Acessing an array from a header file crashes program - c++

Question

I recently finished a course on C++, to test to see what I knew from it I decided to start work on a operating system simulation, it seemed to be going fairly well up till I tried to implement a multiple users system by using a array to store user names, from which the user will select one and it will go into their user. However the problem with it that when I try and modify the array in the header file in the source file the program crashes.
header files:
#ifndef UBUNTU_LOGIN_H
#define UBUNTU_LOGIN_H
#include "Ubuntu_UserDetails.h"
#include <iostream>
#include <string>
#include <vector>
class Ubuntu_Login: protected UserDetails
{
public:
void Ubuntu_UserCreation();
void Ubuntu_login();
};
#endif // UBUNTU_LOGIN_H
#ifndef UBUNTU_USERDETAILS_H
#define UBUNTU_USERDETAILS_H
#include "Ubuntu_login.h"
#include <string>
#include <vector>
class UserDetails
{
protected:
std::string username;
std::string password;
std::string users[0];
};
#endif // UBUNTU_USERDETAILS_H
source files:
#include "Ubuntu_login.h"
#include "Ubuntu_UserDetails.h"
#include <iostream>
#include <vector>
#include <string>
void Ubuntu_Login::Ubuntu_UserCreation()
{
std::cout << "\t _____________" << std::endl;
std::cout << "\t|Create a user|\n" << std::endl;
std::cout << "Enter a username:" << std::endl;
std::getline(std::cin, username);
std::cout << "Enter a password:" << std::endl;
std::getline(std::cin, password);
users[0] = username;
std::cout << users[0] << std::endl;
std::cout << "User created successfully" << std::endl;
}
void Ubuntu_Login::Ubuntu_login()
{
std::cout << "\t ______" << std::endl;
std::cout << "\t|Log in|\n" << std::endl;
std::cout << "Select user" << std::endl;
}
#include "Ubuntu_login.h"
#include <iostream>
int main()
{
Ubuntu_Login UO;
UO.Ubuntu_UserCreation();
UO.Ubuntu_login();
}
when I try to run it I get as far as the initial password entry then the program crashed, I assume this is due to a stack overflow, but I wouldn't know how to solve it. Any help on solving this issue would be greatly appreciated as well as general help on the code as a whole :) .
also the reason I have not included things like storing passwords as hashes or not echoing the password is simply because I don't know the syntax to do it and I wanted to try to do the things I knew how to do first then add additional features later, so don't expect great code :P

Answer 1

Use std::vector<std::string> users
And then just users.push_back(username) to it. Dynamic array is suitable here.

Answer 2

In your Ubuntu_UserDetails.h you have this data member in the UserDetails class:
std::string users[0];
Then in a source file, you have this assignment:
users[0] = username;
If you access an item in an array with index 0, the array must have at least one element. In fact, index 0 refers to the first item in the array.
So, the users array should be defined as having some bigger size.
But, much better, consider using a convenient C++ container class for vectors, like std::vector.
So, in your header file, you can define a data member like this:
// Instead of:
// std::string users[0]
//
std::vector<std::string> userNames; // probably better than "users"
Then you can add new items to the vector using its push_back or emplace_back methods.