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Shapeclass 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Shapeclass 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;}