自学C++中的类时候出现的问题，请帮下忙了。。

#include<iostream>   //题目是这样的，先建立一个抽象的基类就是这个shape，然后派生出point,circle,volum等派生类。最后在主函数里定义shape类的指针pt，使它
#include<string>       可以指向基类和派生类的数据及函数，但是我这个主函数没写2句就错误了。。我定义的shape 指针pt无法指向shape的派生类。。
using namespace std;    特别请教错误啊。。
class shape
{
public:
     virtual float area() {return 0;}
     virtual float volume() {return 0;}
    virtual void shapename() =0;
};
class point :public shape
{
public:
    point(float a=0,float b=0):x(a),y(b){}
    void setnumber()
    {
        cout<<"please input the x"<<endl;
        cin>>x;
        cout<<"please input the y"<<endl;
        cin>>y;
    }
    virtual void shapename()
    {
        cout<<"this porject is a point"<<endl;
    }
    friend ostream &operator <<(ostream &,point &);
protected:
    float x;
    float y;
};
class circle :public point
{
public:
    circle(float a=0,float b=0,float r=0):point(a,b),radius(r){}
    void setradius()
    {cout<<"please input the radius"<<endl;
    cin>>radius;
    }
    virtual float area()
    {
        return(3.14*radius*radius);
    }
    virtual void shapename()
    {
        cout<<"this project is circle"<<endl;
    }
    friend ostream &operator <<(ostream &,circle &);
protected:
    float radius;
};
ostream &operator <<(ostream &output,circle &c1)
{output<<"("<<c1.x<<","<<c1.y<<")"<<c1.radius<<c1.area()<<endl;
return output;
}

class cylinder :public circle
{
public:
    cylinder(float a=0,float b=0,float r=0,float h=0):circle(a,b,r),height(h){}
    void setheight()
    {cout<<"please input the height"<<endl;
    cin>>height;
    }
    virtual float volume()
    {
        return(3.14*radius*radius*height);
    }
    virtual void shapename()
    {
        cout<<"this project is cyliner"<<endl;
    }
    friend ostream &operator <<(ostream &,cylinder &);
protected:
    float height;
};
ostream &operator <<(ostream &output,cylinder &c1)
{output<<"("<<c1.x<<","<<c1.y<<")"<<c1.radius<<c1.area()<<" "<<c1.height<<" "<<c1.volume()<<endl;
return output;
}
int main()
{
 shape *pt;
 pt=&point;//！这就是错误的地方，pt指针无法变为指向point类的。