Re: [Boost-bugs] [Boost C++ Libraries] #8577: Multiple qi::match calls on the same stream corrupt input data

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #8577: Multiple qi::match calls on the same stream corrupt input data
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2017-12-16 23:29:28

#8577: Multiple qi::match calls on the same stream corrupt input data
--------------------------------------+----------------------------
  Reporter:  Vadim Guchenko <yhw@…>   |      Owner:  Joel de Guzman
      Type:  Bugs                     |     Status:  new
 Milestone:  To Be Determined         |  Component:  spirit
   Version:  Boost Development Trunk  |   Severity:  Problem
Resolution:                           |   Keywords:
--------------------------------------+----------------------------

Comment (by Nikita Kniazev <nok.raven@…>):

 > Probably it consumes some characters from the stream to do look-ahead

 It will consume data even if your parser is LL(1).

 > but doesn't return them back to the stream on destruction

 You cannot return read data back to input stream (read about InputIterator
 concept).

 > so if it's the expected qi::match behavior, it's worth to be mentioned
 in the documentation.

 It should be clear to everyone that to parse data from an input stream you
 must pull data from the stream and you cannot place the read data back to
 it, even a single character.

 ----

 All `qi::match` is doing is wrapping your iterators with
 `spirit::basic_istream_iterator`, calling `qi::parse` and setting
 `failbit` if parsing failed. You should not use `qi::match` in loop on the
 same stream at least because it is slow (`spirit::basic_istream_iterator`
 lots of heap allocations). Only use `qi::match` to match all of your
 input.

 Hint: Replace `+qi::lit(' ')` with a skipper (i.e. use `qi::phrase_match`)

-- 
Ticket URL: <https://svn.boost.org/trac10/boost/ticket/8577#comment:2>
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