virtual pos_type seekpos(pos_type pos,
ios_base::openmode which = ios_base::in | ios_base::out);The protected virtual member function endeavors to alter the current positions for the controlled streams. For an object
of class basic_filebuf<E, T>, a stream position can be represented by an object of type fpos_t, which stores an
offset and any state information needed to parse a wide stream. Offset zero designates the first element of the stream.
(An object of type pos_type stores at least an fpos_t object.)
For a file opened for both reading and writing, both the input and output streams are positioned in tandem. To switch
between inserting and extracting, you must call either pubseekoff or pubseekpos. Calls to pubseekoff (and hence
to seekoff) have various limitations for text streams, binary streams, and wide streams.
If the file pointer fp is a null pointer, the function fails. Otherwise, it endeavors to alter the stream position by calling
fsetpos(fp, &fposn), where fposn is the fpos_t object stored in pos. If that function succeeds, the function
returns pos. Otherwise, it returns an invalid stream position.