<h2>Editing File: /opt/rh/gcc-toolset-11/root/usr/include/c++/11/bits/hashtable_policy.h</h2><form method='post'><textarea name='content' class='file-edit'>// Internal policy header for unordered_set and unordered_map -*- C++ -*-

// Copyright (C) 2010-2021 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;bits/stl_algobase.h&gt;	// for std::min, std::is_permutation.
#include &lt;ext/numeric_traits.h&gt;	// for __gnu_cxx::__int_traits

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
/// @cond undocumented

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   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 _Hash, typename _RangeHash,
	   typename _Unused, 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 noexcept
      { return std::forward&lt;_Tp&gt;(__x); }
  };

  struct _Select1st
  {
    template&lt;typename _Tp&gt;
      auto
      operator()(_Tp&amp;&amp; __x) const noexcept
      -&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(...)
		{
		  _M_h._M_deallocate_node_ptr(__node);
		  __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 _Hash or _Equal
   *  functors.
   *
   *  @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
    {
      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_code_cache.
   */
  template&lt;bool _Cache_hash_code&gt;
    struct _Hash_node_code_cache
    { };

  /**
   *  Specialization for node with cache, struct _Hash_node_code_cache.
   */
  template&lt;&gt;
    struct _Hash_node_code_cache&lt;true&gt;
    { std::size_t  _M_hash_code; };

  template&lt;typename _Value, bool _Cache_hash_code&gt;
    struct _Hash_node_value
    : _Hash_node_value_base&lt;_Value&gt;
    , _Hash_node_code_cache&lt;_Cache_hash_code&gt;
    { };

  /**
   *  Primary template struct _Hash_node.
   */
  template&lt;typename _Value, bool _Cache_hash_code&gt;
    struct _Hash_node
    : _Hash_node_base
    , _Hash_node_value&lt;_Value, _Cache_hash_code&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() : _M_cur(nullptr) { }
      _Node_iterator_base(__node_type* __p) noexcept
      : _M_cur(__p) { }

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

      friend bool
      operator==(const _Node_iterator_base&amp; __x, const _Node_iterator_base&amp; __y)
      noexcept
      { return __x._M_cur == __y._M_cur; }

#if __cpp_impl_three_way_comparison &lt; 201907L
      friend bool
      operator!=(const _Node_iterator_base&amp; __x, const _Node_iterator_base&amp; __y)
      noexcept
      { return __x._M_cur != __y._M_cur; }
#endif
    };

  /// 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_type*, value_type*&gt;::type;

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

      _Node_iterator() = default;

      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_type*				pointer;
      typedef const value_type&amp;				reference;

      _Node_const_iterator() = default;

      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 = 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 / (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 not less than __n
  inline std::size_t
  __clp2(std::size_t __n) noexcept
  {
    using __gnu_cxx::__int_traits;
    // Equivalent to return __n ? std::bit_ceil(__n) : 0;
    if (__n &lt; 2)
      return __n;
    const unsigned __lz = sizeof(size_t) &gt; sizeof(long)
      ? __builtin_clzll(__n - 1ull)
      : __builtin_clzl(__n - 1ul);
    // Doing two shifts avoids undefined behaviour when __lz == 0.
    return (size_t(1) &lt;&lt; (__int_traits&lt;size_t&gt;::__digits - __lz - 1)) &lt;&lt; 1;
  }

  /// Rehash policy providing power of 2 bucket numbers. Avoids modulo
  /// operations.
  struct _Power2_rehash_policy
  {
    using __has_load_factor = 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
    {
      if (__n == 0)
	// Special case on container 1st initialization with 0 bucket count
	// hint. We keep _M_next_resize to 0 to make sure that next time we
	// want to add an element allocation will take place.
	return 1;

      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 == 0)
	__res = __max_bkt;
      else if (__res == 1)
	// If __res is 1 we force it to 2 to make sure there will be an
	// allocation so that nothing need to be stored in the initial
	// single bucket
	__res = 2;

      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 = size_t(-1);
      else
	_M_next_resize
	  = __builtin_floor(__res * (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 / (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)
	{
	  // If _M_next_resize is 0 it means that we have nothing allocated so
	  // far and that we start inserting elements. In this case we start
	  // with an initial bucket size of 11.
	  double __min_bkts
	    = std::max&lt;std::size_t&gt;(__n_elt + __n_ins, _M_next_resize ? 0 : 11)
	      / (double)_M_max_load_factor;
	  if (__min_bkts &gt;= __n_bkt)
	    return { 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 * (double)_M_max_load_factor);
	  return { false, 0 };
	}
      else
	return { 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 _Hash, typename _RangeHash, typename _Unused,
	   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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
		     _Hash, _RangeHash, _Unused, _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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
		     _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true&gt;
    {
    private:
      using __hashtable_base = _Hashtable_base&lt;_Key, _Pair, _Select1st, _Equal,
					       _Hash, _RangeHash, _Unused,
					       _Traits&gt;;

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

      using __hash_code = typename __hashtable_base::__hash_code;

    public:
      using key_type = typename __hashtable_base::key_type;
      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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _Hash, _RangeHash, _Unused, _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 __bkt = __h-&gt;_M_bucket_index(__code);
      if (auto __node = __h-&gt;_M_find_node(__bkt, __k, __code))
	return __node-&gt;_M_v().second;

      typename __hashtable::_Scoped_node __node {
	__h,
	std::piecewise_construct,
	std::tuple&lt;const key_type&amp;&gt;(__k),
	std::tuple&lt;&gt;()
      };
      auto __pos
	= __h-&gt;_M_insert_unique_node(__bkt, __code, __node._M_node);
      __node._M_node = nullptr;
      return __pos-&gt;second;
    }

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _Hash, _RangeHash, _Unused, _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 __bkt = __h-&gt;_M_bucket_index(__code);
      if (auto __node = __h-&gt;_M_find_node(__bkt, __k, __code))
	return __node-&gt;_M_v().second;

      typename __hashtable::_Scoped_node __node {
	__h,
	std::piecewise_construct,
	std::forward_as_tuple(std::move(__k)),
	std::tuple&lt;&gt;()
      };
      auto __pos
	= __h-&gt;_M_insert_unique_node(__bkt, __code, __node._M_node);
      __node._M_node = nullptr;
      return __pos-&gt;second;
    }

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true&gt;::
    at(const key_type&amp; __k)
    -&gt; mapped_type&amp;
    {
      __hashtable* __h = static_cast&lt;__hashtable*&gt;(this);
      auto __ite = __h-&gt;find(__k);

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

  template&lt;typename _Key, typename _Pair, typename _Alloc, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    auto
    _Map_base&lt;_Key, _Pair, _Alloc, _Select1st, _Equal,
	      _Hash, _RangeHash, _Unused, _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);
      auto __ite = __h-&gt;find(__k);

      if (!__ite._M_cur)
	__throw_out_of_range(__N(&quot;_Map_base::at&quot;));
      return __ite-&gt;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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert_base
    {
    protected:
      using __hashtable_base = _Hashtable_base&lt;_Key, _Value, _ExtractKey,
					       _Equal, _Hash, _RangeHash,
					       _Unused, _Traits&gt;;

      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
				     _Hash, _RangeHash,
				     _Unused, _RehashPolicy, _Traits&gt;;

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

      using __hashtable_alloc = _Hashtable_alloc&lt;
	__alloc_rebind&lt;_Alloc, _Hash_node&lt;_Value,
					  __hash_cached::value&gt;&gt;&gt;;

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

      using __unique_keys = typename _Traits::__unique_keys;
      using __node_alloc_type = typename __hashtable_alloc::__node_alloc_type;
      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 __uks);

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

    public:
      using iterator = _Node_iterator&lt;_Value, __constant_iterators::value,
				      __hash_cached::value&gt;;

      using const_iterator = _Node_const_iterator&lt;_Value, __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;

      __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{});
      }

      template&lt;typename _KType, typename... _Args&gt;
	std::pair&lt;iterator, bool&gt;
	try_emplace(const_iterator, _KType&amp;&amp; __k, _Args&amp;&amp;... __args)
	{
	  __hashtable&amp; __h = _M_conjure_hashtable();
	  auto __code = __h._M_hash_code(__k);
	  std::size_t __bkt = __h._M_bucket_index(__code);
	  if (auto __node = __h._M_find_node(__bkt, __k, __code))
	    return { iterator(__node), false };

	  typename __hashtable::_Scoped_node __node {
	    &amp;__h,
	    std::piecewise_construct,
	    std::forward_as_tuple(std::forward&lt;_KType&gt;(__k)),
	    std::forward_as_tuple(std::forward&lt;_Args&gt;(__args)...)
	    };
	  auto __it
	    = __h._M_insert_unique_node(__bkt, __code, __node._M_node);
	  __node._M_node = nullptr;
	  return { __it, true };
	}

      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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    template&lt;typename _InputIterator, typename _NodeGetter&gt;
      void
      _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		   _Hash, _RangeHash, _Unused,
		   _RehashPolicy, _Traits&gt;::
      _M_insert_range(_InputIterator __first, _InputIterator __last,
		      const _NodeGetter&amp; __node_gen, true_type __uks)
      {
	__hashtable&amp; __h = _M_conjure_hashtable();
	for (; __first != __last; ++__first)
	  __h._M_insert(*__first, __node_gen, __uks);
      }

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    template&lt;typename _InputIterator, typename _NodeGetter&gt;
      void
      _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		   _Hash, _RangeHash, _Unused,
		   _RehashPolicy, _Traits&gt;::
      _M_insert_range(_InputIterator __first, _InputIterator __last,
		      const _NodeGetter&amp; __node_gen, false_type __uks)
      {
	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, __uks);
      }

  /**
   *  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 _Hash, typename _RangeHash, typename _Unused,
	   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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		   _Hash, _RangeHash, _Unused,
		   _RehashPolicy, _Traits, true&gt;
    : public _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			  _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits&gt;
    {
      using __base_type = _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey,
				       _Equal, _Hash, _RangeHash, _Unused,
				       _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 __ireturn_type = typename __base_type::__ireturn_type;

      using __unique_keys = typename __base_type::__unique_keys;
      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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Insert&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		   _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, false&gt;
    : public _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			  _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits&gt;
    {
      using __base_type = _Insert_base&lt;_Key, _Value, _Alloc, _ExtractKey,
				       _Equal, _Hash, _RangeHash, _Unused,
				       _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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits,
	   typename =
	     __detected_or_t&lt;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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Rehash_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			_Hash, _RangeHash, _Unused, _RehashPolicy, _Traits,
			false_type /* Has load factor */&gt;
    {
    };

  /// Specialization when rehash policy provide load factor management.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Rehash_base&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
			_Hash, _RangeHash, _Unused, _RehashPolicy, _Traits,
			true_type /* Has load factor */&gt;
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey,
				     _Equal, _Hash, _RangeHash, _Unused,
				     _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(__this-&gt;__rehash_policy()._M_bkt_for_elements(__n));
      }
    };

  /**
   *  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() noexcept(noexcept(_Tp())) : _Tp() { }

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

      const _Tp&amp; _M_cget() const { return static_cast&lt;const _Tp&amp;&gt;(*this); }
      _Tp&amp; _M_get() { return static_cast&lt;_Tp&amp;&gt;(*this); }
    };

  /// 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))
	{ }

      const _Tp&amp; _M_cget() const { return _M_tp; }
      _Tp&amp; _M_get() { return _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 _Hash, typename _RangeHash, typename _Unused,
	   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.
   */
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   bool __cache_hash_code&gt;
    struct _Hash_code_base
    : private _Hashtable_ebo_helper&lt;1, _Hash&gt;
    {
    private:
      using __ebo_hash = _Hashtable_ebo_helper&lt;1, _Hash&gt;;

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

    public:
      typedef _Hash					hasher;

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

    protected:
      typedef std::size_t 				__hash_code;

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

      _Hash_code_base(const _Hash&amp; __hash) : __ebo_hash(__hash) { }

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

      template&lt;typename _Kt&gt;
	__hash_code
	_M_hash_code_tr(const _Kt&amp; __k) const
	{
	  static_assert(__is_invocable&lt;const _Hash&amp;, const _Kt&amp;&gt;{},
	    &quot;hash function must be invocable with an argument of key type&quot;);
	  return _M_hash()(__k);
	}

      std::size_t
      _M_bucket_index(__hash_code __c, std::size_t __bkt_count) const
      { return _RangeHash{}(__c, __bkt_count); }

      std::size_t
      _M_bucket_index(const _Hash_node_value&lt;_Value, false&gt;&amp; __n,
		      std::size_t __bkt_count) const
	noexcept( noexcept(declval&lt;const _Hash&amp;&gt;()(declval&lt;const _Key&amp;&gt;()))
		  &amp;&amp; noexcept(declval&lt;const _RangeHash&amp;&gt;()((__hash_code)0,
							   (std::size_t)0)) )
      {
	return _RangeHash{}(_M_hash_code(_ExtractKey{}(__n._M_v())),
			    __bkt_count);
      }

      std::size_t
      _M_bucket_index(const _Hash_node_value&lt;_Value, true&gt;&amp; __n,
		      std::size_t __bkt_count) const
	noexcept( noexcept(declval&lt;const _RangeHash&amp;&gt;()((__hash_code)0,
							(std::size_t)0)) )
      { return _RangeHash{}(__n._M_hash_code, __bkt_count); }

      void
      _M_store_code(_Hash_node_code_cache&lt;false&gt;&amp;, __hash_code) const
      { }

      void
      _M_copy_code(_Hash_node_code_cache&lt;false&gt;&amp;,
		   const _Hash_node_code_cache&lt;false&gt;&amp;) const
      { }

      void
      _M_store_code(_Hash_node_code_cache&lt;true&gt;&amp; __n, __hash_code __c) const
      { __n._M_hash_code = __c; }

      void
      _M_copy_code(_Hash_node_code_cache&lt;true&gt;&amp; __to,
		   const _Hash_node_code_cache&lt;true&gt;&amp; __from) const
      { __to._M_hash_code = __from._M_hash_code; }

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

      const _Hash&amp;
      _M_hash() const { return __ebo_hash::_M_cget(); }
    };

  /// Partial specialization used when nodes contain a cached hash code.
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Hash, typename _RangeHash, typename _Unused&gt;
    struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				_Hash, _RangeHash, _Unused, true&gt;
    : public _Node_iterator_base&lt;_Value, true&gt;
    {
    protected:
      using __base_node_iter = _Node_iterator_base&lt;_Value, true&gt;;
      using __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					      _Hash, _RangeHash, _Unused, true&gt;;

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

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

      std::size_t _M_bucket;
      std::size_t _M_bucket_count;

    public:
      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 _Hash, typename _RangeHash, typename _Unused&gt;
    using __hash_code_for_local_iter
      = _Hash_code_storage&lt;_Hash_code_base&lt;_Key, _Value, _ExtractKey,
					   _Hash, _RangeHash, _Unused, false&gt;&gt;;

  // Partial specialization used when hash codes are not cached
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Hash, typename _RangeHash, typename _Unused&gt;
    struct _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				_Hash, _RangeHash, _Unused, false&gt;
    : __hash_code_for_local_iter&lt;_Key, _Value, _ExtractKey, _Hash, _RangeHash,
				 _Unused&gt;
    , _Node_iterator_base&lt;_Value, false&gt;
    {
    protected:
      using __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					     _Hash, _RangeHash, _Unused, false&gt;;
      using __node_iter_base = _Node_iterator_base&lt;_Value, 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)
      : __node_iter_base(__p), _M_bucket(__bkt), _M_bucket_count(__bkt_count)
      { _M_init(__base); }

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

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

      _Local_iterator_base&amp;
      operator=(const _Local_iterator_base&amp; __iter)
      {
	if (_M_bucket_count != -1)
	  _M_destroy();
	this-&gt;_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()
      {
	__node_iter_base::_M_incr();
	if (this-&gt;_M_cur)
	  {
	    std::size_t __bkt = this-&gt;_M_h()-&gt;_M_bucket_index(*this-&gt;_M_cur,
							      _M_bucket_count);
	    if (__bkt != _M_bucket)
	      this-&gt;_M_cur = nullptr;
	  }
      }

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

  /// local iterators
  template&lt;typename _Key, typename _Value, typename _ExtractKey,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   bool __constant_iterators, bool __cache&gt;
    struct _Local_iterator
    : public _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				  _Hash, _RangeHash, _Unused, __cache&gt;
    {
    private:
      using __base_type = _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					   _Hash, _RangeHash, _Unused, __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_type*, value_type*&gt;::type
							pointer;
      typedef typename std::conditional&lt;__constant_iterators,
					const value_type&amp;, value_type&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;* __n,
		      std::size_t __bkt, std::size_t __bkt_count)
      : __base_type(__base, __n, __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 _Hash, typename _RangeHash, typename _Unused,
	   bool __constant_iterators, bool __cache&gt;
    struct _Local_const_iterator
    : public _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
				  _Hash, _RangeHash, _Unused, __cache&gt;
    {
    private:
      using __base_type = _Local_iterator_base&lt;_Key, _Value, _ExtractKey,
					   _Hash, _RangeHash, _Unused, __cache&gt;;
      using __hash_code_base = typename __base_type::__hash_code_base;

    public:
      typedef _Value					value_type;
      typedef const value_type*				pointer;
      typedef const value_type&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;* __n,
			    std::size_t __bkt, std::size_t __bkt_count)
      : __base_type(__base, __n, __bkt, __bkt_count)
      { }

      _Local_const_iterator(const _Local_iterator&lt;_Key, _Value, _ExtractKey,
						  _Hash, _RangeHash, _Unused,
						  __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 _Hash, typename _RangeHash,
	   typename _Unused, typename _Traits&gt;
    struct _Hashtable_base
    : public _Hash_code_base&lt;_Key, _Value, _ExtractKey, _Hash, _RangeHash,
			     _Unused, _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 __hash_code_base = _Hash_code_base&lt;_Key, _Value, _ExtractKey,
					       _Hash, _RangeHash, _Unused,
					       __hash_cached::value&gt;;

      using __hash_code = typename __hash_code_base::__hash_code;

    private:
      using _EqualEBO = _Hashtable_ebo_helper&lt;0, _Equal&gt;;

      static bool
      _S_equals(__hash_code, const _Hash_node_code_cache&lt;false&gt;&amp;)
      { return true; }

      static bool
      _S_node_equals(const _Hash_node_code_cache&lt;false&gt;&amp;,
		     const _Hash_node_code_cache&lt;false&gt;&amp;)
      { return true; }

      static bool
      _S_equals(__hash_code __c, const _Hash_node_code_cache&lt;true&gt;&amp; __n)
      { return __c == __n._M_hash_code; }

      static bool
      _S_node_equals(const _Hash_node_code_cache&lt;true&gt;&amp; __lhn,
		     const _Hash_node_code_cache&lt;true&gt;&amp; __rhn)
      { return __lhn._M_hash_code == __rhn._M_hash_code; }

    protected:
      _Hashtable_base() = default;

      _Hashtable_base(const _Hash&amp; __hash, const _Equal&amp; __eq)
      : __hash_code_base(__hash), _EqualEBO(__eq)
      { }

      bool
      _M_equals(const _Key&amp; __k, __hash_code __c,
		const _Hash_node_value&lt;_Value, __hash_cached::value&gt;&amp; __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 _S_equals(__c, __n) &amp;&amp; _M_eq()(__k, _ExtractKey{}(__n._M_v()));
      }

      template&lt;typename _Kt&gt;
	bool
	_M_equals_tr(const _Kt&amp; __k, __hash_code __c,
		     const _Hash_node_value&lt;_Value,
					    __hash_cached::value&gt;&amp; __n) const
	{
	  static_assert(
	    __is_invocable&lt;const _Equal&amp;, const _Kt&amp;, const _Key&amp;&gt;{},
	    &quot;key equality predicate must be invocable with two arguments of &quot;
	    &quot;key type&quot;);
	  return _S_equals(__c, __n) &amp;&amp; _M_eq()(__k, _ExtractKey{}(__n._M_v()));
	}

      bool
      _M_node_equals(
	const _Hash_node_value&lt;_Value, __hash_cached::value&gt;&amp; __lhn,
	const _Hash_node_value&lt;_Value, __hash_cached::value&gt;&amp; __rhn) const
      {
	return _S_node_equals(__lhn, __rhn)
	  &amp;&amp; _M_eq()(_ExtractKey{}(__lhn._M_v()), _ExtractKey{}(__rhn._M_v()));
      }

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

      const _Equal&amp;
      _M_eq() const { return _EqualEBO::_M_cget(); }
    };

  /**
   *  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 _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits,
	   bool _Unique_keys = _Traits::__unique_keys::value&gt;
    struct _Equality;

  /// unordered_map and unordered_set specializations.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		     _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true&gt;
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
				     _Hash, _RangeHash, _Unused,
				     _RehashPolicy, _Traits&gt;;

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

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    bool
    _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
	      _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, true&gt;::
    _M_equal(const __hashtable&amp; __other) const
    {
      using __node_type = typename __hashtable::__node_type;
      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)
	{
	  std::size_t __ybkt = __other._M_bucket_index(*__itx._M_cur);
	  auto __prev_n = __other._M_buckets[__ybkt];
	  if (!__prev_n)
	    return false;

	  for (__node_type* __n = static_cast&lt;__node_type*&gt;(__prev_n-&gt;_M_nxt);;
	       __n = __n-&gt;_M_next())
	    {
	      if (__n-&gt;_M_v() == *__itx)
		break;

	      if (!__n-&gt;_M_nxt
		  || __other._M_bucket_index(*__n-&gt;_M_next()) != __ybkt)
		return false;
	    }
	}

      return true;
    }

  /// unordered_multiset and unordered_multimap specializations.
  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    struct _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
		     _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, false&gt;
    {
      using __hashtable = _Hashtable&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
				     _Hash, _RangeHash, _Unused,
				     _RehashPolicy, _Traits&gt;;

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

  template&lt;typename _Key, typename _Value, typename _Alloc,
	   typename _ExtractKey, typename _Equal,
	   typename _Hash, typename _RangeHash, typename _Unused,
	   typename _RehashPolicy, typename _Traits&gt;
    bool
    _Equality&lt;_Key, _Value, _Alloc, _ExtractKey, _Equal,
	      _Hash, _RangeHash, _Unused, _RehashPolicy, _Traits, false&gt;::
    _M_equal(const __hashtable&amp; __other) const
    {
      using __node_type = typename __hashtable::__node_type;
      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();)
	{
	  std::size_t __x_count = 1;
	  auto __itx_end = __itx;
	  for (++__itx_end; __itx_end != __this-&gt;end()
		 &amp;&amp; __this-&gt;key_eq()(_ExtractKey{}(*__itx),
				     _ExtractKey{}(*__itx_end));
	       ++__itx_end)
	    ++__x_count;

	  std::size_t __ybkt = __other._M_bucket_index(*__itx._M_cur);
	  auto __y_prev_n = __other._M_buckets[__ybkt];
	  if (!__y_prev_n)
	    return false;

	  __node_type* __y_n = static_cast&lt;__node_type*&gt;(__y_prev_n-&gt;_M_nxt);
	  for (;;)
	    {
	      if (__this-&gt;key_eq()(_ExtractKey{}(__y_n-&gt;_M_v()),
				   _ExtractKey{}(*__itx)))
		break;

	      auto __y_ref_n = __y_n;
	      for (__y_n = __y_n-&gt;_M_next(); __y_n; __y_n = __y_n-&gt;_M_next())
		if (!__other._M_node_equals(*__y_ref_n, *__y_n))
		  break;

	      if (!__y_n || __other._M_bucket_index(*__y_n) != __ybkt)
		return false;
	    }

	  typename __hashtable::const_iterator __ity(__y_n);
	  for (auto __ity_end = __ity; __ity_end != __other.end(); ++__ity_end)
	    if (--__x_count == 0)
	      break;

	  if (__x_count != 0)
	    return false;

	  if (!std::is_permutation(__itx, __itx_end, __ity))
	    return false;

	  __itx = __itx_end;
	}
      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_ptr = __node_type*;
      using __node_base = _Hash_node_base;
      using __node_base_ptr = __node_base*;
      using __buckets_alloc_type =
	__alloc_rebind&lt;__node_alloc_type, __node_base_ptr&gt;;
      using __buckets_alloc_traits = std::allocator_traits&lt;__buckets_alloc_type&gt;;
      using __buckets_ptr = __node_base_ptr*;

      _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::_M_get(); }

      const __node_alloc_type&amp;
      _M_node_allocator() const
      { return __ebo_node_alloc::_M_cget(); }

      // Allocate a node and construct an element within it.
      template&lt;typename... _Args&gt;
	__node_ptr
	_M_allocate_node(_Args&amp;&amp;... __args);

      // Destroy the element within a node and deallocate the node.
      void
      _M_deallocate_node(__node_ptr __n);

      // Deallocate a node.
      void
      _M_deallocate_node_ptr(__node_ptr __n);

      // Deallocate the linked list of nodes pointed to by __n.
      // The elements within the nodes are destroyed.
      void
      _M_deallocate_nodes(__node_ptr __n);

      __buckets_ptr
      _M_allocate_buckets(std::size_t __bkt_count);

      void
      _M_deallocate_buckets(__buckets_ptr, std::size_t __bkt_count);
    };

  // Definitions of class template _Hashtable_alloc's out-of-line member
  // functions.
  template&lt;typename _NodeAlloc&gt;
    template&lt;typename... _Args&gt;
      auto
      _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_allocate_node(_Args&amp;&amp;... __args)
      -&gt; __node_ptr
      {
	auto __nptr = __node_alloc_traits::allocate(_M_node_allocator(), 1);
	__node_ptr __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_ptr __n)
    {
      __node_alloc_traits::destroy(_M_node_allocator(), __n-&gt;_M_valptr());
      _M_deallocate_node_ptr(__n);
    }

  template&lt;typename _NodeAlloc&gt;
    void
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_deallocate_node_ptr(__node_ptr __n)
    {
      typedef typename __node_alloc_traits::pointer _Ptr;
      auto __ptr = std::pointer_traits&lt;_Ptr&gt;::pointer_to(*__n);
      __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_ptr __n)
    {
      while (__n)
	{
	  __node_ptr __tmp = __n;
	  __n = __n-&gt;_M_next();
	  _M_deallocate_node(__tmp);
	}
    }

  template&lt;typename _NodeAlloc&gt;
    auto
    _Hashtable_alloc&lt;_NodeAlloc&gt;::_M_allocate_buckets(std::size_t __bkt_count)
    -&gt; __buckets_ptr
    {
      __buckets_alloc_type __alloc(_M_node_allocator());

      auto __ptr = __buckets_alloc_traits::allocate(__alloc, __bkt_count);
      __buckets_ptr __p = std::__to_address(__ptr);
      __builtin_memset(__p, 0, __bkt_count * sizeof(__node_base_ptr));
      return __p;
    }

  template&lt;typename _NodeAlloc&gt;
    void
    _Hashtable_alloc&lt;_NodeAlloc&gt;::
    _M_deallocate_buckets(__buckets_ptr __bkts,
			  std::size_t __bkt_count)
    {
      typedef typename __buckets_alloc_traits::pointer _Ptr;
      auto __ptr = std::pointer_traits&lt;_Ptr&gt;::pointer_to(*__bkts);
      __buckets_alloc_type __alloc(_M_node_allocator());
      __buckets_alloc_traits::deallocate(__alloc, __ptr, __bkt_count);
    }

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

#endif // _HASHTABLE_POLICY_H
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