9 Declarations [dcl.dcl]

9.1 Specifiers [dcl.spec]

9.1.6 The inline specifier [dcl.inline]

The inline specifier can be applied only to the declaration or definition of a variable or function.
A function declaration ([dcl.fct], [class.mfct], [class.friend]) with an inline specifier declares an inline function.
The inline specifier indicates to the implementation that inline substitution of the function body at the point of call is to be preferred to the usual function call mechanism.
An implementation is not required to perform this inline substitution at the point of call; however, even if this inline substitution is omitted, the other rules for inline functions specified in this subclause shall still be respected.
A variable declaration with an inline specifier declares an inline variable.
A function defined within a class definition is an inline function.
The inline specifier shall not appear on a block scope declaration.94
If the inline specifier is used in a friend function declaration, that declaration shall be a definition or the function shall have previously been declared inline.
An inline function or variable shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case ([basic.def.odr]).
[Note
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A call to the inline function or a use of the inline variable may be encountered before its definition appears in the translation unit.
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If the definition of a function or variable appears in a translation unit before its first declaration as inline, the program is ill-formed.
If a function or variable with external linkage is declared inline in one translation unit, it shall be declared inline in all translation units in which it appears; no diagnostic is required.
An inline function or variable with external linkage shall have the same address in all translation units.
[Note
:
A static local variable in an inline function with external linkage always refers to the same object.
A type defined within the body of an inline function with external linkage is the same type in every translation unit.
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The inline keyword has no effect on the linkage of a function.