Re: [Boost-users] [fusion] Wrapping a Fusion sequence

Subject: Re: [Boost-users] [fusion] Wrapping a Fusion sequence
From: Joel de Guzman (djowel_at_[hidden])
Date: 2012-08-30 23:05:07

On 8/29/2012 7:10 AM, Agustín K-ballo Bergé wrote:
> I'm looking for a way to create a Fusion sequence wrapper that is itself a Fusion sequence
> and forwards all 'calls' to its wrapped sequence. Something in the lines of
>
>      template< typename Sequence >
>      struct sequence_wrapper
>      {
>          explicit sequence_wrapper( Sequence const& s ) : seq( s ){}
>
>          Sequence seq;
>      };
>
> where sequence_wrapper< Sequence > is a Fusion sequence as well, and works just as
> Sequence would. The reason I need this is that I have several functions that operate on
> Fusion sequences (where all elements satisfy some special requirements), and I would like
> to add some syntax sugar for which I need a custom type to add overloaded operators to. I
> do not need the result of operations on a sequence_wrapper to return a sequence_wrapper as
> well, only the syntax sugar related calls would return a (manually) wrapped sequence. For
> instance, appending elements to a sequence using the comma operator (somewhat of a
> Boost.Assign for Fusion sequences):
>
>      template< typename Sequence, typename T >
>      sequence_wrapper<
>          typename boost::fusion::result_of::push_back<
>              Sequence const&
>            , T
>          >::type
>      > operator,( Sequence const& seq, T const& v )
>      {
>          return
>              sequence_wrapper<
>                  typename boost::fusion::result_of::push_back<
>                      Sequence const&
>                    , T
>                  >::type
>              >( boost::fusion::push_back( seq, v ) )
>              ;
>      }
>
> What would be the best way to achieve this (if it is indeed supported by the library)? I'm
> particularly trying to avoid creating a Fusion sequence from scratch, as I would like to
> use whatever sequence is returned by Fusion operations. Would inheritance + specialization
> of `tag_of` to return the tag of the wrapped sequence just work? Or will I need to define
> a tag of my own and implement all required functions to just forward the call?

I do not know an easy way other than using the Sequence Facade:
http://tinyurl.com/8k5wq5p

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