virtual pos_type seekoff(off_type off, ios_base::seekdir way,
ios_base::openmode mode = 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_stringbuf<E, T, A>, a stream position consists purely of a stream offset. Offset zero designates the
first element of the controlled sequence.
The new position is determined as follows:
way == ios_base::beg, the new position is the beginning of the stream plus off. way == ios_base::cur, the new position is the current stream position plus off. way == ios_base::end, the new position is the end of the stream plus off. If mode & ios_base::in is nonzero, the function alters the next position to read in the input buffer. If mode &
ios_base::out is nonzero, the function alters the next position to write in the output buffer. For a stream to be
affected, its buffer must exist. For a positioning operation to succeed, the resulting stream position must lie within the
controlled sequence. If the function affects both stream positions, way must be ios_base::beg or ios_base::end
and both streams must be positioned at the same element. Otherwise (or if neither position is affected), the positioning
operation fails.
If the function succeeds in altering the stream position(s), it returns the resultant stream position. Otherwise, it fails and returns an invalid stream position.