Re: [boost] [wave] preprocessor directives in output

Subject: Re: [boost] [wave] preprocessor directives in output
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2014-01-21 20:48:28

> >> On 01/21/2014 04:20 PM, pmenso57_at_[hidden] wrote:
> >>> Eric, just be very careful about what macros expand when.  E.g. the
> >> expression:
> >>>
> >>> !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
> >>>
> >>> will get expanded on the first preprocessor pass to something like
> >>>
> >>> !defined(1)
> >>>
> >>> (or whatever it expands to)
> >>
> >>
> >> Thanks for the warning, and that makes sense, but it's not what I'm
> >> seeing. I'm seeing wave spit out:
> >>
> >> # if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
> >>
> >> instead of:
> >>
> >> # if !defined(1)
> >>
> >> Why would that be? Regardless, I can turn off expansion of this macro
> >> during preprocessing with wave's -N switch, so there's a fix should I
> >> ever need it.
> >
> > Turning off the expansion of macros used in #if conditions can cause
> > all kind of trouble. All depends on what is inside the wave pragmas:
> >
> > // this stuff will be seen by wave but not generated
> >
> > #pragma wave option(output:file)
> > // this stuff will be preprocessed and written to 'file'
> > #pragma wave option(output:null)
> >
> > // this stuff will be seen by wave but not generated
> >
> > So if it gets expanded outside of the pragma region all is fine. Did I
> > misunderstand something?
> 
> 
> The code is getting expanded *in* the pragma region, but I think I can
> guess what's happening. I have this:
> 
> #define FUSION_HASH #
> #if defined(__WAVE__) && \
>     defined(BOOST_FUSION_CREATE_PREPROCESSED_FILES)
> FUSION_HASH if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
> #endif
> #if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES) || \
>     (defined(__WAVE__) && \
>      defined(BOOST_FUSION_CREATE_PREPROCESSED_FILES))
>         vector(vector&& rhs)
>             : vec(std::forward<vector_n>(rhs.vec)) {} // more code that
> uses rvalue references #endif #if defined(__WAVE__) && \
>     defined(BOOST_FUSION_CREATE_PREPROCESSED_FILES)
> FUSION_HASH endif
> #endif
> #undef FUSION_HASH
> 
> When wave is preprocessing the files, it spits out:
> 
> # if !defined(BOOST_NO_CXX11_RVALUE_REFERENCES)
>         vector(vector&& rhs)
>             : vec(std::forward<vector_n>(rhs.vec)) {} #endif
> 
> as it should. That's because BOOST_NO_CXX11_RVALUE_REFERENCES is not
> defined, and because it doesn't get replaced with 0, which would only
> happen if wave thought it was evaluating a #if ... which it's not.
> 
> This begs the question of which compiler config wave uses. Does it try to
> emulate a known compiler? If so, which? And what version? It's never
> occurred to me before to wonder what #defines are set when wave processes
> #include <boost/config.hpp>.

I predefines only the wave-specific macros
(http://www.boost.org/doc/libs/1_55_0/libs/wave/doc/predefined_macros.html).
Everything else has to be supplied by you. 

Regards Hartmut
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