What is an 'extern inline' function and when to use?

Question

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?

Answer 1

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?

Answer 2

I am not familiar with this draft feature in C++, and whether it made it into a standard. I think it may be borrowing a feature from C.

---

In C (I think C99?), extern inline is used in a source file, not a header file. to instruct the tooling that the object file produced from this source should be the one that holds the final implementation. Without it, the linker has to choose a site or duplicate the symbol.

For example, in my_alloc.h:

inline char *my_alloc(size_t len) { return (char*)malloc(len); }

And in my_alloc.c:

extern inline char *my_alloc(size_t len);