有一个基类名为“形状”，它有派生类“圆”、 “正方形”和“长方形”。利用多态性概念，以虚函数的形式完成计算“圆”、“正方形”和“长方形”的面积。

[[it] 本帖最后由 huangyi 于 2008-10-25 12:12 编辑 [/it]]

是这样么？？？？
#include <iostream>
using namespace std;

//声明抽象基类Shape
class Shape
{
public:
    virtual double area() const =0;              //纯虚函数
};



//声明Circle（圆形）类
class Circle:public Shape
{
public:
    Circle(double r=0):radius(r){}
    virtual double area() const{return 3.14159*radius*radius;}
protected:
    double radius;
};


//声明Rectangle（矩形）类
class Rectangle:public Shape
{
public:
    Rectangle(double a=0,double b=0):x(a),y(b){}
    virtual double area() const{return x*y;}
protected:
    double x;
    double y;
};


//声明Square（正方形）类
class Square:public Shape
{
public:
    Square(double a=0):x(a){}
    virtual double area()const{return x*x;}
protected:
    double x;
};


int main()
{
    Shape *pt;     //定义基类指针
    
    Circle circle(5.6); //建立Circle类对象circle
    Rectangle rectangle(5,4);
    Square square(6);

    pt=&circle;
    cout<<"The circle's area is "<<circle.area()<<endl;

    pt=&rectangle;
    cout<<"The rectangle's area is "<<rectangle.area()<<endl;

    pt=&square;
    cout<<"The square's area is "<<square.area()<<endl;

    return 0;
}