C++ login (functional) (console)

#include <iostream>
#include <map>
using namespace std;
std::map<std::string, std::string> usermap;

void addUser(std::string username, std::string password) {
    usermap[username] = password;
}
bool registered(std::string username) {
    if (usermap.find(username) != usermap.end()) {
        return true;
    }
    return false;
}
string getPassword(std::string username) {
    if (registered(username)) {
        return usermap[username];
    }
    return NULL;
}
bool passwordIsCorrect(std::string username, std::string password) {
    if (registered(username)) {
        if (usermap[username] == password) {
            return true;
        }
        return false;
    }
    return false;
}


void program() {
    std::string huser;
    std::string hpass;
    int hop;
    std::cout << "\nWelcome!\n1. login\n2. register\n";
    std::cin >> hop;
    if (hop == 1) {
        std::cout << "Enter your username: \n";
            std::cin >> huser;
            if(registered(huser)) {
                std::cout << "Enter your password: \n";
                std::cin >> hpass;
                if (passwordIsCorrect(huser, hpass)) {
                    std::cout << "Logged in!\n";
                    program();
                } else {
                    std::cout << "Invalid password!\n";
                    program();
                }
            } else {
                std::cout << "Invalid username!\n";
                program();
            }
        } else if (hop == 2) {
            std::cout << "Enter a username: \n";
            std::cin >> huser;
            if (registered(huser)) {
                std::cout << "Already registered!\n";
                program();
            } else {
                std::cout << "Enter a password: \n";
                std::cin >> hpass;
                addUser(huser, hpass);
                std::cout << "registered!\n";
                program();
            }
        }
}

int main(int argc, char **argv) {
   program();
    }