Virtual functions and polymorphism

Question

Suppose I have this:

class A
{
    public:
    virtual int hello(A a);
};

class B : public A
{
   public:
   int hello(B b){ bla bla };
};

So,

Answer 1

What you are doing there is overloading not overriding, i.e. it's as if class B is:

class B
{
public:
  int hello(A a) {...}
  int hello(B b) {...}
};

You have two functions of the same name, but with different signatures, which makes them different functions (just like the standard library has different functions for abs(float) and abs(double) etc.)

If you want to override, then you need to have the same signature, i.e. class B's hello needs to take a parameter of type A. That way, when you call hello on an object of class B, it will use class B's hello rather than class A's.

If you actually want class B's hello to only accept objects of type B then what you have is fine, although you probably want to make class A's hello non-virtual as you are not really wanting to override it -- you are defining a new function with new parameters and new behaviour.