[Boost-bugs] [Boost C++ Libraries] #11824: [spirit][qi] skip_flag::dont_postskip not working as expected

Subject: [Boost-bugs] [Boost C++ Libraries] #11824: [spirit][qi] skip_flag::dont_postskip not working as expected
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2015-11-28 08:56:53

#11824: [spirit][qi] skip_flag::dont_postskip not working as expected
-----------------------------------------------+-------------------------
 Reporter:  Daniel Starke <daniel.f.starke@…>  |      Owner:  djowel
     Type:  Bugs                               |     Status:  new
Milestone:  To Be Determined                   |  Component:  spirit
  Version:  Boost 1.59.0                       |   Severity:  Showstopper
 Keywords:  qi skip post-skip                  |
-----------------------------------------------+-------------------------
 Assuming we want to create a primitive which checks if the skip parser is
 called at least once before the next terminal we need to disable post-
 skipping for this purpose. However, disabling post-skipping alone will not
 work if the previous parser failed on a input because all terminals do not
 revert to the position before the skip parser but just after it. A
 workaround could be add an and-prediction in front of each terminal but
 this makes the code unreadable.
 Attached is a possible patch for spirit qi which changes all terminals to
 revert to the position before the skip parser in case of a mismatch.
 Note: This might introduce some performance regression.

 Example:
 {{{
 string input("a b");
 string::const_iterator first(input.begin()), last(input.end());
 phrase_parse(first, last, ((+char_("a", "z")) > no_skip[&blank]) >> char_,
 blank, dont_postskip);
 }}}
 This will fail to match in the current implementation even though it looks
 perfectly fine.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/11824>
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