You may not use inline functions across cpp files like that.
An inline function -must- be defined where it is declared. Otherwise you'll get an unresolved external error in your build.
If your compiler is not from the stone-age, you may not want to explicitly declare your function inline anyway. It should optimize your code. Declaring it inline only -hints- it to be inline, it doesn't guarantee it. (at least in the MSVC compiler)