W.A.P to use of virtual function

#include<iostream.h>

#include<conio.h>

            class cpolygon {

                        protected:

                                    int l,h;

                        public:

                                    void setval(int a, int b){

                                                l=a;

                                                h=b;

                                    }

                                    virtual int area(void) {

                                                return 0;

                                    }

            };

            class crectangle : public cpolygon {

                        public:

                                    int area(void) {

                                                return(l*h);

                                    }

            };

            class ctriangle : public cpolygon {

                        public:

                                    int area(void) {

                                     return((l*h)/2);

                                     }

            };

void main() {

            clrscr();

            crectangle crect;

            ctriangle tangle;

            cpolygon cpoly;

            cpolygon *poly1 = &crect;

            cpolygon *poly2 = &tangle;

            cpolygon *poly3 = &cpoly;

            poly1->setval(4,5);

            poly2->setval(4,5);

            poly3->setval(4,5);

            cout<<"Area : = "<<poly1->area();

            cout<<"\nArea : = "<<poly2->area();

            cout<<"\nArea : = "<<poly3->area();

            getch();

}

OUTPUT:-

Area : = 20

Area : = 10

Area : = 0