Each of the standard categories includes a family of facets.
Some of these implement formatting or parsing of a datum,
for use by standard or users' iostream operators <<
as members put()
Each such member function takes an
argument whose members
specify the format of the corresponding datum ([ios.base]
Those functions which need to use other facets call its member getloc()
to retrieve the locale imbued there.
Formatting facets use the character argument fill
to fill out the specified width where necessary.