Руководство Google по стилю в C++

C++ один из основных языков программирования, используемый в open-source проектах Google. Известно, что C++ очень мощный язык. Вместе с тем это сложный язык и, при неправильном использовании, может быть рассадником багов, затруднить чтение и поддержку кода.

Цель руководства - управлять сложностью кода, описывая в деталях как стоит (или не стоит) писать код на C++. Правила этого руководства упростят управление кодом и увеличат продуктивность кодеров.

Style / Стиль - соглашения, которым следует C++ код. Стиль - это больше, чем форматирование файла с кодом.

Большинство open-source проектов, разрабатываемых Google, соответствуют этому руководству.

Примечание: это руководство не является учебником по C++: предполагается, что вы знакомы с языком.

Цели Руководства по стилю

Зачем нужен этот документ?

Есть несколько основных целей этого документа, внутренних Зачем, лежащих в основе отдельных правил. Используя эти цели можно избежать длинных дискуссий: почему правила такие и зачем им следовать. Если вы понимаете цели каждого правила, то вам легче с ними согласиться или отвергнуть, оценить альтернативы при изменении правил под себя.

Цели руководства следующие::

Правила должны стоить изменений
Преимущества от использования единого стиля должны перевешивать недовольство инженеров по запоминанию и использованию правил. Преимущество оценивается по сравнению с кодовой базой без применения правил, поэтому если ваши люди всё равно не будут применять правила, то выгода будет очень небольшой. Этот принцип объясняет почему некоторые правила отсутствуют: например, goto нарушает многие принципы, однако он практически не используется, поэтому Руководство это не описывает.
Оптимизировано для чтения, не для написания
Наша кодовая база (и большинство отдельных компонентов из неё) будет использоваться продолжительное время. Поэтому, на чтение этого кода будет тратиться существенно больше времени, чем на написание. Мы явно заботимся чтобы нашим инженерам было лего читать, поддерживать, отлаживать код. "Оставляй отладочный/логирующий код" - одно из следствий: когда кусок кода работает "странно" (например, при передаче владения указателем), наличие текстовых подсказок может быть очень полезным (std::unique_ptr явно показывает передачу владения).
Пиши код, похожий на существующий
Использование единого стиля на кодовой базе позволяет переключиться на другие, более важные, вопросы. Также, единый стиль способствует автоматизации. И, конечно, автоформат кода (или выравнивание #include-ов) работает правильно, если он соответствует требованиям утилиты. В остальных случаях из набора правил применяется только одно (наиболее подходящее), а некоторая гибкость в использовании правил позволяет людям меньше спорить.
Пиши код, похожий на используемый в C++ сообщества (по возможности)
Согласованность нашего кода с C++ кодом других организаций и сообществ весьма полезна. Если возможности стандартного C++ или принятые идиомы языка облегчают написание программ, это повод использовать их. Однако, иногда стандарт и идиомы плохо подходят для задачи. В этих случаях (как описано ниже) имеет смысл ограничить или запретить использование некоторых стандартных возможностей. В некоторых случаях создаётся свой решение, но иногда используются внешние библиотеки (вместо стандартной библиотеки C++) и переписывание её под свой стандарт слишком затратно.
Избегайте неожиданных или опасных конструкций
В языке C++ есть неочевидные и даже опасные подходы. Некоторые стили кодирования ограничивают их использование, т.к. их использование несёт большие риски для правильности кода.
Избегайте конструкций, которые средний C++ программист считает заумными и сложно поддерживаемыми
В C++ есть возможности, которые в целом не приветствуются по причине усложнения кода. Однако, в часто используемом коде применение хитрых конструкций более оправданно благодаря многократному использованию, также новые порции кода станут более понятны. В случае сомнений - проконсультируйтесь с лидером проекта. Это очень важно для нашей кодовой базы, т.к. владельцы кода и команда поддержки меняются со временем: даже если сейчас все понимают код, через несколько лет всё может исзмениться.
Учитывайте масштаб кода
С кодовой базой более 100 миллионов строк и тысячами инженеров, ошибки и упрощения могут дорого обойтись. Например, важно избегать замусоривания глобального пространства имён: коллизии имён очень сложно избежать в большой базе кода если всё объявляется в глобальном пространстве имён.
Оптимизируйте по необходимости
Оптимизация производительности иногда важнее, чем следование правилам в кодировании.

Намерение этого документа - обеспечить максимально понятное руководство при разумных ограничениях. Как всегда, здравый смысл никто не отменял. Этой спецификацией мы хотим установить соглашения для всего сообщества Google в C++, не только для отдельных команд или людей. Относитесь со скепсисом к хитрым или необычным конструкциям: отсутствие ограничения не всегда есть разрешение. И, если не можешь решить сам, спроси начальника.

Версия C++

Сейчас код должен соответствовать C++17, т.е. возможности C++2x нежелательны. В дальнейшем, руководство будет корректироваться на более новые версии C++.

Не используйте нестандартные расширения.

Учитывайте совместимость с другим окружением, если собираетесь использовать C++14 and C++17 в свойм проекте.

Заголовочные файлы

Желательно, чтобы каждый .cc файл исходного кода имел парный .h заголовочный файл. Также, есть известные исключения из этого правила, такие как юниттесты или небольшие .cc файлы, содержащие только функцию main().

Правильное использование заголовочных файлов может оказать огромное влияние на читабельность, размер и производительность вашего кода.

Следующие правила позволят избежать частых проблем с заголовочными файлами.

Независимые заголовочники

Заголовочные файлы должны быть самодостаточными (в плане компиляции) и иметь расширение .h. Другие файлы (не заголовочные), предназначенные для включения в код, должны быть с расширением .inc и использоваться в паре с включающим кодом.

Все заголовочные файлы должны быть самодостаточыми. Пользователи и инструменты разработки не должны зависеть от специальных зависимостей при использовании заголовочника. Заголовочник должен иметь блокировка от повторного включения и включать все необходимые файлы.

Предпочтительно размещать определения для шаблонов и inline-функций в одном файле с декларациями. Определения этих конструкций дожны быть включены в каждый .cc файл, использующий их (конструкции), иначе могут быть ошибки линковки на некоторых конфигурациях сборки. Если же декларации и определения находятся в разных файлах, включение одного должно подключать другой. Не выделяйте определения в отдельные заголовочные файлы (-inl.h); Раньше такая практика была очень популярна, сейчас это нежелательно.

Как исключение, если из шаблона создаются все доступные варианты шаблонных аргументов. Или если шаблон реализует функционал, используемый только одним классом - тогда допустимо определять шаблон в одном (и только одном) .cc файле, в котором этот шаблон и используется.

Возможны редкие ситуации, когда заголовочник не самодостаточный. Это может происходить, когда файл подключается в нестандартном месте, например в середине другого файла. В этом случае может отсутствовать блокировка от повторного включения, и дополнительные заголовочники также могут не подключаться. Именуйте такие файлы расширением .inc. Используйте их парой и старайтесь по максимуму соответствовать общим требованиям.

Блокировка от повторного включения

Все заголовочные файлы должны быть с защитой от повторного включения посредством #define. Формат макроопределения должен быть: <PROJECT>_<PATH>_<FILE>_H_.

Для гарантии уникальности, используйте компоненты полного пути к файлу в дереве проекта. Например, файл foo/src/bar/baz.h в проекте foo может иметь следующую блокировку:

#ifndef FOO_BAR_BAZ_H_
#define FOO_BAR_BAZ_H_

...

#endif  // FOO_BAR_BAZ_H_

Предварительное объявление

По возможности, не используйте предварительное объявление. Вместо этого делайте #include необходимых заголовочных файлов.

"Предварительное объявление" - декларация класса, функции, шаблона без соответствующего определения.

Please see Names and Order of Includes for rules about when to #include a header.

Inline Functions

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

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.

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.

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.

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.

Names and Order of Includes

Include headers in the following order: Related header, C system headers, C++ standard library headers, other libraries' headers, your project's headers.

All of a project's header files should be listed as descendants of the project's source directory without use of UNIX directory aliases . (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. A blank line
  3. C system headers (more precisely: headers in angle brackets with the .h extension), e.g. <unistd.h>, <stdlib.h>.
  4. A blank line
  5. C++ standard library headers (without file extension), e.g. <algorithm>, <cstddef>.
  6. A blank line
  7. Other libraries' .h files.
  8. Your project's .h files.

Separate each non-empty group with one blank line.

With the preferred ordering, if the related header 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.

Note that the C headers such as stddef.h are essentially interchangeable with their C++ counterparts (cstddef). Either style is acceptable, but prefer consistency with existing code.

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.

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 <string>
#include <vector>

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

Exception:

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

Scoping

Namespaces

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.

Namespaces subdivide the global scope into distinct, named scopes, and so are useful for preventing name collisions in the global scope.

Namespaces provide a method for preventing name conflicts in large programs while allowing most code to use reasonably short names.

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, and code within each project's namespace can continue to refer to Foo without the prefix.

Inline namespaces automatically place their names in the enclosing scope. Consider the following snippet, for example:

namespace outer {
inline namespace inner {
  void foo();
}  // namespace inner
}  // namespace outer

The expressions outer::inner::foo() and outer::foo() are interchangeable. Inline namespaces are primarily intended for ABI compatibility across versions.

Namespaces can be confusing, because they complicate the mechanics of figuring out what definition a name refers to.

Inline namespaces, in particular, can be confusing because names aren't actually restricted to the namespace where they are declared. They are only useful as part of some larger versioning policy.

In some contexts, it's necessary to repeatedly refer to symbols by their fully-qualified names. For deeply-nested namespaces, this can add a lot of clutter.

Namespaces should be used as follows:

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.

All declarations can be given internal linkage by placing them in unnamed namespaces. Functions and variables can also be given internal linkage by declaring them static. This means that anything you're declaring can't be accessed from another file. If a different file declares something with the same name, then the two entities are completely independent.

Use of internal linkage in .cc files is encouraged for all code that does not need to be referenced elsewhere. Do not use internal linkage in .h files.

Format unnamed namespaces like named namespaces. In the terminating comment, leave the namespace name empty:

namespace {
...
}  // namespace

Nonmember, Static Member, and Global Functions

Prefer placing nonmember functions in a namespace; use completely global functions rarely. Do not use a class simply to group static functions. Static methods of a class should generally be closely related to instances of the class or the class's static data.

Nonmember and static member functions can be useful in some situations. Putting nonmember functions in a namespace avoids polluting the global namespace.

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.

Sometimes it is useful 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. Do not create classes only to group static member functions; this is no different than just giving the function names a common prefix, and such grouping is usually unnecessary anyway.

If you define a nonmember function and it is only needed in its .cc file, use internal linkage to limit its scope.

Local Variables

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

C++ allows you to declare variables anywhere in a function. We encourage you to declare them in as local a scope as possible, and as close to the first use as possible. This makes it easier for the reader to find the declaration and see what type the variable is and what it was initialized to. In particular, initialization should be used instead of declaration and assignment, e.g.:

int i;
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);
}

Static and Global Variables

Objects with static storage duration are forbidden unless they are trivially destructible. Informally this means that the destructor does not do anything, even taking member and base destructors into account. More formally it means that the type has no user-defined or virtual destructor and that all bases and non-static members are trivially destructible. Static function-local variables may use dynamic initialization. Use of dynamic initialization for static class member variables or variables at namespace scope is discouraged, but allowed in limited circumstances; see below for details.

As a rule of thumb: a global variable satisfies these requirements if its declaration, considered in isolation, could be constexpr.

Every object has a storage duration, which correlates with its lifetime. Objects with static storage duration live from the point of their initialization until the end of the program. Such objects appear as variables at namespace scope ("global variables"), as static data members of classes, or as function-local variables that are declared with the static specifier. Function-local static variables are initialized when control first passes through their declaration; all other objects with static storage duration are initialized as part of program start-up. All objects with static storage duration are destroyed at program exit (which happens before unjoined threads are terminated).

Initialization may be dynamic, which means that something non-trivial happens during initialization. (For example, consider a constructor that allocates memory, or a variable that is initialized with the current process ID.) The other kind of initialization is static initialization. The two aren't quite opposites, though: static initialization always happens to objects with static storage duration (initializing the object either to a given constant or to a representation consisting of all bytes set to zero), whereas dynamic initialization happens after that, if required.

Global and static variables are very useful for a large number of applications: named constants, auxiliary data structures internal to some translation unit, command-line flags, logging, registration mechanisms, background infrastructure, etc.

Global and static variables that use dynamic initialization or have non-trivial destructors create complexity that can easily lead to hard-to-find bugs. Dynamic initialization is not ordered across translation units, and neither is destruction (except that destruction happens in reverse order of initialization). When one initialization refers to another variable with static storage duration, it is possible that this causes an object to be accessed before its lifetime has begun (or after its lifetime has ended). Moreover, when a program starts threads that are not joined at exit, those threads may attempt to access objects after their lifetime has ended if their destructor has already run.

Decision on destruction

When destructors are trivial, their execution is not subject to ordering at all (they are effectively not "run"); otherwise we are exposed to the risk of accessing objects after the end of their lifetime. Therefore, we only allow objects with static storage duration if they are trivially destructible. Fundamental types (like pointers and int) are trivially destructible, as are arrays of trivially destructible types. Note that variables marked with constexpr are trivially destructible.

const int kNum = 10;  // allowed

struct X { int n; };
const X kX[] = {{1}, {2}, {3}};  // allowed

void foo() {
  static const char* const kMessages[] = {"hello", "world"};  // allowed
}

// allowed: constexpr guarantees trivial destructor
constexpr std::array<int, 3> kArray = {{1, 2, 3}};
// bad: non-trivial destructor
const std::string kFoo = "foo";

// bad for the same reason, even though kBar is a reference (the
// rule also applies to lifetime-extended temporary objects)
const std::string& kBar = StrCat("a", "b", "c");

void bar() {
  // bad: non-trivial destructor
  static std::map<int, int> kData = {{1, 0}, {2, 0}, {3, 0}};
}

Note that references are not objects, and thus they are not subject to the constraints on destructibility. The constraint on dynamic initialization still applies, though. In particular, a function-local static reference of the form static T& t = *new T; is allowed.

Decision on initialization

Initialization is a more complex topic. This is because we must not only consider whether class constructors execute, but we must also consider the evaluation of the initializer:

int n = 5;    // fine
int m = f();  // ? (depends on f)
Foo x;        // ? (depends on Foo::Foo)
Bar y = g();  // ? (depends on g and on Bar::Bar)

All but the first statement expose us to indeterminate initialization ordering.

The concept we are looking for is called constant initialization in the formal language of the C++ standard. It means that the initializing expression is a constant expression, and if the object is initialized by a constructor call, then the constructor must be specified as constexpr, too:

struct Foo { constexpr Foo(int) {} };

int n = 5;  // fine, 5 is a constant expression
Foo x(2);   // fine, 2 is a constant expression and the chosen constructor is constexpr
Foo a[] = { Foo(1), Foo(2), Foo(3) };  // fine

Constant initialization is always allowed. Constant initialization of static storage duration variables should be marked with constexpr or where possible the ABSL_CONST_INIT attribute. Any non-local static storage duration variable that is not so marked should be presumed to have dynamic initialization, and reviewed very carefully.

By contrast, the following initializations are problematic:

// Some declarations used below.
time_t time(time_t*);      // not constexpr!
int f();                   // not constexpr!
struct Bar { Bar() {} };

// Problematic initializations.
time_t m = time(nullptr);  // initializing expression not a constant expression
Foo y(f());                // ditto
Bar b;                     // chosen constructor Bar::Bar() not constexpr

Dynamic initialization of nonlocal variables is discouraged, and in general it is forbidden. However, we do permit it if no aspect of the program depends on the sequencing of this initialization with respect to all other initializations. Under those restrictions, the ordering of the initialization does not make an observable difference. For example:

int p = getpid();  // allowed, as long as no other static variable
                   // uses p in its own initialization

Dynamic initialization of static local variables is allowed (and common).

Common patterns

thread_local Variables

thread_local variables that aren't declared inside a function must be initialized with a true compile-time constant, and this must be enforced by using the ABSL_CONST_INIT attribute. Prefer thread_local over other ways of defining thread-local data.

Starting with C++11, variables can be declared with the thread_local specifier:

thread_local Foo foo = ...;

Such a variable is actually a collection of objects, so that when different threads access it, they are actually accessing different objects. thread_local variables are much like static storage duration variables in many respects. For instance, they can be declared at namespace scope, inside functions, or as static class members, but not as ordinary class members.

thread_local variable instances are initialized much like static variables, except that they must be initialized separately for each thread, rather than once at program startup. This means that thread_local variables declared within a function are safe, but other thread_local variables are subject to the same initialization-order issues as static variables (and more besides).

thread_local variable instances are destroyed when their thread terminates, so they do not have the destruction-order issues of static variables.

thread_local variables inside a function have no safety concerns, so they can be used without restriction. Note that you can use a function-scope thread_local to simulate a class- or namespace-scope thread_local by defining a function or static method that exposes it:

Foo& MyThreadLocalFoo() {
  thread_local Foo result = ComplicatedInitialization();
  return result;
}

thread_local variables at class or namespace scope must be initialized with a true compile-time constant (i.e. they must have no dynamic initialization). To enforce this, thread_local variables at class or namespace scope must be annotated with ABSL_CONST_INIT (or constexpr, but that should be rare):

ABSL_CONST_INIT thread_local Foo foo = ...;

thread_local should be preferred over other mechanisms for defining thread-local data.

Classes

Classes are the fundamental unit of code in C++. Naturally, we use them extensively. This section lists the main dos and don'ts you should follow when writing a class.

Doing Work in Constructors

Avoid virtual method calls in constructors, and avoid initialization that can fail if you can't signal an error.

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

Constructors should never call virtual functions. If appropriate for your code , terminating the program may be an appropriate error handling response. Otherwise, consider a factory function or Init() method as described in TotW #42 . Avoid Init() methods on objects with no other states that affect which public methods may be called (semi-constructed objects of this form are particularly hard to work with correctly).

Implicit Conversions

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

Implicit conversions allow an object of one type (called the source type) to be used where a different type (called the destination type) is expected, such as when passing an int argument to a function that takes a double parameter.

In addition to the implicit conversions defined by the language, users can define their own, by adding appropriate members to the class definition of the source or destination type. An implicit conversion in the source type is defined by a type conversion operator named after the destination type (e.g. operator bool()). An implicit conversion in the destination type is defined by a constructor that can take the source type as its only argument (or only argument with no default value).

The explicit keyword can be applied to a constructor or (since C++11) a conversion operator, to ensure that it can only be used when the destination type is explicit at the point of use, e.g. with a cast. This applies not only to implicit conversions, but to C++11's list initialization syntax:

class Foo {
  explicit Foo(int x, double y);
  ...
};

void Func(Foo f);
Func({42, 3.14});  // Error
This kind of code isn't technically an implicit conversion, but the language treats it as one as far as explicit is concerned.

Type conversion operators, and constructors that are callable with a single argument, must be marked explicit in the class definition. As an exception, copy and move constructors should not be explicit, since they do not perform type conversion. Implicit conversions can sometimes be necessary and appropriate for types that are designed to transparently wrap other types. In that case, contact your project leads to request a waiver of this rule.

Constructors that cannot be called with a single argument may omit explicit. Constructors that take a single std::initializer_list parameter should also omit explicit, in order to support copy-initialization (e.g. MyType m = {1, 2};).

Copyable and Movable Types

A class's public API must make clear whether the class is copyable, move-only, or neither copyable nor movable. Support copying and/or moving if these operations are clear and meaningful for your type.

A movable type is one that can be initialized and assigned from temporaries.

A copyable type is one that can be initialized or assigned from any other object of the same type (so is also movable by definition), with the stipulation that the value of the source does not change. std::unique_ptr<int> is an example of a movable but not copyable type (since the value of the source std::unique_ptr<int> must be modified during assignment to the destination). int and std::string are examples of movable types that are also copyable. (For int, the move and copy operations are the same; for std::string, there exists a move operation that is less expensive than a copy.)

For user-defined types, the copy behavior is defined by the copy constructor and the copy-assignment operator. Move behavior is defined by the move constructor and the move-assignment operator, if they exist, or by the copy constructor and the copy-assignment operator otherwise.

The copy/move constructors can be implicitly invoked by the compiler in some situations, e.g. when passing objects by value.

Objects of copyable and movable types can be passed and returned by value, which makes APIs simpler, safer, and more general. Unlike when passing objects by pointer or reference, there's no risk of confusion over ownership, lifetime, mutability, and similar issues, and no need to specify them in the contract. It also prevents non-local interactions between the client and the implementation, which makes them easier to understand, maintain, and optimize by the compiler. Further, such objects can be used with generic APIs that require pass-by-value, such as most containers, and they allow for additional flexibility in e.g., type composition.

Copy/move constructors and assignment operators are usually easier to define correctly than alternatives like Clone(), CopyFrom() or Swap(), because they can be generated by the compiler, either implicitly or with = default. They are concise, and ensure that all data members are copied. Copy and move constructors are also generally more efficient, because they don't require heap allocation or separate initialization and assignment steps, and they're eligible for optimizations such as copy elision.

Move operations allow the implicit and efficient transfer of resources out of rvalue objects. This allows a plainer coding style in some cases.

Some types do not need to be copyable, and providing copy operations for such types can be confusing, nonsensical, or outright incorrect. Types representing singleton objects (Registerer), objects tied to a specific scope (Cleanup), or closely coupled to object identity (Mutex) cannot be copied meaningfully. Copy operations for base class types that are to be used polymorphically are hazardous, because use of them can lead to object slicing. Defaulted or carelessly-implemented copy operations can be incorrect, and the resulting bugs can be confusing and difficult to diagnose.

Copy constructors are invoked implicitly, which makes the invocation easy to miss. This may cause confusion for programmers used to languages where pass-by-reference is conventional or mandatory. It may also encourage excessive copying, which can cause performance problems.

Every class's public interface must make clear which copy and move operations the class supports. This should usually take the form of explicitly declaring and/or deleting the appropriate operations in the public section of the declaration.

Specifically, a copyable class should explicitly declare the copy operations, a move-only class should explicitly declare the move operations, and a non-copyable/movable class should explicitly delete the copy operations. Explicitly declaring or deleting all four copy/move operations is permitted, but not required. If you provide a copy or move assignment operator, you must also provide the corresponding constructor.

class Copyable {
 public:
  Copyable(const Copyable& other) = default;
  Copyable& operator=(const Copyable& other) = default;

  // The implicit move operations are suppressed by the declarations above.
};

class MoveOnly {
 public:
  MoveOnly(MoveOnly&& other);
  MoveOnly& operator=(MoveOnly&& other);

  // The copy operations are implicitly deleted, but you can
  // spell that out explicitly if you want:
  MoveOnly(const MoveOnly&) = delete;
  MoveOnly& operator=(const MoveOnly&) = delete;
};

class NotCopyableOrMovable {
 public:
  // Not copyable or movable
  NotCopyableOrMovable(const NotCopyableOrMovable&) = delete;
  NotCopyableOrMovable& operator=(const NotCopyableOrMovable&)
      = delete;

  // The move operations are implicitly disabled, but you can
  // spell that out explicitly if you want:
  NotCopyableOrMovable(NotCopyableOrMovable&&) = delete;
  NotCopyableOrMovable& operator=(NotCopyableOrMovable&&)
      = delete;
};

These declarations/deletions can be omitted only if they are obvious:

A type should not be copyable/movable if the meaning of copying/moving is unclear to a casual user, or if it incurs unexpected costs. Move operations for copyable types are strictly a performance optimization and are a potential source of bugs and complexity, so avoid defining them unless they are significantly more efficient than the corresponding copy operations. If your type provides copy operations, it is recommended that you design your class so that the default implementation of those operations is correct. Remember to review the correctness of any defaulted operations as you would any other code.

Due to the risk of slicing, prefer to avoid providing a public assignment operator or copy/move constructor for a class that's intended to be derived from (and prefer to avoid deriving from a class with such members). If your base class needs to be copyable, provide a public virtual Clone() method, and a protected copy constructor that derived classes can use to implement it.

Structs vs. Classes

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. All fields must be public, and accessed directly rather than through getter/setter methods. The struct must not have invariants that imply relationships between different fields, since direct user access to those fields may break those invariants. Methods should not provide behavior but should only be used to set up the data members, e.g., constructor, destructor, Initialize(), Reset().

If more functionality or invariants are 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 stateless types, such as traits, template metafunctions, and some functors.

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

Structs vs. Pairs and Tuples

Prefer to use a struct instead of a pair or a tuple whenever the elements can have meaningful names.

While using pairs and tuples can avoid the need to define a custom type, potentially saving work when writing code, a meaningful field name will almost always be much clearer when reading code than .first, .second, or std::get<X>. While C++14's introduction of std::get<Type> to access a tuple element by type rather than index (when the type is unique) can sometimes partially mitigate this, a field name is usually substantially clearer and more informative than a type.

Pairs and tuples may be appropriate in generic code where there are not specific meanings for the elements of the pair or tuple. Their use may also be required in order to interoperate with existing code or APIs.

Inheritance

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

When a sub-class inherits from a base class, it includes the definitions of all the data and operations that the base class defines. "Interface inheritance" is inheritance from a pure abstract base class (one with no state or defined methods); all other inheritance is "implementation inheritance".

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.

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.

Multiple inheritance is especially problematic, because it often imposes a higher performance overhead (in fact, the performance drop from single inheritance to multiple inheritance can often be greater than the performance drop from ordinary to virtual dispatch), and because it risks leading to "diamond" inheritance patterns, which are prone to ambiguity, confusion, and outright bugs.

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.

Limit the use of protected to those member functions that might need to be accessed from subclasses. Note that data members should be private.

Explicitly annotate overrides of virtual functions or virtual destructors with exactly one of an override or (less frequently) final specifier. Do not use virtual when declaring an override. Rationale: A function or destructor marked override or final that is not an override of a base class virtual function will not compile, and this helps catch common errors. The specifiers serve as documentation; if no specifier is present, the reader has to check all ancestors of the class in question to determine if the function or destructor is virtual or not.

Multiple inheritance is permitted, but multiple implementation inheritance is strongly discouraged.

Operator Overloading

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

C++ permits user code to declare overloaded versions of the built-in operators using the operator keyword, so long as one of the parameters is a user-defined type. The operator keyword also permits user code to define new kinds of literals using operator"", and to define type-conversion functions such as operator bool().

Operator overloading can make code more concise and intuitive by enabling user-defined types to behave the same as built-in types. Overloaded operators are the idiomatic names for certain operations (e.g. ==, <, =, and <<), and adhering to those conventions can make user-defined types more readable and enable them to interoperate with libraries that expect those names.

User-defined literals are a very concise notation for creating objects of user-defined types.

Define overloaded operators only if their meaning is obvious, unsurprising, and consistent with the corresponding built-in operators. For example, use | as a bitwise- or logical-or, not as a shell-style pipe.

Define operators only on your own types. More precisely, define them in the same headers, .cc files, and namespaces as the types they operate on. That way, the operators are available wherever the type is, minimizing the risk of multiple definitions. If possible, avoid defining operators as templates, because they must satisfy this rule for any possible template arguments. If you define an operator, also define any related operators that make sense, and make sure they are defined consistently. For example, if you overload <, overload all the comparison operators, and make sure < and > never return true for the same arguments.

Prefer to define non-modifying binary operators as non-member functions. If a binary operator is defined as a class member, implicit conversions will apply to the right-hand argument, but not the left-hand one. It will confuse your users if a < b compiles but b < a doesn't.

Don't go out of your way to avoid defining operator overloads. For example, prefer to define ==, =, and <<, rather than Equals(), CopyFrom(), and PrintTo(). Conversely, don't define operator overloads just because other libraries expect them. For example, if your type doesn't have a natural ordering, but you want to store it in a std::set, use a custom comparator rather than overloading <.

Do not overload &&, ||, , (comma), or unary &. Do not overload operator"", i.e. do not introduce user-defined literals. Do not use any such literals provided by others (including the standard library).

Type conversion operators are covered in the section on implicit conversions. The = operator is covered in the section on copy constructors. Overloading << for use with streams is covered in the section on streams. See also the rules on function overloading, which apply to operator overloading as well.

Access Control

Make classes' data members private, unless they are constants. This simplifies reasoning about invariants, at the cost of some easy boilerplate in the form of accessors (usually const) if necessary.

For technical reasons, we allow data members of a test fixture class in a .cc file to be protected when using Google Test).

Declaration Order

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.

Functions

Output Parameters

The output of a C++ function is naturally provided via a return value and sometimes via output parameters.

Prefer using return values over output parameters: they improve readability, and often provide the same or better performance. If output-only parameters are used, they should appear after input parameters.

Parameters 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.

Write Short Functions

Prefer small and focused functions.

We recognize that long functions are sometimes appropriate, so no hard limit is placed on functions length. If a function exceeds about 40 lines, think about whether it can be broken up without harming the structure of the program.

Even if your long function works perfectly now, someone modifying it in a few months may add new behavior. This could result in bugs that are hard to find. Keeping your functions short and simple makes it easier for other people to read and modify your code. Small functions are also easier to test.

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.

Reference Arguments

All parameters passed by lvalue reference must be labeled const.

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).

Defining a parameter as reference avoids ugly code like (*pval)++. Necessary for some applications like copy constructors. Makes it clear, unlike with pointers, that a null pointer is not a possible value.

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

Within function parameter lists all references must be const:

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

In fact it is a very strong convention in Google code that input arguments are values or const references while output arguments are pointers. Input parameters may be const pointers, but we never allow non-const reference parameters except when required by convention, e.g., swap().

However, there are some instances where using const T* is preferable to const T& for input parameters. For example:

Remember that most of the time input parameters are going to be specified as const T&. Using const T* instead communicates to the reader that the input is somehow treated differently. So if you choose const T* rather than const T&, do so for a concrete reason; otherwise it will likely confuse readers by making them look for an explanation that doesn't exist.

Function Overloading

Use overloaded functions (including constructors) only if a reader looking at a call site can get a good idea of what is happening without having to first figure out exactly which overload is being called.

You may write a function that takes a const std::string& and overload it with another that takes const char*. However, in this case consider std::string_view instead.

class MyClass {
 public:
  void Analyze(const std::string &text);
  void Analyze(const char *text, size_t textlen);
};

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.

Overloading based on const or ref qualification may make utility code more usable, more efficient, or both. (See TotW 148 for more.)

If a function is overloaded by the argument types alone, a reader may have to understand C++'s complex matching rules in order to tell what's going on. Also many people are confused by the semantics of inheritance if a derived class overrides only some of the variants of a function.

You may overload a function when there are no semantic differences between variants. These overloads may vary in types, qualifiers, or argument count. However, a reader of such a call must not need to know which member of the overload set is chosen, only that something from the set is being called. If you can document all entries in the overload set with a single comment in the header, that is a good sign that it is a well-designed overload set.

Default Arguments

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.

Often you have a function that uses default values, but occasionally you want to override the defaults. Default parameters allow an easy way to do this without having to define many functions for the rare exceptions. Compared to overloading the function, default arguments have a cleaner syntax, with less boilerplate and a clearer distinction between 'required' and 'optional' arguments.

Defaulted arguments are another way to achieve the semantics of overloaded functions, so all the reasons not to overload functions apply.

The defaults for arguments in a virtual function call are determined by the static type of the target object, and there's no guarantee that all overrides of a given function declare the same defaults.

Default parameters are re-evaluated at each call site, which can bloat the generated code. Readers may also expect the default's value to be fixed at the declaration instead of varying at each call.

Function pointers are confusing in the presence of default arguments, since the function signature often doesn't match the call signature. Adding function overloads avoids these problems.

Default arguments are banned on virtual functions, where they don't work properly, and in cases where the specified default might not evaluate to the same value depending on when it was evaluated. (For example, don't write void f(int n = counter++);.)

In some other cases, default arguments can improve the readability of their function declarations enough to overcome the downsides above, so they are allowed. When in doubt, use overloads.

Trailing Return Type Syntax

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 newer 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.

Trailing return types are the only way to explicitly specify the return type of a lambda expression. In some cases the compiler is able to deduce a lambda's return type, but not in all cases. Even when the compiler can deduce it automatically, sometimes specifying it explicitly would be clearer for readers.

Sometimes it's easier and more readable to specify a return type after the function's parameter list has already appeared. This is particularly true when the return type depends on template parameters. For example:

    template <typename T, typename U>
    auto add(T t, U u) -> decltype(t + u);
  
versus
    template <typename T, typename U>
    decltype(declval<T&>() + declval<U&>()) add(T t, U u);
  

Trailing return type syntax is relatively new and it has no analogue in C++-like languages such as C and Java, so some readers may find it unfamiliar.

Existing code bases have an enormous number of function declarations that aren't going to get changed to use the new syntax, so the realistic choices are using the old syntax only or using a mixture of the two. Using a single version is better for uniformity of style.

In most cases, continue to use the older style of function declaration where the return type goes before the function name. Use the new trailing-return-type form only in cases where it's required (such as lambdas) or where, by putting the type after the function's parameter list, it allows you to write the type in a much more readable way. The latter case should be rare; it's mostly an issue in fairly complicated template code, which is discouraged in most cases.

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.

Ownership and Smart Pointers

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

"Ownership" is a bookkeeping technique for managing dynamically allocated memory (and other resources). The owner of a dynamically allocated object is an object or function that is responsible for ensuring that it is deleted when no longer needed. Ownership can sometimes be shared, in which case the last owner is typically responsible for deleting it. Even when ownership is not shared, it can be transferred from one piece of code to another.

"Smart" pointers are classes that act like pointers, e.g. by overloading the * and -> operators. Some smart pointer types can be used to automate ownership bookkeeping, to ensure these responsibilities are met. std::unique_ptr is a smart pointer type introduced in C++11, which expresses exclusive ownership of a dynamically allocated object; the object is deleted when the std::unique_ptr goes out of scope. It cannot be copied, but can be moved to represent ownership transfer. std::shared_ptr is a smart pointer type that expresses shared ownership of a dynamically allocated object. std::shared_ptrs can be copied; ownership of the object is shared among all copies, and the object is deleted when the last std::shared_ptr is destroyed.

If dynamic allocation is necessary, prefer to keep ownership with the code that allocated it. If other code needs access to the object, consider passing it a copy, or passing a pointer or reference without transferring ownership. Prefer to use std::unique_ptr to make ownership transfer explicit. For example:

std::unique_ptr<Foo> FooFactory();
void FooConsumer(std::unique_ptr<Foo> ptr);

Do not design your code to use shared ownership without a very good reason. One such reason is to avoid expensive copy operations, but you should only do this if the performance benefits are significant, and the underlying object is immutable (i.e. std::shared_ptr<const Foo>). If you do use shared ownership, prefer to use std::shared_ptr.

Never use std::auto_ptr. Instead, use std::unique_ptr.

cpplint

Use cpplint.py to detect style errors.

cpplint.py is a tool that reads a source file and identifies many style errors. It is not perfect, and has both false positives and false negatives, but it is still a valuable tool. False positives can be ignored by putting // NOLINT at the end of the line 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.

Other C++ Features

Rvalue References

Use rvalue references to:

Rvalue references are a type of reference that can only bind to temporary objects. The syntax is similar to traditional reference syntax. For example, void f(std::string&& s); declares a function whose argument is an rvalue reference to a std::string.

When the token '&&' is applied to an unqualified template argument in a function parameter, special template argument deduction rules apply. Such a reference is called forwarding reference.

You may use rvalue references to define move constructors and move assignment operators (as described in Copyable and Movable Types). See the C++ Primer for more information about move semantics and std::move.

You may use rvalue references to define pairs of overloads, one taking Foo&& and the other taking const Foo&. Usually the preferred solution is just to pass by value, but an overloaded pair of functions sometimes yields better performance and is sometimes necessary in generic code that needs to support a wide variety of types. As always: if you're writing more complicated code for the sake of performance, make sure you have evidence that it actually helps.

You may use forwarding references in conjunction with std::forward, to support perfect forwarding.

Friends

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

Friends should usually be defined in the same file so that the reader does not have to look in another file to find uses of the private members of a class. A common use of friend is to have a FooBuilder class be a friend of Foo so that it can construct the inner state of Foo correctly, without exposing this state to the world. In some cases it may be useful to make a unittest class a friend of the class it tests.

Friends extend, but do not break, the encapsulation boundary of a class. In some cases this is better than making a member public when you want to give only one other class access to it. However, most classes should interact with other classes solely through their public members.

Exceptions

We do not use C++ exceptions.

On their face, the benefits of using exceptions outweigh the costs, especially in new projects. However, for existing code, the introduction of exceptions has implications on all dependent code. If exceptions can be propagated beyond a new project, it also becomes problematic to integrate the new project into existing exception-free code. Because most existing C++ code at Google is not prepared to deal with exceptions, it is comparatively difficult to adopt new code that generates exceptions.

Given that Google's existing code is not exception-tolerant, the costs of using exceptions are somewhat greater than the costs in a new project. The conversion process would be slow and error-prone. We don't believe that the available alternatives to exceptions, such as error codes and assertions, introduce a significant burden.

Our advice against using exceptions is not predicated on philosophical or moral grounds, but practical ones. Because we'd like to use our open-source projects at Google and it's difficult to do so if those projects use exceptions, we need to advise against exceptions in Google open-source projects as well. Things would probably be different if we had to do it all over again from scratch.

This prohibition also applies to the exception handling related features added in C++11, such as std::exception_ptr and std::nested_exception.

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

noexcept

Specify noexcept when it is useful and correct.

The noexcept specifier is used to specify whether a function will throw exceptions or not. If an exception escapes from a function marked noexcept, the program crashes via std::terminate.

The noexcept operator performs a compile-time check that returns true if an expression is declared to not throw any exceptions.

You may use noexcept when it is useful for performance if it accurately reflects the intended semantics of your function, i.e. that if an exception is somehow thrown from within the function body then it represents a fatal error. You can assume that noexcept on move constructors has a meaningful performance benefit. If you think there is significant performance benefit from specifying noexcept on some other function, please discuss it with your project leads.

Prefer unconditional noexcept if exceptions are completely disabled (i.e. most Google C++ environments). Otherwise, use conditional noexcept specifiers with simple conditions, in ways that evaluate false only in the few cases where the function could potentially throw. The tests might include type traits check on whether the involved operation might throw (e.g. std::is_nothrow_move_constructible for move-constructing objects), or on whether allocation can throw (e.g. absl::default_allocator_is_nothrow for standard default allocation). Note in many cases the only possible cause for an exception is allocation failure (we believe move constructors should not throw except due to allocation failure), and there are many applications where it’s appropriate to treat memory exhaustion as a fatal error rather than an exceptional condition that your program should attempt to recover from. Even for other potential failures you should prioritize interface simplicity over supporting all possible exception throwing scenarios: instead of writing a complicated noexcept clause that depends on whether a hash function can throw, for example, simply document that your component doesn’t support hash functions throwing and make it unconditionally noexcept.

Run-Time Type Information (RTTI)

Avoid using Run Time Type Information (RTTI).

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

The standard alternatives to RTTI (described below) require modification or redesign of the class hierarchy in question. Sometimes such modifications are infeasible or undesirable, particularly in widely-used or mature code.

RTTI can be useful in some unit tests. For example, it is useful in tests of factory classes where the test has to verify that a newly created object has the expected dynamic type. It is also useful in managing the relationship between objects and their mocks.

RTTI is useful when considering multiple abstract objects. Consider

bool Base::Equal(Base* other) = 0;
bool Derived::Equal(Base* other) {
  Derived* that = dynamic_cast<Derived*>(other);
  if (that == nullptr)
    return false;
  ...
}

Querying the type of an object at run-time frequently means a design problem. Needing to know the type of an object at runtime is often an indication that the design of your class hierarchy is flawed.

Undisciplined use of RTTI makes code hard to maintain. It can lead to type-based decision trees or switch statements scattered throughout the code, all of which must be examined when making further changes.

RTTI has legitimate uses but is prone to abuse, so you must be careful when using it. You may use it freely in unittests, but avoid it when possible in other code. In particular, think twice before using RTTI in new code. If you find yourself needing to write code that behaves differently based on the class of an object, consider one of the following alternatives to querying the type:

When the logic of a program guarantees that a given instance of a base class is in fact an instance of a particular derived class, then a dynamic_cast may be used freely on the object. Usually one can use a static_cast as an alternative in such situations.

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

if (typeid(*data) == typeid(D1)) {
  ...
} else if (typeid(*data) == typeid(D2)) {
  ...
} else if (typeid(*data) == typeid(D3)) {
...

Code such as this usually breaks when additional subclasses are added to the class hierarchy. Moreover, when properties of a subclass change, it is difficult to find and modify all the affected code segments.

Do not hand-implement an RTTI-like workaround. The arguments against RTTI apply just as much to workarounds like class hierarchies with type tags. Moreover, workarounds disguise your true intent.

Casting

Use C++-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).

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

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"). Brace initialization and C++ casts can often help avoid this ambiguity. Additionally, C++ casts are more visible when searching for them.

The C++-style cast syntax is verbose and cumbersome.

Do not use C-style casts. Instead, use these C++-style casts when explicit type conversion is necessary.

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

Streams

Use streams where appropriate, and stick to "simple" usages. Overload << for streaming only for types representing values, and write only the user-visible value, not any implementation details.

Streams are the standard I/O abstraction in C++, as exemplified by the standard header <iostream>. They are widely used in Google code, mostly for debug logging and test diagnostics.

The << and >> stream operators provide an API for formatted I/O that is easily learned, portable, reusable, and extensible. printf, by contrast, doesn't even support std::string, to say nothing of user-defined types, and is very difficult to use portably. printf also obliges you to choose among the numerous slightly different versions of that function, and navigate the dozens of conversion specifiers.

Streams provide first-class support for console I/O via std::cin, std::cout, std::cerr, and std::clog. The C APIs do as well, but are hampered by the need to manually buffer the input.

Use streams only when they are the best tool for the job. This is typically the case when the I/O is ad-hoc, local, human-readable, and targeted at other developers rather than end-users. Be consistent with the code around you, and with the codebase as a whole; if there's an established tool for your problem, use that tool instead. In particular, logging libraries are usually a better choice than std::cerr or std::clog for diagnostic output, and the libraries in absl/strings or the equivalent are usually a better choice than std::stringstream.

Avoid using streams for I/O that faces external users or handles untrusted data. Instead, find and use the appropriate templating libraries to handle issues like internationalization, localization, and security hardening.

If you do use streams, avoid the stateful parts of the streams API (other than error state), such as imbue(), xalloc(), and register_callback(). Use explicit formatting functions (see e.g. absl/strings) rather than stream manipulators or formatting flags to control formatting details such as number base, precision, or padding.

Overload << as a streaming operator for your type only if your type represents a value, and << writes out a human-readable string representation of that value. Avoid exposing implementation details in the output of <<; if you need to print object internals for debugging, use named functions instead (a method named DebugString() is the most common convention).

Preincrement and Predecrement

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

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).

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?

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.

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

In APIs, use const whenever it makes sense. constexpr is a better choice for some uses of const.

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; };).

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.

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.

We strongly recommend using const in APIs (i.e. on function parameters, methods, and non-local variables) wherever it is meaningful and accurate. This provides consistent, mostly compiler-verified documentation of what objects an operation can mutate. Having a consistent and reliable way to distinguish reads from writes is critical to writing thread-safe code, and is useful in many other contexts as well. In particular:

Using const on local variables is neither encouraged nor discouraged.

All of a class's const operations should be safe to invoke concurrently with each other. If that's not feasible, the class must be clearly documented as "thread-unsafe".

Where to put the const

Some people favor the form int const *foo to const int* foo. They argue that this is more readable because it's more consistent: it keeps the rule that const always follows the object it's describing. However, this consistency argument doesn't apply in codebases with few deeply-nested pointer expressions since most const expressions have only one const, and it applies to the underlying value. In such cases, there's no consistency to maintain. Putting the const first is arguably more readable, since it follows English in putting the "adjective" (const) before the "noun" (int).

That said, while we encourage putting const first, we do not require it. But be consistent with the code around you!

Use of constexpr

Use constexpr to define true constants or to ensure constant initialization.

Some variables can be declared constexpr to indicate the variables are true constants, i.e. fixed at compilation/link time. Some functions and constructors can be declared constexpr which enables them to be used in defining a constexpr variable.

Use of constexpr enables definition of constants with floating-point expressions rather than just literals; definition of constants of user-defined types; and definition of constants with function calls.

Prematurely marking something as constexpr may cause migration problems if later on it has to be downgraded. Current restrictions on what is allowed in constexpr functions and constructors may invite obscure workarounds in these definitions.

constexpr definitions enable a more robust specification of the constant parts of an interface. Use constexpr to specify true constants and the functions that support their definitions. Avoid complexifying function definitions to enable their use with constexpr. Do not use constexpr to force inlining.

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 <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.

C++ does not specify the sizes of integer types like int. Typically people assume that short is 16 bits, int is 32 bits, long is 32 bits and long long is 64 bits.

Uniformity of declaration.

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

<cstdint> defines types like int16_t, uint32_t, int64_t, etc. You should always use those in preference to short, unsigned long long and the like, when you need a guarantee on the size of an integer. Of the C integer types, only int should be used. When appropriate, you are welcome to use standard types like size_t and ptrdiff_t.

We use int very often, for integers we know are not going to be too big, e.g., loop counters. Use plain old int for such things. You should assume that an int is at least 32 bits, but don't assume that it has more than 32 bits. If you need a 64-bit integer type, use int64_t or uint64_t.

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

You should not use the unsigned integer types such as uint32_t, unless there is a valid reason such as representing a bit pattern rather than a number, or you need defined overflow modulo 2^N. In particular, do not use unsigned types to say a number will never be negative. Instead, use assertions for this.

If your code is a container that returns a size, be sure to use a type that will accommodate any possible usage of your container. When in doubt, use a larger type rather than a smaller type.

Use care when converting integer types. Integer conversions and promotions can cause undefined behavior, leading to security bugs and other problems.

On Unsigned Integers

Unsigned integers are good for representing bitfields and modular arithmetic. Because of historical accident, the C++ standard also uses unsigned integers to represent the size of containers - many members of the standards body believe this to be a mistake, but it is effectively impossible to fix at this point. The fact that unsigned arithmetic doesn't model the behavior of a simple integer, but is instead defined by the standard to model modular arithmetic (wrapping around on overflow/underflow), means that a significant class of bugs cannot be diagnosed by the compiler. In other cases, the defined behavior impedes optimization.

That said, mixing signedness of integer types is responsible for an equally large class of problems. The best advice we can provide: try to use iterators and containers rather than pointers and sizes, try not to mix signedness, and try to avoid unsigned types (except for representing bitfields or modular arithmetic). Do not use an unsigned type merely to assert that a variable is non-negative.

64-bit Portability

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

Preprocessor Macros

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:

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).

0 and nullptr/NULL

Use nullptr for pointers, and '\0' for chars (and not the 0 literal).

For pointers (address values), use nullptr, as this provides type-safety.

For C++03 projects, prefer NULL to 0. While the values are equivalent, NULL looks more like a pointer to the reader, and some C++ compilers provide special definitions of NULL which enable them to give useful warnings. Never use NULL for numeric (integer or floating-point) values.

Use '\0' for the null character. Using the correct type makes the code more readable.

sizeof

Prefer sizeof(varname) to sizeof(type).

Use sizeof(varname) when you take the size of a particular variable. sizeof(varname) will update appropriately if someone changes the variable type either now or later. You may use sizeof(type) for code unrelated to any particular variable, such as code that manages an external or internal data format where a variable of an appropriate C++ type is not convenient.

struct data;
memset(&data, 0, sizeof(data));
memset(&data, 0, sizeof(Struct));
if (raw_size < sizeof(int)) {
  LOG(ERROR) << "compressed record not big enough for count: " << raw_size;
  return false;
}

Type deduction

Use type deduction only if it makes the code clearer to readers who aren't familiar with the project, or if it makes the code safer. Do not use it merely to avoid the inconvenience of writing an explicit type.

There are several contexts in which C++ allows (or even requires) types to be deduced by the compiler, rather than spelled out explicitly in the code:

Function template argument deduction
A function template can be invoked without explicit template arguments. The compiler deduces those arguments from the types of the function arguments:
template <typename T>
void f(T t);

f(0);  // Invokes f<int>(0)
auto variable declarations
A variable declaration can use the auto keyword in place of the type. The compiler deduces the type from the variable's initializer, following the same rules as function template argument deduction with the same initializer (so long as you don't use curly braces instead of parentheses).
auto a = 42;  // a is an int
auto& b = a;  // b is an int&
auto c = b;   // c is an int
auto d{42};   // d is an int, not a std::initializer_list<int>
auto can be qualified with const, and can be used as part of a pointer or reference type, but it can't be used as a template argument. A rare variant of this syntax uses decltype(auto) instead of auto, in which case the deduced type is the result of applying decltype to the initializer.
Function return type deduction
auto (and decltype(auto)) can also be used in place of a function return type. The compiler deduces the return type from the return statements in the function body, following the same rules as for variable declarations:
auto f() { return 0; }  // The return type of f is int
Lambda expression return types can be deduced in the same way, but this is triggered by omitting the return type, rather than by an explicit auto. Confusingly, trailing return type syntax for functions also uses auto in the return-type position, but that doesn't rely on type deduction; it's just an alternate syntax for an explicit return type.
Generic lambdas
A lambda expression can use the auto keyword in place of one or more of its parameter types. This causes the lambda's call operator to be a function template instead of an ordinary function, with a separate template parameter for each auto function parameter:
// Sort `vec` in increasing order
std::sort(vec.begin(), vec.end(), [](auto lhs, auto rhs) { return lhs > rhs; });
Lambda init captures
Lambda captures can have explicit initializers, which can be used to declare wholly new variables rather than only capturing existing ones:
[x = 42, y = "foo"] { ... }  // x is an int, and y is a const char*
This syntax doesn't allow the type to be specified; instead, it's deduced using the rules for auto variables.
Class template argument deduction
See below.
Structured bindings
When declaring a tuple, struct, or array using auto, you can specify names for the individual elements instead of a name for the whole object; these names are called "structured bindings", and the whole declaration is called a "structured binding declaration". This syntax provides no way of specifying the type of either the enclosing object or the individual names:
auto [iter, success] = my_map.insert({key, value});
if (!success) {
  iter->second = value;
}
The auto can also be qualified with const, &, and &&, but note that these qualifiers technically apply to the anonymous tuple/struct/array, rather than the individual bindings. The rules that determine the types of the bindings are quite complex; the results tend to be unsurprising, except that the binding types typically won't be references even if the declaration declares a reference (but they will usually behave like references anyway).

(These summaries omit many details and caveats; see the links for further information.)

C++ code is usually clearer when types are explicit, especially when type deduction would depend on information from distant parts of the code. In expressions like:

auto foo = x.add_foo();
auto i = y.Find(key);

it may not be obvious what the resulting types are if the type of y isn't very well known, or if y was declared many lines earlier.

Programmers have to understand when type deduction will or won't produce a reference type, or they'll get copies when they didn't mean to.

If a deduced type is used as part of an interface, then a programmer might change its type while only intending to change its value, leading to a more radical API change than intended.

The fundamental rule is: use type deduction only to make the code clearer or safer, and do not use it merely to avoid the inconvenience of writing an explicit type. When judging whether the code is clearer, keep in mind that your readers are not necessarily on your team, or familiar with your project, so types that you and your reviewer experience as as unnecessary clutter will very often provide useful information to others. For example, you can assume that the return type of make_unique<Foo>() is obvious, but the return type of MyWidgetFactory() probably isn't.

These principles applies to all forms of type deduction, but the details vary, as described in the following sections.

Function template argument deduction

Function template argument deduction is almost always OK. Type deduction is the expected default way of interacting with function templates, because it allows function templates to act like infinite sets of ordinary function overloads. Consequently, function templates are almost always designed so that template argument deduction is clear and safe, or doesn't compile.

Local variable type deduction

For local variables, you can use type deduction to make the code clearer by eliminating type information that is obvious or irrelevant, so that the reader can focus on the meaningful parts of the code:

std::unique_ptr<WidgetWithBellsAndWhistles> widget_ptr =
    absl::make_unique<WidgetWithBellsAndWhistles>(arg1, arg2);
absl::flat_hash_map<std::string,
                    std::unique_ptr<WidgetWithBellsAndWhistles>>::const_iterator
    it = my_map_.find(key);
std::array<int, 0> numbers = {4, 8, 15, 16, 23, 42};
auto widget_ptr = absl::make_unique<WidgetWithBellsAndWhistles>(arg1, arg2);
auto it = my_map_.find(key);
std::array numbers = {4, 8, 15, 16, 23, 42};

Types sometimes contain a mixture of useful information and boilerplate, such as it in the example above: it's obvious that the type is an iterator, and in many contexts the container type and even the key type aren't relevant, but the type of the values is probably useful. In such situations, it's often possible to define local variables with explicit types that convey the relevant information:

auto it = my_map_.find(key);
if (it != my_map_.end()) {
  WidgetWithBellsAndWhistles& widget = *it->second;
  // Do stuff with `widget`
}
If the type is a template instance, and the parameters are boilerplate but the template itself is informative, you can use class template argument deduction to suppress the boilerplate. However, cases where this actually provides a meaningful benefit are quite rare. Note that class template argument deduction is also subject to a separate style rule.

Do not use decltype(auto) if a simpler option will work, because it's a fairly obscure feature, so it has a high cost in code clarity.

Return type deduction

Use return type deduction (for both functions and lambdas) only if the function body has a very small number of return statements, and very little other code, because otherwise the reader may not be able to tell at a glance what the return type is. Furthermore, use it only if the function or lambda has a very narrow scope, because functions with deduced return types don't define abstraction boundaries: the implementation is the interface. In particular, public functions in header files should almost never have deduced return types.

Parameter type deduction

auto parameter types for lambdas should be used with caution, because the actual type is determined by the code that calls the lambda, rather than by the definition of the lambda. Consequently, an explicit type will almost always be clearer unless the lambda is explicitly called very close to where it's defined (so that the reader can easily see both), or the lambda is passed to an interface so well-known that it's obvious what arguments it will eventually be called with (e.g. the std::sort example above).

Lambda init captures

Init captures are covered by a more specific style rule, which largely supersedes the general rules for type deduction.

Structured bindings

Unlike other forms of type deduction, structured bindings can actually give the reader additional information, by giving meaningful names to the elements of a larger object. This means that a structured binding declaration may provide a net readability improvement over an explicit type, even in cases where auto would not. Structured bindings are especially beneficial when the object is a pair or tuple (as in the insert example above), because they don't have meaningful field names to begin with, but note that you generally shouldn't use pairs or tuples unless a pre-existing API like insert forces you to.

If the object being bound is a struct, it may sometimes be helpful to provide names that are more specific to your usage, but keep in mind that this may also mean the names are less recognizable to your reader than the field names. We recommend using a comment to indicate the name of the underlying field, if it doesn't match the name of the binding, using the same syntax as for function parameter comments:

auto [/*field_name1=*/ bound_name1, /*field_name2=*/ bound_name2] = ...
As with function parameter comments, this can enable tools to detect if you get the order of the fields wrong.

Class template argument deduction

Use class template argument deduction only with templates that have explicitly opted into supporting it.

Class template argument deduction (often abbreviated "CTAD") occurs when a variable is declared with a type that names a template, and the template argument list is not provided (not even empty angle brackets):

std::array a = {1, 2, 3};  // `a` is a std::array<int, 3>
The compiler deduces the arguments from the initializer using the template's "deduction guides", which can be explicit or implicit.

Explicit deduction guides look like function declarations with trailing return types, except that there's no leading auto, and the function name is the name of the template. For example, the above example relies on this deduction guide for std::array:

namespace std {
template <class T, class... U>
array(T, U...) -> std::array<T, 1 + sizeof...(U)>;
}
Constructors in a primary template (as opposed to a template specialization) also implicitly define deduction guides.

When you declare a variable that relies on CTAD, the compiler selects a deduction guide using the rules of constructor overload resolution, and that guide's return type becomes the type of the variable.

CTAD can sometimes allow you to omit boilerplate from your code.

The implicit deduction guides that are generated from constructors may have undesirable behavior, or be outright incorrect. This is particularly problematic for constructors written before CTAD was introduced in C++17, because the authors of those constructors had no way of knowing about (much less fixing) any problems that their constructors would cause for CTAD. Furthermore, adding explicit deduction guides to fix those problems might break any existing code that relies on the implicit deduction guides.

CTAD also suffers from many of the same drawbacks as auto, because they are both mechanisms for deducing all or part of a variable's type from its initializer. CTAD does give the reader more information than auto, but it also doesn't give the reader an obvious cue that information has been omitted.

Do not use CTAD with a given template unless the template's maintainers have opted into supporting use of CTAD by providing at least one explicit deduction guide (all templates in the std namespace are also presumed to have opted in). This should be enforced with a compiler warning if available.

Uses of CTAD must also follow the general rules on Type deduction.

Lambda expressions

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

Lambda expressions are a concise way of creating anonymous function objects. They're often useful when passing functions as arguments. For example:

std::sort(v.begin(), v.end(), [](int x, int y) {
  return Weight(x) < Weight(y);
});

They further allow capturing variables from the enclosing scope either explicitly by name, or implicitly using a default capture. Explicit captures require each variable to be listed, as either a value or reference capture:

int weight = 3;
int sum = 0;
// Captures `weight` by value and `sum` by reference.
std::for_each(v.begin(), v.end(), [weight, &sum](int x) {
  sum += weight * x;
});

Default captures implicitly capture any variable referenced in the lambda body, including this if any members are used:

const std::vector<int> lookup_table = ...;
std::vector<int> indices = ...;
// Captures `lookup_table` by reference, sorts `indices` by the value
// of the associated element in `lookup_table`.
std::sort(indices.begin(), indices.end(), [&](int a, int b) {
  return lookup_table[a] < lookup_table[b];
});

A variable capture can also have an explicit initializer, which can be used for capturing move-only variables by value, or for other situations not handled by ordinary reference or value captures:

std::unique_ptr<Foo> foo = ...;
[foo = std::move(foo)] () {
  ...
}
Such captures (often called "init captures" or "generalized lambda captures") need not actually "capture" anything from the enclosing scope, or even have a name from the enclosing scope; this syntax is a fully general way to define members of a lambda object:
[foo = std::vector<int>({1, 2, 3})] () {
  ...
}
The type of a capture with an initializer is deduced using the same rules as auto.

Template metaprogramming

Avoid complicated template programming.

Template metaprogramming refers to a family of techniques that exploit the fact that the C++ template instantiation mechanism is Turing complete and can be used to perform arbitrary compile-time computation in the type domain.

Template metaprogramming allows extremely flexible interfaces that are type safe and high performance. Facilities like Google Test, std::tuple, std::function, and Boost.Spirit would be impossible without it.

The techniques used in template metaprogramming are often obscure to anyone but language experts. Code that uses templates in complicated ways is often unreadable, and is hard to debug or maintain.

Template metaprogramming often leads to extremely poor compile time error messages: even if an interface is simple, the complicated implementation details become visible when the user does something wrong.

Template metaprogramming interferes with large scale refactoring by making the job of refactoring tools harder. First, the template code is expanded in multiple contexts, and it's hard to verify that the transformation makes sense in all of them. Second, some refactoring tools work with an AST that only represents the structure of the code after template expansion. It can be difficult to automatically work back to the original source construct that needs to be rewritten.

Template metaprogramming sometimes allows cleaner and easier-to-use interfaces than would be possible without it, but it's also often a temptation to be overly clever. It's best used in a small number of low level components where the extra maintenance burden is spread out over a large number of uses.

Think twice before using template metaprogramming or other complicated template techniques; think about whether the average member of your team will be able to understand your code well enough to maintain it after you switch to another project, or whether a non-C++ programmer or someone casually browsing the code base will be able to understand the error messages or trace the flow of a function they want to call. If you're using recursive template instantiations or type lists or metafunctions or expression templates, or relying on SFINAE or on the sizeof trick for detecting function overload resolution, then there's a good chance you've gone too far.

If you use template metaprogramming, you should expect to put considerable effort into minimizing and isolating the complexity. You should hide metaprogramming as an implementation detail whenever possible, so that user-facing headers are readable, and you should make sure that tricky code is especially well commented. You should carefully document how the code is used, and you should say something about what the "generated" code looks like. Pay extra attention to the error messages that the compiler emits when users make mistakes. The error messages are part of your user interface, and your code should be tweaked as necessary so that the error messages are understandable and actionable from a user point of view.

Boost

Use only approved libraries from the Boost library collection.

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

Boost code is generally very high-quality, is widely portable, and fills many important gaps in the C++ standard library, such as type traits and better binders.

Some Boost libraries encourage coding practices which can hamper readability, such as metaprogramming and other advanced template techniques, and an excessively "functional" style of programming.

In order to maintain a high level of readability for all contributors who might read and maintain code, we only allow an approved subset of Boost features. Currently, the following libraries are permitted:

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

std::hash

Do not define specializations of std::hash.

std::hash<T> is the function object that the C++11 hash containers use to hash keys of type T, unless the user explicitly specifies a different hash function. For example, std::unordered_map<int, std::string> is a hash map that uses std::hash<int> to hash its keys, whereas std::unordered_map<int, std::string, MyIntHash> uses MyIntHash.

std::hash is defined for all integral, floating-point, pointer, and enum types, as well as some standard library types such as string and unique_ptr. Users can enable it to work for their own types by defining specializations of it for those types.

std::hash is easy to use, and simplifies the code since you don't have to name it explicitly. Specializing std::hash is the standard way of specifying how to hash a type, so it's what outside resources will teach, and what new engineers will expect.

std::hash is hard to specialize. It requires a lot of boilerplate code, and more importantly, it combines responsibility for identifying the hash inputs with responsibility for executing the hashing algorithm itself. The type author has to be responsible for the former, but the latter requires expertise that a type author usually doesn't have, and shouldn't need. The stakes here are high because low-quality hash functions can be security vulnerabilities, due to the emergence of hash flooding attacks.

Even for experts, std::hash specializations are inordinately difficult to implement correctly for compound types, because the implementation cannot recursively call std::hash on data members. High-quality hash algorithms maintain large amounts of internal state, and reducing that state to the size_t bytes that std::hash returns is usually the slowest part of the computation, so it should not be done more than once.

Due to exactly that issue, std::hash does not work with std::pair or std::tuple, and the language does not allow us to extend it to support them.

You can use std::hash with the types that it supports "out of the box", but do not specialize it to support additional types. If you need a hash table with a key type that std::hash does not support, consider using legacy hash containers (e.g. hash_map) for now; they use a different default hasher, which is unaffected by this prohibition.

If you want to use the standard hash containers anyway, you will need to specify a custom hasher for the key type, e.g.

std::unordered_map<MyKeyType, Value, MyKeyTypeHasher> my_map;

Consult with the type's owners to see if there is an existing hasher that you can use; otherwise work with them to provide one, or roll your own.

We are planning to provide a hash function that can work with any type, using a new customization mechanism that doesn't have the drawbacks of std::hash.

Other C++ Features

As with Boost, some modern C++ extensions encourage coding practices that hamper readability—for example by removing checked redundancy (such as type names) that may be helpful to readers, or by encouraging template metaprogramming. Other extensions duplicate functionality available through existing mechanisms, which may lead to confusion and conversion costs.

In addition to what's described in the rest of the style guide, the following C++ features may not be used:

Nonstandard Extensions

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

Compilers support various extensions that are not part of standard C++. Such extensions include GCC's __attribute__, intrinsic functions such as __builtin_prefetch, designated initializers (e.g. Foo f = {.field = 3}), inline assembly, __COUNTER__, __PRETTY_FUNCTION__, compound statement expressions (e.g. foo = ({ int x; Bar(&x); x }), variable-length arrays and alloca(), and the "Elvis Operator" a?:b.

Do not use nonstandard extensions. You may use portability wrappers that are implemented using nonstandard extensions, so long as those wrappers are provided by a designated project-wide portability header.

Aliases

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

There are several ways to create names that are aliases of other entities:

typedef Foo Bar;
using Bar = Foo;
using other_namespace::Foo;

In new code, using is preferable to typedef, because it provides a more consistent syntax with the rest of C++ and works with templates.

Like other declarations, aliases declared in a header file are part of that header's public API unless they're in a function definition, in the private portion of a class, or in an explicitly-marked internal namespace. Aliases in such areas or in .cc files are implementation details (because client code can't refer to them), and are not restricted by this rule.

Don't put an alias in your public API just to save typing in the implementation; do so only if you intend it to be used by your clients.

When defining a public alias, document the intent of the new name, including whether it is guaranteed to always be the same as the type it's currently aliased to, or whether a more limited compatibility is intended. This lets the user know whether they can treat the types as substitutable or whether more specific rules must be followed, and can help the implementation retain some degree of freedom to change the alias.

Don't put namespace aliases in your public API. (See also Namespaces).

For example, these aliases document how they are intended to be used in client code:

namespace mynamespace {
// Used to store field measurements. DataPoint may change from Bar* to some internal type.
// Client code should treat it as an opaque pointer.
using DataPoint = foo::Bar*;

// A set of measurements. Just an alias for user convenience.
using TimeSeries = std::unordered_set<DataPoint, std::hash<DataPoint>, DataPointComparator>;
}  // namespace mynamespace

These aliases don't document intended use, and half of them aren't meant for client use:

namespace mynamespace {
// Bad: none of these say how they should be used.
using DataPoint = foo::Bar*;
using std::unordered_set;  // Bad: just for local convenience
using std::hash;           // Bad: just for local convenience
typedef unordered_set<DataPoint, hash<DataPoint>, DataPointComparator> TimeSeries;
}  // namespace mynamespace

However, local convenience aliases are fine in function definitions, private sections of classes, explicitly marked internal namespaces, and in .cc files:

// In a .cc file
using foo::Bar;

Именование

Основные правила стиля кодирования приходятся на именование. Вид имени сразу же (без поиска объявления) говорит нам что это: тип, переменная, функция, константа, макрос и т.д.

Правила именования могут быть произвольными, однако важна их согласованность, и правилам нужно следовать.

Общие принципы именования

Используйте имена, который будут понятны даже людям из другой команды.

Имя должно говорить о цели или применимости объекта. Не экономьте на длине имени, лучше более длинное и более понятное (даже новичкам) имя. Поменьше аббревиатур, особенно если они незнакомы вне проекта. Используйте только известные аббревиатуры (Википедия о них знает?). Не сокращайте слова. В целом, длина имени должна соответствовать размеру области видимости. Например, n - подходящее имя внутри функции в 5 строк, однако при описании класса это может быть коротковато.

class MyClass {
 public:
  int CountFooErrors(const std::vector<Foo>& foos) {
    int n = 0;  // Чёткий смысл для небольшой области видимости
    for (const auto& foo : foos) {
      ...
      ++n;
    }
    return n;
  }
  void DoSomethingImportant() {
    std::string fqdn = ...;  // Известная аббревиатура полного доменного имени
  }
 private:
  const int kMaxAllowedConnections = ...;  // Чёткий смысл для контекста
};
class MyClass {
 public:
  int CountFooErrors(const std::vector<Foo>& foos) {
    int total_number_of_foo_errors = 0;  // Слишком подробное имя для короткой функции
    for (int foo_index = 0; foo_index < foos.size(); ++foo_index) {  // Лучше использовать `i`
      ...
      ++total_number_of_foo_errors;
    }
    return total_number_of_foo_errors;
  }
  void DoSomethingImportant() {
    int cstmr_id = ...;  // Сокращённое слово (удалены буквы)
  }
 private:
  const int kNum = ...;  // Для целого класса очень нечёткое имя
};

Отметим, что типовые имена также допустимы: i для итератора или счётчика, T для параметра шаблона.

В дальнейшем при описании правил "word" / "слово" это всё, что пишется на английском без пробелов, в том числе и аббревиатуры. В слове первая буква может быть заглавной (зависит от стиля: "camel case" или "Pascal case"), остальные буквы - строчные. Например, предпочтительно StartRpc(), нежелательно StartRPC().

Параметры шаблона также следуют правилам своих категорий: type names / имена типов для типов, variable names / имена переменных для переменных.

Имена файлов

Имена файлов должны быть записаны только строчными буквами, для разделения можно использовать подчёркивание (_) или дефис (-). Используйте тот разделитель, который используется в проекте. Если единого подхода нет - используйте "_".

Примеры подходящих имён:

C++ файлы должны заканчиваться на .cc, заголовочные - на .h. Файлы, включаемые как текст должны заканчиваться на .inc (см. также секцию Независимые заголовочники).

Не используйте имена, уже существующие в /usr/include, такие как db.h.

Старайтесь давать файлам специфичные имена. Например, http_server_logs.h лучше чем logs.h. Когда файлы используются парами, лучше давать им одинаковые имена. Например, foo_bar.h и foo_bar.cc (и содержат класс FooBar).

Имена типов

Имена типов начинаются с прописной буквы, каждое новое слово также начинается с прописной буквы. Подчёркивания не используются: MyExcitingClass, MyExcitingEnum.

Имена всех типов - классов, структур, псевдонимов, перечислений, параметров шаблонов - именуются в одинаковом стиле. Имена типов начинаются с прописной буквы, каждое новое слово также начинается с прописной буквы. Подчёркивания не используются. Например:

// classes and structs
class UrlTable { ...
class UrlTableTester { ...
struct UrlTableProperties { ...

// typedefs
typedef hash_map<UrlTableProperties *, std::string> PropertiesMap;

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

// enums
enum UrlTableErrors { ...

Имена переменных

Имена переменных (включая параметры функций) и членов данных пишутся строчными буквами с подчёркиванием между словами. Члены данных классов (не структур) дополняются подчёркиванием в конце имени. Например: a_local_variable, a_struct_data_member, a_class_data_member_.

Имена обычных переменных

Например:

std::string table_name;  // OK - строчные буквы с подчёркиванием
std::string tableName;   // Плохо - смешанный стиль

Члены данных класса

Члены данных классов, статические и нестатические, именуются как обычные переменные с добавлением подчёркивания в конце.

class TableInfo {
  ...
 private:
  std::string table_name_;  // OK - подчёркивание в конце
  static Pool<TableInfo>* pool_;  // OK.
};

Члены данных структуры

Члены данных структуры, статические и нестатические, именуются как обычные переменные. К ним не добавляется символ подчёркивания в конце.

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

См. также Структуры vs Классы, где описано когда использовать структуры, когда классы.

Имена констант

Объекты объявляются как constexpr или const, чтобы значение не менялось в процессе выполнения. Имена констант начинаются с символа "k", далее идёт имя в смешанном стиле (прописные и строчные буквы). Подчёркивание может быть использовано в редких случаях когда прописные буквы не могут использоваться для разделения. Например:

const int kDaysInAWeek = 7;
const int kAndroid8_0_0 = 24;  // Android 8.0.0

Все аналогичные константные объекты со статическим типом хранилища (т.е. статические или глобальные, подробнее тут: Storage Duration) именуются также. Это соглашение является необязательным для переменных в других типах хранилища (например, автоматические константные объекты).

Имена функций

Обычные функции именуются в смешанном стиле (прописные и строчные буквы); функции доступа к переменным (accessor и mutator) должны иметь стиль, похожий на целевую переменную.

Обычно имя функции начинается с прописной буквы и каждое слово в имени пишется с прописной буквы.

AddTableEntry()
DeleteUrl()
OpenFileOrDie()

(Аналогичные правила применяются для констант в области класса или пространства имён (namespace) которые представляют собой часть API и должны выглядеть как функции (и то, что они не функции - некритично))

Accessor-ы и mutator-ы (функции get и set) могут именоваться наподобие соответствующих переменных. Они часто соответствуют реальным переменным-членам, однако это не обязательно. Например, int count() и void set_count(int count).

Именование пространства имён (namespace)

Пространство имён называется строчными буквами. Пространство имён верхнего уровня основывается на имени проекта. Избегайте коллизий ваших имён и других, хорошо известных, пространств имён.

Пространство имён верхнего уровня - это обычно название проекта или команды (которая делала код). Код должен располагаться в директории (или поддиректории) с именем, соответствующим пространству имён.

Не забывайте правило не использовать аббревиатуры - к пространствам имён это также применимо. Коду внутри вряд ли потребуется упоминание пространства имён, поэтому аббревиатуры - это лишнее.

Избегайте использовать для вложенных пространств имён известные названия. Коллизии между именами могут привести к сюрпризам при сборке. В частности, не создавайте вложенных пространств имён с именем std. Рекомендуются уникальные идентификаторы проекта (websearch::index, websearch::index_util) вместо небезопасных к коллизиям websearch::util.

Для internal / внутренних пространств имён коллизии могут возникать при добавлении другого кода (внутренние хелперы имеют свойство повторяться у разных команд). В этом случае хорошо помогает использование имени файла для именования пространства имён. (websearch::index::frobber_internal для использования в frobber.h)

Имена перечислений

Перечисления (как с ограничениями на область видимости (scoped), так и без (unscoped)) должны именоваться либо как константы, либо как macros. Т.е.: либо kEnumName, либо ENUM_NAME.

Предпочтительно именовать отдельные значения в перечислителе как константы. Однако, допустимо именовать как макросы. Имя самого перечисления UrlTableErrorsAlternateUrlTableErrors), это тип. Следовательно, используется смешанный стиль.

enum UrlTableErrors {
  kOk = 0,
  kErrorOutOfMemory,
  kErrorMalformedInput,
};
enum AlternateUrlTableErrors {
  OK = 0,
  OUT_OF_MEMORY = 1,
  MALFORMED_INPUT = 2,
};

Вплоть до января 2009 года стиль именования значений перечисления был как у макросов. Это создавало проблемы дублирования имён макросов и значений перечислений. Применение стиля констант решает проблему и в новом коде предпочтительно использовать стиль констант. Однако, старый код нет необходимости переписывать (пока нет проблем дублирования).

Имена макросов

Вы ведь не собираетесь определять макросы? На всякий случай (если собираетесь), они должны выглядеть так: MY_MACRO_THAT_SCARES_SMALL_CHILDREN_AND_ADULTS_ALIKE.

Пожалуйста прочтите как определять макросы; Обычно, макросы не должны использоваться. Однако, если они вам абсолютно необходимы, именуйте их прописными буквами с символами подчёркивания.

#define ROUND(x) ...
#define PI_ROUNDED 3.0

Исключения из правил именования

Если вам нужно именовать что-то, имеющее аналоги в существующем C или C++ коде, то следуйте используемому в коде стилю.

bigopen()
имя функции, образованное от open()
uint
похож на стандартный тип
bigpos
struct или class, образованный от pos
sparse_hash_map
STL-подобная сущность; следуйте стилю STL
LONGLONG_MAX
константа, такая же как INT_MAX

Комментарии

Комментарии являются обязательными для кода (если вы планируете его читать). Следующие правила описывают, что вы должны комментировать и как. Но помните: хотя комментарии очень важны, идеальный код сам себя документирует. Использование "говорящих" имён для типов и переменных намного лучше, чем непонятные имена, которые потом требуется расписывать в комментариях.

Комментируйте код с учётом его следующих читателей: программистов, которым потребуется разбираться в вашем коде. Учтите, что следующим читателем можете стать вы!

Стиль комментариев

Используйте либо // либо /* */, пока не нарушается единообразие.

Вы можете использовать либо // либо /* */, однако // намного предпочтительнее. Однако, всегда согласовывайте ваш стиль комментариев с уже существующим кодом.

Комментарии в шапке файла

В начало каждого файла вставляйте шапку с лицензией.

Комментарии в файле должны описывать его содержимое. Если файл объявляет, описывает или тестирует одну абстракцию (на которую уже есть комментарий), дополнительное описание в шапке файла не нужно. В ином случае, в начало файла вставляйте описание содержимого.

Правовая информация и список авторов

Каждый файл должен содержать информацию о лицензии. Формат описания зависит от лицензии, используемой в проекте. У каждой лицензии (Apache 2.0, BSD, LGPL, GPL, др.) могут быть свои требования к оформлению.

Если вы делаете значительные изменения в файле, подумайте над удалением прежнего списка авторов. Обновлённые файлы могут уже не содержать упоминание об авторских правах и список авторов.

Содержимое файлов

Если .h объявляет несколько абстракций, комментарий в шапке файла должен в целом описывать содержимое файла и как абстракции связаны друг с другом. Одного, двух предложений в комментарии обычно достаточно. Более детальная информация расписывается в другом месте (не в шапке файла).

Не дублируйте комментарии в .h и .cc файлах - со временем комментарии становятся разными.

Комментарии класса

Каждое объявление класса (кроме совсем очевидных) должно сопровождаться комментарием, для чего класс и как им пользоваться.

// Перебор содержимого GargantuanTable.
// Пример:
//    GargantuanTableIterator* iter = table->NewIterator();
//    for (iter->Seek("foo"); !iter->done(); iter->Next()) {
//      process(iter->key(), iter->value());
//    }
//    delete iter;
class GargantuanTableIterator {
  ...
};

Комментарий к классу должен быть достаточным для понимания: как и когда использовать класс, дополнительные требования для правильного использования класса. Описывайте, если требуется, ограничения (предположения) на синхронизацию в классе. Если экземпляр класса может использоваться из разных потоков, обязательно распишите правила многопоточного использования.

В комментарии к классу также можно привести короткие примеры кода, показывающие как проще использовать класс.

Обычно класс объявляется/определяется в разных файлах (.h и .cc). Комментарии, описывающие использование класса должны быть рядом с определением интерфейса. Комментарии о тонкостях реализации должны быть рядом с кодом самих методов.

Комментарии функции

Комментарии к объявлению функции должны описывать использование функции (кроме самых очевидных случаев). Комментарии к определению функции описывают реализацию.

Объявление функции

Объявление каждой функции должно иметь комментарий (прямо перед объявлением), что функция делает и как ей пользоваться. Комментарий можно опустить, только если функция простая и использование очевидно (например, функции вычитывания значений переменных). Старайтесь начинать комментарии в изъявительном наклонении ("Открывает файл"). Использование повелительного наклонение ("Открыть файл") - не рекомендуется. Комментарий описывает суть функции, а не то, как она это делает.

В комментарии к объявлению функции обратите внимание на следующее:

Пример:

// Возвращает итератор по таблице. Клиент должен удалить 
// итератор после использования. Нельзя использовать итератор
// если соответствующий объект GargantuanTable был удалён.
//
// Итератор изначально указывает на начало таблицы.
//
// Этот метод эквивалентен следующему:
//    Iterator* iter = table->NewIterator();
//    iter->Seek("");
//    return iter;
// Если вы собираетесь сразу же делать новую операцию поиска,
// быстрее будет вызвать NewIterator() и избежать лишней операции поиска.
Iterator* GetIterator() const;

Однако не стоит разжёвывать очевидные вещи.

Когда документируйте перегружаемые функции, делайте основной упор на изменениях по сравнению с исходной функцией. А если изменений нет (что бывает часто), то дополнительные комментарии вообще не нужны.

Комментируя конструкторы и деструкторы, учитывайте, что читатель кода знает их назначение. Поэтому комментарий типа "разрушает этот объект" - бестолковый. Можете описывать, что конструктор делает с аргументами (например, изменение владения на указатели) или какие именно операции по очистке делает деструктор. Если всё и так понятно - ничего не комментируйте. Вообще, обычно деструкторы не имеют комментариев (при объявлении).

Определение функций

Если есть какие-то хитрости в реализации функции, то можно к определению добавить объяснительный комментарий. В нём можно описать трюки с кодом, дать обзор всех этапов вычислений, объяснить выбор той или иной реализации (особенно если есть более лучшие альтернативы). Можете описать принципы синхронизации кусков кода (здесь блокируем, а здесь рыбу заворачиваем).

Отметим что вы не должны повторять комментарий из объявления функции (из .h файла или т.п.). Можно кратко описать, что функция делает, однако основной упор должен быть как она это делает.

Комментарии к переменным

По хорошему, имя переменной должно сразу говорить что это и зачем. Однако, в некоторых случаях требуются дополнительные комментарии.

Член данных класса

Назначение каждого члена класса должно быть очевидно. Если есть неочевидные тонкости (специальные значения, завязки с другими членами, ограничения по времени жизни) - всё это нужно комментировать. Однако, если типа и имени достаточно - комментарии добавлять не нужно.

С другой стороны, полезными будут описания особых (и неочевидных) значений (nullptr или -1). Например:

private:
 // Используется для проверки выхода за границы
 // -1 - показывает, что мы не знаем сколько записей в таблице
 int num_total_entries_;

Глобальные переменные

Ко всем глобальным переменным следует писать комментарий о их назначении и (если не очевидно) почему они должны быть глобальными. Например:

// Общее количество тестов, прогоняемых в регрессионом тесте
const int kNumTestCases = 6;

Комментарии к реализации

Комментируйте реализацию функции или алгоритма в случае наличия неочевидных, интересных, важных кусков кода.

Описательные комментарии

Блоки кода, отличающиеся сложностью или нестандартностью, должны предваряться комментарием. Например:

// Делим результат на 2. Переменная x содержит флаг переноса
for (int i = 0; i < result->size(); ++i) {
  x = (x << 8) + (*result)[i];
  (*result)[i] = x >> 1;
  x &= 1;
}

Построчные комментарии

Строки кода с неочевидным смыслом желательно дополнять комментарием (обычно располагаемым в конце строки). Этот комментраий должен отделяться от кода 2-мя проблами. Например:

// Мапируем блок данных, если объём позволяет
mmap_budget = max<int64>(0, mmap_budget - index_->length());
if (mmap_budget >= data_size_ && !MmapData(mmap_chunk_bytes, mlock))
  return;  // Ошибку уже логировали

Отметим, что здесь 2 комментария на блок кода: один описывает что код делает, другой напоминает, что ошибка уже в логе, если идёт возврат из функции.

Комментарии к аргументам функций

Когда назначение аргумента функции неочевидно, подумайте о следующих вариантах:

Рассмотрим примеры:
// И какое назначение аргументов?
const DecimalNumber product = CalculateProduct(values, 7, false, nullptr);

Попробуем причесать код:

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

Что делать не нужно

Не объясняйте очевидное. В частности, не нужно объяснять вещи, очевидные для человека, знающего C++. Вместо этого, можно описать зачем этот код делает так (или вообще сделайте код само-описываемым).

Сравним:
// Ищём элемент в векторе.  <-- Плохо: очевидно же!
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}
С этим:
// Обрабатывает (Process) "element" пока есть хоть один
auto iter = std::find(v.begin(), v.end(), element);
if (iter != v.end()) {
  Process(element);
}
Само-описывающий код вообще не нуждается в комментариях. Комментарий на код выше может быть вообще очевидным (и не нужным):
if (!IsAlreadyProcessed(element)) {
  Process(element);
}

Пунктуация, орфография и грамматика

Обращайте внимание на пунктуацию, орфографию и грамматику: намного проще читать грамотно написанные комментарии.

Комментарии должны быть написаны как рассказ: с правильной расстановкой прописных букв и знаков препинания. В большинстве случаев законченные предложения легче понимаются, нежели обрывки фраз. Короткие комментарии, такого типа как построчные, могут быть менее формальными, но всё равно должны следовать общему стилю.

Хотя излишнее внимание код-ревьюера к использованию запятых вместо точек с запятой может слегка раздражать, очень важно поддерживать высокий уровень читабельности и понятности кода. Правильная пунктуация, орфография и грамматика этому очень сильно способствует.

Комментарии TODO

Используйте комментарии TODO для временного кода или достаточно хорошего (промежуточного, не идеального) решения.

Комментарий должен включать строку TODO (все буквы прописные), за ней имя, адрес e-mail, ID дефекта или другая информация для идентификации разработчика и сущности проблемы, для которой написан TODO. Цель такого описания - возможность потом найти больше деталей. Наличие TODO с описанием не означает, что указанный программист исправит проблему. Поэтому, когда вы создаёте TODO, обычно там указано Ваше имя.

// TODO(kl@gmail.com): Используйте "*" для объединения.
// TODO(Zeke) Изменить для связывания.
// TODO(bug 12345): удалить фунционал "Последний посетитель".

Если ваш TODO вида "В будущем сделаем по-другому", то указывайте либо конкретную дату ("Исправить в ноябре 2005"), либо событие ("Удалить тот код, когда все клиенты будут обрабатывать XML запросы").

Форматирование

Coding style and formatting are pretty arbitrary, but a project is much easier to follow if everyone uses the same style. Individuals may not agree with every aspect of the formatting rules, and some of the rules may take some getting used to, but it is important that all project contributors follow the style rules so that they can all read and understand everyone's code easily.

To help you format code correctly, we've created a settings file for emacs.

Line Length

Each line of text in your code should be at most 80 characters long.

We recognize that this rule is controversial, but so much existing code already adheres to it, and we feel that consistency is important.

Those who favor this rule argue that it is rude to force them to resize their windows and there is no need for anything longer. Some folks are used to having several code windows side-by-side, and thus don't have room to widen their windows in any case. People set up their work environment assuming a particular maximum window width, and 80 columns has been the traditional standard. Why change it?

Proponents of change argue that a wider line can make code more readable. The 80-column limit is an hidebound throwback to 1960s mainframes; modern equipment has wide screens that can easily show longer lines.

80 characters is the maximum.

A line may exceed 80 characters if it is

Non-ASCII Characters

Non-ASCII characters should be rare, and must use UTF-8 formatting.

You shouldn't hard-code user-facing text in source, even English, so use of non-ASCII characters should be rare. However, in certain cases it is appropriate to include such words in your code. For example, if your code parses data files from foreign sources, it may be appropriate to hard-code the non-ASCII string(s) used in those data files as delimiters. More commonly, unittest code (which does not need to be localized) might contain non-ASCII strings. In such cases, you should use UTF-8, since that is an encoding understood by most tools able to handle more than just ASCII.

Hex encoding is also OK, and encouraged where it enhances readability — for example, "\xEF\xBB\xBF", 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).

Spaces vs. Tabs

Use only spaces, and indent 2 spaces at a time.

We use spaces for indentation. Do not use tabs in your code. You should set your editor to emit spaces when you hit the tab key.

Function Declarations and Definitions

Return type on the same line as function name, parameters on the same line if they fit. 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:

Unused parameters that are obvious from context may be omitted:

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

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:

ABSL_MUST_USE_RESULT bool IsOk();

Lambda Expressions

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());

Floating-point Literals

Floating-point literals should always have a radix point, with digits on both sides, even if they use exponential notation. Readability is improved if all floating-point literals take this familiar form, as this helps ensure that they are not mistaken for integer literals, and that the E/e of the exponential notation is not mistaken for a hexadecimal digit. It is fine to initialize a floating-point variable with an integer literal (assuming the variable type can exactly represent that integer), but note that a number in exponential notation is never an integer literal.

float f = 1.f;
long double ld = -.5L;
double d = 1248e6;
float f = 1.0f;
float f2 = 1;   // Also OK
long double ld = -0.5L;
double d = 1248.0e6;

Function Calls

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);

Braced Initializer List Format

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}};

Conditionals

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 have an accompanying brace.

if (condition)
  DoSomething();  // 2 space indent.

if (condition) {
  DoSomething();  // 2 space indent.
}

However, if one part of an if-else statement uses curly braces, the other part must too:

// Not allowed - curly on IF but not ELSE
if (condition) {
  foo;
} else
  bar;

// Not allowed - curly on ELSE but not IF
if (condition)
  foo;
else {
  bar;
}
// Curly braces around both IF and ELSE required because
// one of the clauses used braces.
if (condition) {
  foo;
} else {
  bar;
}

Loops and Switch Statements

Switch statements may use braces for blocks. Annotate non-trivial fall-through between cases. Braces are optional for single-statement loops. Empty loop bodies should use either 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, treat this as an error. For example:

switch (var) {
  case 0: {  // 2 space indent
    ...      // 4 space indent
    break;
  }
  case 1: {
    ...
    break;
  }
  default: {
    assert(false);
  }
}

Fall-through from one case label to another must be annotated using the ABSL_FALLTHROUGH_INTENDED; macro (defined in absl/base/macros.h). ABSL_FALLTHROUGH_INTENDED; should be placed at a point of execution where a fall-through to the next case label occurs. A common exception is consecutive case labels without intervening code, in which case no annotation is needed.

switch (x) {
  case 41:  // No annotation needed here.
  case 43:
    if (dont_be_picky) {
      // Use this instead of or along with annotations in comments.
      ABSL_FALLTHROUGH_INTENDED;
    } else {
      CloseButNoCigar();
      break;
    }
  case 42:
    DoSomethingSpecial();
    ABSL_FALLTHROUGH_INTENDED;
  default:
    DoSomethingGeneric();
    break;
}

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 either an empty pair of braces or continue with no braces, rather than 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.

Pointer and Reference Expressions

No spaces around period or arrow. Pointer operators do not have trailing spaces.

The following are examples of correctly-formatted pointer and reference expressions:

x = *p;
p = &x;
x = r.y;
x = r->y;

Note that:

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 std::string &str;

// These are fine, space following.
char* c;
const std::string& str;

You should do this consistently within a single file, so, when modifying an existing file, use the style in that file.

It is allowed (if unusual) to declare multiple variables in the same declaration, but it is disallowed if any of those have pointer or reference decorations. Such declarations are easily misread.
// 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 std::string & str;  // Bad - spaces on both sides of &

Boolean Expressions

When you have a boolean expression that is longer than the standard line length, be consistent in how you break up the lines.

In this example, the logical AND operator is always at the end of the lines:

if (this_one_thing > this_other_thing &&
    a_third_thing == a_fourth_thing &&
    yet_another && last_one) {
  ...
}

Note that when the code wraps in this example, both of the && logical AND operators are at the end of the line. This is more common in Google code, though wrapping all operators at the beginning of the line is also allowed. Feel free to insert extra parentheses judiciously because they can be very helpful in increasing readability when used appropriately. Also note that you should always use the punctuation operators, such as && and ~, rather than the word operators, such as and and compl.

Return Values

Do not needlessly surround the return expression with parentheses.

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!

Variable and Array Initialization

Your choice of =, (), or {}.

You may choose between =, (), and {}; the following are all correct:

int x = 3;
int x(3);
int x{3};
std::string name = "Some Name";
std::string name("Some Name");
std::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 containing 100 items: All 1s.
std::vector<int> v{100, 1};  // A vector containing 2 items: 100 and 1.

Also, the brace form prevents narrowing of integral types. This can prevent some types of programming errors.

int pi(3.14);  // OK -- pi == 3.
int pi{3.14};  // Compile error: narrowing conversion.

Preprocessor Directives

The hash mark that starts a preprocessor directive should always be at the beginning of the line.

Even when preprocessor directives are within the body of indented code, the directives should start at the beginning of the line.

// Good - directives at beginning of line
  if (lopsided_score) {
#if DISASTER_PENDING      // Correct -- Starts at beginning of line
    DropEverything();
# if NOTIFY               // OK but not required -- Spaces after #
    NotifyClient();
# endif
#endif
    BackToNormal();
  }
// Bad - indented directives
  if (lopsided_score) {
    #if DISASTER_PENDING  // Wrong!  The "#if" should be at beginning of line
    DropEverything();
    #endif                // Wrong!  Do not indent "#endif"
    BackToNormal();
  }

Class Format

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

The basic format for a class 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:

Constructor Initializer Lists

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) {}

Namespace Formatting

The contents of namespaces are not indented.

Namespaces do not add an extra level of indentation. For example, use:

namespace {

void foo() {  // Correct.  No extra indentation within namespace.
  ...
}

}  // namespace

Do not indent within a namespace:

namespace {

  // Wrong!  Indented when it should not be.
  void foo() {
    ...
  }

}  // namespace

When declaring nested namespaces, put each namespace on its own line.

namespace foo {
namespace bar {

Horizontal Whitespace

Use of horizontal whitespace depends on location. Never put trailing whitespace at the end of a line.

General

void f(bool b) {  // Open braces should always have a space before them.
  ...
int i = 0;  // Semicolons usually have no space before them.
// 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<std::string> x;
y = static_cast<char*>(x);

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

Vertical Whitespace

Minimize use of vertical whitespace.

This is more a principle than a rule: don't use blank lines when you don't have to. In particular, don't put more than one or two blank lines between functions, resist starting functions with a blank line, don't end functions with a blank line, and be sparing with your use of blank lines. A blank line within a block of code serves like a paragraph break in prose: visually separating two thoughts.

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. Use whitespace purposefully to provide separation in that flow.

Some rules of thumb to help when blank lines may be useful:

Exceptions to the Rules

The coding conventions described above are mandatory. However, like all good rules, these sometimes have exceptions, which we discuss here.

Existing Non-conformant Code

You may diverge from the rules when dealing with code that does not conform to this style guide.

If you find yourself modifying code that was written to specifications other than those presented by this guide, you may have to diverge from these rules in order to stay consistent with the local conventions in that code. If you are in doubt about how to do this, ask the original author or the person currently responsible for the code. Remember that consistency includes local consistency, too.

Windows Code

Windows programmers have developed their own set of coding conventions, mainly derived from the conventions in Windows headers and other Microsoft code. We want to make it easy for anyone to understand your code, so we have a single set of guidelines for everyone writing C++ on any platform.

It is worth reiterating a few of the guidelines that you might forget if you are used to the prevalent Windows style:

However, there are just a few rules that we occasionally need to break on Windows:

Parting Words

Use common sense and BE CONSISTENT.

If you are editing code, take a few minutes to look at the code around you and determine its style. If they use spaces around their if clauses, you should, too. If their comments have little boxes of stars around them, make your comments have little boxes of stars around them too.

The point of having style guidelines is to have a common vocabulary of coding so people can concentrate on what you are saying, rather than on how you are saying it. We present global style rules here so people know the vocabulary. But local style is also important. If code you add to a file looks drastically different from the existing code around it, the discontinuity throws readers out of their rhythm when they go to read it. Try to avoid this.

OK, enough writing about writing code; the code itself is much more interesting. Have fun!

Перевод

Кислов Евгений, 2019
email: dev@evgenykislov.com
evgenykislov.com

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Руководство Google по стилю в C++