Overriding vs Virtual

If the function were virtual, then you could do this and still get the output "2":

Parent* a = new Child();
a->say();

This works because a virtual function uses the actual type whereas a non-virtual function uses the declared type. Read up on polymorphism for a better discussion of why you'd want to do this.

Try it with:

Parent *a = new Child();
Parent *b = new Parent();

a->say();
b->say();

Without virtual, both with print '1'. Add virtual, and the child will act like a Child, even though it's being referred to via a pointer to a Parent.

This is the classic question of how polymorphism works I think. The main idea is that you want to abstract the specific type for each object. In other words: You want to be able to call the Child instances without knowing it's a child!

Here is an example: Assuming you have class "Child" and class "Child2" and "Child3" you want to be able to refer to them through their base class (Parent).

Parent* parents[3];
parents[0] = new Child();
parents[1] = new Child2();
parents[2] = new Child3();

for (int i=0; i<3; ++i)
    parents[i]->say();

As you can imagine, this is very powerful. It lets you extend the Parent as many times as you want and functions that take a Parent pointer will still work. For this to work as others mention you need to declare the method as virtual.