Effective C++ 18. Make interfaces easy to use correctly and hard to use incorrectly

Class Month {
public:
explicit Month() {
month = m;
}
private:
int month;
};

Month Jan(Month(1));

// in other file
extern Month Jan;
Data d(Jan, Day(30), Year(1995));   // wrong! unless Jan is initialized before d defined

class Month {
public:
static Month Jan() { return Month(1); }
private:
explicit Month(int m);
};
Date d(Month::Jan(), Day(30), Year(1995));