Initial move from Spirit CVS

[SVN r34896]

include/boost/fusion/sequence/intrinsic/at_key.hpp

@@ -0,0 +1,56 @@
+/*=============================================================================
+    Copyright (c) 2001-2006 Joel de Guzman
+    Copyright (c) 2006 Dan Marsden
+
+    Use, modification and distribution is subject to the Boost Software
+    License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at
+    http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+#if !defined(BOOST_FUSION_AT_KEY_20060304_1755)
+#define BOOST_FUSION_AT_KEY_20060304_1755
+
+#include <boost/type_traits/is_const.hpp>
+#include <boost/fusion/support/tag_of.hpp>
+#include <boost/fusion/support/detail/access.hpp>
+
+namespace boost { namespace fusion
+{
+    namespace extension
+    {
+        template <typename Tag>
+        struct at_key_impl
+        {
+            template <typename Sequence, typename Key>
+            struct apply;
+        };
+    }
+
+    namespace result_of
+    {
+        template <typename Sequence, typename Key>
+        struct at_key
+            : extension::at_key_impl<typename detail::tag_of<Sequence>::type>::
+                template apply<Sequence, Key>
+        {};
+    }
+
+    template <typename Key, typename Sequence>
+    inline typename 
+        lazy_disable_if<
+            is_const<Sequence>
+          , result_of::at_key<Sequence, Key>
+        >::type
+    at_key(Sequence& seq)
+    {
+        return result_of::at_key<Sequence, Key>::call(seq);
+    }
+
+    template <typename Key, typename Sequence>
+    inline typename result_of::at_key<Sequence const, Key>::type
+    at_key(Sequence const& seq)
+    {
+        return result_of::at_key<Sequence const, Key>::call(seq);
+    }
+}}
+
+#endif