C ++-Explanation of Polymorphism

#include <iostream>

#include<stdio.h>using namespace std;class B{public:    void vf()    {        cout << "This is class B" << endl;    }};class D: public B{public:    void vf()    {        cout << "This is class D" << endl;    }};int main(){    B b, *pb;    D d, *pd;    pb = &b;    pb->vf();    printf("%X\n",pb);    pb = &d;    pb->vf();        // no cast needed    printf("%X\n",pb);    pd = (D*)&b;    pd->vf();    // must cast explicitly    printf("%X\n",pd);    pd = &d;    pd->vf();    printf("%X\n",pd);    return 0;}/***** result *******//*This is class B22FF17This is class B22FF16This is class D22FF17This is class D22FF16*/

It is only related to the pointer (type) pd-> vf () and the pointer pd.

/***** The call to a virtual function is determined by the pointer/reference value (Runtime polymorphism) *****/# include <iostream> using namespace std; class B {public: void virtual vf () {cout <"This is class B" <endl ;}; class D: public B {public: void vf () {cout <"This is class D" <endl ;}; int main () {B B, * pb; D d, * pd; pb = & B; pb-> vf (); pb = & d; pb-> vf (); pd = (D *) & B; pd-> vf (); pd = & d; pd-> vf (); return 0 ;} /******* result ******/* This is class BThis is class DThis is class BThis is class D */

#include <iostream>using namespace std;class B{  // virtual function in base class must be defined or be pure  // unless the base class and all its derived classes are not instantiated  public: void virtual vf();};class D: public B{  public: void vf() { cout << "This is class D" << endl; }};main(){  D d, *pd;}/*** result ***//*ERRORundefined reference to `vtable for B'*/