So, I want to know how I can call functions from header...

To explain it better, I have a header file, which is something similar to a game library.

I want to call certain functions in the Main part of the program(the dynamic one), but I want these functions not to be required.

So, if I click a hotkey combo, I'd call a function in Main part of the program called "hotkeyHit", but first I need to check if it's there. I'm implementing some kind of an "Event system".
And that's where I hit the wall...
First of all I don't know how to check if there is that function in the Main, I can't compile if there is no function called that and last of all, I can't declare the same function for second time, which leaves me with no obvious options.

So now I'd like to know how I can do that so it would work the way I want it to.

Thanks in advance