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));