Costs and Benefits

"There are no zero-cost abstractions" (https://www.youtube.com/watch?v=rHIkrotSwcc) is a good CppCon talk. It tells as to look for costs and benefits. For the C++ Core Guideline's Month abstraction here are what I see as the costs and benefits. Your choice whether you feel the benefits outweigh the costs :

#include <iostream>

// Guideline P1 is about expressing ideas directly in code. One part of that is // about using user defined types that express an idea better than say an int. // This file takes the Date/Month example in P1 and expands upon it.

// Neutral 1 : Despite wrapping the unsigned int it is no slower.

struct CalendarType { // Neutral 2 : The user does not know if the value is 0 based or 1 based.

   unsigned int value;

   // Cost 1 : Either the user has to use say month.value or we have to write boiler plate code for required methods.
   // Mitigation 1 : C++ 20 boiler plate for comparison operators is a couple of one liners.

   bool operator==(const CalendarType &other) const = default;
   std::strong_ordering operator<=>(const CalendarType &other) const = default;

};

// Cost 2 : We have a bit of boiler plate code to write. // Mitigation 2 : We've put the common code into a base class.

struct Year : CalendarType { explicit Year(int year) : CalendarType(year) {} };

struct Month : public CalendarType { explicit Month(int month) : CalendarType(month) {} };

struct Day : public CalendarType { explicit Day(int day) : CalendarType(day) {} };

class Date { public: Date(Year year, Month month, Day day) : m_year(year), m_month(month), m_day(day) { }

   Year year() const
   {
      return m_year;
   }

   Month month() const
   {
      return m_month;
   }

   Day day() const
   {
      return m_day;
   }

private: // Cost 3 : To fully understand, the reader needs to look at how Year, Month and Day are implemented.

   Year m_year;
   Month m_month;
   Day m_day;

};

int main() { // Cost 2 :

   Date date1 {Year(1970), Month(4), Day(7)};   // Benefit 1 : It's clear to the reader what each argument is.
   Date date2 {Year(1983), Month(1), Day(12)};
   // Date date3 {7, 4, 1979};                  // Benefit 2 : Code writer can't get them in the wrong order
                                                // (courtesy of explicit this wont compile).


   // (Yes, I've glossed over leap year edge cases)
   bool earlierInTheYear = date2.month() < date1.month() ||
        date2.month() == date1.month() && date2.day() < date1.day();

   std::cout << "1983-01-12 " << (earlierInTheYear ? "is" : "is not")
             << " earlier in the year than 1970-04-07" << std::endl;
}