inline specifier

The inline specifier, when used in a function's decl-specifier-seq, declares the function to be an inline function

A function defined entirely inside a class/struct/union definition, whether it's a member function or a non-member friend function, is implicitly an inline functionunless it is attached to a named module(since C++20)

Explanation

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

• The definition of an inline function or variable (since C++17)
• An inline functionor variable(since C++17) with external linkage (e.g. not declared static

• There may be more 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 functionsand variables(since C++17)) all definitions are identical. For example, an inline functionor an inline variable(since C++17)
• It must be declared inline 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 the inline keyword was to serve as an indicator to the optimizer that inline substitution of a function

Since inline substitution is unobservable in the standard semantics, compilers are free to use inline substitution for any function that's not marked inline, and are free to generate function calls to any function marked inline

Notes

If an inline function or variable (since C++17)

The inline specifier cannot be used with a functionor variable(since C++17)

The inline specifier cannot re-declare a functionor variable(since C++17)

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 of default arguments must be the same at the end of each translation unit.

In C, inline functions do not have to be declared inline in every translation unit (at most one may be non-inline or extern inline

Keywords

inline

Example

Header "example.h":

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

Defect reports

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

See also