新手讲虚函数：怎么实现多态

class B0 //基类B0声明
{public: 
   void display() 
     {cout<<"B0::display()"<<endl;} 
};
class B1: public B0 
{ public:
       void display()  {  cout<<"B1::display()"<<endl;  }
};
class D1: public B1 
{ public:
   void display() {  cout<<"D1::display()"<<endl;  }
};

class B0 //基类B0声明
{public: //外部接口
 virtual void display() //虚成员函数
     {cout<<"B0::display()"<<endl;} 
};
class B1: public B0 //公有派生
{ public:
       void display()  {  cout<<"B1::display()"<<endl;  }
};
class D1: public B1 //公有派生
{ public:
   void display() {  cout<<"D1::display()"<<endl;  }
};