What Does It Mean For a C++ Function To Be Inline?

Question

See title: what does it mean for a C++ function to be inline?

Answer 1

As well as the other (perfectly correct) answers about the performance implications of inline, in C++ you should also note this allows you to safely put a function in a header:

// my_thing.h
inline int do_my_thing(int a, int b) { return a + b; }

// use_my_thing.cpp
#include "my_thing.h"
...
    set_do_thing(&do_my_thing);

// use_my_thing_again.cpp
...
    set_other_do_thing(&do_my_thing);

This is because the compiler only includes the actual body of the function in the first object file that needs a regular callable function to be compiled (normally because it's address was taken, as I showed above).

Without the inline keyword, most compilers would give an error about multiple definition, eg for MSVC:

use_my_thing_again.obj : error LNK2005: "int __cdecl do_my_thing(int,int)" (?do_my_thing@@YAHHH@Z) already defined in use_my_thing.obj
<...>\Scratch.exe : fatal error LNK1169: one or more multiply defined symbols found