In maximally generic code, prefer non-member versions of begin, end, rbegin, etc., over their member function counterparts.

C++11 gives us a way to make explicit that a derived class function is supposed to override a base class version: override.

A brief notes on Clean Architecture

Any function may be deleted, including non-member functions and template instantiations.

Enumerators of scoped enums are visible only within the enum, convert to other types only with a cast, and always support forward-declared because their default underlying type is int.