写一个程序，定义抽象基类Shape,由它派生出3个派生类

写一个程序，定义抽象基类Shape,由它派生出3个派生类，Cirle（圆形、Rectangle（矩形）、Triangle（三角形）。用一个函数printArea 分别输出以上三者的面积，3个图形的数据在定义对象时给定、

 

#include<iostream>
using namespace std;
/*
    圆面积S=3.14*r*r
    矩形面积S=长*宽
    三角形面积S=(底*高)/2
*/
class Shape
{
public:
    virtual float area()const=0;

};
class Circle:public Shape
{
public:
    Circle(double a):r(a){}
    virtual float area()const{return 3.14*r*r;}

private:
    double r;
};

class Rectangle:public Shape
{
public:
    Rectangle(double a,double b):l(a),w(b){}
    virtual float area()const{return l*w;}

private:
    double l;
    double w;
};
class Triangle:public Shape
{
public:
    Triangle(double a,double b):d(a),h(b){}
    virtual float area()const{return (d*h)/2;}

private:
    double d;
    double h;
};
void printArea(const Shape &s1)
{
    cout<<s1.area()<<endl;
}
int main()
{

    Circle c1(6.5);
    cout<<"圆面积=";
    printArea(c1);
    Rectangle r1(3,6);
    cout<<"矩形面积=";
    printArea(r1);
    Triangle t1(4,2);
    cout<<"三角形面积=";
     printArea(t1);


}