形狀類族的中的純虛函數

#include
using namespace std;
class Shape
{
public:
    virtual double area()=0;
};
class Circle:public Shape
{
public:
    Circle(double r)
    {
        radius=r;
    }
    double area()
    {
        return 3.14*radius*radius;
    }
private:
    double radius;
};
class Rectangle:public Shape
{
public:
    Rectangle(double l,double w)
    {
        length=l;
        width=w;
    }
    double area()
    {
        return width*length;
    }
private:
    double length;
    double width;
};
class Triangle:public Shape
{
public:
    Triangle(double h,double b)
    {
        high=h;
        base=b;
    }
     double area()
    {
        return 0.5*high*base;
    }
private:
    double high;
    double base;
};
int main()
{
   Circle c1(12.6),c2(4.9);
   Rectangle r1(4.5,8.4),r2(5.0,2.5);
   Triangle t1(4.5,8.4),t2(3.4,2.8);
   Shape*pt[6]={&c1,&c2,&r1,&r2,&t1,&t2};
   double areas=0.0;
   for(int i=0;i<6;i++)
    {
        areas=areas+pt[i]->area();
    }
    cout<<"totol of all areas="<<areas<<endl;
    return 0;
}