Function pointers to member functions of a parent class

Question

I have been trying to understand function pointers in C++ so that I can successfully use them in one of my projects. I am running into a logic problem though. Say we have two classes: a parent class and a child class which inherits the parent class.

class Parent{
    ...other stuff
   void (Parent::*ptr2func) ();
    ...other stuff
};

Then we have a child class:

class Child : public Parent{
       ...other stuff
       void afunc();
       ...other stuff
};

I wanted to connect the pointer of the parent class to the afunc() function of the child class. In the constructor of the child class is where I tried to do it like so:

  Child::Child()
  {
    ptr2func = &Child::afunc;
  }

This returned an expected error: cannot convert void (Child::*)() to void (Parent::*)() in assignment.

I was afraid that such a thing would happen. But how do I get over this? Can't I link a function pointer to a member function of a parent class to a member function of the child class? As you can understand I am trying to achieve polymorphism through function pointers just for experimenting and without using virtual functions. I was lead to believe that this is possible. What is my mistake? Or is it just not possible?

Answer 1

A void (Parent::*)() takes an object of type Parent. The function Child::afunc requires an object of type Child. Converting the pointer to void (Parent::*)() would therefore allow for an invalid call.

Member function pointers can't really be used to implement polymorphism. The traditional way to implement polymorphism without using virtual is to use regular function pointers:

struct MyBaseClass
{
    void (*function)(MyBaseClass* this_pointer);
};

Answer 2

It would be dangerous if that conversion worked as you would be able to call a function in class Child with a pointer or reference to a Parent which might not be of type Child.

If you are passing in a function and a pointer/reference you could use a boost::bind to achieve it.