Overriding vs Virtual

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.

Solution 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.

Solution 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.