C++中对类的提前引用声明注意事项

//或许，友元是VC++6.0心里永远的痛，对于这个BUG我一直很介意。
//注：这个程序在VC++6.0里是行不通的，在VS2008里是可以的。
#include <iostream>
#include <string>
using namespace std;

class Student; //提前引用声明

//声明Teacher类
class Teacher {
public:
    Teacher(){}
    Teacher(int i,string s,char c,int t=):num(i),name(s),sex(c),theachYears(t){}
    Teacher(Student);
    friend ostream& operator<<(ostream& out,Teacher& te);
private:
    int num;
    string name;
    char sex;//F/M
    int theachYears;
};

//声明Student类
class Student {
public:
    Student(){}
    Student(int i,string s,char c,int g=):num(i),name(s),sex(c),grade(g){}
    friend Teacher::Teacher(Student);
    friend ostream& operator<<(ostream& out,Student& st);
private:
    int num;
    string name;
    char sex;//F/M
    int grade;
};

//注意：根据惯例，我们喜欢在类的声明前直接写成员函数的实现；
//但是，该构造函数的实现不能写在Student类声明之前，
//因为它使用了Student类的成员，提前引用声明在这里将不起作用
Teacher::Teacher(Student st)
    {
    num=st.num;
    name=st.name;
    sex=st.sex;
    theachYears=;
    }

//重载Teacher类的<<  
// 注意：该构造函数的实现不能写在Student类声明之前，原因同上
ostream& operator<<(ostream& out,Teacher& te)
{
    out<<"num="<<te.num<<","<<"name="<<te.name<<","<<"sex="<<te.sex<<","<<"theachYears="<<te.theachYears<<endl;
    return out;
}
//重载Student类的<<
ostream& operator<<(ostream& out,Student& st)
{
    out<<"num="<<st.num<<","<<"name="<<st.name<<","<<"sex="<<st.sex<<","<<"grade="<<st.grade<<endl;
    return out;
}

int main()
{
    Student s(,"xiaoer",'F',);
    cout<<s;
    Teacher t=Teacher(s);
    cout<<t;
    return ;
}