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，3个图形的
#include<iostream>
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 w,double h):width(w),height(h)
{
}
virtual double area() const
{
return width*height;
}
protected:
double width,height;
};
class Triangle:public Shape
{
public:
Triangle(double w,double h):width(w),height(h)
{
}
virtual double area() const
{
return 0.5*width*height;
}
protected:
double width,height;
};
void printArea(const Shape &s)
{
cout<<s.area()<<endl;
}
int main()
{
Circle circle(12.6);
cout<<"area of circle = ";
printArea(circle);
Rectangle rectangle(4.5,8.4);
cout<<"area of rectangle = ";
printArea(rectangle);
Triangle triangle(4.5,8.4);
cout<<"area of triangle = ";
printArea(triangle);
return 0;
}