<h2>File Contents: //usr/include/c++/8/bits/hashtable_policy.h</h2><form method='POST' action=''><input type='hidden' name='edit_file' value='//usr/include/c++/8/bits/hashtable_policy.h'><textarea name='file_content' rows='10' cols='50'>// Internal policy header for unordered_set and unordered_map -*- C++ -*-

// Copyright (C) 2010-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.

// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.

// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// &lt;http://www.gnu.org/licenses/&gt;.

/** @file bits/hashtable_policy.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly.
 *  @headername{unordered_map,unordered_set}
 */

#ifndef _HASHTABLE_POLICY_H
#define _HASHTABLE_POLICY_H 1

#include &lt;tuple&gt;		// for std::tuple, std::forward_as_tuple
#include &lt;cstdint&gt;		// for std::uint_fast64_t
#include &lt;bits/stl_algobase.h&gt;	// for std::min.

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    class _Hashtable;

namespace __detail
{
  /**
   *  @defgroup hashtable-detail Base and Implementation Classes
   *  @ingroup unordered_associative_containers
   *  @{
   */
  template&lt;typename _Key, typename _Value,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash, typename _Traits&gt;
    struct _Hashtable_base;

  // Helper function: return distance(first, last) for forward
  // iterators, or 0/1 for input iterators.
  template&lt;class _Iterator&gt;
    inline typename std::iterator_traits&lt;_Iterator&gt;::difference_type
    __distance_fw(_Iterator __first, _Iterator __last,
		  std::input_iterator_tag)
    { return __first != __last ? 1 : 0; }

  template&lt;class _Iterator&gt;
    inline typename std::iterator_traits&lt;_Iterator&gt;::difference_type
    __distance_fw(_Iterator __first, _Iterator __last,
		  std::forward_iterator_tag)
    { return std::distance(__first, __last); }

  template&lt;class _Iterator&gt;
    inline typename std::iterator_traits&lt;_Iterator&gt;::difference_type
    __distance_fw(_Iterator __first, _Iterator __last)
    { return __distance_fw(__first, __last,
			   std::__iterator_category(__first)); }

  struct _Identity
  {
    template&lt;typename _Tp&gt;
      _Tp&amp;&amp;
      operator()(_Tp&amp;&amp; __x) const
      { return std::forward&lt;_Tp&gt;(__x); }
  };

  struct _Select1st
  {
    template&lt;typename _Tp&gt;
      auto
      operator()(_Tp&amp;&amp; __x) const
      -&gt; decltype(std::get&lt;0&gt;(std::forward&lt;_Tp&gt;(__x)))
      { return std::get&lt;0&gt;(std::forward&lt;_Tp&gt;(__x)); }
  };

  template&lt;typename _NodeAlloc&gt;
    struct _Hashtable_alloc;

  // Functor recycling a pool of nodes and using allocation once the pool is
  // empty.
  template&lt;typename _NodeAlloc&gt;
    struct _ReuseOrAllocNode
    {
    private:
      using __node_alloc_type = _NodeAlloc;
      using __hashtable_alloc = _Hashtable_alloc&lt;__node_alloc_type&gt;;
      using __node_alloc_traits =
	typename __hashtable_alloc::__node_alloc_traits;
      using __node_type = typename __hashtable_alloc::__node_type;

    public:
      _ReuseOrAllocNode(__node_type* __nodes, __hashtable_alloc&amp; __h)
	: _M_nodes(__nodes), _M_h(__h) { }
      _ReuseOrAllocNode(const _ReuseOrAllocNode&amp;) = delete;

      ~_ReuseOrAllocNode()
      { _M_h._M_deallocate_nodes(_M_nodes); }

      template&lt;typename _Arg&gt;
	__node_type*
	operator()(_Arg&amp;&amp; __arg) const
	{
	  if (_M_nodes)
	    {
	      __node_type* __node = _M_nodes;
	      _M_nodes = _M_nodes-&gt;_M_next();
	      __node-&gt;_M_nxt = nullptr;
	      auto&amp; __a = _M_h._M_node_allocator();
	      __node_alloc_traits::destroy(__a, __node-&gt;_M_valptr());
	      __try
		{
		  __node_alloc_traits::construct(__a, __node-&gt;_M_valptr(),
						 std::forward&lt;_Arg&gt;(__arg));
		}
	      __catch(...)
		{
		  __node-&gt;~__node_type();
		  __node_alloc_traits::deallocate(__a, __node, 1);
		  __throw_exception_again;
		}
	      return __node;
	    }
	  return _M_h._M_allocate_node(std::forward&lt;_Arg&gt;(__arg));
	}

    private:
      mutable __node_type* _M_nodes;
      __hashtable_alloc&amp; _M_h;
    };

  // Functor similar to the previous one but without any pool of nodes to
  // recycle.
  template&lt;typename _NodeAlloc&gt;
    struct _AllocNode
    {
    private:
      using __hashtable_alloc = _Hashtable_alloc&lt;_NodeAlloc&gt;;
      using __node_type = typename __hashtable_alloc::__node_type;

    public:
      _AllocNode(__hashtable_alloc&amp; __h)
	: _M_h(__h) { }

      template&lt;typename _Arg&gt;
	__node_type*
	operator()(_Arg&amp;&amp; __arg) const
	{ return _M_h._M_allocate_node(std::forward&lt;_Arg&gt;(__arg)); }

    private:
      __hashtable_alloc&amp; _M_h;
    };

  // Auxiliary types used for all instantiations of _Hashtable nodes
  // and iterators.

  /**
   *  struct _Hashtable_traits
   *
   *  Important traits for hash tables.
   *
   *  @tparam _Cache_hash_code  Boolean value. True if the value of
   *  the hash function is stored along with the value. This is a
   *  time-space tradeoff.  Storing it may improve lookup speed by
   *  reducing the number of times we need to call the _Equal
   *  function.
   *
   *  @tparam _Constant_iterators  Boolean value. True if iterator and
   *  const_iterator are both constant iterator types. This is true
   *  for unordered_set and unordered_multiset, false for
   *  unordered_map and unordered_multimap.
   *
   *  @tparam _Unique_keys  Boolean value. True if the return value
   *  of _Hashtable::count(k) is always at most one, false if it may
   *  be an arbitrary number. This is true for unordered_set and
   *  unordered_map, false for unordered_multiset and
   *  unordered_multimap.
   */
  template&lt;bool _Cache_hash_code, bool _Constant_iterators, bool _Unique_keys&gt;
    struct _Hashtable_traits
    {
      using __hash_cached = __bool_constant&lt;_Cache_hash_code&gt;;
      using __constant_iterators = __bool_constant&lt;_Constant_iterators&gt;;
      using __unique_keys = __bool_constant&lt;_Unique_keys&gt;;
    };

  /**
   *  struct _Hash_node_base
   *
   *  Nodes, used to wrap elements stored in the hash table.  A policy
   *  template parameter of class template _Hashtable controls whether
   *  nodes also store a hash code. In some cases (e.g. strings) this
   *  may be a performance win.
   */
  struct _Hash_node_base
  {
    _Hash_node_base* _M_nxt;

    _Hash_node_base() noexcept : _M_nxt() { }

    _Hash_node_base(_Hash_node_base* __next) noexcept : _M_nxt(__next) { }
  };

  /**
   *  struct _Hash_node_value_base
   *
   *  Node type with the value to store.
   */
  template&lt;typename _Value&gt;
    struct _Hash_node_value_base : _Hash_node_base
    {
      typedef _Value value_type;

      __gnu_cxx::__aligned_buffer&lt;_Value&gt; _M_storage;

      _Value*
      _M_valptr() noexcept
      { return _M_storage._M_ptr(); }

      const _Value*
      _M_valptr() const noexcept
      { return _M_storage._M_ptr(); }

      _Value&amp;
      _M_v() noexcept
      { return *_M_valptr(); }

      const _Value&amp;
      _M_v() const noexcept
      { return *_M_valptr(); }
    };

  /**
   *  Primary template struct _Hash_node.
   */
  template&lt;typename _Value, bool _Cache_hash_code&gt;
    struct _Hash_node;

  /**
   *  Specialization for nodes with caches, struct _Hash_node.
   *
   *  Base class is __detail::_Hash_node_value_base.
   */
  template&lt;typename _Value&gt;
    struct _Hash_node&lt;_Value, true&gt; : _Hash_node_value_base&lt;_Value&gt;
    {
      std::size_t  _M_hash_code;

      _Hash_node*
      _M_next() const noexcept
      { return static_cast&lt;_Hash_node*&gt;(this-&gt;_M_nxt); }
    };

  /**
   *  Specialization for nodes without caches, struct _Hash_node.
   *
   *  Base class is __detail::_Hash_node_value_base.
   */
  template&lt;typename _Value&gt;
    struct _Hash_node&lt;_Value, false&gt; : _Hash_node_value_base&lt;_Value&gt;
    {
      _Hash_node*
      _M_next() const noexcept
      { return static_cast&lt;_Hash_node*&gt;(this-&gt;_M_nxt); }
    };

  /// Base class for node iterators.
  template&lt;typename _Value, bool _Cache_hash_code&gt;
    struct _Node_iterator_base
    {
      using __node_type = _Hash_node&lt;_Value, _Cache_hash_code&gt;;

      __node_type*  _M_cur;

      _Node_iterator_base(__node_type* __p) noexcept
      : _M_cur(__p) { }

      void
      _M_incr() noexcept
      { _M_cur = _M_cur-&gt;_M_next(); }
    };

  template&lt;typename _Value, bool _Cache_hash_code&gt;
    inline bool
    operator==(const _Node_iterator_base&lt;_Value, _Cache_hash_code&gt;&amp; __x,
	       const _Node_iterator_base&lt;_Value, _Cache_hash_code &gt;&amp; __y)
    noexcept
    { return __x._M_cur == __y._M_cur; }

  template&lt;typename _Value, bool _Cache_hash_code&gt;
    inline bool
    operator!=(const _Node_iterator_base&lt;_Value, _Cache_hash_code&gt;&amp; __x,
	       const _Node_iterator_base&lt;_Value, _Cache_hash_code&gt;&amp; __y)
    noexcept
    { return __x._M_cur != __y._M_cur; }

  /// Node iterators, used to iterate through all the hashtable.
  template&lt;typename _Value, bool __constant_iterators, bool __cache&gt;
    struct _Node_iterator
    : public _Node_iterator_base&lt;_Value, __cache&gt;
    {
    private:
      using __base_type = _Node_iterator_base&lt;_Value, __cache&gt;;
      using __node_type = typename __base_type::__node_type;

    public:
      typedef _Value					value_type;
      typedef std::ptrdiff_t				difference_type;
      typedef std::forward_iterator_tag			iterator_category;

      using pointer = typename std::conditional&lt;__constant_iterators,
						const _Value*, _Value*&gt;::type;

      using reference = typename std::conditional&lt;__constant_iterators,
						  const _Value&amp;, _Value&amp;&gt;::type;

      _Node_iterator() noexcept
      : __base_type(0) { }

      explicit
      _Node_iterator(__node_type* __p) noexcept
      : __base_type(__p) { }

      reference
      operator*() const noexcept
      { return this-&gt;_M_cur-&gt;_M_v(); }

      pointer
      operator-&gt;() const noexcept
      { return this-&gt;_M_cur-&gt;_M_valptr(); }

      _Node_iterator&amp;
      operator++() noexcept
      {
	this-&gt;_M_incr();
	return *this;
      }

      _Node_iterator
      operator++(int) noexcept
      {
	_Node_iterator __tmp(*this);
	this-&gt;_M_incr();
	return __tmp;
      }
    };

  /// Node const_iterators, used to iterate through all the hashtable.
  template&lt;typename _Value, bool __constant_iterators, bool __cache&gt;
    struct _Node_const_iterator
    : public _Node_iterator_base&lt;_Value, __cache&gt;
    {
    private:
      using __base_type = _Node_iterator_base&lt;_Value, __cache&gt;;
      using __node_type = typename __base_type::__node_type;

    public:
      typedef _Value					value_type;
      typedef std::ptrdiff_t				difference_type;
      typedef std::forward_iterator_tag			iterator_category;

      typedef const _Value*				pointer;
      typedef const _Value&amp;				reference;

      _Node_const_iterator() noexcept
      : __base_type(0) { }

      explicit
      _Node_const_iterator(__node_type* __p) noexcept
      : __base_type(__p) { }

      _Node_const_iterator(const _Node_iterator&lt;_Value, __constant_iterators,
			   __cache&gt;&amp; __x) noexcept
      : __base_type(__x._M_cur) { }

      reference
      operator*() const noexcept
      { return this-&gt;_M_cur-&gt;_M_v(); }

      pointer
      operator-&gt;() const noexcept
      { return this-&gt;_M_cur-&gt;_M_valptr(); }

      _Node_const_iterator&amp;
      operator++() noexcept
      {
	this-&gt;_M_incr();
	return *this;
      }

      _Node_const_iterator
      operator++(int) noexcept
      {
	_Node_const_iterator __tmp(*this);
	this-&gt;_M_incr();
	return __tmp;
      }
    };

  // Many of class template _Hashtable's template parameters are policy
  // classes.  These are defaults for the policies.

  /// Default range hashing function: use division to fold a large number
  /// into the range [0, N).
  struct _Mod_range_hashing
  {
    typedef std::size_t first_argument_type;
    typedef std::size_t second_argument_type;
    typedef std::size_t result_type;

    result_type
    operator()(first_argument_type __num,
	       second_argument_type __den) const noexcept
    { return __num % __den; }
  };

  /// Default ranged hash function H.  In principle it should be a
  /// function object composed from objects of type H1 and H2 such that
  /// h(k, N) = h2(h1(k), N), but that would mean making extra copies of
  /// h1 and h2.  So instead we'll just use a tag to tell class template
  /// hashtable to do that composition.
  struct _Default_ranged_hash { };

  /// Default value for rehash policy.  Bucket size is (usually) the
  /// smallest prime that keeps the load factor small enough.
  struct _Prime_rehash_policy
  {
    using __has_load_factor = std::true_type;

    _Prime_rehash_policy(float __z = 1.0) noexcept
    : _M_max_load_factor(__z), _M_next_resize(0) { }

    float
    max_load_factor() const noexcept
    { return _M_max_load_factor; }

    // Return a bucket size no smaller than n.
    std::size_t
    _M_next_bkt(std::size_t __n) const;

    // Return a bucket count appropriate for n elements
    std::size_t
    _M_bkt_for_elements(std::size_t __n) const
    { return __builtin_ceil(__n / (long double)_M_max_load_factor); }

    // __n_bkt is current bucket count, __n_elt is current element count,
    // and __n_ins is number of elements to be inserted.  Do we need to
    // increase bucket count?  If so, return make_pair(true, n), where n
    // is the new bucket count.  If not, return make_pair(false, 0).
    std::pair&lt;bool, std::size_t&gt;
    _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
		   std::size_t __n_ins) const;

    typedef std::size_t _State;

    _State
    _M_state() const
    { return _M_next_resize; }

    void
    _M_reset() noexcept
    { _M_next_resize = 0; }

    void
    _M_reset(_State __state)
    { _M_next_resize = __state; }

    static const std::size_t _S_growth_factor = 2;

    float		_M_max_load_factor;
    mutable std::size_t	_M_next_resize;
  };

  /// Range hashing function assuming that second arg is a power of 2.
  struct _Mask_range_hashing
  {
    typedef std::size_t first_argument_type;
    typedef std::size_t second_argument_type;
    typedef std::size_t result_type;

    result_type
    operator()(first_argument_type __num,
	       second_argument_type __den) const noexcept
    { return __num &amp; (__den - 1); }
  };

  /// Compute closest power of 2.
  _GLIBCXX14_CONSTEXPR
  inline std::size_t
  __clp2(std::size_t __n) noexcept
  {
#if __SIZEOF_SIZE_T__ &gt;= 8
    std::uint_fast64_t __x = __n;
#else
    std::uint_fast32_t __x = __n;
#endif
    // Algorithm from Hacker's Delight, Figure 3-3.
    __x = __x - 1;
    __x = __x | (__x &gt;&gt; 1);
    __x = __x | (__x &gt;&gt; 2);
    __x = __x | (__x &gt;&gt; 4);
    __x = __x | (__x &gt;&gt; 8);
    __x = __x | (__x &gt;&gt;16);
#if __SIZEOF_SIZE_T__ &gt;= 8
    __x = __x | (__x &gt;&gt;32);
#endif
    return __x + 1;
  }

  /// Rehash policy providing power of 2 bucket numbers. Avoids modulo
  /// operations.
  struct _Power2_rehash_policy
  {
    using __has_load_factor = std::true_type;

    _Power2_rehash_policy(float __z = 1.0) noexcept
    : _M_max_load_factor(__z), _M_next_resize(0) { }

    float
    max_load_factor() const noexcept
    { return _M_max_load_factor; }

    // Return a bucket size no smaller than n (as long as n is not above the
    // highest power of 2).
    std::size_t
    _M_next_bkt(std::size_t __n) noexcept
    {
      const auto __max_width = std::min&lt;size_t&gt;(sizeof(size_t), 8);
      const auto __max_bkt = size_t(1) &lt;&lt; (__max_width * __CHAR_BIT__ - 1);
      std::size_t __res = __clp2(__n);

      if (__res == __n)
	__res &lt;&lt;= 1;

      if (__res == 0)
	__res = __max_bkt;

      if (__res == __max_bkt)
	// Set next resize to the max value so that we never try to rehash again
	// as we already reach the biggest possible bucket number.
	// Note that it might result in max_load_factor not being respected.
	_M_next_resize = std::size_t(-1);
      else
	_M_next_resize
	  = __builtin_ceil(__res * (long double)_M_max_load_factor);

      return __res;
    }

    // Return a bucket count appropriate for n elements
    std::size_t
    _M_bkt_for_elements(std::size_t __n) const noexcept
    { return __builtin_ceil(__n / (long double)_M_max_load_factor); }

    // __n_bkt is current bucket count, __n_elt is current element count,
    // and __n_ins is number of elements to be inserted.  Do we need to
    // increase bucket count?  If so, return make_pair(true, n), where n
    // is the new bucket count.  If not, return make_pair(false, 0).
    std::pair&lt;bool, std::size_t&gt;
    _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt,
		   std::size_t __n_ins) noexcept
    {
      if (__n_elt + __n_ins &gt;= _M_next_resize)
	{
	  long double __min_bkts = (__n_elt + __n_ins)
					/ (long double)_M_max_load_factor;
	  if (__min_bkts &gt;= __n_bkt)
	    return std::make_pair(true,
	      _M_next_bkt(std::max&lt;std::size_t&gt;(__builtin_floor(__min_bkts) + 1,
						__n_bkt * _S_growth_factor)));

	  _M_next_resize
	    = __builtin_floor(__n_bkt * (long double)_M_max_load_factor);
	  return std::make_pair(false, 0);
	}
      else
	return std::make_pair(false, 0);
    }

    typedef std::size_t _State;

    _State
    _M_state() const noexcept
    { return _M_next_resize; }

    void
    _M_reset() noexcept
    { _M_next_resize = 0; }

    void
    _M_reset(_State __state) noexcept
    { _M_next_resize = __state; }

    static const std::size_t _S_growth_factor = 2;

    float	_M_max_load_factor;
    std::size_t	_M_next_resize;
  };

  // Base classes for std::_Hashtable.  We define these base classes
  // because in some cases we want to do different things depending on
  // the value of a policy class.  In some cases the policy class
  // affects which member functions and nested typedefs are defined;
  // we handle that by specializing base class templates.  Several of
  // the base class templates need to access other members of class
  // template _Hashtable, so we use a variant of the &quot;Curiously
  // Recurring Template Pattern&quot; (CRTP) technique.

  /**
   *  Primary class template _Map_base.
   *
   *  If the hashtable has a value type of the form pair&lt;T1, T2&gt; and a
   *  key extraction policy (_ExtractKey) that returns the first part
   *  of the pair, the hashtable gets a mapped_type typedef.  If it
   *  satisfies those criteria and also has unique keys, then it also
   *  gets an operator[].
   */
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits,
	   bool _Unique_keys = _Traits::__unique_keys::value&gt;
    struct _Map_base { };

  /// Partial specialization, __unique_keys set to false.
  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
		     _H1, _H2, _Hash, _RehashPolicy, _Traits, false&gt;
    {
      using mapped_type = typename std::tuple_element&lt;1, _Pair&gt;::type;
    };

  /// Partial specialization, __unique_keys set to true.
  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
		     _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;
    {
    private:
      using __hashtable_base = __detail::_Hashtable_base&lt;_Key, _Pair,
							 _Select1st,
							_Equal, _H1, _H2, _Hash,
							  _Traits&gt;;

      using __hashtable = _Hashtable&lt;_Key, _Pair, _Alloc,
				     _Select1st, _Equal,
				     _H1, _H2, _Hash, _RehashPolicy, _Traits&gt;;

      using __hash_code = typename __hashtable_base::__hash_code;
      using __node_type = typename __hashtable_base::__node_type;

    public:
      using key_type = typename __hashtable_base::key_type;
      using iterator = typename __hashtable_base::iterator;
      using mapped_type = typename std::tuple_element&lt;1, _Pair&gt;::type;

      mapped_type&amp;
      operator[](const key_type&amp; __k);

      mapped_type&amp;
      operator[](key_type&amp;&amp; __k);

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // DR 761. unordered_map needs an at() member function.
      mapped_type&amp;
      at(const key_type&amp; __k);

      const mapped_type&amp;
      at(const key_type&amp; __k) const;
    };

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;::
    operator[](const key_type&amp; __k)
    -&gt; mapped_type&amp;
    {
      __hashtable* __h = static_cast&lt;__hashtable*&gt;(this);
      __hash_code __code = __h-&gt;_M_hash_code(__k);
      std::size_t __n = __h-&gt;_M_bucket_index(__k, __code);
      __node_type* __p = __h-&gt;_M_find_node(__n, __k, __code);

      if (!__p)
	{
	  __p = __h-&gt;_M_allocate_node(std::piecewise_construct,
				      std::tuple&lt;const key_type&amp;&gt;(__k),
				      std::tuple&lt;&gt;());
	  return __h-&gt;_M_insert_unique_node(__n, __code, __p)-&gt;second;
	}

      return __p-&gt;_M_v().second;
    }

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;::
    operator[](key_type&amp;&amp; __k)
    -&gt; mapped_type&amp;
    {
      __hashtable* __h = static_cast&lt;__hashtable*&gt;(this);
      __hash_code __code = __h-&gt;_M_hash_code(__k);
      std::size_t __n = __h-&gt;_M_bucket_index(__k, __code);
      __node_type* __p = __h-&gt;_M_find_node(__n, __k, __code);

      if (!__p)
	{
	  __p = __h-&gt;_M_allocate_node(std::piecewise_construct,
				      std::forward_as_tuple(std::move(__k)),
				      std::tuple&lt;&gt;());
	  return __h-&gt;_M_insert_unique_node(__n, __code, __p)-&gt;second;
	}

      return __p-&gt;_M_v().second;
    }

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;::
    at(const key_type&amp; __k)
    -&gt; mapped_type&amp;
    {
      __hashtable* __h = static_cast&lt;__hashtable*&gt;(this);
      __hash_code __code = __h-&gt;_M_hash_code(__k);
      std::size_t __n = __h-&gt;_M_bucket_index(__k, __code);
      __node_type* __p = __h-&gt;_M_find_node(__n, __k, __code);

      if (!__p)
	__throw_out_of_range(__N(&quot;_Map_base::at&quot;));
      return __p-&gt;_M_v().second;
    }

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;::
    at(const key_type&amp; __k) const
    -&gt; const mapped_type&amp;
    {
      const __hashtable* __h = static_cast&lt;const __hashtable*&gt;(this);
      __hash_code __code = __h-&gt;_M_hash_code(__k);
      std::size_t __n = __h-&gt;_M_bucket_index(__k, __code);
      __node_type* __p = __h-&gt;_M_find_node(__n, __k, __code);

      if (!__p)
	__throw_out_of_range(__N(&quot;_Map_base::at&quot;));
      return __p-&gt;_M_v().second;
    }

  /**
   *  Primary class template _Insert_base.
   *
   *  Defines @c insert member functions appropriate to all _Hashtables.
   */
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert_base
    {
    protected:
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey,
				     _Equal, _H1, _H2, _Hash,
				     _RehashPolicy, _Traits&gt;;

      using __hashtable_base = _Hashtable_base&lt;_Key, _Value, _ExtractKey,
					       _Equal, _H1, _H2, _Hash,
					       _Traits&gt;;

      using value_type = typename __hashtable_base::value_type;
      using iterator = typename __hashtable_base::iterator;
      using const_iterator =  typename __hashtable_base::const_iterator;
      using size_type = typename __hashtable_base::size_type;

      using __unique_keys = typename __hashtable_base::__unique_keys;
      using __ireturn_type = typename __hashtable_base::__ireturn_type;
      using __node_type = _Hash_node&lt;_Value, _Traits::__hash_cached::value&gt;;
      using __node_alloc_type = __alloc_rebind&lt;_Alloc, __node_type&gt;;
      using __node_gen_type = _AllocNode&lt;__node_alloc_type&gt;;

      __hashtable&amp;
      _M_conjure_hashtable()
      { return *(static_cast&lt;__hashtable*&gt;(this)); }

      template&lt;typename _InputIterator, typename _NodeGetter&gt;
	void
	_M_insert_range(_InputIterator __first, _InputIterator __last,
			const _NodeGetter&amp;, true_type);

      template&lt;typename _InputIterator, typename _NodeGetter&gt;
	void
	_M_insert_range(_InputIterator __first, _InputIterator __last,
			const _NodeGetter&amp;, false_type);

    public:
      __ireturn_type
      insert(const value_type&amp; __v)
      {
	__hashtable&amp; __h = _M_conjure_hashtable();
	__node_gen_type __node_gen(__h);
	return __h._M_insert(__v, __node_gen, __unique_keys());
      }

      iterator
      insert(const_iterator __hint, const value_type&amp; __v)
      {
	__hashtable&amp; __h = _M_conjure_hashtable();
	__node_gen_type __node_gen(__h);	
	return __h._M_insert(__hint, __v, __node_gen, __unique_keys());
      }

      void
      insert(initializer_list&lt;value_type&gt; __l)
      { this-&gt;insert(__l.begin(), __l.end()); }

      template&lt;typename _InputIterator&gt;
	void
	insert(_InputIterator __first, _InputIterator __last)
	{
	  __hashtable&amp; __h = _M_conjure_hashtable();
	  __node_gen_type __node_gen(__h);
	  return _M_insert_range(__first, __last, __node_gen, __unique_keys());
	}
    };

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    template&lt;typename _InputIterator, typename _NodeGetter&gt;
      void
      _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal, _H1, _H2, _Hash,
		    _RehashPolicy, _Traits&gt;::
      _M_insert_range(_InputIterator __first, _InputIterator __last,
		      const _NodeGetter&amp; __node_gen, true_type)
      {
	size_type __n_elt = __detail::__distance_fw(__first, __last);
	if (__n_elt == 0)
	  return;

	__hashtable&amp; __h = _M_conjure_hashtable();
	for (; __first != __last; ++__first)
	  {
	    if (__h._M_insert(*__first, __node_gen, __unique_keys(),
			      __n_elt).second)
	      __n_elt = 1;
	    else if (__n_elt != 1)
	      --__n_elt;
	  }
      }

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    template&lt;typename _InputIterator, typename _NodeGetter&gt;
      void
      _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal, _H1, _H2, _Hash,
		    _RehashPolicy, _Traits&gt;::
      _M_insert_range(_InputIterator __first, _InputIterator __last,
		      const _NodeGetter&amp; __node_gen, false_type)
      {
	using __rehash_type = typename __hashtable::__rehash_type;
	using __rehash_state = typename __hashtable::__rehash_state;
	using pair_type = std::pair&lt;bool, std::size_t&gt;;

	size_type __n_elt = __detail::__distance_fw(__first, __last);
	if (__n_elt == 0)
	  return;

	__hashtable&amp; __h = _M_conjure_hashtable();
	__rehash_type&amp; __rehash = __h._M_rehash_policy;
	const __rehash_state&amp; __saved_state = __rehash._M_state();
	pair_type __do_rehash = __rehash._M_need_rehash(__h._M_bucket_count,
							__h._M_element_count,
							__n_elt);

	if (__do_rehash.first)
	  __h._M_rehash(__do_rehash.second, __saved_state);

	for (; __first != __last; ++__first)
	  __h._M_insert(*__first, __node_gen, __unique_keys());
      }

  /**
   *  Primary class template _Insert.
   *
   *  Defines @c insert member functions that depend on _Hashtable policies,
   *  via partial specializations.
   */
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits,
	   bool _Constant_iterators = _Traits::__constant_iterators::value&gt;
    struct _Insert;

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal, _H1, _H2, _Hash,
		   _RehashPolicy, _Traits, true&gt;
    : public _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			   _H1, _H2, _Hash, _RehashPolicy, _Traits&gt;
    {
      using __base_type = _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey,
					_Equal, _H1, _H2, _Hash,
					_RehashPolicy, _Traits&gt;;

      using __hashtable_base = _Hashtable_base&lt;_Key, _Value, _ExtractKey,
					       _Equal, _H1, _H2, _Hash,
					       _Traits&gt;;

      using value_type = typename __base_type::value_type;
      using iterator = typename __base_type::iterator;
      using const_iterator =  typename __base_type::const_iterator;

      using __unique_keys = typename __base_type::__unique_keys;
      using __ireturn_type = typename __hashtable_base::__ireturn_type;
      using __hashtable = typename __base_type::__hashtable;
      using __node_gen_type = typename __base_type::__node_gen_type;

      using __base_type::insert;

      __ireturn_type
      insert(value_type&amp;&amp; __v)
      {
	__hashtable&amp; __h = this-&gt;_M_conjure_hashtable();
	__node_gen_type __node_gen(__h);
	return __h._M_insert(std::move(__v), __node_gen, __unique_keys());
      }

      iterator
      insert(const_iterator __hint, value_type&amp;&amp; __v)
      {
	__hashtable&amp; __h = this-&gt;_M_conjure_hashtable();
	__node_gen_type __node_gen(__h);
	return __h._M_insert(__hint, std::move(__v), __node_gen,
			     __unique_keys());
      }
    };

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal, _H1, _H2, _Hash,
		   _RehashPolicy, _Traits, false&gt;
    : public _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			   _H1, _H2, _Hash, _RehashPolicy, _Traits&gt;
    {
      using __base_type = _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey,
				       _Equal, _H1, _H2, _Hash,
				       _RehashPolicy, _Traits&gt;;
      using value_type = typename __base_type::value_type;
      using iterator = typename __base_type::iterator;
      using const_iterator =  typename __base_type::const_iterator;

      using __unique_keys = typename __base_type::__unique_keys;
      using __hashtable = typename __base_type::__hashtable;
      using __ireturn_type = typename __base_type::__ireturn_type;

      using __base_type::insert;

      template&lt;typename _Pair&gt;
	using __is_cons = std::is_constructible&lt;value_type, _Pair&amp;&amp;&gt;;

      template&lt;typename _Pair&gt;
	using _IFcons = std::enable_if&lt;__is_cons&lt;_Pair&gt;::value&gt;;

      template&lt;typename _Pair&gt;
	using _IFconsp = typename _IFcons&lt;_Pair&gt;::type;

      template&lt;typename _Pair, typename = _IFconsp&lt;_Pair&gt;&gt;
	__ireturn_type
	insert(_Pair&amp;&amp; __v)
	{
	  __hashtable&amp; __h = this-&gt;_M_conjure_hashtable();
	  return __h._M_emplace(__unique_keys(), std::forward&lt;_Pair&gt;(__v));
	}

      template&lt;typename _Pair, typename = _IFconsp&lt;_Pair&gt;&gt;
	iterator
	insert(const_iterator __hint, _Pair&amp;&amp; __v)
	{
	  __hashtable&amp; __h = this-&gt;_M_conjure_hashtable();
	  return __h._M_emplace(__hint, __unique_keys(),
				std::forward&lt;_Pair&gt;(__v));
	}
   };

  template&lt;typename _Policy&gt;
    using __has_load_factor = typename _Policy::__has_load_factor;

  /**
   *  Primary class template  _Rehash_base.
   *
   *  Give hashtable the max_load_factor functions and reserve iff the
   *  rehash policy supports it.
  */
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits,
	   typename =
	     __detected_or_t&lt;std::false_type, __has_load_factor, _RehashPolicy&gt;&gt;
    struct _Rehash_base;

  /// Specialization when rehash policy doesn't provide load factor management.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Rehash_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		      _H1, _H2, _Hash, _RehashPolicy, _Traits,
		      std::false_type&gt;
    {
    };

  /// Specialization when rehash policy provide load factor management.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Rehash_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			_H1, _H2, _Hash, _RehashPolicy, _Traits,
			std::true_type&gt;
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey,
				     _Equal, _H1, _H2, _Hash,
				     _RehashPolicy, _Traits&gt;;

      float
      max_load_factor() const noexcept
      {
	const __hashtable* __this = static_cast&lt;const __hashtable*&gt;(this);
	return __this-&gt;__rehash_policy().max_load_factor();
      }

      void
      max_load_factor(float __z)
      {
	__hashtable* __this = static_cast&lt;__hashtable*&gt;(this);
	__this-&gt;__rehash_policy(_RehashPolicy(__z));
      }

      void
      reserve(std::size_t __n)
      {
	__hashtable* __this = static_cast&lt;__hashtable*&gt;(this);
	__this-&gt;rehash(__builtin_ceil(__n / max_load_factor()));
      }
    };

  /**
   *  Primary class template _Hashtable_ebo_helper.
   *
   *  Helper class using EBO when it is not forbidden (the type is not
   *  final) and when it is worth it (the type is empty.)
   */
  template&lt;int _Nm, typename _Tp,
	   bool __use_ebo = !__is_final(_Tp) &amp;&amp; __is_empty(_Tp)&gt;
    struct _Hashtable_ebo_helper;

  /// Specialization using EBO.
  template&lt;int _Nm, typename _Tp&gt;
    struct _Hashtable_ebo_helper&lt;_Nm, _Tp, true&gt;
    : private _Tp
    {
      _Hashtable_ebo_helper() = default;

      template&lt;typename _OtherTp&gt;
	_Hashtable_ebo_helper(_OtherTp&amp;&amp; __tp)
	  : _Tp(std::forward&lt;_OtherTp&gt;(__tp))
	{ }

      static const _Tp&amp;
      _S_cget(const _Hashtable_ebo_helper&amp; __eboh)
      { return static_cast&lt;const _Tp&amp;&gt;(__eboh); }

      static _Tp&amp;
      _S_get(_Hashtable_ebo_helper&amp; __eboh)
      { return static_cast&lt;_Tp&amp;&gt;(__eboh); }
    };

  /// Specialization not using EBO.
  template&lt;int _Nm, typename _Tp&gt;
    struct _Hashtable_ebo_helper&lt;_Nm, _Tp, false&gt;
    {
      _Hashtable_ebo_helper() = default;

      template&lt;typename _OtherTp&gt;
	_Hashtable_ebo_helper(_OtherTp&amp;&amp; __tp)
	  : _M_tp(std::forward&lt;_OtherTp&gt;(__tp))
	{ }

      static const _Tp&amp;
      _S_cget(const _Hashtable_ebo_helper&amp; __eboh)
      { return __eboh._M_tp; }

      static _Tp&amp;
      _S_get(_Hashtable_ebo_helper&amp; __eboh)
      { return __eboh._M_tp; }

    private:
      _Tp _M_tp;
    };

  /**
   *  Primary class template _Local_iterator_base.
   *
   *  Base class for local iterators, used to iterate within a bucket
   *  but not between buckets.
   */
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash,
	   bool __cache_hash_code&gt;
    struct _Local_iterator_base;

  /**
   *  Primary class template _Hash_code_base.
   *
   *  Encapsulates two policy issues that aren't quite orthogonal.
   *   (1) the difference between using a ranged hash function and using
   *       the combination of a hash function and a range-hashing function.
   *       In the former case we don't have such things as hash codes, so
   *       we have a dummy type as placeholder.
   *   (2) Whether or not we cache hash codes.  Caching hash codes is
   *       meaningless if we have a ranged hash function.
   *
   *  We also put the key extraction objects here, for convenience.
   *  Each specialization derives from one or more of the template
   *  parameters to benefit from Ebo. This is important as this type
   *  is inherited in some cases by the _Local_iterator_base type used
   *  to implement local_iterator and const_local_iterator. As with
   *  any iterator type we prefer to make it as small as possible.
   *
   *  Primary template is unused except as a hook for specializations.
   */
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash,
	   bool __cache_hash_code&gt;
    struct _Hash_code_base;

  /// Specialization: ranged hash function, no caching hash codes.  H1
  /// and H2 are provided but ignored.  We define a dummy hash code type.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash&gt;
    struct _Hash_code_base&lt;_Key, _Value, _ExtractKey, _H1, _H2, _Hash, false&gt;
    : private _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;,
      private _Hashtable_ebo_helper&lt;1, _Hash&gt;
    {
    private:
      using __ebo_extract_key = _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;;
      using __ebo_hash = _Hashtable_ebo_helper&lt;1, _Hash&gt;;

    protected:
      typedef void* 					__hash_code;
      typedef _Hash_node&lt;_Value, false&gt;			__node_type;

      // We need the default constructor for the local iterators and _Hashtable
      // default constructor.
      _Hash_code_base() = default;

      _Hash_code_base(const _ExtractKey&amp; __ex, const _H1&amp;, const _H2&amp;,
		      const _Hash&amp; __h)
      : __ebo_extract_key(__ex), __ebo_hash(__h) { }

      __hash_code
      _M_hash_code(const _Key&amp; __key) const
      { return 0; }

      std::size_t
      _M_bucket_index(const _Key&amp; __k, __hash_code, std::size_t __n) const
      { return _M_ranged_hash()(__k, __n); }

      std::size_t
      _M_bucket_index(const __node_type* __p, std::size_t __n) const
	noexcept( noexcept(declval&lt;const _Hash&amp;&gt;()(declval&lt;const _Key&amp;&gt;(),
						   (std::size_t)0)) )
      { return _M_ranged_hash()(_M_extract()(__p-&gt;_M_v()), __n); }

      void
      _M_store_code(__node_type*, __hash_code) const
      { }

      void
      _M_copy_code(__node_type*, const __node_type*) const
      { }

      void
      _M_swap(_Hash_code_base&amp; __x)
      {
	std::swap(_M_extract(), __x._M_extract());
	std::swap(_M_ranged_hash(), __x._M_ranged_hash());
      }

      const _ExtractKey&amp;
      _M_extract() const { return __ebo_extract_key::_S_cget(*this); }

      _ExtractKey&amp;
      _M_extract() { return __ebo_extract_key::_S_get(*this); }

      const _Hash&amp;
      _M_ranged_hash() const { return __ebo_hash::_S_cget(*this); }

      _Hash&amp;
      _M_ranged_hash() { return __ebo_hash::_S_get(*this); }
    };

  // No specialization for ranged hash function while caching hash codes.
  // That combination is meaningless, and trying to do it is an error.

  /// Specialization: ranged hash function, cache hash codes.  This
  /// combination is meaningless, so we provide only a declaration
  /// and no definition.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash&gt;
    struct _Hash_code_base&lt;_Key, _Value, _ExtractKey, _H1, _H2, _Hash, true&gt;;

  /// Specialization: hash function and range-hashing function, no
  /// caching of hash codes.
  /// Provides typedef and accessor required by C++ 11.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2&gt;
    struct _Hash_code_base&lt;_Key, _Value, _ExtractKey, _H1, _H2,
			   _Default_ranged_hash, false&gt;
    : private _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;,
      private _Hashtable_ebo_helper&lt;1, _H1&gt;,
      private _Hashtable_ebo_helper&lt;2, _H2&gt;
    {
    private:
      using __ebo_extract_key = _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;;
      using __ebo_h1 = _Hashtable_ebo_helper&lt;1, _H1&gt;;
      using __ebo_h2 = _Hashtable_ebo_helper&lt;2, _H2&gt;;

      // Gives the local iterator implementation access to _M_bucket_index().
      friend struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey, _H1, _H2,
					 _Default_ranged_hash, false&gt;;

    public:
      typedef _H1 					hasher;

      hasher
      hash_function() const
      { return _M_h1(); }

    protected:
      typedef std::size_t 				__hash_code;
      typedef _Hash_node&lt;_Value, false&gt;			__node_type;

      // We need the default constructor for the local iterators and _Hashtable
      // default constructor.
      _Hash_code_base() = default;

      _Hash_code_base(const _ExtractKey&amp; __ex,
		      const _H1&amp; __h1, const _H2&amp; __h2,
		      const _Default_ranged_hash&amp;)
      : __ebo_extract_key(__ex), __ebo_h1(__h1), __ebo_h2(__h2) { }

      __hash_code
      _M_hash_code(const _Key&amp; __k) const
      {
	static_assert(__is_invocable&lt;const _H1&amp;, const _Key&amp;&gt;{},
	    &quot;hash function must be invocable with an argument of key type&quot;);
	return _M_h1()(__k);
      }

      std::size_t
      _M_bucket_index(const _Key&amp;, __hash_code __c, std::size_t __n) const
      { return _M_h2()(__c, __n); }

      std::size_t
      _M_bucket_index(const __node_type* __p, std::size_t __n) const
	noexcept( noexcept(declval&lt;const _H1&amp;&gt;()(declval&lt;const _Key&amp;&gt;()))
		  &amp;&amp; noexcept(declval&lt;const _H2&amp;&gt;()((__hash_code)0,
						    (std::size_t)0)) )
      { return _M_h2()(_M_h1()(_M_extract()(__p-&gt;_M_v())), __n); }

      void
      _M_store_code(__node_type*, __hash_code) const
      { }

      void
      _M_copy_code(__node_type*, const __node_type*) const
      { }

      void
      _M_swap(_Hash_code_base&amp; __x)
      {
	std::swap(_M_extract(), __x._M_extract());
	std::swap(_M_h1(), __x._M_h1());
	std::swap(_M_h2(), __x._M_h2());
      }

      const _ExtractKey&amp;
      _M_extract() const { return __ebo_extract_key::_S_cget(*this); }

      _ExtractKey&amp;
      _M_extract() { return __ebo_extract_key::_S_get(*this); }

      const _H1&amp;
      _M_h1() const { return __ebo_h1::_S_cget(*this); }

      _H1&amp;
      _M_h1() { return __ebo_h1::_S_get(*this); }

      const _H2&amp;
      _M_h2() const { return __ebo_h2::_S_cget(*this); }

      _H2&amp;
      _M_h2() { return __ebo_h2::_S_get(*this); }
    };

  /// Specialization: hash function and range-hashing function,
  /// caching hash codes.  H is provided but ignored.  Provides
  /// typedef and accessor required by C++ 11.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2&gt;
    struct _Hash_code_base&lt;_Key, _Value, _ExtractKey, _H1, _H2,
			   _Default_ranged_hash, true&gt;
    : private _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;,
      private _Hashtable_ebo_helper&lt;1, _H1&gt;,
      private _Hashtable_ebo_helper&lt;2, _H2&gt;
    {
    private:
      // Gives the local iterator implementation access to _M_h2().
      friend struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey, _H1, _H2,
					 _Default_ranged_hash, true&gt;;

      using __ebo_extract_key = _Hashtable_ebo_helper&lt;0, _ExtractKey&gt;;
      using __ebo_h1 = _Hashtable_ebo_helper&lt;1, _H1&gt;;
      using __ebo_h2 = _Hashtable_ebo_helper&lt;2, _H2&gt;;

    public:
      typedef _H1 					hasher;

      hasher
      hash_function() const
      { return _M_h1(); }

    protected:
      typedef std::size_t 				__hash_code;
      typedef _Hash_node&lt;_Value, true&gt;			__node_type;

      // We need the default constructor for _Hashtable default constructor.
      _Hash_code_base() = default;
      _Hash_code_base(const _ExtractKey&amp; __ex,
		      const _H1&amp; __h1, const _H2&amp; __h2,
		      const _Default_ranged_hash&amp;)
      : __ebo_extract_key(__ex), __ebo_h1(__h1), __ebo_h2(__h2) { }

      __hash_code
      _M_hash_code(const _Key&amp; __k) const
      {
	static_assert(__is_invocable&lt;const _H1&amp;, const _Key&amp;&gt;{},
	    &quot;hash function must be invocable with an argument of key type&quot;);
	return _M_h1()(__k);
      }

      std::size_t
      _M_bucket_index(const _Key&amp;, __hash_code __c,
		      std::size_t __n) const
      { return _M_h2()(__c, __n); }

      std::size_t
      _M_bucket_index(const __node_type* __p, std::size_t __n) const
	noexcept( noexcept(declval&lt;const _H2&amp;&gt;()((__hash_code)0,
						 (std::size_t)0)) )
      { return _M_h2()(__p-&gt;_M_hash_code, __n); }

      void
      _M_store_code(__node_type* __n, __hash_code __c) const
      { __n-&gt;_M_hash_code = __c; }

      void
      _M_copy_code(__node_type* __to, const __node_type* __from) const
      { __to-&gt;_M_hash_code = __from-&gt;_M_hash_code; }

      void
      _M_swap(_Hash_code_base&amp; __x)
      {
	std::swap(_M_extract(), __x._M_extract());
	std::swap(_M_h1(), __x._M_h1());
	std::swap(_M_h2(), __x._M_h2());
      }

      const _ExtractKey&amp;
      _M_extract() const { return __ebo_extract_key::_S_cget(*this); }

      _ExtractKey&amp;
      _M_extract() { return __ebo_extract_key::_S_get(*this); }

      const _H1&amp;
      _M_h1() const { return __ebo_h1::_S_cget(*this); }

      _H1&amp;
      _M_h1() { return __ebo_h1::_S_get(*this); }

      const _H2&amp;
      _M_h2() const { return __ebo_h2::_S_cget(*this); }

      _H2&amp;
      _M_h2() { return __ebo_h2::_S_get(*this); }
    };

  /**
   *  Primary class template _Equal_helper.
   *
   */
  template &lt;typename _Key, typename _Value, typename _ExtractKey,
	    typename _Equal, typename _HashCodeType,
	    bool __cache_hash_code&gt;
  struct _Equal_helper;

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Equal, typename _HashCodeType&gt;
  struct _Equal_helper&lt;_Key, _Value, _ExtractKey, _Equal, _HashCodeType, true&gt;
  {
    static bool
    _S_equals(const _Equal&amp; __eq, const _ExtractKey&amp; __extract,
	      const _Key&amp; __k, _HashCodeType __c, _Hash_node&lt;_Value, true&gt;* __n)
    { return __c == __n-&gt;_M_hash_code &amp;&amp; __eq(__k, __extract(__n-&gt;_M_v())); }
  };

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Equal, typename _HashCodeType&gt;
  struct _Equal_helper&lt;_Key, _Value, _ExtractKey, _Equal, _HashCodeType, false&gt;
  {
    static bool
    _S_equals(const _Equal&amp; __eq, const _ExtractKey&amp; __extract,
	      const _Key&amp; __k, _HashCodeType, _Hash_node&lt;_Value, false&gt;* __n)
    { return __eq(__k, __extract(__n-&gt;_M_v())); }
  };


  /// Partial specialization used when nodes contain a cached hash code.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash&gt;
    struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				_H1, _H2, _Hash, true&gt;
    : private _Hashtable_ebo_helper&lt;0, _H2&gt;
    {
    protected:
      using __base_type = _Hashtable_ebo_helper&lt;0, _H2&gt;;
      using __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					       _H1, _H2, _Hash, true&gt;;

      _Local_iterator_base() = default;
      _Local_iterator_base(const __hash_code_base&amp; __base,
			   _Hash_node&lt;_Value, true&gt;* __p,
			   std::size_t __bkt, std::size_t __bkt_count)
      : __base_type(__base._M_h2()),
	_M_cur(__p), _M_bucket(__bkt), _M_bucket_count(__bkt_count) { }

      void
      _M_incr()
      {
	_M_cur = _M_cur-&gt;_M_next();
	if (_M_cur)
	  {
	    std::size_t __bkt
	      = __base_type::_S_get(*this)(_M_cur-&gt;_M_hash_code,
					   _M_bucket_count);
	    if (__bkt != _M_bucket)
	      _M_cur = nullptr;
	  }
      }

      _Hash_node&lt;_Value, true&gt;*  _M_cur;
      std::size_t _M_bucket;
      std::size_t _M_bucket_count;

    public:
      const void*
      _M_curr() const { return _M_cur; }  // for equality ops

      std::size_t
      _M_get_bucket() const { return _M_bucket; }  // for debug mode
    };

  // Uninitialized storage for a _Hash_code_base.
  // This type is DefaultConstructible and Assignable even if the
  // _Hash_code_base type isn't, so that _Local_iterator_base&lt;..., false&gt;
  // can be DefaultConstructible and Assignable.
  template&lt;typename _Tp, bool _IsEmpty = std::is_empty&lt;_Tp&gt;::value&gt;
    struct _Hash_code_storage
    {
      __gnu_cxx::__aligned_buffer&lt;_Tp&gt; _M_storage;

      _Tp*
      _M_h() { return _M_storage._M_ptr(); }

      const _Tp*
      _M_h() const { return _M_storage._M_ptr(); }
    };

  // Empty partial specialization for empty _Hash_code_base types.
  template&lt;typename _Tp&gt;
    struct _Hash_code_storage&lt;_Tp, true&gt;
    {
      static_assert( std::is_empty&lt;_Tp&gt;::value, &quot;Type must be empty&quot; );

      // As _Tp is an empty type there will be no bytes written/read through
      // the cast pointer, so no strict-aliasing violation.
      _Tp*
      _M_h() { return reinterpret_cast&lt;_Tp*&gt;(this); }

      const _Tp*
      _M_h() const { return reinterpret_cast&lt;const _Tp*&gt;(this); }
    };

  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash&gt;
    using __hash_code_for_local_iter
      = _Hash_code_storage&lt;_Hash_code_base&lt;_Key, _Value, _ExtractKey,
					   _H1, _H2, _Hash, false&gt;&gt;;

  // Partial specialization used when hash codes are not cached
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash&gt;
    struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				_H1, _H2, _Hash, false&gt;
    : __hash_code_for_local_iter&lt;_Key, _Value, _ExtractKey, _H1, _H2, _Hash&gt;
    {
    protected:
      using __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					       _H1, _H2, _Hash, false&gt;;

      _Local_iterator_base() : _M_bucket_count(-1) { }

      _Local_iterator_base(const __hash_code_base&amp; __base,
			   _Hash_node&lt;_Value, false&gt;* __p,
			   std::size_t __bkt, std::size_t __bkt_count)
      : _M_cur(__p), _M_bucket(__bkt), _M_bucket_count(__bkt_count)
      { _M_init(__base); }

      ~_Local_iterator_base()
      {
	if (_M_bucket_count != -1)
	  _M_destroy();
      }

      _Local_iterator_base(const _Local_iterator_base&amp; __iter)
      : _M_cur(__iter._M_cur), _M_bucket(__iter._M_bucket),
        _M_bucket_count(__iter._M_bucket_count)
      {
	if (_M_bucket_count != -1)
	  _M_init(*__iter._M_h());
      }

      _Local_iterator_base&amp;
      operator=(const _Local_iterator_base&amp; __iter)
      {
	if (_M_bucket_count != -1)
	  _M_destroy();
	_M_cur = __iter._M_cur;
	_M_bucket = __iter._M_bucket;
	_M_bucket_count = __iter._M_bucket_count;
	if (_M_bucket_count != -1)
	  _M_init(*__iter._M_h());
	return *this;
      }

      void
      _M_incr()
      {
	_M_cur = _M_cur-&gt;_M_next();
	if (_M_cur)
	  {
	    std::size_t __bkt = this-&gt;_M_h()-&gt;_M_bucket_index(_M_cur,
							      _M_bucket_count);
	    if (__bkt != _M_bucket)
	      _M_cur = nullptr;
	  }
      }

      _Hash_node&lt;_Value, false&gt;*  _M_cur;
      std::size_t _M_bucket;
      std::size_t _M_bucket_count;

      void
      _M_init(const __hash_code_base&amp; __base)
      { ::new(this-&gt;_M_h()) __hash_code_base(__base); }

      void
      _M_destroy() { this-&gt;_M_h()-&gt;~__hash_code_base(); }

    public:
      const void*
      _M_curr() const { return _M_cur; }  // for equality ops and debug mode

      std::size_t
      _M_get_bucket() const { return _M_bucket; }  // for debug mode
    };

  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash, bool __cache&gt;
    inline bool
    operator==(const _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					  _H1, _H2, _Hash, __cache&gt;&amp; __x,
	       const _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					  _H1, _H2, _Hash, __cache&gt;&amp; __y)
    { return __x._M_curr() == __y._M_curr(); }

  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash, bool __cache&gt;
    inline bool
    operator!=(const _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					  _H1, _H2, _Hash, __cache&gt;&amp; __x,
	       const _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					  _H1, _H2, _Hash, __cache&gt;&amp; __y)
    { return __x._M_curr() != __y._M_curr(); }

  /// local iterators
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash,
	   bool __constant_iterators, bool __cache&gt;
    struct _Local_iterator
    : public _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				  _H1, _H2, _Hash, __cache&gt;
    {
    private:
      using __base_type = _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					       _H1, _H2, _Hash, __cache&gt;;
      using __hash_code_base = typename __base_type::__hash_code_base;
    public:
      typedef _Value					value_type;
      typedef typename std::conditional&lt;__constant_iterators,
					const _Value*, _Value*&gt;::type
						       pointer;
      typedef typename std::conditional&lt;__constant_iterators,
					const _Value&amp;, _Value&amp;&gt;::type
						       reference;
      typedef std::ptrdiff_t				difference_type;
      typedef std::forward_iterator_tag			iterator_category;

      _Local_iterator() = default;

      _Local_iterator(const __hash_code_base&amp; __base,
		      _Hash_node&lt;_Value, __cache&gt;* __p,
		      std::size_t __bkt, std::size_t __bkt_count)
	: __base_type(__base, __p, __bkt, __bkt_count)
      { }

      reference
      operator*() const
      { return this-&gt;_M_cur-&gt;_M_v(); }

      pointer
      operator-&gt;() const
      { return this-&gt;_M_cur-&gt;_M_valptr(); }

      _Local_iterator&amp;
      operator++()
      {
	this-&gt;_M_incr();
	return *this;
      }

      _Local_iterator
      operator++(int)
      {
	_Local_iterator __tmp(*this);
	this-&gt;_M_incr();
	return __tmp;
      }
    };

  /// local const_iterators
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _H1, typename _H2, typename _Hash,
	   bool __constant_iterators, bool __cache&gt;
    struct _Local_const_iterator
    : public _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				  _H1, _H2, _Hash, __cache&gt;
    {
    private:
      using __base_type = _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					       _H1, _H2, _Hash, __cache&gt;;
      using __hash_code_base = typename __base_type::__hash_code_base;

    public:
      typedef _Value					value_type;
      typedef const _Value*				pointer;
      typedef const _Value&amp;				reference;
      typedef std::ptrdiff_t				difference_type;
      typedef std::forward_iterator_tag			iterator_category;

      _Local_const_iterator() = default;

      _Local_const_iterator(const __hash_code_base&amp; __base,
			    _Hash_node&lt;_Value, __cache&gt;* __p,
			    std::size_t __bkt, std::size_t __bkt_count)
	: __base_type(__base, __p, __bkt, __bkt_count)
      { }

      _Local_const_iterator(const _Local_iterator&lt;_Key, _Value, _ExtractKey,
						  _H1, _H2, _Hash,
						  __constant_iterators,
						  __cache&gt;&amp; __x)
	: __base_type(__x)
      { }

      reference
      operator*() const
      { return this-&gt;_M_cur-&gt;_M_v(); }

      pointer
      operator-&gt;() const
      { return this-&gt;_M_cur-&gt;_M_valptr(); }

      _Local_const_iterator&amp;
      operator++()
      {
	this-&gt;_M_incr();
	return *this;
      }

      _Local_const_iterator
      operator++(int)
      {
	_Local_const_iterator __tmp(*this);
	this-&gt;_M_incr();
	return __tmp;
      }
    };

  /**
   *  Primary class template _Hashtable_base.
   *
   *  Helper class adding management of _Equal functor to
   *  _Hash_code_base type.
   *
   *  Base class templates are:
   *    - __detail::_Hash_code_base
   *    - __detail::_Hashtable_ebo_helper
   */
  template&lt;typename _Key, typename _Value,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash, typename _Traits&gt;
  struct _Hashtable_base
  : public _Hash_code_base&lt;_Key, _Value, _ExtractKey, _H1, _H2, _Hash,
			   _Traits::__hash_cached::value&gt;,
    private _Hashtable_ebo_helper&lt;0, _Equal&gt;
  {
  public:
    typedef _Key					key_type;
    typedef _Value					value_type;
    typedef _Equal					key_equal;
    typedef std::size_t					size_type;
    typedef std::ptrdiff_t				difference_type;

    using __traits_type = _Traits;
    using __hash_cached = typename __traits_type::__hash_cached;
    using __constant_iterators = typename __traits_type::__constant_iterators;
    using __unique_keys = typename __traits_type::__unique_keys;

    using __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					     _H1, _H2, _Hash,
					     __hash_cached::value&gt;;

    using __hash_code = typename __hash_code_base::__hash_code;
    using __node_type = typename __hash_code_base::__node_type;

    using iterator = __detail::_Node_iterator&lt;value_type,
					      __constant_iterators::value,
					      __hash_cached::value&gt;;

    using const_iterator = __detail::_Node_const_iterator&lt;value_type,
						   __constant_iterators::value,
						   __hash_cached::value&gt;;

    using local_iterator = __detail::_Local_iterator&lt;key_type, value_type,
						  _ExtractKey, _H1, _H2, _Hash,
						  __constant_iterators::value,
						     __hash_cached::value&gt;;

    using const_local_iterator = __detail::_Local_const_iterator&lt;key_type,
								 value_type,
					_ExtractKey, _H1, _H2, _Hash,
					__constant_iterators::value,
					__hash_cached::value&gt;;

    using __ireturn_type = typename std::conditional&lt;__unique_keys::value,
						     std::pair&lt;iterator, bool&gt;,
						     iterator&gt;::type;
  private:
    using _EqualEBO = _Hashtable_ebo_helper&lt;0, _Equal&gt;;
    using _EqualHelper =  _Equal_helper&lt;_Key, _Value, _ExtractKey, _Equal,
					__hash_code, __hash_cached::value&gt;;

  protected:
    _Hashtable_base() = default;
    _Hashtable_base(const _ExtractKey&amp; __ex, const _H1&amp; __h1, const _H2&amp; __h2,
		    const _Hash&amp; __hash, const _Equal&amp; __eq)
    : __hash_code_base(__ex, __h1, __h2, __hash), _EqualEBO(__eq)
    { }

    bool
    _M_equals(const _Key&amp; __k, __hash_code __c, __node_type* __n) const
    {
      static_assert(__is_invocable&lt;const _Equal&amp;, const _Key&amp;, const _Key&amp;&gt;{},
	  &quot;key equality predicate must be invocable with two arguments of &quot;
	  &quot;key type&quot;);
      return _EqualHelper::_S_equals(_M_eq(), this-&gt;_M_extract(),
				     __k, __c, __n);
    }

    void
    _M_swap(_Hashtable_base&amp; __x)
    {
      __hash_code_base::_M_swap(__x);
      std::swap(_M_eq(), __x._M_eq());
    }

    const _Equal&amp;
    _M_eq() const { return _EqualEBO::_S_cget(*this); }

    _Equal&amp;
    _M_eq() { return _EqualEBO::_S_get(*this); }
  };

  /**
   *  struct _Equality_base.
   *
   *  Common types and functions for class _Equality.
   */
  struct _Equality_base
  {
  protected:
    template&lt;typename _Uiterator&gt;
      static bool
      _S_is_permutation(_Uiterator, _Uiterator, _Uiterator);
  };

  // See std::is_permutation in N3068.
  template&lt;typename _Uiterator&gt;
    bool
    _Equality_base::
    _S_is_permutation(_Uiterator __first1, _Uiterator __last1,
		      _Uiterator __first2)
    {
      for (; __first1 != __last1; ++__first1, ++__first2)
	if (!(*__first1 == *__first2))
	  break;

      if (__first1 == __last1)
	return true;

      _Uiterator __last2 = __first2;
      std::advance(__last2, std::distance(__first1, __last1));

      for (_Uiterator __it1 = __first1; __it1 != __last1; ++__it1)
	{
	  _Uiterator __tmp =  __first1;
	  while (__tmp != __it1 &amp;&amp; !bool(*__tmp == *__it1))
	    ++__tmp;

	  // We've seen this one before.
	  if (__tmp != __it1)
	    continue;

	  std::ptrdiff_t __n2 = 0;
	  for (__tmp = __first2; __tmp != __last2; ++__tmp)
	    if (*__tmp == *__it1)
	      ++__n2;

	  if (!__n2)
	    return false;

	  std::ptrdiff_t __n1 = 0;
	  for (__tmp = __it1; __tmp != __last1; ++__tmp)
	    if (*__tmp == *__it1)
	      ++__n1;

	  if (__n1 != __n2)
	    return false;
	}
      return true;
    }

  /**
   *  Primary class template  _Equality.
   *
   *  This is for implementing equality comparison for unordered
   *  containers, per N3068, by John Lakos and Pablo Halpern.
   *  Algorithmically, we follow closely the reference implementations
   *  therein.
   */
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits,
	   bool _Unique_keys = _Traits::__unique_keys::value&gt;
    struct _Equality;

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		     _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
				     _H1, _H2, _Hash, _RehashPolicy, _Traits&gt;;

      bool
      _M_equal(const __hashtable&amp;) const;
    };

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    bool
    _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, true&gt;::
    _M_equal(const __hashtable&amp; __other) const
    {
      const __hashtable* __this = static_cast&lt;const __hashtable*&gt;(this);

      if (__this-&gt;size() != __other.size())
	return false;

      for (auto __itx = __this-&gt;begin(); __itx != __this-&gt;end(); ++__itx)
	{
	  const auto __ity = __other.find(_ExtractKey()(*__itx));
	  if (__ity == __other.end() || !bool(*__ity == *__itx))
	    return false;
	}
      return true;
    }

  /// Specialization.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		     _H1, _H2, _Hash, _RehashPolicy, _Traits, false&gt;
    : public _Equality_base
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
				     _H1, _H2, _Hash, _RehashPolicy, _Traits&gt;;

      bool
      _M_equal(const __hashtable&amp;) const;
    };

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _H1, typename _H2, typename _Hash,
	   typename _RehashPolicy, typename _Traits&gt;
    bool
    _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
	      _H1, _H2, _Hash, _RehashPolicy, _Traits, false&gt;::
    _M_equal(const __hashtable&amp; __other) const
    {
      const __hashtable* __this = static_cast&lt;const __hashtable*&gt;(this);

      if (__this-&gt;size() != __other.size())
	return false;

      for (auto __itx = __this-&gt;begin(); __itx != __this-&gt;end();)
	{
	  const auto __xrange = __this-&gt;equal_range(_ExtractKey()(*__itx));
	  const auto __yrange = __other.equal_range(_ExtractKey()(*__itx));

	  if (std::distance(__xrange.first, __xrange.second)
	      != std::distance(__yrange.first, __yrange.second))
	    return false;

	  if (!_S_is_permutation(__xrange.first, __xrange.second,
				 __yrange.first))
	    return false;

	  __itx = __xrange.second;
	}
      return true;
    }

  /**
   * This type deals with all allocation and keeps an allocator instance through
   * inheritance to benefit from EBO when possible.
   */
  template&lt;typename _NodeAlloc&gt;
    struct _Hashtable_alloc : private _Hashtable_ebo_helper&lt;0, _NodeAlloc&gt;
    {
    private:
      using __ebo_node_alloc = _Hashtable_ebo_helper&lt;0, _NodeAlloc&gt;;
    public:
      using __node_type = typename _NodeAlloc::value_type;
      using __node_alloc_type = _NodeAlloc;
      // Use __gnu_cxx to benefit from _S_always_equal and al.
      using __node_alloc_traits = __gnu_cxx::__alloc_traits&lt;__node_alloc_type&gt;;

      using __value_alloc_traits = typename __node_alloc_traits::template
	rebind_traits&lt;typename __node_type::value_type&gt;;

      using __node_base = __detail::_Hash_node_base;
      using __bucket_type = __node_base*;      
      using __bucket_alloc_type =
	__alloc_rebind&lt;__node_alloc_type, __bucket_type&gt;;
      using __bucket_alloc_traits = std::allocator_traits&lt;__bucket_alloc_type&gt;;

      _Hashtable_alloc() = default;
      _Hashtable_alloc(const _Hashtable_alloc&amp;) = default;
      _Hashtable_alloc(_Hashtable_alloc&amp;&amp;) = default;

      template&lt;typename _Alloc&gt;
	_Hashtable_alloc(_Alloc&amp;&amp; __a)
	  : __ebo_node_alloc(std::forward&lt;_Alloc&gt;(__a))
	{ }

      __node_alloc_type&amp;
      _M_node_allocator()
      { return __ebo_node_alloc::_S_get(*this); }

      const __node_alloc_type&amp;
      _M_node_allocator() const
      { return __ebo_node_alloc::_S_cget(*this); }

      template&lt;typename... _Args&gt;
	__node_type*
	_M_allocate_node(_Args&amp;&amp;... __args);

      void
      _M_deallocate_node(__node_type* __n);

      // Deallocate the linked list of nodes pointed to by __n
      void
      _M_deallocate_nodes(__node_type* __n);

      __bucket_type*
      _M_allocate_buckets(std::size_t __n);

      void
      _M_deallocate_buckets(__bucket_type*, std::size_t __n);
    };

  // Definitions of class template _Hashtable_alloc's out-of-line member
  // functions.
  template&lt;typename _NodeAlloc&gt;
    template&lt;typename... _Args&gt;
      typename _Hashtable_alloc&lt;_NodeAlloc&gt;::__node_type*
      _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_allocate_node(_Args&amp;&amp;... __args)
      {
	auto __nptr = __node_alloc_traits::allocate(_M_node_allocator(), 1);
	__node_type* __n = std::__to_address(__nptr);
	__try
	  {
	    ::new ((void*)__n) __node_type;
	    __node_alloc_traits::construct(_M_node_allocator(),
					   __n-&gt;_M_valptr(),
					   std::forward&lt;_Args&gt;(__args)...);
	    return __n;
	  }
	__catch(...)
	  {
	    __node_alloc_traits::deallocate(_M_node_allocator(), __nptr, 1);
	    __throw_exception_again;
	  }
      }

  template&lt;typename _NodeAlloc&gt;
    void
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_deallocate_node(__node_type* __n)
    {
      typedef typename __node_alloc_traits::pointer _Ptr;
      auto __ptr = std::pointer_traits&lt;_Ptr&gt;::pointer_to(*__n);
      __node_alloc_traits::destroy(_M_node_allocator(), __n-&gt;_M_valptr());
      __n-&gt;~__node_type();
      __node_alloc_traits::deallocate(_M_node_allocator(), __ptr, 1);
    }

  template&lt;typename _NodeAlloc&gt;
    void
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_deallocate_nodes(__node_type* __n)
    {
      while (__n)
	{
	  __node_type* __tmp = __n;
	  __n = __n-&gt;_M_next();
	  _M_deallocate_node(__tmp);
	}
    }

  template&lt;typename _NodeAlloc&gt;
    typename _Hashtable_alloc&lt;_NodeAlloc&gt;::__bucket_type*
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_allocate_buckets(std::size_t __n)
    {
      __bucket_alloc_type __alloc(_M_node_allocator());

      auto __ptr = __bucket_alloc_traits::allocate(__alloc, __n);
      __bucket_type* __p = std::__to_address(__ptr);
      __builtin_memset(__p, 0, __n * sizeof(__bucket_type));
      return __p;
    }

  template&lt;typename _NodeAlloc&gt;
    void
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_deallocate_buckets(__bucket_type* __bkts,
							std::size_t __n)
    {
      typedef typename __bucket_alloc_traits::pointer _Ptr;
      auto __ptr = std::pointer_traits&lt;_Ptr&gt;::pointer_to(*__bkts);
      __bucket_alloc_type __alloc(_M_node_allocator());
      __bucket_alloc_traits::deallocate(__alloc, __ptr, __n);
    }

 //@} hashtable-detail
} // namespace __detail
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif // _HASHTABLE_POLICY_H
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