【初学者求教】关于C++ 类的问题

#include<iostream>
#include <string>
#include <vector>
using namespace std;

/*

 * The definition of Student.

 */
class Student {
public:
    Student(int id, const string &_name)
    : stu_no(id), name(_name) {};
    Student(const Student &stu)
    : stu_no(stu.stu_no), name(stu.name) {};
    ~Student() {};

    friend ostream &operator <<(ostream &out, const Student &stu);

private:
    int stu_no;
    string name;
};

ostream &operator <<(ostream &out, const Student &stu)
{
    return out << stu.stu_no << "\t" << stu.name;
}

/*

 * The definition of Class.

 */
class Class {
public:
    Class(const string &_name)
    : name(_name) {};
    Class(const Class &cls)
    : name(cls.name), member(cls.member) {};
    ~Class() {};

    void show_capacity()
    { cout << name << " has " << member.size() << " students." << endl; }
    void show_member();

    void add(const Student &stu) { member.push_back(stu); };

private:
    string name;
    vector<Student> member;
};

void Class::show_member()
{
    vector<Student>::iterator itr = member.begin();
    for (; itr != member.end(); ++itr) {
    cout << *itr << endl;
    }
}



int main(int argc, char *argv[])
{
    Class cls("1-A");

    cls.add(Student(1, "abc"));
    cls.add(Student(2, "def"));
    cls.add(Student(3, "ghi"));
    cls.add(Student(4, "jkl"));
    cls.add(Student(5, "mno"));

    cls.show_capacity();
    cls.show_member();

    return 0;
}