【C++】error: passing ‘const xxx’ as ‘this’ argument discards qualifiers [-fpermissive]

错误原因

将“const xxx”作为“this”参数传递会丢弃限定符
当函数参数为const的对象，需要给函数添加限定符const，使函数不改变该对象内容。

修改前

class UserProfile{
public:
    enum ulevel{base,middle,high,expert};
    UserProfile() = default;
    UserProfile(string login,ulevel = base);

    bool operator!=(const UserProfile& rhs);
    bool operator==(const UserProfile&);
    //错误处
    string login() {return _login;}
    string user_name() {return _user_name;}
    int login_count() {return _times_logged;}
    int guess_count() {return _guesses;}
    int guess_correct() {return _correct_guesses;}
    double guess_average() const;
    string level() const; 
};


ostream& operator<<(ostream &os,const UserProfile& rhs)
{
    os<<rhs.login()<<" "
        <<rhs.level()<<" "
        <<rhs.login_count()<<" "
        << rhs.guess_count() << ' '
        << rhs.guess_correct() << ' '
        << rhs.guess_average() << endl;
   return os;
}

修改后

(给对应的类成员函数添加const限定符)

class UserProfile{
public:
    enum ulevel{base,middle,high,expert};
    UserProfile() = default;
    UserProfile(string login,ulevel = base);

    bool operator!=(const UserProfile& rhs);
    bool operator==(const UserProfile&);
    //错误处
    string login() const {return _login;}
    string user_name() const {return _user_name;}
    int login_count() const {return _times_logged;}
    int guess_count() const {return _guesses;}
    int guess_correct() const {return _correct_guesses;}
    double guess_average() const;
    string level() const; 
};