基类派生

/* C++代码 */


#include <iostream>
#include<iomanip>
using namespace std;
class Shape
{
    public:
         void printArea();
};
class Circle:public Shape
{
    private:
         double radius;
    public:
          Circle(double r):radius(r){}
          friend void printArea(Circle);

};
void printArea(Circle c)
          {
              cout<<3.14*c.radius*c.radius<<endl;
          }
class Rectangle:public Shape
{
    private:
          double length,width;
    public:
          Rectangle(double l,double w):length(l),width(w){}
          friend void printArea(Rectangle);

};
void printArea(Rectangle a)
          {
              cout<<a.length*a.width<<endl;
          }
class Triangle:public Shape
{
    private:
          double side,high;
    public:
          Triangle(double s,double h):side(s),high(h){}
          friend void printArea(Triangle);

};
void printArea(Triangle b)
{
              cout<<0.5*b.side*b.high<<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;
}