Re: [boost] [fusion] list/detail/at_impl.hpp, Suggest recrusive elem_at_c instead of recursive apply

From: Joel de Guzman (joel_at_[hidden])
Date: 2006-05-17 02:10:44

Larry Evans wrote:
> In boost/fusion/.../list/detail/at_impl.hpp
> 
> the apply is recursive; however, the only reason for the
> recursion is to "find the right element type".  The other
> reason for apply is the same as that of the apply
> in.../vector/detail/at_impl.hpp.  "find the right element
> type" in vector's at_impl is done with mpl::at.  The
> similarity between list and vector at_impl would be more
> apparent if:
> 
>              template <typename Sequence, int N>
>              struct elem_at_c
>              {
>                  typedef typename
>                      mpl::eval_if<
>                          mpl::bool_<N == 0>
>                        , mpl::identity<typename Sequence::car_type>
>                        , elem_at_c<typename Sequence::cdr_type, N-1>
>                      >::type
>                  type;
>              };
> 
> were used to find the right element type.  Then the frist part of
> list::at_impl::apply would be:
> 
>              template <typename Sequence, typename N>
>              struct apply
>              {
>                  typedef elem_at_c<Sequence, N::value> element;
>                  typedef typename
>                      mpl::eval_if<
>                          is_const<Sequence>
>                        , detail::cref_result<element>
>                        , detail::ref_result<element>
>                      >::type
>                  type;
> 
> which is very similar to the counterpart in vector:
> 
>              template <typename Sequence, typename N>
>              struct apply
>              {
>                  typedef mpl::at<typename Sequence::types, N> element;
>                  typedef typename
>                      mpl::eval_if<
>                          is_const<Sequence>
>                        , detail::cref_result<element>
>                        , detail::ref_result<element>
>                      >::type
>                  type;

Looks good to me. Do you have a patch?

Regards,
-- 
Joel de Guzman
http://www.boost-consulting.com
http://spirit.sf.net