在例12.3的基础上作以下修改，并作必要的讨论。
(1)把构造函数修改为带参数的函数，在建立对象时初始化。
(2)先不将析构函数声明为virtual，在main函数中另设一个指向Circle类对象的指针变量，使它指向gradl。运行程序，分析结果。
(3)不作第(2)点的修改而将析构函数声明为virtual，运行程序，分析结果。

以下是此题的【c++源代码】
*******************（1）*****************
#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 radus;
};

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

*******************（2）*****************
#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(int a,int b,int r):Point(a,b),radius(r){}
  ~Circle(){cout<<"executing Circle destructor"<<endl;}
 private:
  float radus;
};

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

*******************（3）*****************
#include <iostream>
using namespace std;
class Point
{public:
  Point(float a,float b):x(a),y(b){}
  virtual ~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){}
  virtual ~Circle(){cout<<"executing Circle destructor"<<endl;}
 private:
  float radus;
};

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

来源：c++程序设计第三版谭浩强课后答案

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