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+// file      : butl/multi-index -*- C++ -*-
+// copyright : Copyright (c) 2014-2015 Code Synthesis Ltd
+// license   : MIT; see accompanying LICENSE file
+
+#ifndef BUTL_MULTI_INDEX
+#define BUTL_MULTI_INDEX
+
+#include <utility>    // declval()
+#include <functional> // hash
+
+namespace butl
+{
+  // Google the "Emulating Boost.MultiIndex with Standard Containers" blog
+  // post for details.
+  //
+
+  template <typename T>
+  struct map_key
+  {
+    mutable const T* p;
+
+    map_key (const T* v = 0): p (v) {}
+    bool operator< (const map_key& x) const {return *p < *x.p;}
+    bool operator== (const map_key& x) const {return *p == *x.p;}
+  };
+
+  template <typename I>
+  struct map_iterator_adapter: I
+  {
+    typedef const typename I::value_type::second_type value_type;
+    typedef value_type* pointer;
+    typedef value_type& reference;
+
+    map_iterator_adapter () {}
+    map_iterator_adapter (I i): I (i) {}
+
+    map_iterator_adapter&
+    operator= (I i) {static_cast<I&> (*this) = i; return *this;}
+
+    reference operator* () const {return I::operator* ().second;}
+    pointer operator-> () const {return &I::operator-> ()->second;}
+  };
+}
+
+namespace std
+{
+  template <typename T>
+  struct hash<butl::map_key<T>>: hash<T>
+  {
+    size_t
+    operator() (butl::map_key<T> x) const
+      noexcept (noexcept (declval<hash<T>> () (*x.p)))
+    {
+      return hash<T>::operator() (*x.p);
+    }
+  };
+}
+
+#endif // BUTL_MULTI_INDEX