C++中對類的提前引用聲明注意事項

//或許，友元是VC++6.0心里永遠的痛，對於這個BUG我一直很介意。
//注：這個程序在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; //提前引用聲明

//聲明Teacher類
class Teacher {
public:
    Teacher(){}
    Teacher(int i,string s,char c,int t=0):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;
};

//聲明Student類
class Student {
public:
    Student(){}
    Student(int i,string s,char c,int g=0):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：根據慣例，我們喜歡在類的聲明前直接寫成員函數的實現；
//但是，該構造函數的實現不能寫在Student類聲明之前，
//因為它使用了Student類的成員，提前引用聲明在這里將不起作用
Teacher::Teacher(Student st)
    {
    num=st.num;
    name=st.name;
    sex=st.sex;
    theachYears=0;
    }

//重載Teacher類的<<  
// 注意：該構造函數的實現不能寫在Student類聲明之前，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;
}
//重載Student類的<<   
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(1,"xiaoer",'F',2);
    cout<<s;
    Teacher t=Teacher(s);
    cout<<t;
    return 0;
}