Virtual function in OOP

#include<iostream.h>
#include<conio.h>

class A
{
    public:

        virtual void show()           //this function enables us to execute different functions by the same function call
    {
    cout<<"parent class A:    ";
    }
};

class B:public A
{
    public:

        void show()
    {
    cout<<"child class B:    ";
    }
};

class C:public A
{
    public:

        void show()
    {
    cout<<"child class C:    ";
    }
};

        void main()

{
    A obj1;
    B obj2;
    C obj3;
    A * ptr;
    ptr = & obj1;          //ampercent is used to access address of variable and store in a pointer
    ptr -> show();
    ptr = & obj2;
   ptr -> show();
    ptr= & obj3;
    ptr -> show();
    getch();
}