virtual streampos seekpos(streampos sp,
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 strstreambuf, 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 which & ios_base::in is nonzero and the input buffer exists, the function alters the next position to read in the
input buffer. (If which & ios_base::out is nonzero and the output buffer exists, the function also sets the next
position to write to match the next position to read.) Otherwise, if which & ios_base::out is nonzero and the
output buffer exists, the function alters the next position to write in the output buffer. Otherwise, the positioning
operation fails. For a positioning operation to succeed, the resulting stream position must lie within the controlled
sequence.
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.