Theinline specifier, when used in a function'sdecl-specifier-seq, declares the function to be aninline function.

A function defined entirely inside aclass/struct/union definition, whether it's a member function or a non-memberfriend function, is implicitly an inline function unless it is attached to anamed module(since C++20).

[edit]Explanation

An inline function or inline variable(since C++17) has the following properties:

• The definition of an inline function or variable(since C++17) must be reachable in the translation unit where it is accessed (not necessarily before the point of access).
• An inline function or variable(since C++17) withexternal linkage (e.g. not declaredstatic) has the following additional properties:

• There may bemore than one definition of an inline function or variable(since C++17) in the program as long as each definition appears in a different translation unit and (for non-static inline functions and variables(since C++17)) all definitions are identical. For example, an inline function or an inline variable(since C++17) may be defined in a header file that is included in multiple source files.
• It must be declaredinline in every translation unit.
• It has the same address in every translation unit.

In an inline function,

• Function-local static objects in all function definitions are shared across all translation units (they all refer to the same object defined in one translation unit).
• Types defined in all function definitions are also the same in all translation units.

The original intent of theinline keyword was to serve as an indicator to the optimizer thatinline substitution of a function is preferred over function call, that is, instead of executing the function call CPU instruction to transfer control to the function body, a copy of the function body is executed without generating the call. This avoids overhead created by the function call (passing the arguments and retrieving the result) but it may result in a larger executable as the code for the function has to be repeated multiple times.

Since inline substitution is unobservable in the standard semantics, compilers are free to use inline substitution for any function that's not markedinline, and are free to generate function calls to any function markedinline. Those optimization choices do not change the rules regarding multiple definitions and shared statics listed above.

[edit]Notes

If an inline function or variable(since C++17) with external linkage is defined differently in different translation units, the program is ill-formed, no diagnostic required.

Theinline specifier cannot be used with a function or variable(since C++17) declaration at block scope (inside another function).

Theinline specifier cannot re-declare a function or variable(since C++17) that was already defined in the translation unit as non-inline.

The implicitly-generated member functions and any member function declared as defaulted on its first declaration are inline just like any other function defined inside a class definition.

If an inline function is declared in different translation units, the accumulated sets ofdefault arguments must be the same at the end of each translation unit.

In C, inline functions do not have to be declaredinline in every translation unit (at most one may be non-inline orexterninline), the function definitions do not have to be identical (but the behavior of the program is unspecified if it depends on which one is called), and the function-local statics are distinct between different definitions of the same function.

[edit]Keywords

inline

[edit]Example

Header "example.h":

#ifndef EXAMPLE_H#define EXAMPLE_H #include <atomic> // function included in multiple source files must be inlineinlineint sum(int a,int b){return a+ b;} // variable with external linkage included in multiple source files must be inlineinlinestd::atomic<int> counter(0); #endif

Source file #1:

#include "example.h" int a(){++counter;return sum(1,2);}

Source file #2:

#include "example.h" int b(){++counter;return sum(3,4);}

[edit]Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

[edit]See also

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