const error_category& generic_category() noexcept;
Returns: A reference to an object of a type derived from class error_category. All calls to this function shall return references to the same object.
Remarks: The object's default_error_condition and equivalent virtual functions shall behave as specified for the class error_category. The object's name virtual function shall return a pointer to the string "generic".
const error_category& system_category() noexcept;
Returns: A reference to an object of a type derived from class error_category. All calls to this function shall return references to the same object.
Remarks: The object's equivalent virtual functions shall behave as specified for class error_category. The object's name virtual function shall return a pointer to the string "system". The object's default_error_condition virtual function shall behave as follows:
If the argument ev corresponds to a POSIX errno value posv, the function shall return error_condition(posv, generic_category()). Otherwise, the function shall return error_condition(ev, system_category()). What constitutes correspondence for any given operating system is unspecified. [ Note: The number of potential system error codes is large and unbounded, and some may not correspond to any POSIX errno value. Thus implementations are given latitude in determining correspondence. — end note ]