category() member shall return std::system_category() for
errors originating from the operating system, or a reference to an
implementation-defined error_category object for errors originating elsewhere.
The implementation shall define the possible values of value() for each of these
For operating systems that are based on POSIX,
implementations should define the std::system_category() values as
identical to the POSIX errno values, with additional values as defined by the
operating system's documentation.
Implementations for operating systems that are not
based on POSIX should define values identical to the operating system's
For errors that do not originate from the operating system, the implementation
may provide enums for the associated values.