C++关于父类指针指向子类对象的问题

#if 1
#include <iostream>
#include <string>
using namespace std;

class Stu
{
public:
Stu(const int num, const string& na): number(num), name(na){}
~Stu()
{}
private:
int number;
string name;
};

class Graduate : public Stu
{
public:
Graduate(const int num, const string& na, const int ag):Stu(num, na), age(ag){}
~Graduate()
{}
private:
int age;
};

int main()
{
int number = 1001;
string aa("ddd");
Stu stud(1001, "ding");
Stu *student = &stud;
Graduate gra(1001,"wei", 20);

student = &gra;
}

#endif