C++ program to demonstrate abstract class

#include<iostream>
using namespace std;
class shape
{
public:
double ar;
virtual void area()=0; //pure virtual function

};
class circle:public shape
{
double rad;
public:
circle(double r)
{
rad=r;
}
void area()
{
ar=3.14*rad*rad;
cout<<"area of circle "<<ar<<endl;
}
};
class triangle:public shape
{
double height,base;
public:
triangle(double h,double b)
{
height=h;
base=b;
}
void area()
{
ar=0.5*base*height;
cout<<"area of triangle "<<ar<<endl;
}
};
class rectangle:public shape
{
double lenght,bredth;
public:
rectangle(double l,double b)
{
lenght=l;
bredth=b;
}
void area()
{
ar=lenght*bredth;
cout<<"area of rectangle "<<ar<<endl;
}
};
int main()
{
shape *s[3];
circle c(10.2);
triangle t(12.5,8.4);
rectangle r(5.0,8.0);

s[0]=&c;
s[1]=&t;
s[2]=&r;

for(int i=0;i<3;i++)
s[i]->area();
return 0;
}