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。

圆的半径
矩形的边长
三角形的底与高

圆的面积
矩形的面积
三角形的面积

12.6
4.5 8.4
4.5 8.4

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

#include <iostream> 
#include <iomanip> 
using namespace std; 
class Shape 
{ 
public: 
    virtual double area() const=0; 
}; 
  

class Circle:public Shape 
{ 
public: 
Circle(double r):radius(r){} 
virtual double area() const{return 3.14159*radius*radius;} 
protected: 
    double radius; 
}; 


class Rectangle:public Shape 
{ 
public: 
Rectangle(double r,double l):width(r),height(l){} 
virtual double area() const{return width*height;} 
protected: 
    double width; 
    double height; 
}; 


class Triangle:public Shape 
{ 
public: 
Triangle(double r,double l):width(r),heigth(l){} 
virtual double area() const{return 0.5*width*heigth;} 
protected: 
    double width; 
    double heigth; 
}; 


void printArea(Shape &a) 
{ 
    cout<<a.area()<<endl; 
   
} 
  
  
  
  
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; 
}