Re: [boost] DRYing BOOST_FUSION_ADAPT_STRUCT with decltype/Boost.TypeOf ?

Subject: Re: [boost] DRYing BOOST_FUSION_ADAPT_STRUCT with decltype/Boost.TypeOf ?
From: Joel de Guzman (djowel_at_[hidden])
Date: 2013-12-19 17:46:24

On 12/18/13, 9:35 AM, Damien Buhl wrote:
> Hi dear Boost Fusion developers,
>
> I was asking myself if there were already plan to improve BOOST_FUSION_ADAPT_STRUCT to benefit from C++11 features like decltype ? This would be interesting to avoid the risk encountered that the types definitions can get out-of-sync. And would avoid typing type declarations twice.
>
> Currently there is BOOST_FUSION_DEFINE_STRUCT to avoid repeating the type information for each field. However this is something that makes most developer I know unhappy, as it obliges to modify the readable C++ aggregate types definition (i.e. Could be public headers) with this macro.
>
> Then there is the nice choice of BOOST_FUSION_ADAPT_STRUCT which obliges retyping the type and adds the risk that some field type information could get out-of-sync between the struct definition and the BOOST_FUSION_ADAPT_STRUCT listing.
>
> I think it would be valuable to add macros that would enable to select the fields that we want to adapt to a Random Access Sequence without requiring to repeat the type information. I would find more comfortable to write the example provided here http://www.boost.org/doc/libs/1_55_0/libs/fusion/doc/html/fusion/adapted/adapt_struct.html as follow :
>
> namespace demo
> {
>      struct employee
>      {
>          std::string name;
>          int age;
>      };
> }
>
> // demo::employee is now a Fusion sequence
> BOOST_FUSION_ADAPT_STRUCT_EXT(
>      demo::employee,
>      (name)
>      (age))
>
> And a naïve implementation of BOOST_FUSION_ADAPT_STRUCT_EXT could be:
>
> #include <boost/typeof/typeof.hpp>
> #include <boost/fusion/include/adapt_struct.hpp>
> #include <boost/preprocessor/seq/for_each.hpp>
>
> #define ATTRIBUTE_DEFINITION(r, data, elem)                                     \
>      (BOOST_TYPEOF(data::elem), elem)                                            \
>
>
> #define BOOST_FUSION_ADAPT_STRUCT_EXT(NAME, ATTRIBUTES)                         \
>      BOOST_FUSION_ADAPT_STRUCT(NAME,                                             \
>          BOOST_PP_SEQ_FOR_EACH(ATTRIBUTE_DEFINITION, NAME, ATTRIBUTES))
>
> #endif
>
> The same could certainly be achieved for BOOST_FUSION_ADAPT_TPL_STRUCT, while I didn't check.
>
> Thanks for the really great job done in Boost.Fusion.

Thanks, Damien. I like the idea. Could you 1) try it out and
2) submit a patch (or pull request). It would also be super if
you can add in the doc patch.

Regards,
-- 
Joel de Guzman
http://www.ciere.com
http://boost-spirit.com
http://www.cycfi.com/