C++ - ‘_itoa’ was not declared in this scope - c++

Question

In the UserProfile.h, I declared the class UserProfile...
#include <iostream>
#include <map>
#include <string>
using namespace std;
class UserProfile
{
...
}
then, I defined the function UserProfile() and operator<< in the UserProfile.cpp
#include "UserProfile.h"
#include <cstdlib>
inline UserProfile::UserProfile()
: _login("guest"), _user_level(Beginner),
_times_logged(1), _guesses(0), _correct_guesses(0)
{
static int id = 0;
char buffer[16];
_itoa(id++, buffer, 10);
_login += buffer;
}
...
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;
}
However, when I try to compile them, the g++ report error:
g++ UserProfile.cpp E44.cpp -o main
UserProfile.cpp: In constructor ‘UserProfile::UserProfile()’:
UserProfile.cpp:11:27: error: ‘_itoa’ was not declared in this scope
_itoa(id++, buffer, 10);
But I have included "cstdlib"... so strange...
Also, when I want to use << in my main cpp, the g++ report error too:
#include "UserProfile.h"
int main()
{
UserProfile anon;
cout << anon;
...
}
g++ report:
E44.cpp: In function ‘int main()’:
E44.cpp:6:10: error: no match for ‘operator<<’ (operand types are ‘std::ostream {aka std::basic_ostream<char>}’ and ‘UserProfile’)
cout << anon;
I am so confused... Did I do something wrong??

Answer 1

The correct name of the function is itoa not _itoa. However, it cannot be used in C++. cplusplus.com says:
This function is not defined in ANSI-C and is not part of C++, but is supported by some compilers.
Instead, use std::to_string or string streams.
As far as the compile time error regarding operator<< is concerned, that is because you define the operator in another translation unit than the one with the main function which will be compiled separately I guess and you don't declare it in the header file. In order for this to work you need to declare the overloaded operator, preferably in the same header file as the class' header file UserProfile.h.
ostream& operator<<(ostream &os, const UserProfile &rhs);
Moreover, you may want to make it a friend of your class to directly access fields and not using methods. That doesn't break encapsulation because you implement both the class and the overloaded operator.
class UserProfile
{
friend ostream& operator<<(ostream &os, const UserProfile &rhs);
};