重载 << 运算符时如何解决此错误消息

Question

当我尝试重载 << 运算符时，出现错误“此运算符函数的参数过多”？

这是我写的代码：

class studentrecord{
string firstname, lastname, grade;        
studentrecord(string firstname, string lastname, string grade){
    this -> firstname = firstname;
    this -> lastname = lastname;
    this -> grade = grade;
}

ostream& operator<<(ostream& os, const studentrecord& studentrecord) {
    os << "\n\t" << studentrecord.firstname << "\t" << studentrecord.lastname << "\t" << studentrecord.grade;
    return os;
}
};

谢谢您的帮助！

Answer 1

@DesmondGold 指出了答案，只需使用“朋友”关键字即可。

这个function的正确使用可以看这个页面：

为你自己的类重载 << 运算符

代码变为：

class studentrecord{
string firstname, lastname, grade;        
studentrecord(string firstname, string lastname, string grade){
    this -> firstname = firstname;
    this -> lastname = lastname;
    this -> grade = grade;
}

friend ostream& operator<<(ostream& os, const studentrecord& studentrecord) {
    os << "\n\t" << studentrecord.firstname << "\t" << studentrecord.lastname << "\t" << studentrecord.grade;
    return os;
}
};