由Shape派生出的抽象基类

Description

编写一个程序，声明抽象基类Shape，由它派生出3个派生类： Circle(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（结果保留两位小数），3个图形的数据在定义对象时给定。

Input

圆的半径
矩形的边长
三角形的底与高

Output

圆的面积
矩形的面积
三角形的面积

Sample Input

12.64.5 8.44.5 8.4

Sample Output

area of circle = 498.76area of rectangle = 37.80area of triangle = 18.90

Code

#include <iostream>#include <iomanip>using namespace std;class Shape{public:    virtual double area()=0;//定义一个纯虚函数};void printArea(Shape &shape){    cout<<shape.area()<<endl;}class Circle:public Shape{protected:    double r;public:    Circle(double a):r(a){}    virtual double area()    {        return (3.14159*r*r);    }};class Rectangle:public Shape{protected:    double a;    double b;public:    Rectangle(double x,double y):a(x),b(y){}    virtual double area()    {        return (a*b);    }};class Triangle:public Shape{protected:    double w;    double h;public:    Triangle(double x,double y):w(x),h(y){}    virtual double area()    {        return (w*h/2);    }};int main(){    float r,a,b,w,h;    cout<<fixed<<setprecision(2);    cin>>r;    Circle circle(r);    cout<<"area of circle = ";    printArea(circle);    cin>>a>>b;    Rectangle rectangle(a,b);    cout<<"area of rectangle = ";    printArea(rectangle);    cin>>w>>h;    Triangle triangle(w,h);    cout<<"area of triangle = ";    printArea(triangle);    return 0;}