1 /*=============================================================================
2     Copyright (c) 2001-2011 Joel de Guzman
3     Copyright (c) 2006 Dan Marsden
4 
5     Distributed under the Boost Software License, Version 1.0. (See accompanying
6     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
7 ==============================================================================*/
8 #if !defined(FUSION_VALUE_AT_IMPL_20060124_2129)
9 #define FUSION_VALUE_AT_IMPL_20060124_2129
10 
11 #include <boost/fusion/support/config.hpp>
12 #include <boost/fusion/container/vector/convert.hpp>
13 #include <boost/fusion/algorithm/transformation/transform.hpp>
14 #include <boost/fusion/sequence/intrinsic/value_at.hpp>
15 #include <boost/type_traits/remove_reference.hpp>
16 #include <boost/fusion/support/unused.hpp>
17 #include <boost/mpl/eval_if.hpp>
18 #include <boost/mpl/identity.hpp>
19 #include <boost/type_traits/is_same.hpp>
20 #include <boost/config.hpp>
21 
22 namespace boost { namespace fusion {
23 
24     struct zip_view_tag;
25 
26     namespace detail
27     {
28         template<typename N>
29         struct poly_value_at
30         {
31             template<typename T>
32             struct result;
33 
34             template<typename N1, typename Seq>
35             struct result<poly_value_at<N1>(Seq)>
36                 : mpl::eval_if<is_same<Seq, unused_type const&>,
37                                mpl::identity<unused_type>,
38                                result_of::value_at<typename remove_reference<Seq>::type, N> >
39             {};
40 
41             // never called, but needed for decltype-based result_of (C++0x)
42 #ifndef BOOST_NO_CXX11_RVALUE_REFERENCES
43             template<typename Seq>
44             BOOST_FUSION_GPU_ENABLED
45             typename result<poly_value_at(Seq)>::type
46             operator()(Seq&&) const;
47 #endif
48         };
49     }
50 
51     namespace extension
52     {
53         template<typename Tag>
54         struct value_at_impl;
55 
56         template<>
57         struct value_at_impl<zip_view_tag>
58         {
59             template<typename Sequence, typename N>
60             struct apply
61             {
62                 typedef typename result_of::transform<
63                     typename Sequence::sequences,
64                     detail::poly_value_at<N> >::type values;
65                 typedef typename result_of::as_vector<values>::type type;
66             };
67         };
68     }
69 }}
70 
71 #endif
72