From 1ad2e59e072d726abe3dad78fe130af19c1445b8 Mon Sep 17 00:00:00 2001
From: Kohei Takahashi <flast@flast.jp>
Date: Tue, 20 Oct 2015 20:12:14 +0900
Subject: [PATCH] Make fusion::at<vector<...>,N> O(1).

---
 .../fusion/container/vector/detail/config.hpp |  3 +-
 .../container/vector/detail/value_at_impl.hpp | 24 ++++----------
 .../boost/fusion/container/vector/vector.hpp  | 32 ++++++++++++-------
 3 files changed, 30 insertions(+), 29 deletions(-)

diff --git a/include/boost/fusion/container/vector/detail/config.hpp b/include/boost/fusion/container/vector/detail/config.hpp
index 475fe641..84f4605d 100644
--- a/include/boost/fusion/container/vector/detail/config.hpp
+++ b/include/boost/fusion/container/vector/detail/config.hpp
@@ -13,7 +13,8 @@
 
 #if (defined(BOOST_NO_CXX11_VARIADIC_TEMPLATES) \
   || defined(BOOST_NO_CXX11_RVALUE_REFERENCES) \
-  || defined(BOOST_NO_CXX11_TEMPLATE_ALIASES))  \
+  || defined(BOOST_NO_CXX11_TEMPLATE_ALIASES)  \
+  || defined(BOOST_NO_CXX11_DECLTYPE)) \
   || (defined(__WAVE__) && defined(BOOST_FUSION_CREATE_PREPROCESSED_FILES))
 # if defined(BOOST_FUSION_HAS_VARIADIC_VECTOR)
 #   undef BOOST_FUSION_HAS_VARIADIC_VECTOR
diff --git a/include/boost/fusion/container/vector/detail/value_at_impl.hpp b/include/boost/fusion/container/vector/detail/value_at_impl.hpp
index 220807c7..3d748afd 100644
--- a/include/boost/fusion/container/vector/detail/value_at_impl.hpp
+++ b/include/boost/fusion/container/vector/detail/value_at_impl.hpp
@@ -21,8 +21,9 @@
 ///////////////////////////////////////////////////////////////////////////////
 // C++11 interface
 ///////////////////////////////////////////////////////////////////////////////
-#include <cstddef>
 #include <boost/fusion/container/vector/vector_fwd.hpp>
+#include <boost/type_traits/declval.hpp>
+#include <boost/type_traits/remove_cv.hpp>
 
 namespace boost { namespace fusion
 {
@@ -42,23 +43,12 @@ namespace boost { namespace fusion
         template <>
         struct value_at_impl<vector_tag>
         {
-            template <typename V, std::size_t N>
-            struct apply_impl;
-
-            template <typename H, typename ...T>
-            struct apply_impl<vector<H, T...>, 0>
-            {
-                typedef H type;
-            };
-
-            template <typename H, typename ...T, std::size_t N>
-            struct apply_impl<vector<H, T...>, N>
-                : apply_impl<vector<T...>, N - 1>
-            {};
-
             template <typename Sequence, typename N>
-            struct apply : apply_impl<typename Sequence::type_sequence, N::value>
-            {};
+            struct apply
+            {
+                typedef typename boost::remove_cv<Sequence>::type seq;
+                typedef decltype(seq::template value_at_impl<N::value>(boost::declval<seq*>())) type;
+            };
         };
     }
 }}
diff --git a/include/boost/fusion/container/vector/vector.hpp b/include/boost/fusion/container/vector/vector.hpp
index 9c5032f9..4ca4b237 100644
--- a/include/boost/fusion/container/vector/vector.hpp
+++ b/include/boost/fusion/container/vector/vector.hpp
@@ -230,27 +230,37 @@ namespace boost { namespace fusion
                 assign(std::forward<Sequence>(seq), detail::index_sequence<M...>());
             }
 
-            typedef extension::value_at_impl<vector_tag> value_at_impl;
+            template <std::size_t N, typename U>
+            static BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+            auto at_detail(store<N, U>* this_) -> decltype(this_->get())
+            {
+                return this_->get();
+            }
+
+            template <std::size_t N, typename U>
+            static BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+            auto at_detail(store<N, U> const* this_) -> decltype(this_->get())
+            {
+                return this_->get();
+            }
 
             template <typename J>
             BOOST_CXX14_CONSTEXPR BOOST_FUSION_GPU_ENABLED
-            typename value_at_impl::template apply<vector_data, J>::type&
-            at_impl(J)
+            auto at_impl(J) -> decltype(at_detail<J::value>(this))
             {
-                typedef typename value_at_impl::template apply<vector_data, J>::type U;
-                typedef store<J::value, U> S;
-                return static_cast<S*>(this)->get();
+                return at_detail<J::value>(this);
             }
 
             template <typename J>
             BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
-            typename value_at_impl::template apply<vector_data, J>::type const&
-            at_impl(J) const
+            auto at_impl(J) const -> decltype(at_detail<J::value>(this))
             {
-                typedef typename value_at_impl::template apply<vector_data, J>::type U;
-                typedef store<J::value, U> S;
-                return static_cast<S const*>(this)->get();
+                return at_detail<J::value>(this);
             }
+
+            template <std::size_t N, typename U>
+            static BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
+            U value_at_impl(store<N, U>*);
         };
 
         template <typename V, typename... T>