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There is such thing and it's in the latest C++ draft:
At § 7.1.2 .4:
An inline function shall be defined in every translation unit in which it is odr-used and shall have exactly the same definition in every case (3.2). [ Note: A call to the inline function may be encountered before its definition appears in the translation unit. — end note ] If the definition of a function appears in a translation unit before its first declaration as inline, the program is ill-formed. If a function 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 with external linkage shall have the same address in all translation units. A static local variable in an extern inline function always refers to the same object. A type defined within the body of an extern inline function is the same type in every translation unit.
Some insights on what is this and when to use it?
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The extern-inline module supports the use of C99-style extern inline functions so that the code still runs on compilers that do not support this feature correctly. C code ordinarily should not use inline .
Inline function in C++ is an enhancement feature that improves the execution time and speed of the program. The main advantage of inline functions is that you can use them with C++ classes as well.
Similarly, if you define a function as extern inline , or redeclare an inline function as extern , the function simply becomes a regular, external function and is not inlined. End of C only. Beginning of C++ only.
Advantages :- 1) It does not require function calling overhead. 2) It also save overhead of variables push/pop on the stack, while function calling. 3) It also save overhead of return call from a function. 4) It increases locality of reference by utilizing instruction cache.
extern is redundant for functions, so it is pointless to declare a function extern inline. If, for example you declared a function inline at global scope, the rules of this section would apply. Likewise if you declared a class at global scope and defined a member function within the class definition, as such a function would be implicitly inline.
The question of when you should declare a function inline is a can of worms I'm not inclined to open here. See: When should I write the keyword 'inline' for a function/method?
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