ошибка LNK2019: ссылка на неразрешенный внешний символ в функции _main visual c++

У кого-нибудь есть идеи, как это исправить? Я посмотрел в Интернете и изменил настройки своего визуального С++, но он все еще не работает.

class store
{
public:
    int MainMenu();
    store();
private:
    int main;
};

class customer:store
{
public:
    int CustomerMenu();
    customer();
private:
    int cmenu;
};

class employee:store
{
public:
    int EmployeeMenu();
    employee();
private:
    int emenu;

};

int main()
{
    int main;
    store a;
    customer b;
employee c;
a.MainMenu();
if(main = 1)
{
    c.EmployeeMenu();
}
else if(main = 2)
{
    b.CustomerMenu();
}
else
{
    exit(EXIT_SUCCESS);
}
}

int MainMenu()
{
    int main;
cout << "Choose an option: " << endl;
cout << " 1. Administration menu" << endl;
cout << " 2. Customer menu" << endl;
cout << " 3. Exit the program" << endl;
cin >> main;
return main;
}

int CustomerMenu()
{
int cmenu;
cout << " 1. Search Video" << endl;
cout << " 2. View Video Titles" << endl;
cout << " 3. Rent Video" << endl;
cout << " 4. Exit to the Main Menu" << endl;
cout << " 5. Exit the program" << endl;
cin >> cmenu;
return cmenu;

}

int EmployeeMenu()
{
int emenu;
    cout << " 1.  Store Information menu" << endl;
    cout << " 2.  Merchandise Information menu" << endl;
    cout << " 3.  Category Information menu" << endl;
    cout << " 4.  Customer Information menu" << endl;
    cout << " 5.  Employee Information menu" << endl;
    cout << " 6.  Rent a Video" << endl;
    cout << " 7.  Restock Video" << endl;
    cout << " 8.  Sales menu" << endl;
    cout << " 9.  Exit to Main Menu" << endl;
    cout << " 10. Exit the program" << endl;
cin >> emenu;
return emenu;

}

store::store()
{
main = 0;
}

customer::customer()
{
cmenu = 0;
}

employee::employee()
{
emenu = 0;
}

Это дает мне:

Store.obj : error LNK2019: unresolved external symbol "public: int __thiscall customer::CustomerMenu(void)" (?CustomerMenu@customer@@QAEHXZ) referenced in function _main
1>Store.obj : error LNK2019: unresolved external symbol "public: int __thiscall employee::EmployeeMenu(void)" (?EmployeeMenu@employee@@QAEHXZ) referenced in function _main
1>Store.obj : error LNK2019: unresolved external symbol "public: int __thiscall store::MainMenu(void)" (?MainMenu@store@@QAEHXZ) referenced in function _main