M0

#include <iostream>
using namespace std;
class B
{
    public:
       void display()
         { cout<<"Content of base class.\n"; }
};

class D : public B
{
    public:
       void display()
         { cout<<"Content of derived class.\n"; }
};

int main()
{
    B *b;
    D d;
    b->display();

    b = &d;    /* Address of object d in pointer variable */
    b->display();
    return 0;
}
Note: An object(either normal or pointer) of derived class is type compatible with pointer to base class. So, b = &d; is allowed in above program.

Output

Content of base class.
Content of base class.

In above program, even if the object of derived class d is put in pointer to base class, display( ) of the base class is executed( member function of the class that matches the type of pointer

he base class virtual which makes that function existing in appearance only but, you can't call that function. In order to make a function virtual, you have to add keyword virtual in front of a function.

/* Example to demonstrate the working of virtual function in C++ programming. */

#include <iostream>
using namespace std;
class B
{
    public:
     virtual void display()      /* Virtual function */
         { cout<<"Content of base class.\n"; }
};

class D1 : public B
{
    public:
       void display()
         { cout<<"Content of first derived class.\n"; }
};

class D2 : public B
{
    public:
       void display()
         { cout<<"Content of second derived class.\n"; }
};

int main()
{
    B *b;
    D1 d1;
    D2 d2;

/* b->display();  // You cannot use this code here because the function of base class is virtual. */

    b = &d1;
    b->display();   /* calls display() of class derived D1 */
    b = &d2;
    b->display();   /* calls display() of class derived D2 */
    return 0;
}
Output

Content of first derived class.
Content of second derived class.
After the function of base class is made virtual, code b->display( ) will call the display( ) of the derived class depending upon the content of pointer.

In this program, display( ) function of two different classes are called with same code which is one of the example of polymorphism in C++ programming using virtual functions.