Overriding vs Virtual

Question

What is the purpose of using the reserved word virtual in front of functions? If I want a child class to override a parent function, I just declare the same function such as void draw(){}.

class Parent {  public:     void say() {         std::cout << "1";     } };  class Child : public Parent { public:     void say()     {         std::cout << "2";     } };  int main() {     Child* a = new Child();     a->say();     return 0; } 

The output is 2.

So again, why would the reserved word virtual be necessary in the header of say() ?

Thanks a bunch.

Answer 1

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.

Answer 2

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.