Re: [Boost-bugs] [Boost C++ Libraries] #6126: Signed integer members of Boost.Fusion adapted ADTs are not output correctly with Boost.Spirit.Karma rules

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #6126: Signed integer members of Boost.Fusion adapted ADTs are not output correctly with Boost.Spirit.Karma rules
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-02-06 15:09:17

#6126: Signed integer members of Boost.Fusion adapted ADTs are not output
correctly with Boost.Spirit.Karma rules
-------------------------------+--------------------------------------------
  Reporter:  t0rt1e@…          |       Owner:  hkaiser                                                                                                             
      Type:  Bugs              |      Status:  new                                                                                                                 
 Milestone:  To Be Determined  |   Component:  spirit                                                                                                              
   Version:  Boost 1.48.0      |    Severity:  Regression                                                                                                          
Resolution:                    |    Keywords:  patch proposed, Boost.Spirit.Karma, Boost.Fusion, BOOST_FUSION_ADAPT_CLASS, BOOST_FUSION_ADAPT_ADT, short, int, long
-------------------------------+--------------------------------------------

Comment (by t0rt1e@…):

 Replying to [comment:17 hkaiser]:
 > Replying to [comment:16 Seth Heeren <bugs@…>]:
 > > Replying to [comment:15 hkaiser]:
 > > >
 > > > That's excellent news! Thanks. That allows to revert the patch
 above.
 [...]
 > > Apart from specializing extract_from_attribute for adt_attribute_proxy
 I don't see an easy way to resolve this. And I wouldn't think that this
 approach is entirely trivial, either (it might be enough due to the
 implicit conversion of the proxy to its exposed type?)
 >
 > Yep, that's what I think needs to be done, at least as a first step.

 Since reimplementing the boost::rational getters to return by reference
 instead by value isn't an option for me, I had a look to the Karma sources
 to try Seth's suggestion. {{{extract_from_attribute}}} has been already
 specialized for {{{adt_attribute_proxy}}} in
 {{{boost/spirit/home/support/adapt_adt_attributes.hpp:192}}}. Commenting
 out this specialization, will generate a compiler warning:
 {{{
 boost/spirit/home/karma/detail/extract_from.hpp:71:20: warning: returning
 reference to local temporary object
             return extract_from<Exposed>(fusion::at_c<0>(attr), ctx);
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 }}}
 This also happens if the return {{{type}}} {{{typedef}}} is changed to
 {{{fusion::extension::adt_attribute_proxy<T, N, Const>}}} in
 {{{boost/spirit/home/support/adapt_adt_attributes.hpp:192}}} and the proxy
 object is directly returned {{{call()}}} like suggested by Seth. I tried
 to further follow the {{{extract_from}}} to see where the proxy gets
 converted to a const ref to the contained type, but unfortunately got
 lost. So, I'm stuck again.

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/6126#comment:19>
Boost C++ Libraries <http://www.boost.org/>
Boost provides free peer-reviewed portable C++ source libraries.