求一个多层派生时的构造函数程序问题？

#include<iostream>
#include<string>
using namespace std;
class Student
{
public:
    Student(int n,string nam):num(n),name(nam){}
    void display()
    {
        cout<<"num:"<<num<<endl;
        cout<<"name:"<<name<<endl;
    }
protected:
    int num;
    string name;
};

class Student1:public Student
{
public:
    Student1(int n,string nam,int a):Student(n,nam),age(a){}
    void show()
    {
        display();
        cout<<"age:"<<age<<endl;
    }
protected:
    int age;
};

class Student2:public Student1
{
    Student2(int n,string nam,int a,int s):Student1(n,nam,a),score(s){}
    void show_all()
    {
        show();
        cout<<"score:"<<score<<endl;
    }
protected:
    int score;
};

void main()
{
    Student2 stud(10010,"Zhangcan",17,89);
    stud.show_all();
}
帮忙各位高手给看看这个程序！谢谢！