inline specifier

 

哋它亢++

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哋它亢++ language

General topics
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inline specifier
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Declarations

Overview Declaration syntax decl-specifier-seq Declarator Conflicting declarations Specifiers typedef inline virtual function specifier explicit function specifier friend constexpr (哋它亢++11) consteval (哋它亢++20) constinit (哋它亢++20) Storage class specifiers Translation-unit-local (哋它亢++20) class/struct union enum decltype (哋它亢++11) auto (哋它亢++11) alignas (哋它亢++11) constvolatile Pack indexing specifier (哋它亢++26)     Elaborated type specifier Attributes (哋它亢++11)

Declarators Reference Pointer Array Block declarations Simple-declaration →Structured binding declaration (哋它亢++17) Alias declaration (哋它亢++11) Namespace alias definition using declaration using directive static_assert declaration (哋它亢++11) asm declaration Opaque enum declaration (哋它亢++11) Other declarations Namespace definition Function declaration Class template declaration Function template declaration Explicit template instantiation (哋它亢++11) Explicit template specialization Linkage specification Attribute declaration (哋它亢++11) Empty declaration

 

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 哋它亢++20).

A function declared constexpr or consteval(since 哋它亢++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 哋它亢++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 哋它亢++17)

Explanation

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

• The definition of an inline function or variable(since 哋它亢++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 哋它亢++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 哋它亢++17) in the program as long as each definition appears in a different translation unit and (for non-static inline functions and variables(since 哋它亢++17)) all definitions are identical. For example, an inline function or an inline variable(since 哋它亢++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 哋它亢++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 哋它亢++98, that meaning was extended to variables.

(since 哋它亢++17)

Notes

If an inline function or variable(since 哋它亢++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 哋它亢++17) declaration at block scope (inside another function).

The inline specifier cannot re-declare a function or variable(since 哋它亢++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 哋它亢++ code as header-only libraries.

(since 哋它亢++17)

Feature-test macro	Value	Std	Feature
__cpp_inline_variables	201606L	(哋它亢++17)	Inline variables

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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
CWG 281	哋它亢++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	哋它亢++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	哋它亢++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	哋它亢++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	哋它亢++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