousands of engineers, some mistakes and simplifications for one engineer can become costly for many. For instance it‘s particularly important to avoid polluting the global namespace: name collisions across a codebase of hundreds of millions of lines are difficult to work with and hard to avoid if everyone puts things into the global namespace.

Concede to optimization when necessary

Performance optimizations can sometimes be necessary and appropriate, even when they conflict with the other principles of this document.

The intent of this document is to provide maximal guidance with reasonable restriction. As always, common sense and good taste should prevail. By this we specifically refer to the established conventions of the entire Google C++ community, not just your personal preferences or those of your team. Be skeptical about and reluctant to use clever or unusual constructs: the absence of a prohibition is not the same as a license to proceed.  Use your judgment, and if you are unsure, please don‘t hesitate to ask your project leads to get additional input.

Header files should be self-contained (compile on their own) and end in .h.  Non-header files that are meant for inclusion should end in .inc and be used sparingly.

All header files should be self-contained. Users and refactoring tools should not have to adhere to special conditions to include the header. Specifically, a header should have header guards and include all other headers it needs.

Prefer placing the definitions for template and inline functions in the same file as their declarations.  The definitions of these constructs must be included into every .cc file that uses them, or the program may fail to link in some build configurations.  If declarations and definitions are in different files, including the former should transitively include the latter.  Do not move these definitions to separately included header files (-inl.h); this practice was common in the past, but is no longer allowed.

As an exception, a template that is explicitly instantiated for all relevant sets of template arguments, or that is a private implementation detail of a class, is allowed to be defined in the one and only .cc file that instantiates the template.

There are rare cases where a file designed to be included is not self-contained.  These are typically intended to be included at unusual locations, such as the middle of another file.  They might not use header guards, and might not include their prerequisites.  Name such files with the .inc extension.  Use sparingly, and prefer self-contained headers when possible.

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_

Avoid using forward declarations where possible.  Just #include the headers you need.

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

Use standard order for readability and to avoid hidden dependencies: Related header, 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.
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 usually in the same directory (e.g. base/basictypes_test.cc and base/basictypes.h), but may sometimes 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.

You should include all the headers that define the symbols you rely upon, except in the unusual case of forward declaration. If you rely on symbols from bar.h, don‘t count on the fact that you included foo.h which (currently) includes bar.h: include bar.h yourself, unless foo.h explicitly demonstrates its intent to provide you the symbols of bar.h.  However, any includes present in the related header do not need to be included again in the related cc (i.e., foo.cc can rely on foo.h‘s includes).

For example, the includes in google-awesome-project/src/foo/internal/fooserver.cc might look like this:

#include "foo/server/fooserver.h"

#include <sys/types.h>
#include <unistd.h>

#include <hash_map>
#include <vector>

#include "base/basictypes.h"
#include "base/commandlineflags.h"
#include "foo/server/bar.h"

Sometimes, system-specific code needs conditional includes. Such code can put conditional includes after other includes. Of course, keep your system-specific code small and localized. Example:

#include "foo/public/fooserver.h"

#include "base/port.h"  // For LANG_CXX11.

#ifdef LANG_CXX11
#include <initializer_list>
#endif  // LANG_CXX11

With few exceptions, place code in a namespace. Namespaces should have unique names based on the project name, and possibly its path. Do not use using-directives (e.g. using namespace foo). Do not use inline namespaces. For unnamed namespaces, see Unnamed Namespaces and Static Variables.

When definitions in a .cc file do not need to be referenced outside that file, place them in an unnamed namespace or declare them static. Do not use either of these constructs in .h files.

Prefer placing nonmember functions in a namespace; use completely global functions rarely. Prefer grouping functions with a namespace instead of using a class as if it were a namespace. Static methods of a class should generally be closely related to instances of the class or the class‘s static data.

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

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

std::vector<int> v;
v.push_back(1);  // Prefer initializing using brace initialization.
v.push_back(2);

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

Variables needed for if, while and for statements should normally be declared within those statements, so that such variables are confined to those scopes.  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);
}

Variables of class type with     static storage duration are forbidden: they cause hard-to-find bugs due  to indeterminate order of construction and destruction. However, such  variables are allowed if they are constexpr: they have no  dynamic initialization or 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 non-local static 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. However, a static POD variable within function scope may be initialized with the result of a function, since its initialization order is well-defined and does not occur until control passes through its declaration.

Likewise, global and static variables are destroyed when the program terminates, regardless of whether the termination is by returning from main() or by calling exit(). 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.

One way to alleviate the destructor problem is to terminate the program by calling quick_exit() instead of exit(). The difference is that quick_exit() does not invoke destructors and does not invoke any handlers that were registered by calling atexit(). If you have a handler that needs to run when a program terminates via quick_exit() (flushing logs, for example), you can register it using at_quick_exit(). (If you have a handler that needs to run at both exit() and quick_exit(), you need to register it in both places.)

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

Avoid virtual method calls in constructors, and avoid initialization that can fail if you can‘t signal an error.

Do not define implicit conversions. Use the explicit keyword for conversion operators and single-argument constructors.

Support copying and/or moving if these operations are clear and meaningful for your type. Otherwise, disable the implicitly generated special functions that perform copies and moves.

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.

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

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.

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

Overload operators judiciously. Do not create user-defined literals.

Make data members private, unless they are static const (and follow the naming convention for constants). For technical reasons, we allow data members of a test fixture class to be protected when using Google Test).

Group similar declarations together, placing public parts earlier.

A class definition should usually start with a public: section, followed by protected:, then private:.  Omit sections that would be empty.

Within each section, generally prefer grouping similar kinds of declarations together, and generally prefer the following order: types (including typedef, using, and nested structs and classes), constants, factory functions, constructors, assignment operators, destructor, all other methods, data members.

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.

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 pointers to non-const. 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.

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.

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.

All parameters passed by reference must be labeled const.

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.

Default arguments are allowed on non-virtual functions when the default is guaranteed to always have the same value. Follow the same restrictions as for function overloading, and prefer overloaded functions if the readability gained with default arguments doesn‘t outweigh the downsides below.

Use trailing return types only where using the ordinary syntax (leading  return types) is impractical or much less readable.

C++ allows two different forms of function declarations. In the older  form, the return type appears before the function name. For example:

int foo(int x);

The new form, introduced in C++11, uses the auto  keyword before the function name and a trailing return type after  the argument list. For example, the declaration above could  equivalently be written:

auto foo(int x) -> int;

The trailing return type is in the function‘s scope. This doesn‘t  make a difference for a simple case like int but it matters  for more complicated cases, like types declared in class scope or  types written in terms of the function parameters.

Prefer to have single, fixed owners for dynamically allocated objects. Prefer to transfer ownership with smart pointers.

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 or // NOLINTNEXTLINE in the previous 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.

Use rvalue references only to define move constructors and move assignment operators, or for perfect forwarding.

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.

We do not use C++ exceptions.

Avoid using Run Time Type Information (RTTI).

Use C++-style casts like static_cast<float>(double_value), or brace initialization for conversion of arithmetic types like int64 y = int64{1} << 42. Do not use cast formats like int y = (int)x or int y = int(x) (but the latter is okay when invoking a constructor of a class type).

Use streams where appropriate, and stick to "simple" usages.

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

Use const whenever it makes sense. With C++11, constexpr is a better choice for some uses of const.

In C++11, use constexpr to define true constants or to ensure constant initialization.

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.

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
  #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 %td

  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. Note also  that spaces are required around the macro identifier to  separate it from the string literal. 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 = http://www.mamicode.com/0x123456789LL;>
  

Avoid defining macros, especially in headers; prefer inline functions, enums, and const variables. Name macros with a project-specific prefix. Do not use macros to define pieces of a C++ API.

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.

The problems introduced by macros are especially severe when they are used to define pieces of a C++ API, and still more so for public APIs. Every error message from the compiler when developers incorrectly use that interface now must explain how the macros formed the interface. Refactoring and analysis tools have a dramatically harder time updating the interface. As a consequence, we specifically disallow using macros in this way. For example, avoid patterns like:

class WOMBAT_TYPE(Foo) {
  // ...

 public:
  EXPAND_PUBLIC_WOMBAT_API(Foo)

  EXPAND_WOMBAT_COMPARISONS(Foo, ==, <)
};

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. If you need to use a macro to define an interface, contact  your project leads to request a waiver of this rule.

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

Exporting macros from headers (i.e. defining them in a header without #undefing them before the end of the header) is extremely strongly discouraged. If you do export a macro from a header, it must have a globally unique name. To achieve this, it must be named with a prefix consisting of your project‘s namespace name (but upper case).

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, NULL, and 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.

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;
}

Use auto to avoid type names that are noisy, obvious, or unimportant - cases where the type doesn‘t aid in clarity for the reader. Continue to use manifest type declarations when it helps readability.

You may use braced initializer lists.

In C++03, aggregate types (arrays and structs with no constructor) could be initialized with braced initializer lists.

struct Point { int x; int y; };
Point p = {1, 2};

In C++11, this syntax was generalized, and any object type can now be created with a braced initializer list, known as a braced-init-list in the C++ grammar. Here are a few examples of its use.

// Vector takes a braced-init-list of elements.
std::vector<string> v{"foo", "bar"};

// Basically the same, ignoring some small technicalities.
// You may choose to use either form.
std::vector<string> v = {"foo", "bar"};

// Usable with ‘new‘ expressions.
auto p = new vector<string>{"foo", "bar"};

// A map can take a list of pairs. Nested braced-init-lists work.
std::map<int, string> m = {{1, "one"}, {2, "2"}};

// A braced-init-list can be implicitly converted to a return type.
std::vector<int> test_function() { return {1, 2, 3}; }

// Iterate over a braced-init-list.
for (int i : {-1, -2, -3}) {}

// Call a function using a braced-init-list.
void TestFunction2(std::vector<int> v) {}
TestFunction2({1, 2, 3});

A user-defined type can also define a constructor and/or assignment operator that take std::initializer_list<T>, which is automatically created from braced-init-list:

class MyType {
 public:
  // std::initializer_list references the underlying init list.
  // It should be passed by value.
  MyType(std::initializer_list<int> init_list) {
    for (int i : init_list) append(i);
  }
  MyType& operator=(std::initializer_list<int> init_list) {
    clear();
    for (int i : init_list) append(i);
  }
};
MyType m{2, 3, 5, 7};

Finally, brace initialization can also call ordinary constructors of data types, even if they do not have std::initializer_list<T> constructors.

double d{1.23};
// Calls ordinary constructor as long as MyOtherType has no
// std::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 a std::initializer_list<double>

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

See Braced_Initializer_List_Format for formatting.

Use lambda expressions where appropriate. Prefer explicit captures when the lambda will escape the current scope.

Avoid complicated template programming.

Use only approved libraries from the Boost library collection.

Do not define specializations of std::hash.

Use libraries and language extensions from C++11 when appropriate. Consider portability to other environments before using C++11 features in your project.

Nonstandard extensions to C++ may not be used unless otherwise specified.

Public aliases are for the benefit of an API‘s user, and should be clearly documented.

Names should be descriptive; avoid 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.

Note that certain universally-known abbreviations are OK, such as i for an iteration variable and T for a template parameter.

Template parameters should follow the naming style for their category: type template parameters should follow the rules for type names, and non-type template parameters should follow the rules for variable names.

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. Files that rely on being textually included at specific points should end in .inc (see also the section on self-contained headers).

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.

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, type aliases, enums, and type template parameters — 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;

// using aliases
using PropertiesMap = hash_map<UrlTableProperties *, string>;

// enums
enum UrlTableErrors { ...

The names of variables (including function parameters) and data members are all lowercase, with underscores between words. Data members of classes (but not structs) additionally have trailing underscores. For instance: a_local_variable, a_struct_data_member, a_class_data_member_.

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 of classes, both static and non-static, are named like ordinary nonmember variables, but with a trailing underscore.

class TableInfo {
  ...
 private:
  string table_name_;  // OK - underscore at end.
  string tablename_;   // OK.
  static Pool<TableInfo>* pool_;  // OK.
};

Struct Data Members

Data members of structs, both static and non-static, are named like ordinary nonmember variables. They do not have the trailing underscores that data members in classes have.

struct UrlTableProperties {
  string name;
  int num_entries;
  static Pool<UrlTableProperties>* pool;
};

See Structs vs. Classes for a discussion of when to use a struct versus a class.

Variables declared constexpr or const, and whose value is fixed for  the duration of the program, are named with a leading "k" followed  by mixed case.  For example:

All such variables with static storage duration (i.e. statics and globals,  see     Storage Duration for details) should be named this way.  This  convention is optional for variables of other storage classes, e.g. automatic  variables, otherwise the usual variable naming rules apply.

Regular functions have mixed case; accessors and mutators may be named like variables.

Ordinarily, functions should start with a capital letter and have a capital letter for each new word (a.k.a. "Camel Case" or "Pascal case"). Such names should not have underscores. Prefer to capitalize acronyms as single words (i.e. StartRpc(), not StartRPC()).

AddTableEntry()
DeleteUrl()
OpenFileOrDie()

(The same naming rule applies to class- and namespace-scope constants that are exposed as part of an API and that are intended to look like functions, because the fact that they‘re objects rather than functions is an unimportant implementation detail.)

Accessors and mutators (get and set functions) may be named like variables. These often correspond to actual member variables, but this is not required. For example, int count() and void set_count(int count).

The name of a top-level namespace should usually be the name of the project or team whose code is contained in that namespace. The code in that namespace should usually be in a directory whose basename matches the namespace name (or subdirectories thereof).

Keep in mind that the rule against abbreviated names applies to namespaces just as much as variable names. Code inside the namespace seldom needs to mention the namespace name, so there‘s usually no particular need for abbreviation anyway.

Avoid nested namespaces that match well-known top-level namespaces. Collisions between namespace names can lead to surprising build breaks because of name lookup rules. In particular, do not create any nested std namespaces. Prefer unique project identifiers (websearch::index, websearch::index_util) over collision-prone names like websearch::util.

For internal namespaces, be wary of other code being added to the same internal namespace causing a collision (internal helpers within a team tend to be related and may lead to collisions). In such a situation, using the filename to make a unique internal name is helpful (websearch::index::frobber_internal for use in frobber.h)

Enumerators (for both scoped and unscoped enums) 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.

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

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

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.

Start each file with license boilerplate.

File comments describe the contents of a file. If a file declares, implements, or tests exactly one abstraction that is documented by a comment at the point of declaration, file comments are not required. All other files must have file comments.

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

If a .h declares multiple abstractions, the file-level comment should broadly describe the contents of the file, and how the abstractions are related. A 1 or 2 sentence file-level comment may be sufficient. The detailed documentation about individual abstractions belongs with those abstractions, not at the file level.

Do not duplicate comments in both the .h and the .cc. Duplicated comments diverge.

Every non-obvious class declaration should have an accompanying comment that describes what it is for and how it should be used.

// Iterates over the contents of a GargantuanTable.
// Example:
//    GargantuanTableIterator* iter = table->NewIterator();
//    for (iter->Seek("foo"); !iter->done(); iter->Next()) {
//      process(iter->key(), iter->value());
//    }
//    delete iter;
class GargantuanTableIterator {
  ...
};

The class comment should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class. 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.

The class comment is often a good place for a small example code snippet demonstrating a simple and focused usage of the class.

When sufficiently separated (e.g. .h and .cc files), comments describing the use of the class should go together with its interface definition; comments about the class operation and implementation should accompany the implementation of the class‘s methods.

Declaration comments describe use of the function (when it is non-obvious); comments at the definition of a function describe operation.

Function Declarations

Almost every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments may be omitted only if the function is simple and obvious (e.g. simple accessors for obvious properties of the class).  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.
//
// 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 documenting function overrides, focus on the specifics of the override itself, rather than repeating the comment from the overridden function.  In many of these cases, the override needs no additional documentation and thus no comment is required.

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.

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

The purpose of each class data member (also called an instance variable or member variable) must be clear. If there are any invariants (special values, relationships between members, lifetime requirements) not clearly expressed by the type and name, they must be commented. However, if the type and name suffice (int num_events_;), no comment is needed.

In particular, add comments to describe the existence and meaning of sentinel values, such as nullptr or -1, when they are not obvious. For example:

private:
 // Used to bounds-check table accesses. -1 means
 // that we don‘t yet know how many entries the table has.
 int num_total_entries_;

Global Variables

All global variables should have a comment describing what they are, what they are used for, and (if unclear) why it needs to be global. For example:

// The total number of tests cases that we run through in this regression test.
const int kNumTestCases = 6;

In your implementation you should have comments in tricky, non-obvious, interesting, or important parts of your code.

Explanatory Comments

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;
  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();  // 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.
}
std::vector<string> list{
                    // Comments in braced lists describe the next element...
                    "First item",
                    // .. and should be aligned appropriately.
                    "Second item"};
DoSomething(); /* For trailing block comments, one space is fine. */

Function Argument Comments

When the meaning of a function argument is nonobvious, consider one of the following remedies:

• If the argument is a literal constant, and the same constant is  used in multiple function calls in a way that tacitly assumes they‘re  the same, you should use a named constant to make that constraint  explicit, and to guarantee that it holds.
• Consider changing the function signature to replace a bool  argument with an enum argument. This will make the argument  values self-describing.
• For functions that have several configuration options, consider  defining a single class or struct to hold all the options  ,  and pass an instance of that.  This approach has several advantages. Options are referenced by name  at the call site, which clarifies their meaning. It also reduces  function argument count, which makes function calls easier to read and  write. As an added benefit, you don‘t have to change call sites when  you add another option.  
• Replace large or complex nested expressions with named variables.
• As a last resort, use comments to clarify argument meanings at the  call site.

// What are these arguments?
const DecimalNumber product = CalculateProduct(values, 7, false, nullptr);

versus:

ProductOptions options;
options.set_precision_decimals(7);
options.set_use_cache(ProductOptions::kDontUseCache);
const DecimalNumber product =
    CalculateProduct(values, options, /*completion_callback=*/nullptr);

Don‘ts

Do not state the obvious. In particular, don‘t literally describe what code does, unless the behavior is nonobvious to a reader who understands C++ well. Instead, provide higher level comments that describe why the code does what it does, or make the code self describing.

// Find the element in the vector.  <-- Bad: obvious!
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}

// Process "element" unless it was already processed.
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}

if (!IsAlreadyProcessed(element)) {
  Process(element);
}

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 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.

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, bug ID, or other identifier of the person or issue with the best context about the problem referenced by the TODO. The main purpose is to have a consistent TODO that can be searched to find out how to get more details upon request. A TODO is not a commitment that the person referenced will fix the problem. Thus when you create a TODO with a name, 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.
// TODO(bug 12345): remove the "Last visitors" feature

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.").

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.

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.

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", or, even more simply, u8"\uFEFF", is the Unicode zero-width no-break space character, which would be invisible if included in the source as straight UTF-8.

Use the u8 prefix to guarantee that a string literal containing \uXXXX escape sequences is encoded as UTF-8. Do not use it for strings containing non-ASCII characters encoded as UTF-8, because that will produce incorrect output if the compiler does not interpret the source file as UTF-8.

You shouldn‘t use the C++11 char16_t and char32_t character types, since they‘re for non-UTF-8 text. For similar reasons you also shouldn‘t use wchar_t (unless you‘re writing code that interacts with the Windows API, which uses wchar_t extensively).

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.

Return type on the same line as function name, parameters on the same line if they fit. Wrap parameter lists which do not fit on a single line as you would wrap arguments in a function call.

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:

• Choose good parameter names.
• Parameter names may be omitted only if the parameter is unused and its  purpose is obvious.
• If you cannot fit the return type and the function  name on a single line, break between them.
• If you break after the return type of a function  declaration or definition, do not indent.
• 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.
• The open curly brace is always on the end of the last line of the function  declaration, not the start of the next line.
• The close curly brace is either on the last line by  itself or 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 aligned if possible.
• Default indentation is 2 spaces.
• Wrapped parameters have a 4 space indent.

Unused parameters that are obvious from context may be omitted:

class Foo {
 public:
  Foo(Foo&&);
  Foo(const Foo&);
  Foo& operator=(Foo&&);
  Foo& operator=(const Foo&);
};

Unused parameters that might not be obvious should comment out the variable name in the function definition:

class Shape {
 public:
  virtual void Rotate(double radians) = 0;
};

class Circle : public Shape {
 public:
  void Rotate(double radians) override;
};

void Circle::Rotate(double /*radians*/) {}

// Bad - if someone wants to implement later, it‘s not clear what the
// variable means.
void Circle::Rotate(double) {}

Attributes, and macros that expand to attributes, appear at the very beginning of the function declaration or definition, before the return type:

MUST_USE_RESULT bool IsOK();

Format parameters and bodies as for any other function, and capture lists like other comma-separated lists.

For by-reference captures, do not leave a space between the ampersand (&) and the variable name.

int x = 0;
auto x_plus_n = [&x](int n) -> int { return x + n; }

Short lambdas may be written inline as function arguments.

std::set<int> blacklist = {7, 8, 9};
std::vector<int> digits = {3, 9, 1, 8, 4, 7, 1};
digits.erase(std::remove_if(digits.begin(), digits.end(), [&blacklist](int i) {
               return blacklist.find(i) != blacklist.end();
             }),
             digits.end());

Either write the call all on a single line, wrap the arguments at the parenthesis, or start the arguments on a new line indented by four spaces and continue at that 4 space indent. In the absence of other considerations, use the minimum number of lines, including placing multiple arguments on each line where appropriate.

Function calls have the following format:

bool result = 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 result = DoSomething(averyveryveryverylongargument1,
                          argument2, argument3);

Arguments may optionally all be placed on subsequent lines with a four space indent:

if (...) {
  ...
  ...
  if (...) {
    bool result = DoSomething(
        argument1, argument2,  // 4 space indent
        argument3, argument4);
    ...
  }

Put multiple arguments on a single line to reduce the number of lines necessary for calling a function unless there is a specific readability problem. Some find that formatting with strictly one argument on each line is more readable and simplifies editing of the arguments. However, we prioritize for the reader over the ease of editing arguments, and most readability problems are better addressed with the following techniques.

If having multiple arguments in a single line decreases readability due to the complexity or confusing nature of the expressions that make up some arguments, try creating variables that capture those arguments in a descriptive name:

int my_heuristic = scores[x] * y + bases[x];
bool result = DoSomething(my_heuristic, x, y, z);

Or put the confusing argument on its own line with an explanatory comment:

bool result = DoSomething(scores[x] * y + bases[x],  // Score heuristic.
                          x, y, z);

If there is still a case where one argument is significantly more readable on its own line, then put it on its own line. The decision should be specific to the argument which is made more readable rather than a general policy.

Sometimes arguments form a structure that is important for readability. In those cases, feel free to format the arguments according to that structure:

// Transform the widget by a 3x3 matrix.
my_widget.Transform(x1, x2, x3,
                    y1, y2, y3,
                    z1, z2, z3);

Format a braced initializer list exactly like you would format a function call in its place.

If the braced list follows a name (e.g. a type or variable name), format as if the {} were the parentheses of a function call with that name. If there is no name, assume a zero-length name.

// Examples of braced init list on a single line.
return {foo, bar};
functioncall({foo, bar});
std::pair<int, int> p{foo, bar};

// When you have to wrap.
SomeFunction(
    {"assume a zero-length name before {"},
    some_other_function_parameter);
SomeType variable{
    some, other, values,
    {"assume a zero-length name before {"},
    SomeOtherType{
        "Very long string requiring the surrounding breaks.",
        some, other values},
    SomeOtherType{"Slightly shorter string",
                  some, other, values}};
SomeType variable{
    "This is too long to fit all in one line"};
MyType m = {  // Here, you could also break before {.
    superlongvariablename1,
    superlongvariablename2,
    {short, interior, list},
    {interiorwrappinglist,
     interiorwrappinglist2}};

Prefer no spaces inside parentheses. The if and else keywords belong on separate lines.

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 {.

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;
}

Switch statements may use braces for blocks. Annotate non-trivial fall-through between cases. Braces are optional for single-statement loops. Empty loop bodies should use empty braces 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: {
    assert(false);
  }
}

Braces are optional for single-statement loops.

for (int i = 0; i < kSomeNumber; ++i)
  printf("I love you\n");

for (int i = 0; i < kSomeNumber; ++i) {
  printf("I take it back\n");
}

Empty loop bodies should use an empty pair of braces or continue, but not a single semicolon.

while (condition) {
  // Repeat test until it returns false.
}
for (int i = 0; i < kSomeNumber; ++i) {}  // Good - one newline is also OK.
while (condition) continue;  // Good - continue indicates no logic.

while (condition);  // Bad - looks like part of do/while loop.

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;
const string& str;

// Fine if helpful for readability.
int x, y;

int x, *y;  // Disallowed - no & or * in multiple declaration
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.

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.

Do not needlessly surround the return expression with parentheses.

Use parentheses in return expr; only where you would use them in x = expr;.

return result;                  // No parentheses in the simple case.
// Parentheses OK to make a complex expression more readable.
return (some_long_condition &&
        another_condition);

return (value);                // You wouldn‘t write var = (value);
return(result);                // return is not a function!

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 a braced initialization list {...} on a type with an std::initializer_list constructor. A nonempty braced-init-list prefers the std::initializer_list constructor whenever possible. Note that empty braces {} are special, and will call a default constructor if available. To force the non-std::initializer_list constructor, use parentheses instead of braces.

std::vector<int> v(100, 1);  // A vector of 100 1s.
std::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.

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();
  }

Sections in public, protected and private order, each indented one space.

The basic format for a class definition (lacking the comments, see Class Comments for a discussion of what comments are needed) is:

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_;
};

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 can be all on one line or with subsequent lines indented four spaces.

The acceptable formats for initializer lists are:

// When everything fits on one line:
MyClass::MyClass(int var) : some_var_(var) {
  DoSomething();
}

// If the signature and initializer list are not all on one line,
// you must wrap before the colon and indent 4 spaces:
MyClass::MyClass(int var)
    : some_var_(var), some_other_var_(var + 1) {
  DoSomething();
}

// When the list spans multiple lines, put each member on its own line
// and align them:
MyClass::MyClass(int var)
    : some_var_(var),             // 4 space indent
      some_other_var_(var + 1) {  // lined up
  DoSomething();
}

// As with any other code block, the close curly can be on the same
// line as the open curly, if it fits.
MyClass::MyClass(int var)
    : some_var_(var) {}

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 {

  // 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 {

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.
// Spaces inside braces for braced-init-list are optional.  If you use them,
// put them on both sides!
int x[] = { 0 };
int x[] = {0};

// 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) {
// Loops and conditions may have spaces inside parentheses, but this
// is rare.  Be consistent.
switch ( i ) {
if ( test ) {
for ( int i = 0; i < 5; ++i ) {
// For loops always have a space after the semicolon.  They may have a space
// before the semicolon, but this is rare.
for ( ; i < 5 ; ++i) {
  ...

// Range-based for loops always have a space before and after the colon.
for (auto x : counts) {
  ...
}
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

// Assignment operators always have spaces around them.
x = 0;

// Other binary operators usually have spaces around them, but it‘s
// OK to remove spaces around factors.  Parentheses should have no
// internal padding.
v = w * x + y / z;
v = w*x + y/z;
v = w * (x + z);

// No spaces separating unary operators and their arguments.
x = -5;
++x;
if (x && !y)
  ...

Templates and Casts

// No spaces inside the angle brackets (< and >), before
// <, or between >( in a cast
std::vector<string> x;
y = static_cast<char*>(x);

// Spaces between type and pointer are OK, but be consistent.
std::vector<char *> x;

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.

Existing Non-conformant 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:

• 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.