using char_type = CHAR_T;
using int_type = INT_T;
Requires: For a certain character container type char_type, a related container type INT_T shall be a type or class which can represent all of the valid characters converted from the corresponding char_type values, as well as an end-of-file value, eof(). The type int_type represents a character container type which can hold end-of-file to be used as a return type of the iostream class member functions.224
using off_type = implementation-defined;
using pos_type = implementation-defined;
using state_type = STATE_T;
If eof() can be held in char_type then some iostreams operations may give surprising results.