Define a different function for each instance of a class - c++

Question

Say I have this class:
class Object {
public:
int x;
int y;
void update(SDL_Event);
void start();
};
I want to be able to make start and update change behavior from instance to instance.
I have tried using function pointers, like so:
class Object {
public:
int x;
int y;
void (*update)(SDL_Event);
void (*start)(void);
};
void teststart() {
x++;
return;
}
And in my main(), I do:
testlevel.objs[1].start = teststart;
But then, I cannot reference variables from inside the class.
I am sure there is something obvious I am overlooking, but searching online yields no results for when the pointer is in the class.
Maybe function pointers are not the right answer?

Answer 1

You would need to pass your functions a pointer/reference to the Object instance, eg:
class Object {
public:
int x;
int y;
void (*doupdate)(Object*, SDL_Event);
void (*dostart)(Object*);
void update(SDL_Event event) { if (doupdate) doupdate(this, event); }
void start() { if (dostart) dostart(this); }
};
void teststart(Object* obj) {
obj->x++;
}
void testupdate(Object* obj, SDL_Event event) {
// update obj as needed...
}
Object &obj = testlevel.objs[1];
obj.dostart = teststart;
obj.doupdate = testupdate;
Otherwise, use std::function with capturing lambdas instead:
#include <functional>
class Object {
public:
int x;
int y;
std::function<void(SDL_Event)> update;
std::function<void()> start;
};
Object *obj = &(testlevel.objs[1]);
obj->start = [obj](){
obj->x++;
};
obj->update = [obj](SDL_Event event){
// update obj as needed...
};