void Date::showDate(ostream &out) const {
    out << year << "-" << month << "-" << day;
};
void EuropeanDate::showDate(ostream &out) const {
    out << month << "/" << day << "/" << year;
};
void WeekDate::showDate(ostream &out) const {
    out << *weekDay << " ";
    Date::showDate(out);
};



    cout << "Date Class: ";
    dateobj.showDate(cout);
    cout << endl << "European Date Class: ";
    euDateobj.showDate(cout);
    cout << endl << "Week Date Class: ";
    weekDateobj.showDate(cout);
    cout << endl;