zip_view/detail/value_at_impl.hpp

0001 /*=============================================================================
0002     Copyright (c) 2001-2011 Joel de Guzman
0003     Copyright (c) 2006 Dan Marsden
0004
0005     Distributed under the Boost Software License, Version 1.0. (See accompanying
0006     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
0007 ==============================================================================*/
0008 #if !defined(FUSION_VALUE_AT_IMPL_20060124_2129)
0009 #define FUSION_VALUE_AT_IMPL_20060124_2129
0010
0011 #include <boost/fusion/support/config.hpp>
0012 #include <boost/fusion/container/vector/convert.hpp>
0013 #include <boost/fusion/algorithm/transformation/transform.hpp>
0014 #include <boost/fusion/sequence/intrinsic/value_at.hpp>
0015 #include <boost/type_traits/remove_reference.hpp>
0016 #include <boost/fusion/support/unused.hpp>
0017 #include <boost/mpl/eval_if.hpp>
0018 #include <boost/mpl/identity.hpp>
0019 #include <boost/type_traits/is_same.hpp>
0020 #include <boost/config.hpp>
0021
0022 namespace boost { namespace fusion {
0023
0024     struct zip_view_tag;
0025
0026     namespace detail
0027     {
0028         template<typename N>
0029         struct poly_value_at
0030         {
0031             template<typename T>
0032             struct result;
0033
0034             template<typename N1, typename Seq>
0035             struct result<poly_value_at<N1>(Seq)>
0036                 : mpl::eval_if<is_same<Seq, unused_type const&>,
0037                                mpl::identity<unused_type>,
0038                                result_of::value_at<typename remove_reference<Seq>::type, N> >
0039             {};
0040
0041             // never called, but needed for decltype-based result_of (C++0x)
0042 #ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
0043             template<typename Seq>
0044             BOOST_FUSION_GPU_ENABLED
0045             typename result<poly_value_at(Seq)>::type
0046             operator()(Seq&&) const;
0047 #endif
0048         };
0049     }
0050
0051     namespace extension
0052     {
0053         template<typename Tag>
0054         struct value_at_impl;
0055
0056         template<>
0057         struct value_at_impl<zip_view_tag>
0058         {
0059             template<typename Sequence, typename N>
0060             struct apply
0061             {
0062                 typedef typename result_of::transform<
0063                     typename Sequence::sequences,
0064                     detail::poly_value_at<N> >::type values;
0065                 typedef typename result_of::as_vector<values>::type type;
0066             };
0067         };
0068     }
0069 }}
0070
0071 #endif