How can I call non-existent functions in C++ - c++

Question

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

Answer 1

Some example clode below to hopefully explain my comment. In this, the methods in the CBaseHandler class are intentionally empty. Your Event Handler takes a reference to a CBaseHandler and in this example tries to handle a keypress and a mouseclick. However, the concrete implementation of the handler, CHandler only implements the Keypress handler (in this case just by printing a string, but you can do whatever you want)
When the Event Handler runs, only the Keypress will do anything. Presumably you'll be wanting to call that in a loop of some description in real life.
The event handler loop in your library can now take any implementation of an event handler for whatever purposes you need.
#include <iostream>
class CBaseHandler
{
public:
virtual void handleKeyPress(int i)
{
}
virtual void handleMouseClick(int i)
{
}
};
class CHandler : public CBaseHandler
{
public:
virtual void handleKeyPress(int i)
{
std::cout << "Derived - KP" << std::endl;
}
};
void DoHandle(CBaseHandler& handler)
{
handler.handleKeyPress(4);
handler.handleMouseClick(0);
}
int main() {
CHandler myhandler;
DoHandle(myhandler);
}

Answer 2

If you have a function declared in header file but not defined anywhere, this will compile fine. However if you attempt to use this function anywhere in the code, the linker will complain when you try to compile it as it won't be able to find the definition. So you could simply add dummy definition which does nothing.
header.h
void myFunction();
source.cpp (1)
int main()
{
myFunction(); // linker error on compile
return 0;
}
source.cpp (2)
void myFunction() {}; // dummy definition
int main()
{
myFunction(); // no problem
return 0;
}

Answer 3

Ok, so after getting some help from you guys and figuring it out online, I've found a solution that works for me.
I've set it up like so:
Main code:
void mouseClicked();
void main(){
setMouseClicked(mouseClicked);
}
void mouseClicked();
Header code:
void (*mc)() = NULL;
void setMouseClicked(void (*fun)(void)){
mc = fun;
}
and then in the event function I do this:
void handleMousePress(int button, int state, int x, int y){
mouseX = x;
mouseY = y;
mouseButton = button;
if(state == GLUT_DOWN){
if(mc != NULL)
(*mc)();
}
}
I've worked on a compromise by having a declaration of the function in the main part of the code and having a setter for that function, but with that I've added an ability for renaming functions that are called when the event is triggered.
So once again, thanks for your suggestions, but at the end I've found that pointers to functions work the best.