1 /*=============================================================================
2     Copyright (c) 2001-2011 Joel de Guzman
3 
4     Distributed under the Boost Software License, Version 1.0. (See accompanying
5     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 ==============================================================================*/
7 #if !defined(FUSION_AT_IMPL_07172005_0726)
8 #define FUSION_AT_IMPL_07172005_0726
9 
10 #include <boost/fusion/support/config.hpp>
11 #include <boost/fusion/support/detail/access.hpp>
12 #include <boost/type_traits/is_const.hpp>
13 #include <boost/type_traits/add_const.hpp>
14 #include <boost/mpl/if.hpp>
15 #include <boost/mpl/bool.hpp>
16 
17 namespace boost { namespace fusion
18 {
19     namespace detail
20     {
21         template <typename Cons>
22         struct cons_deref
23         {
24             typedef typename Cons::car_type type;
25         };
26 
27         template <typename Cons, int I>
28         struct cons_advance
29         {
30             typedef typename
31                 cons_advance<Cons, I-1>::type::cdr_type
32             type;
33         };
34 
35         template <typename Cons>
36         struct cons_advance<Cons, 0>
37         {
38             typedef Cons type;
39         };
40 
41         template <typename Cons>
42         struct cons_advance<Cons, 1>
43         {
44             typedef typename Cons::cdr_type type;
45         };
46 
47         template <typename Cons>
48         struct cons_advance<Cons, 2>
49         {
50 #if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
51             typedef typename Cons::cdr_type::cdr_type type;
52 #else
53             typedef typename Cons::cdr_type _a;
54             typedef typename _a::cdr_type type;
55 #endif
56         };
57 
58         template <typename Cons>
59         struct cons_advance<Cons, 3>
60         {
61 #if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
62             typedef typename Cons::cdr_type::cdr_type::cdr_type type;
63 #else
64             typedef typename Cons::cdr_type _a;
65             typedef typename _a::cdr_type _b;
66             typedef typename _b::cdr_type type;
67 #endif
68         };
69 
70         template <typename Cons>
71         struct cons_advance<Cons, 4>
72         {
73 #if BOOST_WORKAROUND(BOOST_MSVC, > 1400) // VC8 and above
74             typedef typename Cons::cdr_type::cdr_type::cdr_type::cdr_type type;
75 #else
76             typedef typename Cons::cdr_type _a;
77             typedef typename _a::cdr_type _b;
78             typedef typename _b::cdr_type _c;
79             typedef typename _c::cdr_type type;
80 #endif
81         };
82     }
83 
84     struct cons_tag;
85 
86     namespace extension
87     {
88         template <typename Tag>
89         struct at_impl;
90 
91         template <>
92         struct at_impl<cons_tag>
93         {
94             template <typename Sequence, typename N>
95             struct apply
96             {
97                 typedef typename detail::cons_deref<
98                     typename detail::cons_advance<Sequence, N::value>::type>::type
99                 element;
100 
101                 typedef typename
102                     mpl::if_<
103                         is_const<Sequence>
104                       , typename detail::cref_result<element>::type
105                       , typename detail::ref_result<element>::type
106                     >::type
107                 type;
108 
109                 template <typename Cons, int N2>
110                 BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
111                 static type
callboost::fusion::extension::at_impl::apply112                 call(Cons& s, mpl::int_<N2>)
113                 {
114                     return call(s.cdr, mpl::int_<N2-1>());
115                 }
116 
117                 template <typename Cons>
118                 BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
119                 static type
callboost::fusion::extension::at_impl::apply120                 call(Cons& s, mpl::int_<0>)
121                 {
122                     return s.car;
123                 }
124 
125                 BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
126                 static type
callboost::fusion::extension::at_impl::apply127                 call(Sequence& s)
128                 {
129                     return call(s, mpl::int_<N::value>());
130                 }
131             };
132         };
133     }
134 }}
135 
136 #endif
137