Re: [Boost-users] [fusion] is there anything resembling a joint_view of 2 maps?

Subject: Re: [Boost-users] [fusion] is there anything resembling a joint_view of 2 maps?
From: er (erwann.rogard_at_[hidden])
Date: 2009-10-22 17:34:37

> Either way, for now you can workaround your issue by providing your own 
> extension implementations for 
> fusion::extension::category_of_impl<fusion::joint_view_tag>, 
> fusion::extension::has_key_impl<fusion::joint_view_tag> and 
> fusion::extension::at_key_impl<fusion::joint_view_tag>, or, even better, 
> wrap the joint_view up in another view which implements both (forward 
> and associative) extension backends.
> 

Thanks, but the workaround you are suggesting is not obvious to me. 
Maybe you can help? I have started here :

         template <>
         template<typename Seq1,typename Seq2,typename Key>
         struct at_key_impl<joint_view_tag>::apply<
             joint_view<Seq1,Seq2>,Key
         >
         {
             typedef joint_view<Seq1,Seq2> jv_;

             typedef fusion::result_of::has_key<Seq1,Key> has_key1_;
             typedef typename mpl::if_<
                 has_key1_,
                 Seq1,
                 Seq2
             >::type seq_;

             typedef typename result_of::at_key<seq_,Key>::type type;

             static type
             call(jv_& m)
             {
                 // return m.at_impl(mpl::identity<Key>());
                ????

             }
         };