unordered_map.h

Go to the documentation of this file.

// unordered_map implementation -*- C++ -*-

// Copyright (C) 2010-2019 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
// <http://www.gnu.org/licenses/>.

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

#ifndef _UNORDERED_MAP_H
#define _UNORDERED_MAP_H

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
_GLIBCXX_BEGIN_NAMESPACE_CONTAINER

  /// Base types for unordered_map.
  template<bool _Cache>
    using __umap_traits = __detail::_Hashtable_traits<_Cache, false, true>;

  template<typename _Key,
           typename _Tp,
           typename _Hash = hash<_Key>,
           typename _Pred = std::equal_to<_Key>,
           typename _Alloc = std::allocator<std::pair<const _Key, _Tp> >,
           typename _Tr = __umap_traits<__cache_default<_Key, _Hash>::value>>
    using __umap_hashtable = _Hashtable<_Key, std::pair<const _Key, _Tp>,
                                        _Alloc, __detail::_Select1st,
                                        _Pred, _Hash,
                                        __detail::_Mod_range_hashing,
                                        __detail::_Default_ranged_hash,
                                        __detail::_Prime_rehash_policy, _Tr>;

  /// Base types for unordered_multimap.
  template<bool _Cache>
    using __ummap_traits = __detail::_Hashtable_traits<_Cache, false, false>;

  template<typename _Key,
           typename _Tp,
           typename _Hash = hash<_Key>,
           typename _Pred = std::equal_to<_Key>,
           typename _Alloc = std::allocator<std::pair<const _Key, _Tp> >,
           typename _Tr = __ummap_traits<__cache_default<_Key, _Hash>::value>>
    using __ummap_hashtable = _Hashtable<_Key, std::pair<const _Key, _Tp>,
                                         _Alloc, __detail::_Select1st,
                                         _Pred, _Hash,
                                         __detail::_Mod_range_hashing,
                                         __detail::_Default_ranged_hash,
                                         __detail::_Prime_rehash_policy, _Tr>;

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    class unordered_multimap;

  /**
   *  @brief A standard container composed of unique keys (containing
   *  at most one of each key value) that associates values of another type
   *  with the keys.
   *
   *  @ingroup unordered_associative_containers
   *
   *  @tparam  _Key    Type of key objects.
   *  @tparam  _Tp     Type of mapped objects.
   *  @tparam  _Hash   Hashing function object type, defaults to hash<_Value>.
   *  @tparam  _Pred   Predicate function object type, defaults
   *                   to equal_to<_Value>.
   *  @tparam  _Alloc  Allocator type, defaults to 
   *                   std::allocator<std::pair<const _Key, _Tp>>.
   *
   *  Meets the requirements of a <a href="tables.html#65">container</a>, and
   *  <a href="tables.html#xx">unordered associative container</a>
   *
   * The resulting value type of the container is std::pair<const _Key, _Tp>.
   *
   *  Base is _Hashtable, dispatched at compile time via template
   *  alias __umap_hashtable.
   */
  template<typename _Key, typename _Tp,
           typename _Hash = hash<_Key>,
           typename _Pred = equal_to<_Key>,
           typename _Alloc = allocator<std::pair<const _Key, _Tp>>>
    class unordered_map
    {
      typedef __umap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc>  _Hashtable;
      _Hashtable _M_h;

    public:
      // typedefs:
      //@{
      /// Public typedefs.
      typedef typename _Hashtable::key_type     key_type;
      typedef typename _Hashtable::value_type   value_type;
      typedef typename _Hashtable::mapped_type  mapped_type;
      typedef typename _Hashtable::hasher       hasher;
      typedef typename _Hashtable::key_equal    key_equal;
      typedef typename _Hashtable::allocator_type allocator_type;
      //@}

      //@{
      ///  Iterator-related typedefs.
      typedef typename _Hashtable::pointer              pointer;
      typedef typename _Hashtable::const_pointer        const_pointer;
      typedef typename _Hashtable::reference            reference;
      typedef typename _Hashtable::const_reference      const_reference;
      typedef typename _Hashtable::iterator             iterator;
      typedef typename _Hashtable::const_iterator       const_iterator;
      typedef typename _Hashtable::local_iterator       local_iterator;
      typedef typename _Hashtable::const_local_iterator const_local_iterator;
      typedef typename _Hashtable::size_type            size_type;
      typedef typename _Hashtable::difference_type      difference_type;
      //@}

#if __cplusplus > 201402L
      using node_type = typename _Hashtable::node_type;
      using insert_return_type = typename _Hashtable::insert_return_type;
#endif

      //construct/destroy/copy

      /// Default constructor.
      unordered_map() = default;

      /**
       *  @brief  Default constructor creates no elements.
       *  @param __n  Minimal initial number of buckets.
       *  @param __hf  A hash functor.
       *  @param __eql  A key equality functor.
       *  @param __a  An allocator object.
       */
      explicit
      unordered_map(size_type __n,
                    const hasher& __hf = hasher(),
                    const key_equal& __eql = key_equal(),
                    const allocator_type& __a = allocator_type())
      : _M_h(__n, __hf, __eql, __a)
      { }

      /**
       *  @brief  Builds an %unordered_map from a range.
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *  @param __n  Minimal initial number of buckets.
       *  @param __hf  A hash functor.
       *  @param __eql  A key equality functor.
       *  @param __a  An allocator object.
       *
       *  Create an %unordered_map consisting of copies of the elements from
       *  [__first,__last).  This is linear in N (where N is
       *  distance(__first,__last)).
       */
      template<typename _InputIterator>
        unordered_map(_InputIterator __first, _InputIterator __last,
                      size_type __n = 0,
                      const hasher& __hf = hasher(),
                      const key_equal& __eql = key_equal(),
                      const allocator_type& __a = allocator_type())
        : _M_h(__first, __last, __n, __hf, __eql, __a)
        { }

      /// Copy constructor.
      unordered_map(const unordered_map&) = default;

      /// Move constructor.
      unordered_map(unordered_map&&) = default;

      /**
       *  @brief Creates an %unordered_map with no elements.
       *  @param __a An allocator object.
       */
      explicit
      unordered_map(const allocator_type& __a)
        : _M_h(__a)
      { }

      /*
       *  @brief Copy constructor with allocator argument.
       * @param  __uset  Input %unordered_map to copy.
       * @param  __a  An allocator object.
       */
      unordered_map(const unordered_map& __umap,
                    const allocator_type& __a)
      : _M_h(__umap._M_h, __a)
      { }

      /*
       *  @brief  Move constructor with allocator argument.
       *  @param  __uset Input %unordered_map to move.
       *  @param  __a    An allocator object.
       */
      unordered_map(unordered_map&& __umap,
                    const allocator_type& __a)
      : _M_h(std::move(__umap._M_h), __a)
      { }

      /**
       *  @brief  Builds an %unordered_map from an initializer_list.
       *  @param  __l  An initializer_list.
       *  @param __n  Minimal initial number of buckets.
       *  @param __hf  A hash functor.
       *  @param __eql  A key equality functor.
       *  @param  __a  An allocator object.
       *
       *  Create an %unordered_map consisting of copies of the elements in the
       *  list. This is linear in N (where N is @a __l.size()).
       */
      unordered_map(initializer_list<value_type> __l,
                    size_type __n = 0,
                    const hasher& __hf = hasher(),
                    const key_equal& __eql = key_equal(),
                    const allocator_type& __a = allocator_type())
      : _M_h(__l, __n, __hf, __eql, __a)
      { }

      unordered_map(size_type __n, const allocator_type& __a)
      : unordered_map(__n, hasher(), key_equal(), __a)
      { }

      unordered_map(size_type __n, const hasher& __hf,
                    const allocator_type& __a)
      : unordered_map(__n, __hf, key_equal(), __a)
      { }

      template<typename _InputIterator>
        unordered_map(_InputIterator __first, _InputIterator __last,
                      size_type __n,
                      const allocator_type& __a)
        : unordered_map(__first, __last, __n, hasher(), key_equal(), __a)
        { }

      template<typename _InputIterator>
        unordered_map(_InputIterator __first, _InputIterator __last,
                      size_type __n, const hasher& __hf,
                      const allocator_type& __a)
          : unordered_map(__first, __last, __n, __hf, key_equal(), __a)
        { }

      unordered_map(initializer_list<value_type> __l,
                    size_type __n,
                    const allocator_type& __a)
      : unordered_map(__l, __n, hasher(), key_equal(), __a)
      { }

      unordered_map(initializer_list<value_type> __l,
                    size_type __n, const hasher& __hf,
                    const allocator_type& __a)
      : unordered_map(__l, __n, __hf, key_equal(), __a)
      { }

      /// Copy assignment operator.
      unordered_map&
      operator=(const unordered_map&) = default;

      /// Move assignment operator.
      unordered_map&
      operator=(unordered_map&&) = default;

      /**
       *  @brief  %Unordered_map list assignment operator.
       *  @param  __l  An initializer_list.
       *
       *  This function fills an %unordered_map with copies of the elements in
       *  the initializer list @a __l.
       *
       *  Note that the assignment completely changes the %unordered_map and
       *  that the resulting %unordered_map's size is the same as the number
       *  of elements assigned.
       */
      unordered_map&
      operator=(initializer_list<value_type> __l)
      {
        _M_h = __l;
        return *this;
      }

      ///  Returns the allocator object used by the %unordered_map.
      allocator_type
      get_allocator() const noexcept
      { return _M_h.get_allocator(); }

      // size and capacity:

      ///  Returns true if the %unordered_map is empty.
      _GLIBCXX_NODISCARD bool
      empty() const noexcept
      { return _M_h.empty(); }

      ///  Returns the size of the %unordered_map.
      size_type
      size() const noexcept
      { return _M_h.size(); }

      ///  Returns the maximum size of the %unordered_map.
      size_type
      max_size() const noexcept
      { return _M_h.max_size(); }

      // iterators.

      /**
       *  Returns a read/write iterator that points to the first element in the
       *  %unordered_map.
       */
      iterator
      begin() noexcept
      { return _M_h.begin(); }

      //@{
      /**
       *  Returns a read-only (constant) iterator that points to the first
       *  element in the %unordered_map.
       */
      const_iterator
      begin() const noexcept
      { return _M_h.begin(); }

      const_iterator
      cbegin() const noexcept
      { return _M_h.begin(); }
      //@}

      /**
       *  Returns a read/write iterator that points one past the last element in
       *  the %unordered_map.
       */
      iterator
      end() noexcept
      { return _M_h.end(); }

      //@{
      /**
       *  Returns a read-only (constant) iterator that points one past the last
       *  element in the %unordered_map.
       */
      const_iterator
      end() const noexcept
      { return _M_h.end(); }

      const_iterator
      cend() const noexcept
      { return _M_h.end(); }
      //@}

      // modifiers.

      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_map.
       *
       *  @param __args  Arguments used to generate a new pair instance (see
       *                std::piecewise_contruct for passing arguments to each
       *                part of the pair constructor).
       *
       *  @return  A pair, of which the first element is an iterator that points
       *           to the possibly inserted pair, and the second is a bool that
       *           is true if the pair was actually inserted.
       *
       *  This function attempts to build and insert a (key, value) %pair into
       *  the %unordered_map.
       *  An %unordered_map relies on unique keys and thus a %pair is only
       *  inserted if its first element (the key) is not already present in the
       *  %unordered_map.
       *
       *  Insertion requires amortized constant time.
       */
      template<typename... _Args>
        std::pair<iterator, bool>
        emplace(_Args&&... __args)
        { return _M_h.emplace(std::forward<_Args>(__args)...); }

      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_map.
       *
       *  @param  __pos  An iterator that serves as a hint as to where the pair
       *                should be inserted.
       *  @param  __args  Arguments used to generate a new pair instance (see
       *                 std::piecewise_contruct for passing arguments to each
       *                 part of the pair constructor).
       *  @return An iterator that points to the element with key of the
       *          std::pair built from @a __args (may or may not be that
       *          std::pair).
       *
       *  This function is not concerned about whether the insertion took place,
       *  and thus does not return a boolean like the single-argument emplace()
       *  does.
       *  Note that the first parameter is only a hint and can potentially
       *  improve the performance of the insertion process. A bad hint would
       *  cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      template<typename... _Args>
        iterator
        emplace_hint(const_iterator __pos, _Args&&... __args)
        { return _M_h.emplace_hint(__pos, std::forward<_Args>(__args)...); }

#if __cplusplus > 201402L
      /// Extract a node.
      node_type
      extract(const_iterator __pos)
      {
        __glibcxx_assert(__pos != end());
        return _M_h.extract(__pos);
      }

      /// Extract a node.
      node_type
      extract(const key_type& __key)
      { return _M_h.extract(__key); }

      /// Re-insert an extracted node.
      insert_return_type
      insert(node_type&& __nh)
      { return _M_h._M_reinsert_node(std::move(__nh)); }

      /// Re-insert an extracted node.
      iterator
      insert(const_iterator, node_type&& __nh)
      { return _M_h._M_reinsert_node(std::move(__nh)).position; }

#define __cpp_lib_unordered_map_try_emplace 201411
      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_map.
       *
       *  @param __k    Key to use for finding a possibly existing pair in
       *                the unordered_map.
       *  @param __args  Arguments used to generate the .second for a 
       *                new pair instance.
       *
       *  @return  A pair, of which the first element is an iterator that points
       *           to the possibly inserted pair, and the second is a bool that
       *           is true if the pair was actually inserted.
       *
       *  This function attempts to build and insert a (key, value) %pair into
       *  the %unordered_map.
       *  An %unordered_map relies on unique keys and thus a %pair is only
       *  inserted if its first element (the key) is not already present in the
       *  %unordered_map.
       *  If a %pair is not inserted, this function has no effect.
       *
       *  Insertion requires amortized constant time.
       */
      template <typename... _Args>
        pair<iterator, bool>
        try_emplace(const key_type& __k, _Args&&... __args)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              __i = emplace(std::piecewise_construct,
                            std::forward_as_tuple(__k),
                            std::forward_as_tuple(
                              std::forward<_Args>(__args)...))
                .first;
              return {__i, true};
            }
          return {__i, false};
        }

      // move-capable overload
      template <typename... _Args>
        pair<iterator, bool>
        try_emplace(key_type&& __k, _Args&&... __args)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              __i = emplace(std::piecewise_construct,
                            std::forward_as_tuple(std::move(__k)),
                            std::forward_as_tuple(
                              std::forward<_Args>(__args)...))
                .first;
              return {__i, true};
            }
          return {__i, false};
        }

      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_map.
       *
       *  @param  __hint  An iterator that serves as a hint as to where the pair
       *                should be inserted.
       *  @param __k    Key to use for finding a possibly existing pair in
       *                the unordered_map.
       *  @param __args  Arguments used to generate the .second for a 
       *                new pair instance.
       *  @return An iterator that points to the element with key of the
       *          std::pair built from @a __args (may or may not be that
       *          std::pair).
       *
       *  This function is not concerned about whether the insertion took place,
       *  and thus does not return a boolean like the single-argument emplace()
       *  does. However, if insertion did not take place,
       *  this function has no effect.
       *  Note that the first parameter is only a hint and can potentially
       *  improve the performance of the insertion process. A bad hint would
       *  cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      template <typename... _Args>
        iterator
        try_emplace(const_iterator __hint, const key_type& __k,
                    _Args&&... __args)
        {
          iterator __i = find(__k);
          if (__i == end())
            __i = emplace_hint(__hint, std::piecewise_construct,
                               std::forward_as_tuple(__k),
                               std::forward_as_tuple(
                                 std::forward<_Args>(__args)...));
          return __i;
        }

      // move-capable overload
      template <typename... _Args>
        iterator
        try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
        {
          iterator __i = find(__k);
          if (__i == end())
            __i = emplace_hint(__hint, std::piecewise_construct,
                               std::forward_as_tuple(std::move(__k)),
                               std::forward_as_tuple(
                                 std::forward<_Args>(__args)...));
          return __i;
        }
#endif // C++17

      //@{
      /**
       *  @brief Attempts to insert a std::pair into the %unordered_map.

       *  @param __x Pair to be inserted (see std::make_pair for easy
       *             creation of pairs).
       *
       *  @return  A pair, of which the first element is an iterator that 
       *           points to the possibly inserted pair, and the second is 
       *           a bool that is true if the pair was actually inserted.
       *
       *  This function attempts to insert a (key, value) %pair into the
       *  %unordered_map. An %unordered_map relies on unique keys and thus a
       *  %pair is only inserted if its first element (the key) is not already
       *  present in the %unordered_map.
       *
       *  Insertion requires amortized constant time.
       */
      std::pair<iterator, bool>
      insert(const value_type& __x)
      { return _M_h.insert(__x); }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2354. Unnecessary copying when inserting into maps with braced-init
      std::pair<iterator, bool>
      insert(value_type&& __x)
      { return _M_h.insert(std::move(__x)); }

      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair&&>::value,
                      pair<iterator, bool>>
        insert(_Pair&& __x)
        { return _M_h.emplace(std::forward<_Pair>(__x)); }
      //@}

      //@{
      /**
       *  @brief Attempts to insert a std::pair into the %unordered_map.
       *  @param  __hint  An iterator that serves as a hint as to where the
       *                 pair should be inserted.
       *  @param  __x  Pair to be inserted (see std::make_pair for easy creation
       *               of pairs).
       *  @return An iterator that points to the element with key of
       *           @a __x (may or may not be the %pair passed in).
       *
       *  This function is not concerned about whether the insertion took place,
       *  and thus does not return a boolean like the single-argument insert()
       *  does.  Note that the first parameter is only a hint and can
       *  potentially improve the performance of the insertion process.  A bad
       *  hint would cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      iterator
      insert(const_iterator __hint, const value_type& __x)
      { return _M_h.insert(__hint, __x); }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2354. Unnecessary copying when inserting into maps with braced-init
      iterator
      insert(const_iterator __hint, value_type&& __x)
      { return _M_h.insert(__hint, std::move(__x)); }

      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair&&>::value, iterator>
        insert(const_iterator __hint, _Pair&& __x)
        { return _M_h.emplace_hint(__hint, std::forward<_Pair>(__x)); }
      //@}

      /**
       *  @brief A template function that attempts to insert a range of
       *  elements.
       *  @param  __first  Iterator pointing to the start of the range to be
       *                   inserted.
       *  @param  __last  Iterator pointing to the end of the range.
       *
       *  Complexity similar to that of the range constructor.
       */
      template<typename _InputIterator>
        void
        insert(_InputIterator __first, _InputIterator __last)
        { _M_h.insert(__first, __last); }

      /**
       *  @brief Attempts to insert a list of elements into the %unordered_map.
       *  @param  __l  A std::initializer_list<value_type> of elements
       *               to be inserted.
       *
       *  Complexity similar to that of the range constructor.
       */
      void
      insert(initializer_list<value_type> __l)
      { _M_h.insert(__l); }


#if __cplusplus > 201402L
#define __cpp_lib_unordered_map_insertion 201411
      /**
       *  @brief Attempts to insert a std::pair into the %unordered_map.
       *  @param __k    Key to use for finding a possibly existing pair in
       *                the map.
       *  @param __obj  Argument used to generate the .second for a pair 
       *                instance.
       *
       *  @return  A pair, of which the first element is an iterator that 
       *           points to the possibly inserted pair, and the second is 
       *           a bool that is true if the pair was actually inserted.
       *
       *  This function attempts to insert a (key, value) %pair into the
       *  %unordered_map. An %unordered_map relies on unique keys and thus a
       *  %pair is only inserted if its first element (the key) is not already
       *  present in the %unordered_map.
       *  If the %pair was already in the %unordered_map, the .second of 
       *  the %pair is assigned from __obj.
       *
       *  Insertion requires amortized constant time.
       */
      template <typename _Obj>
        pair<iterator, bool>
        insert_or_assign(const key_type& __k, _Obj&& __obj)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              __i = emplace(std::piecewise_construct,
                            std::forward_as_tuple(__k),
                            std::forward_as_tuple(std::forward<_Obj>(__obj)))
                .first;
              return {__i, true};
            }
          (*__i).second = std::forward<_Obj>(__obj);
          return {__i, false};
        }

      // move-capable overload
      template <typename _Obj>
        pair<iterator, bool>
        insert_or_assign(key_type&& __k, _Obj&& __obj)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              __i = emplace(std::piecewise_construct,
                            std::forward_as_tuple(std::move(__k)),
                            std::forward_as_tuple(std::forward<_Obj>(__obj)))
                .first;
              return {__i, true};
            }
          (*__i).second = std::forward<_Obj>(__obj);
          return {__i, false};
        }

      /**
       *  @brief Attempts to insert a std::pair into the %unordered_map.
       *  @param  __hint  An iterator that serves as a hint as to where the
       *                  pair should be inserted.
       *  @param __k    Key to use for finding a possibly existing pair in
       *                the unordered_map.
       *  @param __obj  Argument used to generate the .second for a pair 
       *                instance.
       *  @return An iterator that points to the element with key of
       *           @a __x (may or may not be the %pair passed in).
       *
       *  This function is not concerned about whether the insertion took place,
       *  and thus does not return a boolean like the single-argument insert()
       *  does.         
       *  If the %pair was already in the %unordered map, the .second of
       *  the %pair is assigned from __obj.
       *  Note that the first parameter is only a hint and can
       *  potentially improve the performance of the insertion process.  A bad
       *  hint would cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      template <typename _Obj>
        iterator
        insert_or_assign(const_iterator __hint, const key_type& __k,
                         _Obj&& __obj)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              return emplace_hint(__hint, std::piecewise_construct,
                                  std::forward_as_tuple(__k),
                                  std::forward_as_tuple(
                                    std::forward<_Obj>(__obj)));
            }
          (*__i).second = std::forward<_Obj>(__obj);
          return __i;
        }

      // move-capable overload
      template <typename _Obj>
        iterator
        insert_or_assign(const_iterator __hint, key_type&& __k, _Obj&& __obj)
        {
          iterator __i = find(__k);
          if (__i == end())
            {
              return emplace_hint(__hint, std::piecewise_construct,
                                  std::forward_as_tuple(std::move(__k)),
                                  std::forward_as_tuple(
                                    std::forward<_Obj>(__obj)));
            }
          (*__i).second = std::forward<_Obj>(__obj);
          return __i;
        }
#endif

      //@{
      /**
       *  @brief Erases an element from an %unordered_map.
       *  @param  __position  An iterator pointing to the element to be erased.
       *  @return An iterator pointing to the element immediately following
       *          @a __position prior to the element being erased. If no such
       *          element exists, end() is returned.
       *
       *  This function erases an element, pointed to by the given iterator,
       *  from an %unordered_map.
       *  Note that this function only erases the element, and that if the
       *  element is itself a pointer, the pointed-to memory is not touched in
       *  any way.  Managing the pointer is the user's responsibility.
       */
      iterator
      erase(const_iterator __position)
      { return _M_h.erase(__position); }

      // LWG 2059.
      iterator
      erase(iterator __position)
      { return _M_h.erase(__position); }
      //@}

      /**
       *  @brief Erases elements according to the provided key.
       *  @param  __x  Key of element to be erased.
       *  @return  The number of elements erased.
       *
       *  This function erases all the elements located by the given key from
       *  an %unordered_map. For an %unordered_map the result of this function
       *  can only be 0 (not present) or 1 (present).
       *  Note that this function only erases the element, and that if the
       *  element is itself a pointer, the pointed-to memory is not touched in
       *  any way.  Managing the pointer is the user's responsibility.
       */
      size_type
      erase(const key_type& __x)
      { return _M_h.erase(__x); }

      /**
       *  @brief Erases a [__first,__last) range of elements from an
       *  %unordered_map.
       *  @param  __first  Iterator pointing to the start of the range to be
       *                  erased.
       *  @param __last  Iterator pointing to the end of the range to
       *                be erased.
       *  @return The iterator @a __last.
       *
       *  This function erases a sequence of elements from an %unordered_map.
       *  Note that this function only erases the elements, and that if
       *  the element is itself a pointer, the pointed-to memory is not touched
       *  in any way.  Managing the pointer is the user's responsibility.
       */
      iterator
      erase(const_iterator __first, const_iterator __last)
      { return _M_h.erase(__first, __last); }

      /**
       *  Erases all elements in an %unordered_map.
       *  Note that this function only erases the elements, and that if the
       *  elements themselves are pointers, the pointed-to memory is not touched
       *  in any way.  Managing the pointer is the user's responsibility.
       */
      void
      clear() noexcept
      { _M_h.clear(); }

      /**
       *  @brief  Swaps data with another %unordered_map.
       *  @param  __x  An %unordered_map of the same element and allocator
       *  types.
       *
       *  This exchanges the elements between two %unordered_map in constant
       *  time.
       *  Note that the global std::swap() function is specialized such that
       *  std::swap(m1,m2) will feed to this function.
       */
      void
      swap(unordered_map& __x)
      noexcept( noexcept(_M_h.swap(__x._M_h)) )
      { _M_h.swap(__x._M_h); }

#if __cplusplus > 201402L
      template<typename, typename, typename>
        friend class std::_Hash_merge_helper;

      template<typename _H2, typename _P2>
        void
        merge(unordered_map<_Key, _Tp, _H2, _P2, _Alloc>& __source)
        {
          using _Merge_helper = _Hash_merge_helper<unordered_map, _H2, _P2>;
          _M_h._M_merge_unique(_Merge_helper::_S_get_table(__source));
        }

      template<typename _H2, typename _P2>
        void
        merge(unordered_map<_Key, _Tp, _H2, _P2, _Alloc>&& __source)
        { merge(__source); }

      template<typename _H2, typename _P2>
        void
        merge(unordered_multimap<_Key, _Tp, _H2, _P2, _Alloc>& __source)
        {
          using _Merge_helper = _Hash_merge_helper<unordered_map, _H2, _P2>;
          _M_h._M_merge_unique(_Merge_helper::_S_get_table(__source));
        }

      template<typename _H2, typename _P2>
        void
        merge(unordered_multimap<_Key, _Tp, _H2, _P2, _Alloc>&& __source)
        { merge(__source); }
#endif // C++17

      // observers.

      ///  Returns the hash functor object with which the %unordered_map was
      ///  constructed.
      hasher
      hash_function() const
      { return _M_h.hash_function(); }

      ///  Returns the key comparison object with which the %unordered_map was
      ///  constructed.
      key_equal
      key_eq() const
      { return _M_h.key_eq(); }

      // lookup.

      //@{
      /**
       *  @brief Tries to locate an element in an %unordered_map.
       *  @param  __x  Key to be located.
       *  @return  Iterator pointing to sought-after element, or end() if not
       *           found.
       *
       *  This function takes a key and tries to locate the element with which
       *  the key matches.  If successful the function returns an iterator
       *  pointing to the sought after element.  If unsuccessful it returns the
       *  past-the-end ( @c end() ) iterator.
       */
      iterator
      find(const key_type& __x)
      { return _M_h.find(__x); }

      const_iterator
      find(const key_type& __x) const
      { return _M_h.find(__x); }
      //@}

      /**
       *  @brief  Finds the number of elements.
       *  @param  __x  Key to count.
       *  @return  Number of elements with specified key.
       *
       *  This function only makes sense for %unordered_multimap; for
       *  %unordered_map the result will either be 0 (not present) or 1
       *  (present).
       */
      size_type
      count(const key_type& __x) const
      { return _M_h.count(__x); }

#if __cplusplus > 201703L
      /**
       *  @brief  Finds whether an element with the given key exists.
       *  @param  __x  Key of elements to be located.
       *  @return  True if there is any element with the specified key.
       */
      bool
      contains(const key_type& __x) const
      { return _M_h.find(__x) != _M_h.end(); }
#endif

      //@{
      /**
       *  @brief Finds a subsequence matching given key.
       *  @param  __x  Key to be located.
       *  @return  Pair of iterators that possibly points to the subsequence
       *           matching given key.
       *
       *  This function probably only makes sense for %unordered_multimap.
       */
      std::pair<iterator, iterator>
      equal_range(const key_type& __x)
      { return _M_h.equal_range(__x); }

      std::pair<const_iterator, const_iterator>
      equal_range(const key_type& __x) const
      { return _M_h.equal_range(__x); }
      //@}

      //@{
      /**
       *  @brief  Subscript ( @c [] ) access to %unordered_map data.
       *  @param  __k  The key for which data should be retrieved.
       *  @return  A reference to the data of the (key,data) %pair.
       *
       *  Allows for easy lookup with the subscript ( @c [] )operator.  Returns
       *  data associated with the key specified in subscript.  If the key does
       *  not exist, a pair with that key is created using default values, which
       *  is then returned.
       *
       *  Lookup requires constant time.
       */
      mapped_type&
      operator[](const key_type& __k)
      { return _M_h[__k]; }

      mapped_type&
      operator[](key_type&& __k)
      { return _M_h[std::move(__k)]; }
      //@}

      //@{
      /**
       *  @brief  Access to %unordered_map data.
       *  @param  __k  The key for which data should be retrieved.
       *  @return  A reference to the data whose key is equal to @a __k, if
       *           such a data is present in the %unordered_map.
       *  @throw  std::out_of_range  If no such data is present.
       */
      mapped_type&
      at(const key_type& __k)
      { return _M_h.at(__k); }

      const mapped_type&
      at(const key_type& __k) const
      { return _M_h.at(__k); }
      //@}

      // bucket interface.

      /// Returns the number of buckets of the %unordered_map.
      size_type
      bucket_count() const noexcept
      { return _M_h.bucket_count(); }

      /// Returns the maximum number of buckets of the %unordered_map.
      size_type
      max_bucket_count() const noexcept
      { return _M_h.max_bucket_count(); }

      /*
       * @brief  Returns the number of elements in a given bucket.
       * @param  __n  A bucket index.
       * @return  The number of elements in the bucket.
       */
      size_type
      bucket_size(size_type __n) const
      { return _M_h.bucket_size(__n); }

      /*
       * @brief  Returns the bucket index of a given element.
       * @param  __key  A key instance.
       * @return  The key bucket index.
       */
      size_type
      bucket(const key_type& __key) const
      { return _M_h.bucket(__key); }
      
      /**
       *  @brief  Returns a read/write iterator pointing to the first bucket
       *         element.
       *  @param  __n The bucket index.
       *  @return  A read/write local iterator.
       */
      local_iterator
      begin(size_type __n)
      { return _M_h.begin(__n); }

      //@{
      /**
       *  @brief  Returns a read-only (constant) iterator pointing to the first
       *         bucket element.
       *  @param  __n The bucket index.
       *  @return  A read-only local iterator.
       */
      const_local_iterator
      begin(size_type __n) const
      { return _M_h.begin(__n); }

      const_local_iterator
      cbegin(size_type __n) const
      { return _M_h.cbegin(__n); }
      //@}

      /**
       *  @brief  Returns a read/write iterator pointing to one past the last
       *         bucket elements.
       *  @param  __n The bucket index.
       *  @return  A read/write local iterator.
       */
      local_iterator
      end(size_type __n)
      { return _M_h.end(__n); }

      //@{
      /**
       *  @brief  Returns a read-only (constant) iterator pointing to one past
       *         the last bucket elements.
       *  @param  __n The bucket index.
       *  @return  A read-only local iterator.
       */
      const_local_iterator
      end(size_type __n) const
      { return _M_h.end(__n); }

      const_local_iterator
      cend(size_type __n) const
      { return _M_h.cend(__n); }
      //@}

      // hash policy.

      /// Returns the average number of elements per bucket.
      float
      load_factor() const noexcept
      { return _M_h.load_factor(); }

      /// Returns a positive number that the %unordered_map tries to keep the
      /// load factor less than or equal to.
      float
      max_load_factor() const noexcept
      { return _M_h.max_load_factor(); }

      /**
       *  @brief  Change the %unordered_map maximum load factor.
       *  @param  __z The new maximum load factor.
       */
      void
      max_load_factor(float __z)
      { _M_h.max_load_factor(__z); }

      /**
       *  @brief  May rehash the %unordered_map.
       *  @param  __n The new number of buckets.
       *
       *  Rehash will occur only if the new number of buckets respect the
       *  %unordered_map maximum load factor.
       */
      void
      rehash(size_type __n)
      { _M_h.rehash(__n); }

      /**
       *  @brief  Prepare the %unordered_map for a specified number of
       *          elements.
       *  @param  __n Number of elements required.
       *
       *  Same as rehash(ceil(n / max_load_factor())).
       */
      void
      reserve(size_type __n)
      { _M_h.reserve(__n); }

      template<typename _Key1, typename _Tp1, typename _Hash1, typename _Pred1,
               typename _Alloc1>
        friend bool
        operator==(const unordered_map<_Key1, _Tp1, _Hash1, _Pred1, _Alloc1>&,
                   const unordered_map<_Key1, _Tp1, _Hash1, _Pred1, _Alloc1>&);
    };

#if __cpp_deduction_guides >= 201606

  template<typename _InputIterator,
           typename _Hash = hash<__iter_key_t<_InputIterator>>,
           typename _Pred = equal_to<__iter_key_t<_InputIterator>>,
           typename _Allocator = allocator<__iter_to_alloc_t<_InputIterator>>,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireNotAllocator<_Pred>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(_InputIterator, _InputIterator,
                  typename unordered_map<int, int>::size_type = {},
                  _Hash = _Hash(), _Pred = _Pred(), _Allocator = _Allocator())
    -> unordered_map<__iter_key_t<_InputIterator>,
                     __iter_val_t<_InputIterator>,
                     _Hash, _Pred, _Allocator>;

  template<typename _Key, typename _Tp, typename _Hash = hash<_Key>,
           typename _Pred = equal_to<_Key>,
           typename _Allocator = allocator<pair<const _Key, _Tp>>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireNotAllocator<_Pred>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(initializer_list<pair<_Key, _Tp>>,
                  typename unordered_map<int, int>::size_type = {},
                  _Hash = _Hash(), _Pred = _Pred(), _Allocator = _Allocator())
    -> unordered_map<_Key, _Tp, _Hash, _Pred, _Allocator>;

  template<typename _InputIterator, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(_InputIterator, _InputIterator,
                  typename unordered_map<int, int>::size_type, _Allocator)
    -> unordered_map<__iter_key_t<_InputIterator>,
                     __iter_val_t<_InputIterator>,
                     hash<__iter_key_t<_InputIterator>>,
                     equal_to<__iter_key_t<_InputIterator>>,
                     _Allocator>;

  template<typename _InputIterator, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(_InputIterator, _InputIterator, _Allocator)
    -> unordered_map<__iter_key_t<_InputIterator>,
                     __iter_val_t<_InputIterator>,
                     hash<__iter_key_t<_InputIterator>>,
                     equal_to<__iter_key_t<_InputIterator>>,
                     _Allocator>;

  template<typename _InputIterator, typename _Hash, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(_InputIterator, _InputIterator,
                  typename unordered_map<int, int>::size_type,
                  _Hash, _Allocator)
    -> unordered_map<__iter_key_t<_InputIterator>,
                     __iter_val_t<_InputIterator>, _Hash,
                     equal_to<__iter_key_t<_InputIterator>>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Allocator,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(initializer_list<pair<_Key, _Tp>>,
                  typename unordered_map<int, int>::size_type,
                  _Allocator)
    -> unordered_map<_Key, _Tp, hash<_Key>, equal_to<_Key>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Allocator,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(initializer_list<pair<_Key, _Tp>>, _Allocator)
    -> unordered_map<_Key, _Tp, hash<_Key>, equal_to<_Key>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Hash, typename _Allocator,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireAllocator<_Allocator>>
    unordered_map(initializer_list<pair<_Key, _Tp>>,
                  typename unordered_map<int, int>::size_type,
                  _Hash, _Allocator)
    -> unordered_map<_Key, _Tp, _Hash, equal_to<_Key>, _Allocator>;

#endif

  /**
   *  @brief A standard container composed of equivalent keys
   *  (possibly containing multiple of each key value) that associates
   *  values of another type with the keys.
   *
   *  @ingroup unordered_associative_containers
   *
   *  @tparam  _Key    Type of key objects.
   *  @tparam  _Tp     Type of mapped objects.
   *  @tparam  _Hash   Hashing function object type, defaults to hash<_Value>.
   *  @tparam  _Pred   Predicate function object type, defaults
   *                   to equal_to<_Value>.
   *  @tparam  _Alloc  Allocator type, defaults to
   *                   std::allocator<std::pair<const _Key, _Tp>>.
   *
   *  Meets the requirements of a <a href="tables.html#65">container</a>, and
   *  <a href="tables.html#xx">unordered associative container</a>
   *
   * The resulting value type of the container is std::pair<const _Key, _Tp>.
   *
   *  Base is _Hashtable, dispatched at compile time via template
   *  alias __ummap_hashtable.
   */
  template<typename _Key, typename _Tp,
           typename _Hash = hash<_Key>,
           typename _Pred = equal_to<_Key>,
           typename _Alloc = allocator<std::pair<const _Key, _Tp>>>
    class unordered_multimap
    {
      typedef __ummap_hashtable<_Key, _Tp, _Hash, _Pred, _Alloc>  _Hashtable;
      _Hashtable _M_h;

    public:
      // typedefs:
      //@{
      /// Public typedefs.
      typedef typename _Hashtable::key_type     key_type;
      typedef typename _Hashtable::value_type   value_type;
      typedef typename _Hashtable::mapped_type  mapped_type;
      typedef typename _Hashtable::hasher       hasher;
      typedef typename _Hashtable::key_equal    key_equal;
      typedef typename _Hashtable::allocator_type allocator_type;
      //@}

      //@{
      ///  Iterator-related typedefs.
      typedef typename _Hashtable::pointer              pointer;
      typedef typename _Hashtable::const_pointer        const_pointer;
      typedef typename _Hashtable::reference            reference;
      typedef typename _Hashtable::const_reference      const_reference;
      typedef typename _Hashtable::iterator             iterator;
      typedef typename _Hashtable::const_iterator       const_iterator;
      typedef typename _Hashtable::local_iterator       local_iterator;
      typedef typename _Hashtable::const_local_iterator const_local_iterator;
      typedef typename _Hashtable::size_type            size_type;
      typedef typename _Hashtable::difference_type      difference_type;
      //@}

#if __cplusplus > 201402L
      using node_type = typename _Hashtable::node_type;
#endif

      //construct/destroy/copy

      /// Default constructor.
      unordered_multimap() = default;

      /**
       *  @brief  Default constructor creates no elements.
       *  @param __n  Mnimal initial number of buckets.
       *  @param __hf  A hash functor.
       *  @param __eql  A key equality functor.
       *  @param __a  An allocator object.
       */
      explicit
      unordered_multimap(size_type __n,
                         const hasher& __hf = hasher(),
                         const key_equal& __eql = key_equal(),
                         const allocator_type& __a = allocator_type())
      : _M_h(__n, __hf, __eql, __a)
      { }

      /**
       *  @brief  Builds an %unordered_multimap from a range.
       *  @param  __first An input iterator.
       *  @param  __last  An input iterator.
       *  @param __n      Minimal initial number of buckets.
       *  @param __hf     A hash functor.
       *  @param __eql    A key equality functor.
       *  @param __a      An allocator object.
       *
       *  Create an %unordered_multimap consisting of copies of the elements
       *  from [__first,__last).  This is linear in N (where N is
       *  distance(__first,__last)).
       */
      template<typename _InputIterator>
        unordered_multimap(_InputIterator __first, _InputIterator __last,
                           size_type __n = 0,
                           const hasher& __hf = hasher(),
                           const key_equal& __eql = key_equal(),
                           const allocator_type& __a = allocator_type())
        : _M_h(__first, __last, __n, __hf, __eql, __a)
        { }

      /// Copy constructor.
      unordered_multimap(const unordered_multimap&) = default;

      /// Move constructor.
      unordered_multimap(unordered_multimap&&) = default;

      /**
       *  @brief Creates an %unordered_multimap with no elements.
       *  @param __a An allocator object.
       */
      explicit
      unordered_multimap(const allocator_type& __a)
      : _M_h(__a)
      { }

      /*
       *  @brief Copy constructor with allocator argument.
       * @param  __uset  Input %unordered_multimap to copy.
       * @param  __a  An allocator object.
       */
      unordered_multimap(const unordered_multimap& __ummap,
                         const allocator_type& __a)
      : _M_h(__ummap._M_h, __a)
      { }

      /*
       *  @brief  Move constructor with allocator argument.
       *  @param  __uset Input %unordered_multimap to move.
       *  @param  __a    An allocator object.
       */
      unordered_multimap(unordered_multimap&& __ummap,
                         const allocator_type& __a)
      : _M_h(std::move(__ummap._M_h), __a)
      { }

      /**
       *  @brief  Builds an %unordered_multimap from an initializer_list.
       *  @param  __l  An initializer_list.
       *  @param __n  Minimal initial number of buckets.
       *  @param __hf  A hash functor.
       *  @param __eql  A key equality functor.
       *  @param  __a  An allocator object.
       *
       *  Create an %unordered_multimap consisting of copies of the elements in
       *  the list. This is linear in N (where N is @a __l.size()).
       */
      unordered_multimap(initializer_list<value_type> __l,
                         size_type __n = 0,
                         const hasher& __hf = hasher(),
                         const key_equal& __eql = key_equal(),
                         const allocator_type& __a = allocator_type())
      : _M_h(__l, __n, __hf, __eql, __a)
      { }

      unordered_multimap(size_type __n, const allocator_type& __a)
      : unordered_multimap(__n, hasher(), key_equal(), __a)
      { }

      unordered_multimap(size_type __n, const hasher& __hf,
                         const allocator_type& __a)
      : unordered_multimap(__n, __hf, key_equal(), __a)
      { }

      template<typename _InputIterator>
        unordered_multimap(_InputIterator __first, _InputIterator __last,
                           size_type __n,
                           const allocator_type& __a)
        : unordered_multimap(__first, __last, __n, hasher(), key_equal(), __a)
        { }

      template<typename _InputIterator>
        unordered_multimap(_InputIterator __first, _InputIterator __last,
                           size_type __n, const hasher& __hf,
                           const allocator_type& __a)
        : unordered_multimap(__first, __last, __n, __hf, key_equal(), __a)
        { }

      unordered_multimap(initializer_list<value_type> __l,
                         size_type __n,
                         const allocator_type& __a)
      : unordered_multimap(__l, __n, hasher(), key_equal(), __a)
      { }

      unordered_multimap(initializer_list<value_type> __l,
                         size_type __n, const hasher& __hf,
                         const allocator_type& __a)
      : unordered_multimap(__l, __n, __hf, key_equal(), __a)
      { }

      /// Copy assignment operator.
      unordered_multimap&
      operator=(const unordered_multimap&) = default;

      /// Move assignment operator.
      unordered_multimap&
      operator=(unordered_multimap&&) = default;

      /**
       *  @brief  %Unordered_multimap list assignment operator.
       *  @param  __l  An initializer_list.
       *
       *  This function fills an %unordered_multimap with copies of the
       *  elements in the initializer list @a __l.
       *
       *  Note that the assignment completely changes the %unordered_multimap
       *  and that the resulting %unordered_multimap's size is the same as the
       *  number of elements assigned.
       */
      unordered_multimap&
      operator=(initializer_list<value_type> __l)
      {
        _M_h = __l;
        return *this;
      }

      ///  Returns the allocator object used by the %unordered_multimap.
      allocator_type
      get_allocator() const noexcept
      { return _M_h.get_allocator(); }

      // size and capacity:

      ///  Returns true if the %unordered_multimap is empty.
      _GLIBCXX_NODISCARD bool
      empty() const noexcept
      { return _M_h.empty(); }

      ///  Returns the size of the %unordered_multimap.
      size_type
      size() const noexcept
      { return _M_h.size(); }

      ///  Returns the maximum size of the %unordered_multimap.
      size_type
      max_size() const noexcept
      { return _M_h.max_size(); }

      // iterators.

      /**
       *  Returns a read/write iterator that points to the first element in the
       *  %unordered_multimap.
       */
      iterator
      begin() noexcept
      { return _M_h.begin(); }

      //@{
      /**
       *  Returns a read-only (constant) iterator that points to the first
       *  element in the %unordered_multimap.
       */
      const_iterator
      begin() const noexcept
      { return _M_h.begin(); }

      const_iterator
      cbegin() const noexcept
      { return _M_h.begin(); }
      //@}

      /**
       *  Returns a read/write iterator that points one past the last element in
       *  the %unordered_multimap.
       */
      iterator
      end() noexcept
      { return _M_h.end(); }

      //@{
      /**
       *  Returns a read-only (constant) iterator that points one past the last
       *  element in the %unordered_multimap.
       */
      const_iterator
      end() const noexcept
      { return _M_h.end(); }

      const_iterator
      cend() const noexcept
      { return _M_h.end(); }
      //@}

      // modifiers.

      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_multimap.
       *
       *  @param __args  Arguments used to generate a new pair instance (see
       *                std::piecewise_contruct for passing arguments to each
       *                part of the pair constructor).
       *
       *  @return  An iterator that points to the inserted pair.
       *
       *  This function attempts to build and insert a (key, value) %pair into
       *  the %unordered_multimap.
       *
       *  Insertion requires amortized constant time.
       */
      template<typename... _Args>
        iterator
        emplace(_Args&&... __args)
        { return _M_h.emplace(std::forward<_Args>(__args)...); }

      /**
       *  @brief Attempts to build and insert a std::pair into the
       *  %unordered_multimap.
       *
       *  @param  __pos  An iterator that serves as a hint as to where the pair
       *                should be inserted.
       *  @param  __args  Arguments used to generate a new pair instance (see
       *                 std::piecewise_contruct for passing arguments to each
       *                 part of the pair constructor).
       *  @return An iterator that points to the element with key of the
       *          std::pair built from @a __args.
       *
       *  Note that the first parameter is only a hint and can potentially
       *  improve the performance of the insertion process. A bad hint would
       *  cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      template<typename... _Args>
        iterator
        emplace_hint(const_iterator __pos, _Args&&... __args)
        { return _M_h.emplace_hint(__pos, std::forward<_Args>(__args)...); }

      //@{
      /**
       *  @brief Inserts a std::pair into the %unordered_multimap.
       *  @param __x Pair to be inserted (see std::make_pair for easy
       *             creation of pairs).
       *
       *  @return  An iterator that points to the inserted pair.
       *
       *  Insertion requires amortized constant time.
       */
      iterator
      insert(const value_type& __x)
      { return _M_h.insert(__x); }

      iterator
      insert(value_type&& __x)
      { return _M_h.insert(std::move(__x)); }

      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair&&>::value, iterator>
        insert(_Pair&& __x)
        { return _M_h.emplace(std::forward<_Pair>(__x)); }
      //@}

      //@{
      /**
       *  @brief Inserts a std::pair into the %unordered_multimap.
       *  @param  __hint  An iterator that serves as a hint as to where the
       *                 pair should be inserted.
       *  @param  __x  Pair to be inserted (see std::make_pair for easy creation
       *               of pairs).
       *  @return An iterator that points to the element with key of
       *           @a __x (may or may not be the %pair passed in).
       *
       *  Note that the first parameter is only a hint and can potentially
       *  improve the performance of the insertion process.  A bad hint would
       *  cause no gains in efficiency.
       *
       *  See
       *  https://gcc.gnu.org/onlinedocs/libstdc++/manual/associative.html#containers.associative.insert_hints
       *  for more on @a hinting.
       *
       *  Insertion requires amortized constant time.
       */
      iterator
      insert(const_iterator __hint, const value_type& __x)
      { return _M_h.insert(__hint, __x); }

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2354. Unnecessary copying when inserting into maps with braced-init
      iterator
      insert(const_iterator __hint, value_type&& __x)
      { return _M_h.insert(__hint, std::move(__x)); }

      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair&&>::value, iterator>
        insert(const_iterator __hint, _Pair&& __x)
        { return _M_h.emplace_hint(__hint, std::forward<_Pair>(__x)); }
      //@}

      /**
       *  @brief A template function that attempts to insert a range of
       *  elements.
       *  @param  __first  Iterator pointing to the start of the range to be
       *                   inserted.
       *  @param  __last  Iterator pointing to the end of the range.
       *
       *  Complexity similar to that of the range constructor.
       */
      template<typename _InputIterator>
        void
        insert(_InputIterator __first, _InputIterator __last)
        { _M_h.insert(__first, __last); }

      /**
       *  @brief Attempts to insert a list of elements into the
       *  %unordered_multimap.
       *  @param  __l  A std::initializer_list<value_type> of elements
       *               to be inserted.
       *
       *  Complexity similar to that of the range constructor.
       */
      void
      insert(initializer_list<value_type> __l)
      { _M_h.insert(__l); }

#if __cplusplus > 201402L
      /// Extract a node.
      node_type
      extract(const_iterator __pos)
      {
        __glibcxx_assert(__pos != end());
        return _M_h.extract(__pos);
      }

      /// Extract a node.
      node_type
      extract(const key_type& __key)
      { return _M_h.extract(__key); }

      /// Re-insert an extracted node.
      iterator
      insert(node_type&& __nh)
      { return _M_h._M_reinsert_node_multi(cend(), std::move(__nh)); }

      /// Re-insert an extracted node.
      iterator
      insert(const_iterator __hint, node_type&& __nh)
      { return _M_h._M_reinsert_node_multi(__hint, std::move(__nh)); }
#endif // C++17

      //@{
      /**
       *  @brief Erases an element from an %unordered_multimap.
       *  @param  __position  An iterator pointing to the element to be erased.
       *  @return An iterator pointing to the element immediately following
       *          @a __position prior to the element being erased. If no such
       *          element exists, end() is returned.
       *
       *  This function erases an element, pointed to by the given iterator,
       *  from an %unordered_multimap.
       *  Note that this function only erases the element, and that if the
       *  element is itself a pointer, the pointed-to memory is not touched in
       *  any way.  Managing the pointer is the user's responsibility.
       */
      iterator
      erase(const_iterator __position)
      { return _M_h.erase(__position); }

      // LWG 2059.
      iterator
      erase(iterator __position)
      { return _M_h.erase(__position); }
      //@}

      /**
       *  @brief Erases elements according to the provided key.
       *  @param  __x  Key of elements to be erased.
       *  @return  The number of elements erased.
       *
       *  This function erases all the elements located by the given key from
       *  an %unordered_multimap.
       *  Note that this function only erases the element, and that if the
       *  element is itself a pointer, the pointed-to memory is not touched in
       *  any way.  Managing the pointer is the user's responsibility.
       */
      size_type
      erase(const key_type& __x)
      { return _M_h.erase(__x); }

      /**
       *  @brief Erases a [__first,__last) range of elements from an
       *  %unordered_multimap.
       *  @param  __first  Iterator pointing to the start of the range to be
       *                  erased.
       *  @param __last  Iterator pointing to the end of the range to
       *                be erased.
       *  @return The iterator @a __last.
       *
       *  This function erases a sequence of elements from an
       *  %unordered_multimap.
       *  Note that this function only erases the elements, and that if
       *  the element is itself a pointer, the pointed-to memory is not touched
       *  in any way.  Managing the pointer is the user's responsibility.
       */
      iterator
      erase(const_iterator __first, const_iterator __last)
      { return _M_h.erase(__first, __last); }

      /**
       *  Erases all elements in an %unordered_multimap.
       *  Note that this function only erases the elements, and that if the
       *  elements themselves are pointers, the pointed-to memory is not touched
       *  in any way.  Managing the pointer is the user's responsibility.
       */
      void
      clear() noexcept
      { _M_h.clear(); }

      /**
       *  @brief  Swaps data with another %unordered_multimap.
       *  @param  __x  An %unordered_multimap of the same element and allocator
       *  types.
       *
       *  This exchanges the elements between two %unordered_multimap in
       *  constant time.
       *  Note that the global std::swap() function is specialized such that
       *  std::swap(m1,m2) will feed to this function.
       */
      void
      swap(unordered_multimap& __x)
      noexcept( noexcept(_M_h.swap(__x._M_h)) )
      { _M_h.swap(__x._M_h); }

#if __cplusplus > 201402L
      template<typename, typename, typename>
        friend class std::_Hash_merge_helper;

      template<typename _H2, typename _P2>
        void
        merge(unordered_multimap<_Key, _Tp, _H2, _P2, _Alloc>& __source)
        {
          using _Merge_helper
            = _Hash_merge_helper<unordered_multimap, _H2, _P2>;
          _M_h._M_merge_multi(_Merge_helper::_S_get_table(__source));
        }

      template<typename _H2, typename _P2>
        void
        merge(unordered_multimap<_Key, _Tp, _H2, _P2, _Alloc>&& __source)
        { merge(__source); }

      template<typename _H2, typename _P2>
        void
        merge(unordered_map<_Key, _Tp, _H2, _P2, _Alloc>& __source)
        {
          using _Merge_helper
            = _Hash_merge_helper<unordered_multimap, _H2, _P2>;
          _M_h._M_merge_multi(_Merge_helper::_S_get_table(__source));
        }

      template<typename _H2, typename _P2>
        void
        merge(unordered_map<_Key, _Tp, _H2, _P2, _Alloc>&& __source)
        { merge(__source); }
#endif // C++17

      // observers.

      ///  Returns the hash functor object with which the %unordered_multimap
      ///  was constructed.
      hasher
      hash_function() const
      { return _M_h.hash_function(); }

      ///  Returns the key comparison object with which the %unordered_multimap
      ///  was constructed.
      key_equal
      key_eq() const
      { return _M_h.key_eq(); }

      // lookup.

      //@{
      /**
       *  @brief Tries to locate an element in an %unordered_multimap.
       *  @param  __x  Key to be located.
       *  @return  Iterator pointing to sought-after element, or end() if not
       *           found.
       *
       *  This function takes a key and tries to locate the element with which
       *  the key matches.  If successful the function returns an iterator
       *  pointing to the sought after element.  If unsuccessful it returns the
       *  past-the-end ( @c end() ) iterator.
       */
      iterator
      find(const key_type& __x)
      { return _M_h.find(__x); }

      const_iterator
      find(const key_type& __x) const
      { return _M_h.find(__x); }
      //@}

      /**
       *  @brief  Finds the number of elements.
       *  @param  __x  Key to count.
       *  @return  Number of elements with specified key.
       */
      size_type
      count(const key_type& __x) const
      { return _M_h.count(__x); }

#if __cplusplus > 201703L
      /**
       *  @brief  Finds whether an element with the given key exists.
       *  @param  __x  Key of elements to be located.
       *  @return  True if there is any element with the specified key.
       */
      bool
      contains(const key_type& __x) const
      { return _M_h.find(__x) != _M_h.end(); }
#endif

      //@{
      /**
       *  @brief Finds a subsequence matching given key.
       *  @param  __x  Key to be located.
       *  @return  Pair of iterators that possibly points to the subsequence
       *           matching given key.
       */
      std::pair<iterator, iterator>
      equal_range(const key_type& __x)
      { return _M_h.equal_range(__x); }

      std::pair<const_iterator, const_iterator>
      equal_range(const key_type& __x) const
      { return _M_h.equal_range(__x); }
      //@}

      // bucket interface.

      /// Returns the number of buckets of the %unordered_multimap.
      size_type
      bucket_count() const noexcept
      { return _M_h.bucket_count(); }

      /// Returns the maximum number of buckets of the %unordered_multimap.
      size_type
      max_bucket_count() const noexcept
      { return _M_h.max_bucket_count(); }

      /*
       * @brief  Returns the number of elements in a given bucket.
       * @param  __n  A bucket index.
       * @return  The number of elements in the bucket.
       */
      size_type
      bucket_size(size_type __n) const
      { return _M_h.bucket_size(__n); }

      /*
       * @brief  Returns the bucket index of a given element.
       * @param  __key  A key instance.
       * @return  The key bucket index.
       */
      size_type
      bucket(const key_type& __key) const
      { return _M_h.bucket(__key); }
      
      /**
       *  @brief  Returns a read/write iterator pointing to the first bucket
       *         element.
       *  @param  __n The bucket index.
       *  @return  A read/write local iterator.
       */
      local_iterator
      begin(size_type __n)
      { return _M_h.begin(__n); }

      //@{
      /**
       *  @brief  Returns a read-only (constant) iterator pointing to the first
       *         bucket element.
       *  @param  __n The bucket index.
       *  @return  A read-only local iterator.
       */
      const_local_iterator
      begin(size_type __n) const
      { return _M_h.begin(__n); }

      const_local_iterator
      cbegin(size_type __n) const
      { return _M_h.cbegin(__n); }
      //@}

      /**
       *  @brief  Returns a read/write iterator pointing to one past the last
       *         bucket elements.
       *  @param  __n The bucket index.
       *  @return  A read/write local iterator.
       */
      local_iterator
      end(size_type __n)
      { return _M_h.end(__n); }

      //@{
      /**
       *  @brief  Returns a read-only (constant) iterator pointing to one past
       *         the last bucket elements.
       *  @param  __n The bucket index.
       *  @return  A read-only local iterator.
       */
      const_local_iterator
      end(size_type __n) const
      { return _M_h.end(__n); }

      const_local_iterator
      cend(size_type __n) const
      { return _M_h.cend(__n); }
      //@}

      // hash policy.

      /// Returns the average number of elements per bucket.
      float
      load_factor() const noexcept
      { return _M_h.load_factor(); }

      /// Returns a positive number that the %unordered_multimap tries to keep
      /// the load factor less than or equal to.
      float
      max_load_factor() const noexcept
      { return _M_h.max_load_factor(); }

      /**
       *  @brief  Change the %unordered_multimap maximum load factor.
       *  @param  __z The new maximum load factor.
       */
      void
      max_load_factor(float __z)
      { _M_h.max_load_factor(__z); }

      /**
       *  @brief  May rehash the %unordered_multimap.
       *  @param  __n The new number of buckets.
       *
       *  Rehash will occur only if the new number of buckets respect the
       *  %unordered_multimap maximum load factor.
       */
      void
      rehash(size_type __n)
      { _M_h.rehash(__n); }

      /**
       *  @brief  Prepare the %unordered_multimap for a specified number of
       *          elements.
       *  @param  __n Number of elements required.
       *
       *  Same as rehash(ceil(n / max_load_factor())).
       */
      void
      reserve(size_type __n)
      { _M_h.reserve(__n); }

      template<typename _Key1, typename _Tp1, typename _Hash1, typename _Pred1,
               typename _Alloc1>
        friend bool
        operator==(const unordered_multimap<_Key1, _Tp1,
                                            _Hash1, _Pred1, _Alloc1>&,
                   const unordered_multimap<_Key1, _Tp1,
                                            _Hash1, _Pred1, _Alloc1>&);
    };

#if __cpp_deduction_guides >= 201606

  template<typename _InputIterator,
           typename _Hash = hash<__iter_key_t<_InputIterator>>,
           typename _Pred = equal_to<__iter_key_t<_InputIterator>>,
           typename _Allocator = allocator<__iter_to_alloc_t<_InputIterator>>,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireNotAllocator<_Pred>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(_InputIterator, _InputIterator,
                       unordered_multimap<int, int>::size_type = {},
                       _Hash = _Hash(), _Pred = _Pred(),
                       _Allocator = _Allocator())
    -> unordered_multimap<__iter_key_t<_InputIterator>,
                          __iter_val_t<_InputIterator>, _Hash, _Pred,
                          _Allocator>;

  template<typename _Key, typename _Tp, typename _Hash = hash<_Key>,
           typename _Pred = equal_to<_Key>,
           typename _Allocator = allocator<pair<const _Key, _Tp>>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireNotAllocator<_Pred>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(initializer_list<pair<_Key, _Tp>>,
                       unordered_multimap<int, int>::size_type = {},
                       _Hash = _Hash(), _Pred = _Pred(),
                       _Allocator = _Allocator())
    -> unordered_multimap<_Key, _Tp, _Hash, _Pred, _Allocator>;

  template<typename _InputIterator, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(_InputIterator, _InputIterator,
                       unordered_multimap<int, int>::size_type, _Allocator)
    -> unordered_multimap<__iter_key_t<_InputIterator>,
                          __iter_val_t<_InputIterator>,
                          hash<__iter_key_t<_InputIterator>>,
                          equal_to<__iter_key_t<_InputIterator>>, _Allocator>;

  template<typename _InputIterator, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(_InputIterator, _InputIterator, _Allocator)
    -> unordered_multimap<__iter_key_t<_InputIterator>,
                          __iter_val_t<_InputIterator>,
                          hash<__iter_key_t<_InputIterator>>,
                          equal_to<__iter_key_t<_InputIterator>>, _Allocator>;

  template<typename _InputIterator, typename _Hash, typename _Allocator,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(_InputIterator, _InputIterator,
                       unordered_multimap<int, int>::size_type, _Hash,
                       _Allocator)
    -> unordered_multimap<__iter_key_t<_InputIterator>,
                          __iter_val_t<_InputIterator>, _Hash,
                          equal_to<__iter_key_t<_InputIterator>>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Allocator,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(initializer_list<pair<_Key, _Tp>>,
                       unordered_multimap<int, int>::size_type,
                       _Allocator)
    -> unordered_multimap<_Key, _Tp, hash<_Key>, equal_to<_Key>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Allocator,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(initializer_list<pair<_Key, _Tp>>, _Allocator)
    -> unordered_multimap<_Key, _Tp, hash<_Key>, equal_to<_Key>, _Allocator>;

  template<typename _Key, typename _Tp, typename _Hash, typename _Allocator,
           typename = _RequireNotAllocatorOrIntegral<_Hash>,
           typename = _RequireAllocator<_Allocator>>
    unordered_multimap(initializer_list<pair<_Key, _Tp>>,
                       unordered_multimap<int, int>::size_type,
                       _Hash, _Allocator)
    -> unordered_multimap<_Key, _Tp, _Hash, equal_to<_Key>, _Allocator>;

#endif

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline void
    swap(unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
         unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    noexcept(noexcept(__x.swap(__y)))
    { __x.swap(__y); }

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline void
    swap(unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
         unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    noexcept(noexcept(__x.swap(__y)))
    { __x.swap(__y); }

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline bool
    operator==(const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
               const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    { return __x._M_h._M_equal(__y._M_h); }

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline bool
    operator!=(const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
               const unordered_map<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    { return !(__x == __y); }

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline bool
    operator==(const unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
               const unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    { return __x._M_h._M_equal(__y._M_h); }

  template<class _Key, class _Tp, class _Hash, class _Pred, class _Alloc>
    inline bool
    operator!=(const unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __x,
               const unordered_multimap<_Key, _Tp, _Hash, _Pred, _Alloc>& __y)
    { return !(__x == __y); }

_GLIBCXX_END_NAMESPACE_CONTAINER

#if __cplusplus > 201402L
  // Allow std::unordered_map access to internals of compatible maps.
  template<typename _Key, typename _Val, typename _Hash1, typename _Eq1,
           typename _Alloc, typename _Hash2, typename _Eq2>
    struct _Hash_merge_helper<
      _GLIBCXX_STD_C::unordered_map<_Key, _Val, _Hash1, _Eq1, _Alloc>,
      _Hash2, _Eq2>
    {
    private:
      template<typename... _Tp>
        using unordered_map = _GLIBCXX_STD_C::unordered_map<_Tp...>;
      template<typename... _Tp>
        using unordered_multimap = _GLIBCXX_STD_C::unordered_multimap<_Tp...>;

      friend unordered_map<_Key, _Val, _Hash1, _Eq1, _Alloc>;

      static auto&
      _S_get_table(unordered_map<_Key, _Val, _Hash2, _Eq2, _Alloc>& __map)
      { return __map._M_h; }

      static auto&
      _S_get_table(unordered_multimap<_Key, _Val, _Hash2, _Eq2, _Alloc>& __map)
      { return __map._M_h; }
    };

  // Allow std::unordered_multimap access to internals of compatible maps.
  template<typename _Key, typename _Val, typename _Hash1, typename _Eq1,
           typename _Alloc, typename _Hash2, typename _Eq2>
    struct _Hash_merge_helper<
      _GLIBCXX_STD_C::unordered_multimap<_Key, _Val, _Hash1, _Eq1, _Alloc>,
      _Hash2, _Eq2>
    {
    private:
      template<typename... _Tp>
        using unordered_map = _GLIBCXX_STD_C::unordered_map<_Tp...>;
      template<typename... _Tp>
        using unordered_multimap = _GLIBCXX_STD_C::unordered_multimap<_Tp...>;

      friend unordered_multimap<_Key, _Val, _Hash1, _Eq1, _Alloc>;

      static auto&
      _S_get_table(unordered_map<_Key, _Val, _Hash2, _Eq2, _Alloc>& __map)
      { return __map._M_h; }

      static auto&
      _S_get_table(unordered_multimap<_Key, _Val, _Hash2, _Eq2, _Alloc>& __map)
      { return __map._M_h; }
    };
#endif // C++17

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif /* _UNORDERED_MAP_H */