cpp/io/basic filebuf/underflow

Reads more data into the input area.

Behaves like the base class, except that to read the data from the associated character sequence (the file) into the get area, first reads the bytes from the file into a temporary buffer (allocated as large as necessary), then uses of the imbued locale to convert the external (typically, multibyte) representation to the internal form which is then used to populate the get area. The conversion may be skipped if the locale's returns

Parameters
(none)

Return value
(the first character of the pending sequence) in case of success, or in case of failure.