C+编程：编写一个程序,定义抽象基类Shape，由它派生出5个派生类：Cricle（圆形),Square（正方形)， Rectangle（矩形）， Trapezoid （梯形）,Triangle（三角形）。用虚函数分别计算图形面积，并求所有图形面积的和．要求用基类指针数组，使它的每一个元素指向一个派生类对象。

以下是此题的【c++源代码】
#include <iostream>
using namespace std;
//定义抽象基类Shape
class Shape
{public:
 virtual double area() const =0;                                //纯虚函数
};

//定义Circle(圆形)类
class Circle:public Shape
{public:
Circle(double r):radius(r){}                                     //结构函数
  virtual double area() const {return 3.14159*radius*radius;};   //定义虚函数
 protected:
  double radius;                                                 //半径
};

//定义Square(正方形)类
class Square:public Shape
{public:
  Square(double s):side(s){}                                     //结构函数    
  virtual double area() const {return side*side;}                //定义虚函数
 protected:
  double side;
};

//定义Rectangle(矩形)类
class Rectangle:public Shape
{public:
 Rectangle(double w,double h):width(w),height(h){}               //结构函数
 virtual double area() const {return width*height;}              //定义虚函数
 protected:
  double width,height;                                           //宽与高
};

//定义Trapezoid(梯形)类
class Trapezoid:public Shape
{public:
  Trapezoid(double t,double b,double h):top(t),bottom(t),height(h){} //结构函数
  virtual double area() const {return 0.5*(top+bottom)*height;}      //定义虚函数
 protected:
  double top,bottom,height;                                          //上底、下底与高
};

//定义Triangle(三角形)类
class Triangle:public Shape
{public:
  Triangle(double w,double h):width(w),height(h){}                //结构函数
  virtual double area() const {return 0.5*width*height;}          //定义虚函数
 protected:
  double width,height;                                            //宽与高
};

int main()
{
 Circle circle(12.6);                                             //建立Circle类对象circle
 Square square(3.5);                                              //建立Square类对象square
 Rectangle rectangle(4.5,8.4);                                    //建立Rectangle类对象rectangle
 Trapezoid trapezoid(2.0,4.5,3.2);                                //建立Trapezoid类对象trapezoid
 Triangle triangle(4.5,8.4);                                      //建立Triangle类对象
 Shape *pt[5]={&circle,&square,&rectangle,&trapezoid,&triangle}; 
                                              //定义基类指针数组pt，使它每一个元素指向一个派生类对象
 double areas=0.0;                                                //areas为总面积
 for(int i=0;i<5;i++)
  {areas=areas+pt[i]->area();}
 cout<<"totol of all areas="<<areas<<endl;   //输出总面积
 return 0;
}

来源：c++程序设计第三版谭浩强课后答案

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