指向基类对象的指针变量也可以指向派生类对象

#include 
    
     #include 
     
      using namespace std;class Student//声明Student类{public:    Student(int, string,float);    void display( );private:    int num;    string name;    float score;};Student::Student(int n, string nam,float s){    num=n;    name=nam;    score=s;}void Student::display( ){    cout<
      
       <<"num:"<
       
        <
        
         display( );    pt=&grad1;    pt->display( );    return 0;}