第六章

#include <iostream>
using namespace std;
class Point

{public:
  Point(float a,float b):x(a),y(b){}
  ~Point(){cout<<"executing Point destructor"<<endl;}
 private:
  float x;
  float y;
};

class Circle:public Point
{public:
 Circle(float a,float b,float r):Point(a,b),radius(r){}
  ~Circle(){cout<<"executing Circle destructor"<<endl;}
 private:
  float radius;
};

int main()
{Point *p=new Circle(2.5,1.8,4.5);
 delete p;
 return 0;
}

#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Rectangle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
};

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
};

//输出面积的函数
void printArea(const Shape &s)
{cout<<s.area()<<endl;}                                            //输出s的面积

int main()
{
 Circle circle(12.6);                                             //建立Circle类对象circle
 cout<<"area of circle    =";          
 printArea(circle);                                               //输出circle的面积
 Rectangle rectangle(4.5,8.4);                                    //建立Rectangle类对象rectangle
 cout<<"area of rectangle =";                                     
 printArea(rectangle);                                            //输出rectangle的面积
 Triangle triangle(4.5,8.4); //建立Triangle类对象
 cout<<"area of triangle  =";
 printArea(triangle);           //输出triangle的面积
 return 0;
}

#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;
}