For some standard facets a standard “…_byname
derived from it, implements the virtual function semantics
equivalent to that facet of the locale
constructed by locale(const char*)
with the same name.
Each such facet provides a constructor that takes
a const char*
argument, which names the locale, and
argument, which is passed to the base class constructor.
Each such facet also provides a constructor that takes
which has the same effect as calling the first constructor
with the two arguments str.c_str()
If there is no “…_byname
” version of a facet,
the base class implements named locale semantics itself
by reference to other facets.