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_KEY_05052005_0229)
9 #define FUSION_VALUE_AT_KEY_05052005_0229
10 
11 #include <boost/fusion/support/config.hpp>
12 #include <boost/mpl/int.hpp>
13 #include <boost/mpl/empty_base.hpp>
14 #include <boost/mpl/if.hpp>
15 #include <boost/mpl/or.hpp>
16 #include <boost/fusion/sequence/intrinsic_fwd.hpp>
17 #include <boost/fusion/sequence/intrinsic/has_key.hpp>
18 #include <boost/fusion/iterator/value_of_data.hpp>
19 #include <boost/fusion/algorithm/query/find.hpp>
20 #include <boost/fusion/support/tag_of.hpp>
21 #include <boost/fusion/support/category_of.hpp>
22 
23 namespace boost { namespace fusion
24 {
25     // Special tags:
26     struct sequence_facade_tag;
27     struct boost_array_tag; // boost::array tag
28     struct mpl_sequence_tag; // mpl sequence tag
29     struct std_pair_tag; // std::pair tag
30 
31     namespace extension
32     {
33         template <typename Tag>
34         struct value_at_key_impl
35         {
36             template <typename Seq, typename Key>
37             struct apply
38               : result_of::value_of_data<
39                     typename result_of::find<Seq, Key>::type
40                 >
41             {};
42         };
43 
44         template <>
45         struct value_at_key_impl<sequence_facade_tag>
46         {
47             template <typename Sequence, typename Key>
48             struct apply : Sequence::template value_at_key<Sequence, Key> {};
49         };
50 
51         template <>
52         struct value_at_key_impl<boost_array_tag>;
53 
54         template <>
55         struct value_at_key_impl<mpl_sequence_tag>;
56 
57         template <>
58         struct value_at_key_impl<std_pair_tag>;
59     }
60 
61     namespace detail
62     {
63         template <typename Sequence, typename N, typename Tag>
64         struct value_at_key_impl
65             : mpl::if_<
66                   mpl::or_<
67                       typename extension::has_key_impl<Tag>::template apply<Sequence, N>
68                     , traits::is_unbounded<Sequence>
69                   >
70                 , typename extension::value_at_key_impl<Tag>::template apply<Sequence, N>
71                 , mpl::empty_base
72               >::type
73         {};
74     }
75 
76     namespace result_of
77     {
78         template <typename Sequence, typename N>
79         struct value_at_key
80             : detail::value_at_key_impl<Sequence, N, typename detail::tag_of<Sequence>::type>
81         {};
82     }
83 }}
84 
85 #endif
86 
87