vc中dynamic_cast，下列程序转换指针类型，怎么不成功？？/display是虚函数，graduatestudent是student的子类

void main()
{
   GraduateStudent gs;  
   Student *ps;
   ps=dynamic_cast<Student*>(&gs);
   if(ps!=NULL)   
     ps->display();
   else
    cout<<"ok"<<endl;
}