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 function unless it is attached to a named module (since C++20).

Explanation

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

1. 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).
2. An inline function or variable (since C++17) with external linkage (e.g. not declared static) has the following additional properties:

1. 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 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.
2. It must be declared inline in every translation unit.
3. 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 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 this meaning of the keyword inline is non-binding, 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. Those optimization choices do not change the rules regarding multiple definitions and shared statics listed above.

Notes

If an inline function or variable (since C++17) with external linkage is defined differently in different translation units, the behavior is undefined.

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

The inline 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 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), 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.

Example

#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

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