Defining methods outside of a class's scope via function pointers - c++

Question

Preface
I am trying to create an event system. An event will do nothing by default. If an object needs to opt into the event system, then all that is required is that the programmer defines a callable function for the listener.
In theory, this sounds great, but in reality I am not sure if it is possible to implement this sort of thing within C++. Here is an example of what I imagine the code to look like.
Outline
#include <iostream>
enum Color{ red, green, blue };
class Event {
public:
// Would be called within the context of a loop.
// Each time we go through the loop, we would have different input.
void listen(const Color& color) {
// If the callable is defined, then call it.
if(callable) callable(color);
}
// We would pass in function to be called within listen()
void defineAction(void(*function)(const Color &color)) {
callable = function;
}
private:
void (*callable)(const Color& color) = nullptr;
};
class Greeter: public Event {
void greet() { std::cout << "Hello" << std::endl; }
};
int main(int argc, char** argv) {
Greeter greeter;
// This doesn't work for two reasons. We cannot define a function here,
// and scoping rules wont allow for an implicit this->greet().
// Ideally, I would like to do this inline with a lambda
/*void greetIfRed(const Color& color) {
if(color == red)
// call greet().
}
greeter.defineAction(&greetIfRed);
*/
}
Expanding on this idea
Basically, the Event class will receive something that will be changing each time we go through the event loop. I chose color for this example, but it could be mouse coordinates, an integer, or something else. That type will need to be strictly defined before hand.
In my design, I don't want it to matter what classes we have within the event loop. We could have a greeter, a player, or something that just says farewell. I imagine that the event loop would look something like this. (psuedocode)
while(event) {
greeter.listen(event)
player.listen(event)
text.listen(event) // e.g.,the text could change color if the mouse is over it,
}
The Question
Is it possible to define a method outside a class's scope via a function pointer that utilizes said class's methods? This would need to work for a sub class. The class Greet and the function greetIfRed capture the functionality that I am trying to achieve.

Answer 1

So, I experimented with many suggestions, and I managed to create a working example without many changes. I compiled this with C++14.
#include <iostream>
#include <functional>
enum Color{ red, green, blue };
class Event {
public:
void listen(const Color& color) {
if(callable) callable(color);
}
void defineAction( std::function<void(const Color& color)> function) {
callable = function;
}
protected:
std::function<void(const Color& color)> callable = nullptr;
};
class Greeter: public Event {
public:
void greet() { std::cout << "Hello" << std::endl; }
};
int main()
{
Greeter greeter;
greeter.defineAction([&greeter](const Color& color){
if(color == red)
greeter.greet();
});
greeter.listen(red);
greeter.listen(green);
}