Thread: Pointer to member function as callback function

Hello

Let me define the problem:

You have a class with a function that enumerates elements in a list. For each object the function calls the supplied callback function:

Code:

void CMyClass::EnumObjects(void (*EnumObjectCallback)(OBJECT))
{
    for each object in the objects list{
        EnumObjectCallback(object);
    }
}

Now, this would work great for a case where the callbackfunction wouldn't be in a class

Code:

void PrintObject(OBJECT);
-----
pMyClass->EnumObjects(&PrintObject);

But when the callback function is in another class, there will be problem.

Code:

void CAnotherClass:rintObject(OBJECT);
-----
pMyClass->EnumObjects(&pAnotherClass->PrintObject);

This is where the problem is. Since I wont be knowing the classtype of the class I cant define the enum finction as EnumObjects(void(CAnotherClass::*EnumObjectCallbac k)(OBJECT)) that would otherwise be an solution. But then I could liklewise write EnumObjects(CAnotherClass *pAnotherClass), and this would beat the purpose of the callback.

So the question is:
How do I use pointer to member function as callback functions?

Note that this is NOT a question about STL, since most experienced programmers out there will see that the EnumObjects function does exactly the same thing as std::for_each algo. You don't need to point that out! BTW the for_each algo even have the same callback problem! Also, it is not a question about linked lists and containers, unless your super linked list solves the problem!

Nobody knows??

Hi ninebit,

in order to invoke a member function of an instance of a class you will need a pointer to that member function and a pointer (or a reference) to the right instance. If you are a bit cofused I'll give you an example:

Code:

class Object{
public:
	void ouch();
};

void Object::ouch()
{
	cout << "ouch";
}

class B
{
public:
	void first(Object* o)
	{
		o->ouch();
	}
	void second(Object* o)
	{
		o->ouch(); cout << "second";
	}
};
class A
{
public:
	Object o1;
	Object o2;

	typedef void (*externalfunc)(Object*);
	void enumobj(externalfunc func)
	{
		func(&o1);
		func(&o2);
	}

	typedef void (B::*member_of_B_func)(Object*);
	void enumobjB(B*instanceB, member_of_B_func func)
	{
		(instanceB->*func)(&o1);
		(instanceB->*func)(&o2);
	}
};

so you can easily use something similar to:

Code:

	A a;
	B b;
	a.enumobjB(&b, B::first);
	a.enumobjB(&b, B::second);

the tricky moment here is the use of the operator
'->*' or '.*'

damyan

So there really isn't a way to use pointer to member functions directly without knowing at design time which class to use... I'm amazed that a OOP language can't do this... Or does OOP really mean "do not use that! its old!"?

Well actually there is another OOP solution: templates.

By templateate(?) what you did there in the code, i can get a step closer.

Thanx for the help mate! Happy coding!

Yes. The templates are the solution.
Here is one possible implementation of the above example using a template member function:

Code:

	template <class T>
	void enumobj(T* pInstance_of_T, void (T::*member_func_of_T)(Object*))
	{
		(pInstance_of_T->*member_func_of_T)(&o1);
		(pInstance_of_T->*member_func_of_T)(&o2);
	}

damyan

What does bumping own thread mean?


I'll look into template functors! Thanx for the help!

There is a way to do this. But it's a bit complicated. (well not really, rather far fetched)

here goes:

the callback function in the class should be declared static. This is because normal functions also have to know which class there in (this variable). But static functions do not. Then if you still want to be able to acces data in the calling class, you have to give an extra parameter: the this var of the class.

So you'll have something like this:

Code:

void callEnum(void (*fnc)(void *, int), void * user)
{
  for (int i=0; i<10; i++)
  {
    (*fnc)(user, i);
  }
}

class callBack
{
  static void fnc(void *, int);
  void fncThis(int);
  void enum();
};

void callBack::fnc(void * pntr, int val)
{
  ((callBack *)pntr)->fncThis(val);
}

void callBack::fncThis(int val)
{
  printf("%i\n", val);
}

void callBack::enumFnc()
{
  callEnum(fnc, this);
}

call callBack.enumFnc() to print 0 to 9.


HtH.

Joren