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inline specifier

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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).

A function declared constexpr or consteval(since C++20) on its first declaration is implicitly an inline function.

A deleted function is implicitly an inline function: its (deleted) definition can appear in more than one translation unit.

(since C++11)


The inline specifier, when used in a decl-specifier-seq of a variable with static storage duration (static class member or namespace-scope variable), declares the variable to be an inline variable.

A static data member declared constexpr on its first declaration is implicitly an inline variable.

(since C++17)

Contents

[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) with external linkage (e.g. not declared static) has the following additional properties:
  • 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.
  • 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.

Inline const variables at namespace scope have external linkage by default (unlike the non-inline non-volatile const-qualified variables).

(since C++17)

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 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. Those optimization choices do not change the rules regarding multiple definitions and shared statics listed above.

Because the meaning of the keyword inline for functions came to mean "multiple definitions are permitted" rather than "inlining is preferred" since C++98, that meaning was extended to variables.

(since C++17)

[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.

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.

See static data members for additional rules about inline static members.

Inline variables eliminate the main obstacle to packaging C++ code as header-only libraries.

(since C++17)
Feature-test macro Value Std Feature
__cpp_inline_variables 201606L (C++17) Inline variables

[edit] 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);
}

[edit] Defect reports

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

DR Applied to Behavior as published Correct behavior
CWG 281 C++98 a friend function declaration could use the inline specifier
even if the friended function is not an inline function
prohibit such uses
CWG 317 C++98 a function could be declared inline even if it has a non-inline
definition in the same translation unit before the declaration
the program is ill-
formed in this case
CWG 765 C++98 a type defined in an inline function might
be different in different translation units
such types are the same
in all translation units
CWG 1823 C++98 string literals in all definitions of an inline
function were shared across all translation units
the requirement is removed due to
consistency and implementations
CWG 2531 C++17 a static data member could be implicitly inline even if
it was not declared constexpr on its first declaration
it is not implicitly
inline in this case

[edit] See also

C documentation for inline