virtual pos_type seekpos(pos_type sp,
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 by sp.
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. 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.