C++ multithreading - condition variable supplying data in a wrong way

I've got this multithreading project where I'm supposed to create a simulation of a hotel.

I've got this receptionist struct and their job is to constantly find if there's a free room (done in check_free_rooms method, accommodate_guests is his thread method). If he found a number_to_check_in, is_a_room_ready is set to true and one of guest threads waiting for a room is notified.

Every guest has a field being a reference to the receptionist (there is only one in the hotel) and the guests are waiting on the receptionist's condition variable receptionist.cv to be notified with the condition of receptionist.is_a_room_ready becoming true. Then, if I understand correctly, one random guest should get a room and the other ones should patiently wait for another notification from the receptionist.

struct Guest;

struct Room
{
    Room() {}
    int id;
    int guest_id;
    std::atomic<bool> is_ready_for_guest{true};

    void guest_arrives(int guest_id)
    {
        this->guest_id = guest_id;
        this->is_ready_for_guest = false;
    }
    void guest_leaves(int guest_id)
    {
        this->guest_id = -1;
    }
};

struct Receptionist
{
    Receptionist(std::vector<Room> &rooms) : rooms(rooms) {}
    std::vector<Room> &rooms;

    std::mutex mx;
    std::condition_variable cv;
    std::atomic<bool> is_a_room_ready{false};
    int number_to_check_in = 0;

    void check_free_rooms()
    {
        std::unique_lock<std::mutex> lock_receptionist(mx);
        do
        {
            this->number_to_check_in = std::experimental::randint(0, (int)rooms.size() - 1); //find an empty room
        } while (!rooms[this->number_to_check_in].is_ready_for_guest);
        is_a_room_ready = true;
        cv.notify_one();
    }

    void accommodate_guests()
    {
        while (true)
        {
            check_free_rooms();
            std::this_thread::sleep_for(std::chrono::milliseconds(100));
        }
    }
};

struct Guest
{
    Guest(int id, Receptionist &receptionist, Coffee_machine &coffee_machine,
          Swimming_pool &swimming_pool) : id(id), receptionist(receptionist),
                                          coffee_machine(coffee_machine), swimming_pool(swimming_pool) {}
    int id;
    int room_id;

    Receptionist &receptionist;
    Coffee_machine coffee_machine;
    Swimming_pool swimming_pool;

    void check_in()
    {
        std::unique_lock<std::mutex> lock_receptionist(receptionist.mx);
        while (!receptionist.is_a_room_ready)
        {
            receptionist.cv.wait(lock_receptionist);
        }
        receptionist.is_a_room_ready = false;
        this->room_id = receptionist.number_to_check_in;           //assign room to guest
        receptionist.rooms[this->room_id].guest_arrives(this->id); //assign guest to room && room becomes occupied
        {
            std::lock_guard<std::mutex> writing_lock(mx_writing);
            std::cout << "Guest " << this->id << " accomodated in room " << this->room_id << std::endl;
        }
        std::this_thread::sleep_for(std::chrono::milliseconds(20));
    }
};

int main()
{
    std::vector<Room> rooms(10);
    for (int i = 0; i < 10; i++)
    {
        rooms[i].id = i;
    }

    Receptionist receptionist(rooms);

    std::vector<Guest> guests;
    for (int i = 0; i < 15; i++)
    {
        guests.emplace_back(Guest(i, receptionist, coffee_machine, swimming_pool));
    }

    std::vector<std::thread> threadList;

    threadList.emplace_back(std::thread(&Receptionist::accommodate_guests, std::ref(receptionist)));
    for (Guest guest : guests)
    {
        threadList.emplace_back(std::thread(&Guest::have_holiday, std::ref(guest)));
    }

    for (std::thread &t : threadList)
    {
        t.join();
    }

    return 0;
}

However, the lines that appear in the terminal after a guest gets their room are not quite what I imagined. Guest ids range from 0 to 14 and room ids range from 0 to 9 but only rooms seem to be more or less ok in the outputs. I've got no idea why guest ids are a random large int and not the ones I assigned while object construction.

I'm not that very experienced with multithreading and condition variables and have virtually no clue how to solve this problem as I gave it a lot thinking and came up with nothing. I'd very much appreciate any help.

Some example outputs:

Guest 5 accomodated in room 32657
Guest -624431120 accomodated in room 7
Guest -624431120 accomodated in room 9
Guest -624431120 accomodated in room 8
Guest -624431120 accomodated in room 5
Guest -624431120 accomodated in room 4
Guest -624431120 accomodated in room 3
Guest -624431120 accomodated in room 2
Guest -624431120 accomodated in room 0
Guest -624431120 accomodated in room 6
^C

or

Guest 4 accomodated in room 32539
Guest -497561616 accomodated in room 1
Guest -497561616 accomodated in room 9
Guest -497561616 accomodated in room 5
Guest -497561616 accomodated in room 8
Guest -497561616 accomodated in room 6
Guest -497561616 accomodated in room 7
Guest -497561616 accomodated in room 3
Guest -497561616 accomodated in room 4
Guest -497561616 accomodated in room 0
^C

or

Guest 4 accomodated in room 32746
Guest -1510756368 accomodated in room 8
Guest -1510756368 accomodated in room 1
Guest -1510756368 accomodated in room 7
Guest -1510756368 accomodated in room 4
Guest -1510756368 accomodated in room 2
Guest -1510756368 accomodated in room 5
Guest -1510756368 accomodated in room 9
Guest -1510756368 accomodated in room 6
Guest -1510756368 accomodated in room 3
^C