using char_type = CHAR_T;
is used to refer to the character container type
in the implementation of the library classes defined in [string.classes]
and Clause [input.output].
using int_type = INT_T;
For a certain character container type
a related container type
shall be a type or class which can represent all of the
valid characters converted from the corresponding
values, as well as an end-of-file value,
represents a character container type
which can hold end-of-file to be used as a return type
of the iostream class member functions.
using off_type = implementation-defined;
using pos_type = implementation-defined;
using state_type = STATE_T;