In order to support file I/O and multibyte/wide character conversion,
conversions are performed using members of a facet, referred to as
a_codecvt in following subclauses, obtained as if by
const codecvt<charT, char, typename traits::state_type>& a_codecvt =
use_facet<codecvt<charT, char, typename traits::state_type>>(getloc());