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Re: [Patch] Fix seek to beg after grow bug

Nathan Myers wrote:

This doesn't seem quite right. I think the right code is
more like just

__size_type __len = std::max(_M_string.size(),
__size_type(this->_M_out_cur - this->_M_out_beg));

In other words, either cur and beg are both 0, or (cur - beg) is at least as large as the string.

No, this cannot be ok.
Look at the testcase: _M_out_cur == _M_out_beg in that case and _M_out_end,
which points one-past-end, gives the current string length, as _M_out_end -
_M_out_cur which is _bigger_ than _M_string.size().
Indeed, the code continues as:

   return __string_type(this->_M_out_beg, this->_M_out_beg + __len);

We have to be sure, too, that in seek{pos,off}, the string length gets updated like the above before _M_out_cur gets moved, because sometimes _M_out_cur is our only record of how long the string really has become.

Humm. The seeks do not change any lenght by themselves, but they change _M_out_cur,
but _always_ via _M_out_cur_move which is:

_M_out_cur_move(off_type __n) // argument needs to be +-
bool __testin = _M_in_cur;

_M_out_cur += __n;
if (__testin && _M_buf_unified)
_M_in_cur += __n;
if (_M_out_cur > _M_out_end)
_M_out_end = _M_out_cur;
// NB: in | out buffers drag the _M_in_end pointer along...
if (__testin)
_M_in_end += __n;

Therefore, you can see that _M_out_cur _cannot_ become bigger than _M_out_end.

Really, it seems to me that _M_out_end is _always_ equal to the string end,
and in fact, as such is used (under the name epptr, but this is another story :)
in the seek operation which we already privately discussed about...

Likewise, whenever the buffer gets lengthened, the string capacity needs to be updated along with _M_out_end.

This is done correctly, if not with optimal efficiency by str(), it seems.

Do you agree??


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