形状类族中的純虚函数

#include<iostream>
using namespace std;
class Shape
{
public:
    virtual double area()const=0;
};
class Circle:public Shape
{
public:
    virtual double area()const
    {
        return 3.14*R*R;
    }
    Circle(double r):R(r){}
private:
    double R;
};
class Rectangle:public Shape
{
public:
    Rectangle(double a,double b):t1(a),t2(b){}
    virtual double area()const
    {
        return t1*t2;
    }
private:
    double t1,t2;
};
class Triangle:public Shape
{
public:
    Triangle(double a,double b):h(a),c(b){}
    virtual double area()const
 {
     return h*c;
 }
private:
    double h,c;
};
int main()
{
    Circle c1(12.6),c2(4.9);//建立Circle类对象c1,c2,参数为圆半径
    Rectangle r1(4.5,8.4),r2(5.0,2.5);//建立Rectangle类对象r1,r2,参数为矩形长、宽
    Triangle t1(4.5,8.4),t2(3.4,2.8); //建立Triangle类对象t1,t2，参数为三角形底边长与高
    Shape *pt[6]= {&c1,&c2,&r1,&r2,&t1,&t2}; //定义基类指针数组pt，使它每一个元素指向一个派生类对象
    double areas=0.0; //areas为总面积
    for(int i=0; i<6; i++)
    {
        areas=areas + pt[i]->area();
    }
    cout<<"totol of all areas="<<areas<<endl;   //输出总面积
    return 0;
}