The mysterious _M_output_unshift

[Paolo Carlini]

Hi,

I'm currently trying to understand in better detail basic_filebuf
(in particular seekoff) in order to simplify _M_*_cur_move, the
last remaining CPU hog in the implementation of sputc(), sbumpc(),
etc.

As part of this I would really appreciate some help about the
rationale behind basic_filebuf::_M_output_unshift, currently
completely empty, and also about this code in basic_filebuf::close:

#if 0
      // XXX not done
      if (_M_last_overflowed)
        {
          _M_output_unshift();
          _M_really_overflow(__eof);
        }
#endif


Thank you very much in advance!
Paolo.