From 79cf0c4bfb4117b3864420ff80bb4b3a5ce11d74 Mon Sep 17 00:00:00 2001
From: Daniel James <daniel@calamity.org.uk>
Date: Thu, 23 Feb 2017 20:14:27 +0000
Subject: [PATCH] pair_traits for getting pair types without instantiating.

I was having some problems in an abandoned prototype with incomplete
types, I'm not sure I will have this problem now, but I'm keeping this
anyway, as it seems useful.
---
 .../boost/unordered/detail/implementation.hpp | 22 +++++++++++++++++--
 include/boost/unordered/detail/map.hpp        |  4 ++--
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/include/boost/unordered/detail/implementation.hpp b/include/boost/unordered/detail/implementation.hpp
index 7e1be8cd..225fbee2 100644
--- a/include/boost/unordered/detail/implementation.hpp
+++ b/include/boost/unordered/detail/implementation.hpp
@@ -337,6 +337,23 @@ struct compressed
     compressed& operator=(compressed const&);
 };
 
+////////////////////////////////////////////////////////////////////////////
+// pair_traits
+//
+// Used to get the types from a pair without instantiating it.
+
+template <typename Pair> struct pair_traits
+{
+    typedef typename Pair::first_type first_type;
+    typedef typename Pair::second_type second_type;
+};
+
+template <typename T1, typename T2> struct pair_traits<std::pair<T1, T2> >
+{
+    typedef T1 first_type;
+    typedef T2 second_type;
+};
+
 #if defined(BOOST_MSVC)
 #pragma warning(push)
 #pragma warning(disable : 4512) // assignment operator could not be generated.
@@ -3186,10 +3203,11 @@ template <class ValueType> struct set_extractor
 #endif
 };
 
-template <class Key, class ValueType> struct map_extractor
+template <class ValueType> struct map_extractor
 {
     typedef ValueType value_type;
-    typedef typename boost::remove_const<Key>::type key_type;
+    typedef typename boost::remove_const<typename boost::unordered::detail::
+            pair_traits<ValueType>::first_type>::type key_type;
 
     static key_type const& extract(value_type const& v) { return v.first; }
 
diff --git a/include/boost/unordered/detail/map.hpp b/include/boost/unordered/detail/map.hpp
index dc23931b..6758036c 100644
--- a/include/boost/unordered/detail/map.hpp
+++ b/include/boost/unordered/detail/map.hpp
@@ -29,7 +29,7 @@ template <typename A, typename K, typename M, typename H, typename P> struct map
     typedef typename pick::link_pointer link_pointer;
 
     typedef boost::unordered::detail::table_impl<types> table;
-    typedef boost::unordered::detail::map_extractor<K, value_type> extractor;
+    typedef boost::unordered::detail::map_extractor<value_type> extractor;
 
     typedef typename boost::unordered::detail::pick_policy<K>::type policy;
 
@@ -62,7 +62,7 @@ struct multimap
     typedef typename pick::link_pointer link_pointer;
 
     typedef boost::unordered::detail::grouped_table_impl<types> table;
-    typedef boost::unordered::detail::map_extractor<K, value_type> extractor;
+    typedef boost::unordered::detail::map_extractor<value_type> extractor;
 
     typedef typename boost::unordered::detail::pick_policy<K>::type policy;