新人求助，Point &p=NULL;

#include<iostream>
using namespace std;
class Point
{
    public:
    Point(){}
    Point(int X,int Y)
    {
        x=X;
        y=Y;
    }

   virtual void set(int X,int Y)
    {
       x=X;
       y=Y;
    }
     
    virtual void display()
     {
         cout<<"x="<<x<<endl;
         cout<<"y="<<y<<endl;
     }
    protected:
    int x;
    int y;
};
class Circle:public Point
{
    public:
    Circle(){}
    Circle(int X,int Y,int Radius):Point(X,Y)
    {
        x=X;
        y=Y;
        radius=Radius;
    }
    void set(int X,int Y,int Radius)
    {
        x=X;
        y=Y;
        radius=Radius;
    }
    void display()
        {
         cout<<"x="<<x<<endl;
         cout<<"y="<<y<<endl;
         cout<<"半径="<<radius<<endl;
         }
    protected:
    int radius;
};
class Cylinder:public Circle
{
    public:
    Cylinder(){}
    Cylinder(int X,int Y,int Radius, int Height):Circle(X,Y,Radius)
    {
        x=X;
        y=Y;
        radius=Radius;
        height=Height;
    }
    void set(int X,int Y,int Radius, int Height)
    {
        x=X;
        y=Y;
        radius=Radius;
        height=Height;

    }
    void display()
    {
         cout<<"x="<<x<<endl;
         cout<<"y="<<y<<endl;
         cout<<"半径="<<radius<<endl;
         cout<<"高="<<height<<endl;
    }
    private:
    int height;
};
int main()
{
    Point &p=NULL;
    Point t;
   
    t.set(1,2);
    t.display();

    Circle c;
    Cylinder d;

    p=&c;
    c->set(1,2,3);
    c->display();

    p=&d;
    d->set(1,2,3,4);
    d->display();
   
    return 0;
}