Нужна помощь с ошибкой о статической переменной и статическом методе

Писал программу с класами, и всё было хорошо до тех пор, пока я не стал делать статичную переменную и статичный метод в классе. Выдаёт такая ошибка:

Снизу прикреплю код, чтобы было понятней

#include <iostream>
#include <ctime>
#include <fstream>
#include <string>
#include <iomanip>
#include <Windows.h>
#pragma warning(disable:4996);
using namespace std;

class bus_ticket
{
public:
    bus_ticket();
    bus_ticket(int, int, int);
    bus_ticket(const bus_ticket&);
    ~bus_ticket();
    void set_inf(int);
    void get_inf();
    void set_inf_to_file(bus_ticket);
    void get_inf_from_file();
    int selling();
    void selling(int);
    void selling(bus_ticket);
    string passenger;
    static int get_count()
    {
        return count;
    }

private:
    time_t current_date_and_time;
    string departure;
    string arrival;
    int ticket_number;
    int seat_number;
    int bus_number;
    bool status;
    static int count;

protected:
    char control[100];
};

bus_ticket::bus_ticket()
{
    ticket_number = 1000 + rand() % 9000;
    seat_number = 10 + rand() % 40;
    bus_number = 1000 + rand() % 9000;
    current_date_and_time = time(NULL);
    count++;
}

bus_ticket::bus_ticket(int ticket_number_1, int seat_number_1, int bus_number_1)
{
    ticket_number = ticket_number_1;
    seat_number = seat_number_1;
    bus_number = bus_number_1;
    current_date_and_time = time(NULL);
    count++;
}

bus_ticket::bus_ticket(const bus_ticket& other)
{
    this->ticket_number = other.ticket_number;
    this->bus_number = other.bus_number;
    this->seat_number = other.seat_number;
    this->passenger = other.passenger;
    this->departure = other.departure;
    this->arrival = other.arrival;
    this->current_date_and_time = other.current_date_and_time;
    count++;
}

void bus_ticket::set_inf(int i)
{
    cout << endl << "Enter " << i + 1 << " passenger name: ";
    cin >> passenger;
    cout << "Enter departure: ";
    cin >> departure;
    cout << "Enter arrival: ";
    cin >> arrival;
    cout << endl << endl;
}

void bus_ticket::get_inf()
{
    char buffer[80];
    tm* timeinfo = localtime(&current_date_and_time);
    const char* format = "%A, %B %d, %Y %H:%M:%S";
    strftime(buffer, 80, format, timeinfo);
    cout << "Passenger: " << passenger << endl;
    cout << "Ticket number: " << ticket_number << endl;
    cout << "Date and time of purchase: " << buffer << endl;
    cout << "Seat number: " << seat_number << endl;
    cout << "Bus number: " << bus_number << endl;
    cout << "Departure: " << departure << endl;
    cout << "Arrival: " << arrival << endl;
    cout << "Current number of tickets: " << endl << endl;
}

void bus_ticket::set_inf_to_file(bus_ticket ticket)
{
    ofstream fn("text.txt");
    if (!fn.is_open())
    {
        cout << "File does not exist!" << endl;
    }
    else
    {
        fn.write((char*)&ticket, sizeof(bus_ticket));
    }
    fn.close();
}

void bus_ticket::get_inf_from_file()
{
    ifstream fn("text1.txt");
    if (!fn.is_open())
    {
        cout << "File does not exist!" << endl;
    }
    else
    {
        string str;
        cout << endl;
        while (!fn.eof())
        {
            getline(fn, str);
            cout << str << endl;
        }
        cout << endl;
    }
    fn.close();
}

int bus_ticket::selling()
{
    return status;
}

void bus_ticket::selling(int i)
{
    set_inf(i);
    status = 1;
}

void bus_ticket::selling(bus_ticket object)
{
    object.get_inf_from_file();
    status = 1;
}

bus_ticket::~bus_ticket()
{
    count--;
}

int main()
{
    SetConsoleOutputCP(1251);
    SetConsoleCP(1251);
    srand(time(NULL));
    int a, b;
    cout << "Enter count of passengers: ";
    cin >> a;
    bus_ticket* object = new bus_ticket[a];
    for (int i = 0; i < a; i++)
    {
        cout << "For entering info about passenger from keyboard press 1, from file - press 2: ";
        cin >> b;
        switch (b)
        {
        case 1:
            object[i].selling(i);
            object[i].get_inf();
            break;
        case 2:
            object[i].selling(object[i]);
            break;
        }
    }
    return 0;
}