19 Diagnostics library [diagnostics]

19.5 System error support [syserr]

19.5.3 Class error_­category [syserr.errcat]

19.5.3.5 Error category objects [syserr.errcat.objects]

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 1:
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]