c++ - 自定义类集的重载提取运算符

Question

我做了一个自定义类 user ，并做了一个set<user> users在另一个类(class) dbase .我已经成功地重载了 user 类的提取运算符现在我正在尝试重载 dbase 类的提取运算符.

这是我的代码:

ostream& operator<< (ostream &out, dbase &db) {
    set<user>::iterator it;
    for(it=db.users.begin(); it!=db.users.end(); it++)
        out<<(*it)<<endl;    //error reported on this line
    return out;
}

但我一直收到这个错误:

error: cannot bind 'std::ostream {aka std::basic_ostream<char>}' lvalue to 'std::basic_ostream<char>&&'

这是 operator<<类声明user :

ostream& operator<<(ostream &out, user &u) {
    cout<<endl;
    cout<<"Username: "<<u.uname;
    cout<<endl;
    return out;
}

这是完整的代码:(ideone)

#include <iostream>
#include <set>
#include <string>
#include <sstream>
using namespace std;

template <class T>
string to_string(T x) {
    stringstream ss;
    ss<<x;
    return ss.str();
}

class date {
    int dd,mm,yyyy;
    string str_date;
    public:
        string get_date() {
            return to_string(dd)+"-"+to_string(mm)+"-"+to_string(yyyy);
        }
        friend ostream& operator << (ostream&,date&);
        friend istream& operator >> (istream&,date&);
};

ostream& operator<< (ostream &out, date &d) {
    out<<d.get_date();
    return out;
}

istream& operator>> (istream &in, date &d) {
    cin>>d.dd>>d.mm>>d.yyyy;
    return in;
}

class user {
    string uname, pwd;
    string full_name, contact;
    static unsigned long long int id;
    date dob;

    public:
        bool operator < (const user &u) const {
            if(uname.compare(u.uname)<0) return true;
            return false;
        }
        void set_uname(string u) {
            uname = u;
        }
        void set_attrib() {
            string temp_pass;
            while(1) {
                cout<<"Full Name: ";
                cin>>full_name;
                cout<<"Password: ";
                cin>>temp_pass;
                cout<<"Confirm Password: ";
                cin>>pwd;
                if(pwd.compare(temp_pass))
                    cout<<"Passwords do not match. Try Again."<<endl;
                else break;
            }
            cout<<"Contact Number: ";
            cin>>contact;
            cout<<"Date of Birth(dd mm yyyy): ";
            cin>>dob;
        }
        friend ostream& operator<<(ostream&, user&);
};

ostream& operator<<(ostream &out, user &u) {
    cout<<endl;
    cout<<"Username: "<<u.uname;
    cout<<endl;
    return out;
}

class dbase {
    set<user> users;
    public:
        void add_user() {
            string uname;
            user u;
            while(1) {
                cout<<"Username: ";
                cin>>uname;
                u.set_uname(uname);
                if(users.find(u)==users.end()) {
                    u.set_uname(uname);
                    u.set_attrib();
                    users.insert(u);
                    break;
                }
                else
                    cout<<"Username already exists. Please try a different username."<<endl;
            }
        }
        friend ostream& operator << (ostream&, dbase&);
};

ostream& operator<< (ostream &out, dbase &db) {
    set<user>::iterator it;
    user u;
    for(it=db.users.begin(); it!=db.users.end(); it++)
        out<<(*it)<<endl;
    return out;
}

int main() {

    return 0;
}

Answer 1

所以...

第 1 课

请发布一个完整的可编译示例。在这种情况下，您没有提供足够的信息让我们轻松调试您的代码。

对于您的特定问题，该代码可能如下所示:

#include <iostream>
#include <set>
using namespace std;

class user {
    public:
        user(int id = 0): id(id) {}
        int id;
};

class dbase {
    public:
        set<user> users;
};

bool operator<(user const & lhs, user const & rhs) {
    return lhs.id < rhs.id;
}

ostream& operator<< (ostream &out, user & user) {
    return out << "user(" << user.id << ")";
}

ostream& operator<< (ostream &out, dbase &db) {
    set<user>::iterator it;
    user u;
    for(it=db.users.begin(); it!=db.users.end(); it++)
        cout<<*it<<endl;
    return out;
}

int main() {
    dbase db;
    db.users.emplace(3);
    db.users.emplace(4);
    cout << db << endl;
}

第 2 课

请发布您的编译器报告的完整错误消息。在这种情况下，它实际上是:

se.cc: In function ‘std::ostream& operator<<(std::ostream&, dbase&)’:
se.cc:28:16: error: cannot bind ‘std::ostream {aka std::basic_ostream<char>}’ lvalue to ‘std::basic_ostream<char>&&’
         cout<<*it<<endl;
                ^
In file included from /opt/gcc/4.8.1/include/c++/4.8.1/iostream:39:0,
                 from se.cc:1:
/opt/gcc/4.8.1/include/c++/4.8.1/ostream:602:5: error:   initializing argument 1 of ‘std::basic_ostream<_CharT, _Traits>& std::operator<<(std::basic_ostream<_CharT, _Traits>&&, const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Tp = user]’
     operator<<(basic_ostream<_CharT, _Traits>&& __os, const _Tp& __x)
     ^

请注意，我没有以任何方式编辑错误消息。消息中可能有您遗漏的其他信息，这些信息可能对尝试回答您问题的人有所帮助。

第 3 课

代替:

ostream& operator<< (ostream &out, user & user) { ... }

做:

ostream& operator<< (ostream &out, user const & user) { ... }

第 4 课

当重载流提取或流插入运算符时，您可能永远不应该使用 cout或 cerr .相反，您已将一个流对象传递到您的函数中。在您的情况下，您已使用 out 的典型(预期)参数名称命名它.

因此，您的两个流插入运算符应该具有 cout 的每个实例替换为 out .

这将导致您的代码在您扩展代码以使用诸如使用 fstream 将用户数据库写入文件，或者如果您想将消息打印到 cerr 时正确 工作。

第 5 课

只是要注意。这是一个问题，因为 *it返回对 const User 的引用.所以你现有的 operator<<重载不匹配，因为它需要用户的非常量版本。