如何编写程序，实现由定义抽象基类Shape（形状）派生出三个派生类

定义抽象基类Shape，由它派生出3个派生类: Circle(圆形)、Rectangle(矩形)、Tr足毂忍珩iangle(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> usingnamespacestd; classShape { public:

virtualfloatprintArea()const{return0.0;}; }; classCircle:publicShape {

public: Circle(float=0); virtualfloatprintArea()const{return3.14159*radius*radius;}

protected: floatradius; }; Circle::Circle(floatr):radius(r) { } classRectangle:publicShape { public: Rectangle(float=0,float=0);

virtualfloatprintArea()const; protected: floatheight; floatwidth; };

Rectangle::Rectangle(floatw,floath):width(w),height(h){ }

floatRectangle::printArea()const { returnwidth*height; }

classTriangle:publicShape { public: Triangle(float=0,float=0);

virtualfloatprintArea()const; protected: floatheight; floatwidth;