重载自定义类集的提取操作符

我做了一个自定义类user，并在另一个类dbase中做了一个set<user> users。我已经成功地重载了类user的提取操作符，现在我正试图重载类dbase的提取操作符。

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;
}

但是我一直得到这个错误:

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

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

#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;
}