Define a different function for each instance of a class-CodePudding

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?

CodePudding user response：

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...
};