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Google C++ Style Guide

Background

C++ is one of the main development languages used by many of Google's open-source projects. As every C++ programmer knows, the language has many powerful features, but this power brings with it complexity, which in turn can make code more bug-prone and harder to read and maintain.

The goal of this guide is to manage this complexity by describing in detail the dos and don'ts of writing C++ code . These rules exist to keep the code base manageable while still allowing coders to use C++ language features productively.

Style, also known as readability, is what we call the conventions that govern our C++ code. The term Style is a bit of a misnomer, since these conventions cover far more than just source file formatting.

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

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

Goals of the Style Guide

Why do we have this document?

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

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

Style rules should pull their weightThe benefit of a style rule must be large enough to justify asking all of our engineers to remember it. The benefit is measured relative to the codebase we would get without the rule, so a rule against a very harmful practice may still have a small benefit if people are unlikely to do it anyway. This principle mostly explains the rules we dont have, rather than the rules we do: for example, goto contravenes many of the following principles, but is already vanishingly rare, so the Style Guide doesnt discuss it.Optimize for the reader, not the writerOur codebase [and most individual components submitted to it] is expected to continue for quite some time. As a result, more time will be spent reading most of our code than writing it. We explicitly choose to optimize for the experience of our average software engineer reading, maintaining, and debugging code in our codebase rather than ease when writing said code. "Leave a trace for the reader" is a particularly common sub-point of this principle: When something surprising or unusual is happening in a snippet of code [for example, transfer of pointer ownership], leaving textual hints for the reader at the point of use is valuable [std::unique_ptr demonstrates the ownership transfer unambiguously at the call site].Be consistent with existing codeUsing one style consistently through our codebase lets us focus on other [more important] issues. Consistency also allows for automation: tools that format your code or adjust your #includes only work properly when your code is consistent with the expectations of the tooling. In many cases, rules that are attributed to "Be Consistent" boil down to "Just pick one and stop worrying about it"; the potential value of allowing flexibility on these points is outweighed by the cost of having people argue over them. However, there are limits to consistency; it is a good tie breaker when there is no clear technical argument, nor a long-term direction. It applies more heavily locally [per file, or for a tightly-related set of interfaces]. Consistency should not generally be used as a justification to do things in an old style without considering the benefits of the new style, or the tendency of the codebase to converge on newer styles over time.Be consistent with the broader C++ community when appropriateConsistency with the way other organizations use C++ has value for the same reasons as consistency within our code base. If a feature in the C++ standard solves a problem, or if some idiom is widely known and accepted, that's an argument for using it. However, sometimes standard features and idioms are flawed, or were just designed without our codebase's needs in mind. In those cases [as described below] it's appropriate to constrain or ban standard features. In some cases we prefer a homegrown or third-party library over a library defined in the C++ Standard, either out of perceived superiority or insufficient value to transition the codebase to the standard interface.Avoid surprising or dangerous constructsC++ has features that are more surprising or dangerous than one might think at a glance. Some style guide restrictions are in place to prevent falling into these pitfalls. There is a high bar for style guide waivers on such restrictions, because waiving such rules often directly risks compromising program correctness.Avoid constructs that our average C++ programmer would find tricky or hard to maintainC++ has features that may not be generally appropriate because of the complexity they introduce to the code. In widely used code, it may be more acceptable to use trickier language constructs, because any benefits of more complex implementation are multiplied widely by usage, and the cost in understanding the complexity does not need to be paid again when working with new portions of the codebase. When in doubt, waivers to rules of this type can be sought by asking your project leads. This is specifically important for our codebase because code ownership and team membership changes over time: even if everyone that works with some piece of code currently understands it, such understanding is not guaranteed to hold a few years from now.Be mindful of our scaleWith a codebase of 100+ million lines and thousands of engineers, some mistakes and simplifications for one engineer can become costly for many. For instance it's particularly important to avoid polluting the global namespace: name collisions across a codebase of hundreds of millions of lines are difficult to work with and hard to avoid if everyone puts things into the global namespace.Concede to optimization when necessaryPerformance optimizations can sometimes be necessary and appropriate, even when they conflict with the other principles of this document.

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

C++ Version

Currently, code should target C++17, i.e., should not use C++2x features, with the exception of designated initializers. The C++ version targeted by this guide will advance [aggressively] over time.

Do not use non-standard extensions.

Header Files

In general, every .cc file should have an associated .h file. There are some common exceptions, such as unit tests and small .cc files containing just a main[] function.

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

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

Self-contained Headers

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

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

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

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

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

The #define Guard

All header files should have #define guards to prevent multiple inclusion. The format of the symbol name should be ___H_.

#ifndef FOO_BAR_BAZ_H_ #define FOO_BAR_BAZ_H_ ... #endif // FOO_BAR_BAZ_H_

Include What You Use

If a source or header file refers to a symbol defined elsewhere, the file should directly include a header file which properly intends to provide a declaration or definition of that symbol. It should not include header files for any other reason.

Do not rely on transitive inclusions. This allows people to remove no-longer-needed #include statements from their headers without breaking clients. This also applies to related headers - foo.cc should include bar.h if it uses a symbol from it even if foo.h includes bar.h.

Forward Declarations

Avoid using forward declarations where possible. Instead, include the headers you need.

A "forward declaration" is a declaration of an entity without an associated definition.

// In a C++ source file: class B; void FuncInB[]; extern int variable_in_b; ABSL_DECLARE_FLAG[flag_in_b];

• Forward declarations can save compile time, as #includes force the compiler to open more files and process more input.
• Forward declarations can save on unnecessary recompilation. #includes can force your code to be recompiled more often, due to unrelated changes in the header.

• Forward declarations can hide a dependency, allowing user code to skip necessary recompilation when headers change.
• A forward declaration as opposed to an #include statement makes it difficult for automatic tooling to discover the module defining the symbol.
• A forward declaration may be broken by subsequent changes to the library. Forward declarations of functions and templates can prevent the header owners from making otherwise-compatible changes to their APIs, such as widening a parameter type, adding a template parameter with a default value, or migrating to a new namespace.
• Forward declaring symbols from namespace std:: yields undefined behavior.
• It can be difficult to determine whether a forward declaration or a full #include is needed. Replacing an #include with a forward declaration can silently change the meaning of code:// b.h: struct B {}; struct D : B {}; // good_user.cc: #include "b.h" void f[B*]; void f[void*]; void test[D* x] { f[x]; } // calls f[B*] If the #include was replaced with forward decls for B and D, test[] would call f[void*].
• Forward declaring multiple symbols from a header can be more verbose than simply #includeing the header.
• Structuring code to enable forward declarations [e.g., using pointer members instead of object members] can make the code slower and more complex.

Try to avoid forward declarations of entities defined in another project.

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., , .
4. A blank line
5. C++ standard library headers [without file extension], e.g., , .
6. A blank line
Other libraries' .h files.
A blank line
7. 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.

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

#include "foo/server/fooserver.h" #include #include #include #include #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 #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 Internal Linkage.

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:

• Follow the rules on Namespace Names.
• Terminate multi-line namespaces with comments as shown in the given examples.

• Namespaces wrap the entire source file after includes, gflags definitions/declarations and forward declarations of classes from other namespaces.

  // In the .h file namespace mynamespace { // All declarations are within the namespace scope. // Notice the lack of indentation. class MyClass { public: ... void Foo[]; }; } // namespace mynamespace // In the .cc file namespace mynamespace { // Definition of functions is within scope of the namespace. void MyClass::Foo[] { ... } } // namespace mynamespace

  More complex .cc files might have additional details, like flags or using-declarations.

  #include "a.h" ABSL_FLAG[bool, someflag, false, "dummy flag"]; namespace mynamespace { using ::foo::Bar; ...code for mynamespace... // Code goes against the left margin. } // namespace mynamespace
• To place generated protocol message code in a namespace, use the package specifier in the .proto file. See Protocol Buffer Packages for details.
• Do not declare anything in namespace std, including forward declarations of standard library classes. Declaring entities in namespace std is undefined behavior, i.e., not portable. To declare entities from the standard library, include the appropriate header file.

• You may not use a using-directive to make all names from a namespace available.

  // Forbidden -- This pollutes the namespace. using namespace foo;

• Do not use Namespace aliases at namespace scope in header files except in explicitly marked internal-only namespaces, because anything imported into a namespace in a header file becomes part of the public API exported by that file.

  // Shorten access to some commonly used names in .cc files. namespace baz = ::foo::bar::baz; // Shorten access to some commonly used names [in a .h file]. namespace librarian { namespace impl { // Internal, not part of the API. namespace sidetable = ::pipeline_diagnostics::sidetable; } // namespace impl inline void my_inline_function[] { // namespace alias local to a function [or method]. namespace baz = ::foo::bar::baz; ... } } // namespace librarian
• Do not use inline namespaces.

Internal Linkage

When definitions in a .cc file do not need to be referenced outside that file, give them internal linkage by placing them in an unnamed namespace or declaring 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 members. 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 members; this is no different than just giving the 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 v; v.push_back[1]; // Prefer initializing using brace initialization. v.push_back[2]; std::vector 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 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 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

• Global strings: if you require a named global or static string constant, consider using a constexpr variable of string_view, character array, or character pointer, pointing to a string literal. String literals have static storage duration already and are usually sufficient. See TotW #140.
• Maps, sets, and other dynamic containers: if you require a static, fixed collection, such as a set to search against or a lookup table, you cannot use the dynamic containers from the standard library as a static variable, since they have non-trivial destructors. Instead, consider a simple array of trivial types, e.g., an array of arrays of ints [for a "map from int to int"], or an array of pairs [e.g., pairs of int and const char*]. For small collections, linear search is entirely sufficient [and efficient, due to memory locality]; consider using the facilities from absl/algorithm/container.h for the standard operations. If necessary, keep the collection in sorted order and use a binary search algorithm. If you do really prefer a dynamic container from the standard library, consider using a function-local static pointer, as described below.
• Smart pointers [unique_ptr, shared_ptr]: smart pointers execute cleanup during destruction and are therefore forbidden. Consider whether your use case fits into one of the other patterns described in this section. One simple solution is to use a plain pointer to a dynamically allocated object and never delete it [see last item].
• Static variables of custom types: if you require static, constant data of a type that you need to define yourself, give the type a trivial destructor and a constexpr constructor.
• If all else fails, you can create an object dynamically and never delete it by using a function-local static pointer or reference [e.g., static const auto& impl = *new T[args...];].

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.

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 not destroyed before their thread terminates, so they do not have the destruction-order issues of static variables.

• Thread-local data is inherently safe from races [because only one thread can ordinarily access it], which makes thread_local useful for concurrent programming.
• thread_local is the only standard-supported way of creating thread-local data.

• Accessing a thread_local variable may trigger execution of an unpredictable and uncontrollable amount of other code.
• thread_local variables are effectively global variables, and have all the drawbacks of global variables other than lack of thread-safety.
• The memory consumed by a thread_local variable scales with the number of running threads [in the worst case], which can be quite large in a program.
• Non-static data members cannot be thread_local.
• thread_local may not be as efficient as certain compiler intrinsics.

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.

• No need to worry about whether the class has been initialized or not.
• Objects that are fully initialized by constructor call can be const and may also be easier to use with standard containers or algorithms.

• If the work calls virtual functions, these calls will not get dispatched to the subclass implementations. Future modification to your class can quietly introduce this problem even if your class is not currently subclassed, causing much confusion.
• There is no easy way for constructors to signal errors, short of crashing the program [not always appropriate] or using exceptions [which are forbidden].
• If the work fails, we now have an object whose initialization code failed, so it may be an unusual state requiring a bool IsValid[] state checking mechanism [or similar] which is easy to forget to call.
• You cannot take the address of a constructor, so whatever work is done in the constructor cannot easily be handed off to, for example, another thread.

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 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 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.

• Implicit conversions can make a type more usable and expressive by eliminating the need to explicitly name a type when it's obvious.
• Implicit conversions can be a simpler alternative to overloading, such as when a single function with a string_view parameter takes the place of separate overloads for std::string and const char*.
• List initialization syntax is a concise and expressive way of initializing objects.

• Implicit conversions can hide type-mismatch bugs, where the destination type does not match the user's expectation, or the user is unaware that any conversion will take place.
• Implicit conversions can make code harder to read, particularly in the presence of overloading, by making it less obvious what code is actually getting called.
• Constructors that take a single argument may accidentally be usable as implicit type conversions, even if they are not intended to do so.
• When a single-argument constructor is not marked explicit, there's no reliable way to tell whether it's intended to define an implicit conversion, or the author simply forgot to mark it.
• Implicit conversions can lead to call-site ambiguities, especially when there are bidirectional implicit conversions. This can be caused either by having two types that both provide an implicit conversion, or by a single type that has both an implicit constructor and an implicit type conversion operator.
• List initialization can suffer from the same problems if the destination type is implicit, particularly if the list has only a single element.

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 be interchangeable, for example when objects of two types are just different representations of the same underlying value. 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 is an example of a movable but not copyable type [since the value of the source std::unique_ptr 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. A copyable class may also declare move operations in order to support efficient moves. 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. // You may explicitly declare move operations to support efficient moves. }; class MoveOnly { public: MoveOnly[MoveOnly&& other] = default; MoveOnly& operator=[MoveOnly&& other] = default; // 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:

• If the class has no private section, like a struct or an interface-only base class, then the copyability/movability can be determined by the copyability/movability of any public data members.
• If a base class clearly isn't copyable or movable, derived classes naturally won't be either. An interface-only base class that leaves these operations implicit is not sufficient to make concrete subclasses clear.
• Note that if you explicitly declare or delete either the constructor or assignment operation for copy, the other copy operation is not obvious and must be declared or deleted. Likewise for move operations.

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.

To eliminate the risk of slicing, prefer to make base classes abstract, by making their constructors protected, by declaring their destructors protected, or by giving them one or more pure virtual member functions. Prefer to avoid deriving from concrete classes.

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. All fields must be public. The struct must not have invariants that imply relationships between different fields, since direct user access to those fields may break those invariants. Constructors, destructors, and helper methods may be present; however, these methods must not require or enforce any invariants.

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. While C++14's introduction of std::get 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. You may use final on classes when you don't intend to support using them as base classes.

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., ==, 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 auto add[T t, U u] -> decltype[t + u]; versus template decltype[declval[] + declval[]] 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

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 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.

• It's virtually impossible to manage dynamically allocated memory without some sort of ownership logic.
• Transferring ownership of an object can be cheaper than copying it [if copying it is even possible].
• Transferring ownership can be simpler than 'borrowing' a pointer or reference, because it reduces the need to coordinate the lifetime of the object between the two users.
• Smart pointers can improve readability by making ownership logic explicit, self-documenting, and unambiguous.
• Smart pointers can eliminate manual ownership bookkeeping, simplifying the code and ruling out large classes of errors.
• For const objects, shared ownership can be a simple and efficient alternative to deep copying.

• Ownership must be represented and transferred via pointers [whether smart or plain]. Pointer semantics are more complicated than value semantics, especially in APIs: you have to worry not just about ownership, but also aliasing, lifetime, and mutability, among other issues.
• The performance costs of value semantics are often overestimated, so the performance benefits of ownership transfer might not justify the readability and complexity costs.
• APIs that transfer ownership force their clients into a single memory management model.
• Code using smart pointers is less explicit about where the resource releases take place.
• std::unique_ptr expresses ownership transfer using move semantics, which are relatively new and may confuse some programmers.
• Shared ownership can be a tempting alternative to careful ownership design, obfuscating the design of a system.
• Shared ownership requires explicit bookkeeping at run-time, which can be costly.
• In some cases [e.g., cyclic references], objects with shared ownership may never be deleted.
• Smart pointers are not perfect substitutes for plain pointers.

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 FooFactory[]; void FooConsumer[std::unique_ptr 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]. 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.

Other C++ Features

Rvalue References

Use rvalue references only in certain special cases listed below.

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.

• Defining a move constructor [a constructor taking an rvalue reference to the class type] makes it possible to move a value instead of copying it. If v1 is a std::vector, for example, then auto v2[std::move[v1]] will probably just result in some simple pointer manipulation instead of copying a large amount of data. In many cases this can result in a major performance improvement.
• Rvalue references make it possible to implement types that are movable but not copyable, which can be useful for types that have no sensible definition of copying but where you might still want to pass them as function arguments, put them in containers, etc.
• std::move is necessary to make effective use of some standard-library types, such as std::unique_ptr.
• Forwarding references which use the rvalue reference token, make it possible to write a generic function wrapper that forwards its arguments to another function, and works whether or not its arguments are temporary objects and/or const. This is called 'perfect forwarding'.

• Rvalue references are not yet widely understood. Rules like reference collapsing and the special deduction rule for forwarding references are somewhat obscure.
• Rvalue references are often misused. Using rvalue references is counter-intuitive in signatures where the argument is expected to have a valid specified state after the function call, or where no move operation is performed.

Do not use rvalue references [or apply the && qualifier to methods], except as follows:

• You may use them to define move constructors and move assignment operators [as described in Copyable and Movable Types].
• You may use them to define &&-qualified methods that logically "consume" *this, leaving it in an unusable or empty state. Note that this applies only to method qualifiers [which come after the closing parenthesis of the function signature]; if you want to "consume" an ordinary function parameter, prefer to pass it by value.
• You may use forwarding references in conjunction with std::forward, to support perfect forwarding.
• You may use them to define pairs of overloads, such as 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.

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.

• Exceptions allow higher levels of an application to decide how to handle "can't happen" failures in deeply nested functions, without the obscuring and error-prone bookkeeping of error codes.
Exceptions are used by most other modern languages. Using them in C++ would make it more consistent with Python, Java, and the C++ that others are familiar with.
• Some third-party C++ libraries use exceptions, and turning them off internally makes it harder to integrate with those libraries.
• Exceptions are the only way for a constructor to fail. We can simulate this with a factory function or an Init[] method, but these require heap allocation or a new "invalid" state, respectively.
• Exceptions are really handy in testing frameworks.

• When you add a throw statement to an existing function, you must examine all of its transitive callers. Either they must make at least the basic exception safety guarantee, or they must never catch the exception and be happy with the program terminating as a result. For instance, if f[] calls g[] calls h[], and h throws an exception that f catches, g has to be careful or it may not clean up properly.
• More generally, exceptions make the control flow of programs difficult to evaluate by looking at code: functions may return in places you don't expect. This causes maintainability and debugging difficulties. You can minimize this cost via some rules on how and where exceptions can be used, but at the cost of more that a developer needs to know and understand.
• Exception safety requires both RAII and different coding practices. Lots of supporting machinery is needed to make writing correct exception-safe code easy. Further, to avoid requiring readers to understand the entire call graph, exception-safe code must isolate logic that writes to persistent state into a "commit" phase. This will have both benefits and costs [perhaps where you're forced to obfuscate code to isolate the commit]. Allowing exceptions would force us to always pay those costs even when they're not worth it.
• Turning on exceptions adds data to each binary produced, increasing compile time [probably slightly] and possibly increasing address space pressure.
• The availability of exceptions may encourage developers to throw them when they are not appropriate or recover from them when it's not safe to do so. For example, invalid user input should not cause exceptions to be thrown. We would need to make the style guide even longer to document these restrictions!

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 exception handling related features 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.

• Specifying move constructors as noexcept improves performance in some cases, e.g., std::vector::resize[] moves rather than copies the objects if T's move constructor is noexcept.
• Specifying noexcept on a function can trigger compiler optimizations in environments where exceptions are enabled, e.g., compiler does not have to generate extra code for stack-unwinding, if it knows that no exceptions can be thrown due to a noexcept specifier.

• In projects following this guide that have exceptions disabled it is hard to ensure that noexcept specifiers are correct, and hard to define what correctness even means.
• It's hard, if not impossible, to undo noexcept because it eliminates a guarantee that callers may be relying on, in ways that are hard to detect.

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 its 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 doesnt 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[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:

• Virtual methods are the preferred way of executing different code paths depending on a specific subclass type. This puts the work within the object itself.
• If the work belongs outside the object and instead in some processing code, consider a double-dispatch solution, such as the Visitor design pattern. This allows a facility outside the object itself to determine the type of class using the built-in type system.

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

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

if [typeid[*data] == typeid[D1]] { ... } else if [typeid[*data] == typeid[D2]] { ... } else if [typeid[*data] == typeid[D3]] { ...

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

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

Casting

Use C++-style casts like static_cast[double_value], or brace initialization for conversion of arithmetic types like int64_t y = int64_t{1} std::array; }

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.

Designated Initializers

Use designated initializers only in their C++20-compliant form.

Designated initializers are a syntax that allows for initializing an aggregate ["plain old struct"] by naming its fields explicitly:

struct Point { float x = 0.0; float y = 0.0; float z = 0.0; }; Point p = { .x = 1.0, .y = 2.0, // z will be 0.0 };

The explicitly listed fields will be initialized as specified, and others will be initialized in the same way they would be in a traditional aggregate initialization expression like Point{1.0, 2.0}.

Designated initializers can make for convenient and highly readable aggregate expressions, especially for structs with less straightforward ordering of fields than the Point example above.

While designated initializers have long been part of the C standard and supported by C++ compilers as an extension, only recently have they made it into the C++ standard, being added as part of C++20.

The rules in the C++ standard are stricter than in C and compiler extensions, requiring that the designated initializers appear in the same order as the fields appear in the struct definition. So in the example above, it is legal according to C++20 to initialize x and then z, but not y and then x.

Use designated initializers only in the form that is compatible with the C++20 standard: with initializers in the same order as the corresponding fields appear in the struct definition.

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 lookup_table = ...; std::vector 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 = 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[{1, 2, 3}]] [] { ... }

The type of a capture with an initializer is deduced using the same rules as auto.

• Lambdas are much more concise than other ways of defining function objects to be passed to STL algorithms, which can be a readability improvement.
• Appropriate use of default captures can remove redundancy and highlight important exceptions from the default.
• Lambdas, std::function, and std::bind can be used in combination as a general purpose callback mechanism; they make it easy to write functions that take bound functions as arguments.

• Variable capture in lambdas can be a source of dangling-pointer bugs, particularly if a lambda escapes the current scope.
• Default captures by value can be misleading because they do not prevent dangling-pointer bugs. Capturing a pointer by value doesn't cause a deep copy, so it often has the same lifetime issues as capture by reference. This is especially confusing when capturing this by value, since the use of this is often implicit.
• Captures actually declare new variables [whether or not the captures have initializers], but they look nothing like any other variable declaration syntax in C++. In particular, there's no place for the variable's type, or even an auto placeholder [although init captures can indicate it indirectly, e.g., with a cast]. This can make it difficult to even recognize them as declarations.
• Init captures inherently rely on type deduction, and suffer from many of the same drawbacks as auto, with the additional problem that the syntax doesn't even cue the reader that deduction is taking place.
• It's possible for use of lambdas to get out of hand; very long nested anonymous functions can make code harder to understand.

• Use lambda expressions where appropriate, with formatting as described below.
• Prefer explicit captures if the lambda may escape the current scope. For example, instead of:{ Foo foo; ... executor->Schedule[[&] { Frobnicate[foo]; }] ... } // BAD! The fact that the lambda makes use of a reference to `foo` and // possibly `this` [if `Frobnicate` is a member function] may not be // apparent on a cursory inspection. If the lambda is invoked after // the function returns, that would be bad, because both `foo` // and the enclosing object could have been destroyed. prefer to write:{ Foo foo; ... executor->Schedule[[&foo] { Frobnicate[foo]; }] ... } // BETTER - The compile will fail if `Frobnicate` is a member // function, and it's clearer that `foo` is dangerously captured by // reference.
• Use default capture by reference [[&]] only when the lifetime of the lambda is obviously shorter than any potential captures.
• Use default capture by value [[=]] only as a means of binding a few variables for a short lambda, where the set of captured variables is obvious at a glance, and which does not result in capturing this implicitly. [That means that a lambda that appears in a non-static class member function and refers to non-static class members in its body must capture this explicitly or via [&].] Prefer not to write long or complex lambdas with default capture by value.
• Use captures only to actually capture variables from the enclosing scope. Do not use captures with initializers to introduce new names, or to substantially change the meaning of an existing name. Instead, declare a new variable in the conventional way and then capture it, or avoid the lambda shorthand and define a function object explicitly.
• See the section on type deduction for guidance on specifying the parameter and return types.

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 GoogleTest, 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.

Other C++ Features

As with Boost, some modern C++ extensions encourage coding practices that hamper readabilityfor 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:

• Compile-time rational numbers [], because of concerns that it's tied to a more template-heavy interface style.
• The and headers, because many compilers do not support those features reliably.
• The header, which does not have sufficient support for testing, and suffers from inherent security vulnerabilities.

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, 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.

• Nonstandard extensions may provide useful features that do not exist in standard C++.
• Important performance guidance to the compiler can only be specified using extensions.

• Nonstandard extensions do not work in all compilers. Use of nonstandard extensions reduces portability of code.
• Even if they are supported in all targeted compilers, the extensions are often not well-specified, and there may be subtle behavior differences between compilers.
• Nonstandard extensions add to the language features that a reader must know to understand the code.

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.

• Aliases can improve readability by simplifying a long or complicated name.
• Aliases can reduce duplication by naming in one place a type used repeatedly in an API, which might make it easier to change the type later.

• When placed in a header where client code can refer to them, aliases increase the number of entities in that header's API, increasing its complexity.
• Clients can easily rely on unintended details of public aliases, making changes difficult.
• It can be tempting to create a public alias that is only intended for use in the implementation, without considering its impact on the API, or on maintainability.
• Aliases can create risk of name collisions
• Aliases can reduce readability by giving a familiar construct an unfamiliar name
• Type aliases can create an unclear API contract: it is unclear whether the alias is guaranteed to be identical to the type it aliases, to have the same API, or only to be usable in specified narrow ways

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; } // 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 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;

Inclusive Language

In all code, including naming and comments, use inclusive language and avoid terms that other programmers might find disrespectful or offensive [such as "master" and "slave", "blacklist" and "whitelist", or "redline"], even if the terms also have an ostensibly neutral meaning. Similarly, use gender-neutral language unless you're referring to a specific person [and using their pronouns]. For example, use "they"/"them"/"their" for people of unspecified gender [even when singular], and "it"/"its" for software, computers, and other things that aren't people.

Naming

The most important consistency rules are those that govern naming. The style of a name immediately informs us what sort of thing the named entity is: a type, a variable, a function, a constant, a macro, etc., without requiring us to search for the declaration of that entity. The pattern-matching engine in our brains relies a great deal on these naming rules.

Naming rules are pretty arbitrary, but we feel that consistency is more important than individual preferences in this area, so regardless of whether you find them sensible or not, the rules are the rules.

General Naming Rules

Optimize for readability using names that would be clear even to people on a different team.

Use names that describe the purpose or intent of the object. Do not worry about saving horizontal space as it is far more important to make your code immediately understandable by a new reader. Minimize the use of abbreviations that would likely be unknown to someone outside your project [especially acronyms and initialisms]. Do not abbreviate by deleting letters within a word. As a rule of thumb, an abbreviation is probably OK if it's listed in Wikipedia. Generally speaking, descriptiveness should be proportional to the name's scope of visibility. For example, n may be a fine name within a 5-line function, but within the scope of a class, it's likely too vague.

class MyClass { public: int CountFooErrors[const std::vector& foos] { int n = 0; // Clear meaning given limited scope and context for [const auto& foo : foos] { ... ++n; } return n; } void DoSomethingImportant[] { std::string fqdn = ...; // Well-known abbreviation for Fully Qualified Domain Name } private: const int kMaxAllowedConnections = ...; // Clear meaning within context }; class MyClass { public: int CountFooErrors[const std::vector& foos] { int total_number_of_foo_errors = 0; // Overly verbose given limited scope and context for [int foo_index = 0; foo_index < foos.size[]; ++foo_index] { // Use idiomatic `i` ... ++total_number_of_foo_errors; } return total_number_of_foo_errors; } void DoSomethingImportant[] { int cstmr_id = ...; // Deletes internal letters } private: const int kNum = ...; // Unclear meaning within broad scope };

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

For the purposes of the naming rules below, a "word" is anything that you would write in English without internal spaces. This includes abbreviations, such as acronyms and initialisms. For names written in mixed case [also sometimes referred to as "camel case" or "Pascal case"], in which the first letter of each word is capitalized, prefer to capitalize abbreviations as single words, e.g., StartRpc[] rather than StartRPC[].

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

File Names

Filenames should be all lowercase and can include underscores [_] or dashes [-]. Follow the convention that your project uses. If there is no consistent local pattern to follow, prefer "_".

Examples of acceptable file names:

• my_useful_class.cc
• my-useful-class.cc
• myusefulclass.cc
• myusefulclass_test.cc // _unittest and _regtest are deprecated.

C++ files should end in .cc and header files should end in .h. Files that rely on being textually included at specific points should end in .inc [see also the section on self-contained headers].

Do not use filenames that already exist in /usr/include, such as db.h.

In general, make your filenames very specific. For example, use http_server_logs.h rather than logs.h. A very common case is to have a pair of files called, e.g., foo_bar.h and foo_bar.cc, defining a class called FooBar.

Type Names

Type names start with a capital letter and have a capital letter for each new word, with no underscores: MyExcitingClass, MyExcitingEnum.

The names of all types classes, structs, type aliases, enums, and type template parameters have the same naming convention. Type names should start with a capital letter and have a capital letter for each new word. No underscores. For example:

// classes and structs class UrlTable { ... class UrlTableTester { ... struct UrlTableProperties { ... // typedefs typedef hash_map PropertiesMap; // using aliases using PropertiesMap = hash_map; // enums enum class UrlTableError { ...

Variable Names

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

Common Variable names

For example:

std::string table_name; // OK - lowercase with underscore. std::string tableName; // Bad - mixed case.

Class Data Members

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

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

Struct Data Members

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

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

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

Constant Names

Variables declared constexpr or const, and whose value is fixed for the duration of the program, are named with a leading "k" followed by mixed case. Underscores can be used as separators in the rare cases where capitalization cannot be used for separation. For example:

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

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

Function Names

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

Ordinarily, functions should start with a capital letter and have a capital letter for each new word.

AddTableEntry[] DeleteUrl[] OpenFileOrDie[]

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

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

Namespace Names

Namespace names are all lower-case, with words separated by underscores. Top-level namespace names are based on the project name . Avoid collisions between nested namespaces and well-known top-level namespaces.

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

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

Avoid nested namespaces that match well-known top-level namespaces. Collisions between namespace names can lead to surprising build breaks because of name lookup rules. In particular, do not create any nested std namespaces. Prefer unique project identifiers [websearch::index, websearch::index_util] over collision-prone names like websearch::util. Also avoid overly deep nesting namespaces [TotW #130].

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

Enumerator Names

Enumerators [for both scoped and unscoped enums] should be named like constants, not like macros. That is, use kEnumName not ENUM_NAME.

enum class UrlTableError { kOk = 0, kOutOfMemory, kMalformedInput, }; enum class AlternateUrlTableError { OK = 0, OUT_OF_MEMORY = 1, MALFORMED_INPUT = 2, };

Until January 2009, the style was to name enum values like macros. This caused problems with name collisions between enum values and macros. Hence, the change to prefer constant-style naming was put in place. New code should use constant-style naming.

Macro Names

You're not really going to define a macro, are you? If you do, they're like this: MY_MACRO_THAT_SCARES_SMALL_CHILDREN_AND_ADULTS_ALIKE.

Please see the description of macros; in general macros should not be used. However, if they are absolutely needed, then they should be named with all capitals and underscores.

#define ROUND[x] ... #define PI_ROUNDED 3.0

Exceptions to Naming Rules

If you are naming something that is analogous to an existing C or C++ entity then you can follow the existing naming convention scheme.

bigopen[]function name, follows form of open[]uinttypedefbigposstruct or class, follows form of possparse_hash_mapSTL-like entity; follows STL naming conventionsLONGLONG_MAXa constant, as in INT_MAX

Comments

Comments are absolutely vital to keeping our code readable. The following rules describe what you should comment and where. But remember: while comments are very important, the best code is self-documenting. Giving sensible names to types and variables is much better than using obscure names that you must then explain through comments.

When writing your comments, write for your audience: the next contributor who will need to understand your code. Be generous the next one may be you!

Comment Style

Use either the // or /* */ syntax, as long as you are consistent.

You can use either the // or the /* */ syntax; however, // is much more common. Be consistent with how you comment and what style you use where.

File Comments

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

Legal Notice and Author Line

If you make significant changes to a file with an author line, consider deleting the author line. New files should usually not contain copyright notice or author line.

File Contents

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

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

Class Comments

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

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

The class comment should provide the reader with enough information to know how and when to use the class, as well as any additional considerations necessary to correctly use the class. Document the synchronization assumptions the class makes, if any. If an instance of the class can be accessed by multiple threads, take extra care to document the rules and invariants surrounding multithreaded use.

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

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

Function Comments

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

Function Declarations

Almost every function declaration should have comments immediately preceding it that describe what the function does and how to use it. These comments may be omitted only if the function is simple and obvious [e.g., simple accessors for obvious properties of the class]. Private methods and functions declared in `.cc` files are not exempt. Function comments should be written with an implied subject of This function and should start with the verb phrase; for example, "Opens the file", rather than "Open the file". In general, these comments do not describe how the function performs its task. Instead, that should be left to comments in the function definition.

Types of things to mention in comments at the function declaration:

• What the inputs and outputs are. If function argument names are provided in `backticks`, then code-indexing tools may be able to present the documentation better.
• For class member functions: whether the object remembers reference arguments beyond the duration of the method call, and whether it will free them or not.
• If the function allocates memory that the caller must free.
• Whether any of the arguments can be a null pointer.
• If there are any performance implications of how a function is used.
• If the function is re-entrant. What are its synchronization assumptions?

Here is an example:

// Returns an iterator for this table, positioned at the first entry // lexically greater than or equal to `start_word`. If there is no // such entry, returns a null pointer. The client must not use the // iterator after the underlying GargantuanTable has been destroyed. // // This method is equivalent to: // std::unique_ptr iter = table->NewIterator[]; // iter->Seek[start_word]; // return iter; std::unique_ptr GetIterator[absl::string_view start_word] const;

However, do not be unnecessarily verbose or state the completely obvious.

When documenting function overrides, focus on the specifics of the override itself, rather than repeating the comment from the overridden function. In many of these cases, the override needs no additional documentation and thus no comment is required.

When commenting constructors and destructors, remember that the person reading your code knows what constructors and destructors are for, so comments that just say something like "destroys this object" are not useful. Document what constructors do with their arguments [for example, if they take ownership of pointers], and what cleanup the destructor does. If this is trivial, just skip the comment. It is quite common for destructors not to have a header comment.

Function Definitions

If there is anything tricky about how a function does its job, the function definition should have an explanatory comment. For example, in the definition comment you might describe any coding tricks you use, give an overview of the steps you go through, or explain why you chose to implement the function in the way you did rather than using a viable alternative. For instance, you might mention why it must acquire a lock for the first half of the function but why it is not needed for the second half.

Note you should not just repeat the comments given with the function declaration, in the .h file or wherever. It's okay to recapitulate briefly what the function does, but the focus of the comments should be on how it does it.

Variable Comments

In general the actual name of the variable should be descriptive enough to give a good idea of what the variable is used for. In certain cases, more comments are required.

Class Data Members

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

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

private: // Used to bounds-check table accesses. -1 means // that we don't yet know how many entries the table has. int num_total_entries_;

Global Variables

All global variables should have a comment describing what they are, what they are used for, and [if unclear] why they need to be global. For example:

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

Implementation Comments

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

Explanatory Comments

Tricky or complicated code blocks should have comments before them.

Function Argument Comments

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

• If the argument is a literal constant, and the same constant is used in multiple function calls in a way that tacitly assumes they're the same, you should use a named constant to make that constraint explicit, and to guarantee that it holds.
• Consider changing the function signature to replace a bool argument with an enum argument. This will make the argument values self-describing.
• For functions that have several configuration options, consider defining a single class or struct to hold all the options , and pass an instance of that. This approach has several advantages. Options are referenced by name at the call site, which clarifies their meaning. It also reduces function argument count, which makes function calls easier to read and write. As an added benefit, you don't have to change call sites when you add another option.
• Replace large or complex nested expressions with named variables.
• As a last resort, use comments to clarify argument meanings at the call site.

Consider the following example:// What are these arguments? const DecimalNumber product = CalculateProduct[values, 7, false, nullptr];

versus:

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

Don'ts

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

Compare this:// Find the element in the vector. '

You should do this consistently within a single file. 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 int* x, *y; // Disallowed - no & or * in multiple declaration; inconsistent spacing 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, but be careful about overuse. 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

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 v[100, 1]; // A vector containing 100 items: All 1s. std::vector 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:

• Any base class name should be on the same line as the subclass name, subject to the 80-column limit.
• The public:, protected:, and private: keywords should be indented one space.
• Except for the first instance, these keywords should be preceded by a blank line. This rule is optional in small classes.
• Do not leave a blank line after these keywords.
• The public section should be first, followed by the protected and finally the private section.
• See Declaration Order for rules on ordering declarations within each of these sections.

Constructor Initializer Lists

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

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

Horizontal Whitespace

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

General

int i = 0; // Two spaces before end-of-line comments. 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 // [ in a cast std::vector x; y = static_cast[x]; // Spaces between type and pointer are OK, but be consistent. std::vector 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:

• Blank lines at the beginning or end of a function do not help readability.
• Blank lines inside a chain of if-else blocks may well help readability.
• A blank line before a comment line usually helps readability the introduction of a new comment suggests the start of a new thought, and the blank line makes it clear that the comment goes with the following thing instead of the preceding.
• Blank lines immediately inside a declaration of a namespace or block of namespaces may help readability by visually separating the load-bearing content from the [largely non-semantic] organizational wrapper. Especially when the first declaration inside the namespace[s] is preceded by a comment, this becomes a special case of the previous rule, helping the comment to "attach" to the subsequent declaration.

Exceptions to the Rules

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

Windows Code

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

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

• Do not use Hungarian notation [for example, naming an integer iNum]. Use the Google naming conventions, including the .cc extension for source files.
• Windows defines many of its own synonyms for primitive types, such as DWORD, HANDLE, etc. It is perfectly acceptable, and encouraged, that you use these types when calling Windows API functions. Even so, keep as close as you can to the underlying C++ types. For example, use const TCHAR * instead of LPCTSTR.
• When compiling with Microsoft Visual C++, set the compiler to warning level 3 or higher, and treat all warnings as errors.
• Do not use #pragma once; instead use the standard Google include guards. The path in the include guards should be relative to the top of your project tree.
• In fact, do not use any nonstandard extensions, like #pragma and __declspec, unless you absolutely must. Using __declspec[dllimport] and __declspec[dllexport] is allowed; however, you must use them through macros such as DLLIMPORT and DLLEXPORT, so that someone can easily disable the extensions if they share the code.

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

• Normally we strongly discourage the use of multiple implementation inheritance; however, it is required when using COM and some ATL/WTL classes. You may use multiple implementation inheritance to implement COM or ATL/WTL classes and interfaces.
• Although you should not use exceptions in your own code, they are used extensively in the ATL and some STLs, including the one that comes with Visual C++. When using the ATL, you should define _ATL_NO_EXCEPTIONS to disable exceptions. You should investigate whether you can also disable exceptions in your STL, but if not, it is OK to turn on exceptions in the compiler. [Note that this is only to get the STL to compile. You should still not write exception handling code yourself.]
• The usual way of working with precompiled headers is to include a header file at the top of each source file, typically with a name like StdAfx.h or precompile.h. To make your code easier to share with other projects, avoid including this file explicitly [except in precompile.cc], and use the /FI compiler option to include the file automatically.
• Resource headers, which are usually named resource.h and contain only macros, do not need to conform to these style guidelines.