stl_map.h

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// Map implementation -*- C++ -*-
 
// Copyright (C) 2001-2020 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/>.
 
/*
 *
 * Copyright (c) 1994
 * Hewlett-Packard Company
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Hewlett-Packard Company makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 *
 *
 * Copyright (c) 1996,1997
 * Silicon Graphics Computer Systems, Inc.
 *
 * Permission to use, copy, modify, distribute and sell this software
 * and its documentation for any purpose is hereby granted without fee,
 * provided that the above copyright notice appear in all copies and
 * that both that copyright notice and this permission notice appear
 * in supporting documentation.  Silicon Graphics makes no
 * representations about the suitability of this software for any
 * purpose.  It is provided "as is" without express or implied warranty.
 */
 
/** @file bits/stl_map.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{map}
 */
 
#ifndef _STL_MAP_H
#define _STL_MAP_H 1
 
#include <bits/functexcept.h>
#include <bits/concept_check.h>
#if __cplusplus >= 201103L
#include <initializer_list>
#include <tuple>
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
_GLIBCXX_BEGIN_NAMESPACE_CONTAINER
 
  template <typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    class multimap;
 
  /**
   *  @brief A standard container made up of (key,value) pairs, which can be
   *  retrieved based on a key, in logarithmic time.
   *
   *  @ingroup associative_containers
   *
   *  @tparam _Key  Type of key objects.
   *  @tparam  _Tp  Type of mapped objects.
   *  @tparam _Compare  Comparison function object type, defaults to less<_Key>.
   *  @tparam _Alloc  Allocator type, defaults to
   *                  allocator<pair<const _Key, _Tp>.
   *
   *  Meets the requirements of a <a href="tables.html#65">container</a>, a
   *  <a href="tables.html#66">reversible container</a>, and an
   *  <a href="tables.html#69">associative container</a> (using unique keys).
   *  For a @c map<Key,T> the key_type is Key, the mapped_type is T, and the
   *  value_type is std::pair<const Key,T>.
   *
   *  Maps support bidirectional iterators.
   *
   *  The private tree data is declared exactly the same way for map and
   *  multimap; the distinction is made entirely in how the tree functions are
   *  called (*_unique versus *_equal, same as the standard).
  */
  template <typename _Key, typename _Tp, typename _Compare = std::less<_Key>,
            typename _Alloc = std::allocator<std::pair<const _Key, _Tp> > >
    class map
    {
    public:
      typedef _Key                                      key_type;
      typedef _Tp                                       mapped_type;
      typedef std::pair<const _Key, _Tp>                value_type;
      typedef _Compare                                  key_compare;
      typedef _Alloc                                    allocator_type;
 
    private:
#ifdef _GLIBCXX_CONCEPT_CHECKS
      // concept requirements
      typedef typename _Alloc::value_type               _Alloc_value_type;
# if __cplusplus < 201103L
      __glibcxx_class_requires(_Tp, _SGIAssignableConcept)
# endif
      __glibcxx_class_requires4(_Compare, bool, _Key, _Key,
                                _BinaryFunctionConcept)
      __glibcxx_class_requires2(value_type, _Alloc_value_type, _SameTypeConcept)
#endif
 
#if __cplusplus >= 201103L
#if __cplusplus > 201703L || defined __STRICT_ANSI__
      static_assert(is_same<typename _Alloc::value_type, value_type>::value,
          "std::map must have the same value_type as its allocator");
#endif
#endif
 
    public:
      class value_compare
      : public std::binary_function<value_type, value_type, bool>
      {
        friend class map<_Key, _Tp, _Compare, _Alloc>;
      protected:
        _Compare comp;
 
        value_compare(_Compare __c)
        : comp(__c) { }
 
      public:
        bool operator()(const value_type& __x, const value_type& __y) const
        { return comp(__x.first, __y.first); }
      };
 
    private:
      /// This turns a red-black tree into a [multi]map.
      typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
        rebind<value_type>::other _Pair_alloc_type;
 
      typedef _Rb_tree<key_type, value_type, _Select1st<value_type>,
                       key_compare, _Pair_alloc_type> _Rep_type;
 
      /// The actual tree structure.
      _Rep_type _M_t;
 
      typedef __gnu_cxx::__alloc_traits<_Pair_alloc_type> _Alloc_traits;
 
    public:
      // many of these are specified differently in ISO, but the following are
      // "functionally equivalent"
      typedef typename _Alloc_traits::pointer            pointer;
      typedef typename _Alloc_traits::const_pointer      const_pointer;
      typedef typename _Alloc_traits::reference          reference;
      typedef typename _Alloc_traits::const_reference    const_reference;
      typedef typename _Rep_type::iterator               iterator;
      typedef typename _Rep_type::const_iterator         const_iterator;
      typedef typename _Rep_type::size_type              size_type;
      typedef typename _Rep_type::difference_type        difference_type;
      typedef typename _Rep_type::reverse_iterator       reverse_iterator;
      typedef typename _Rep_type::const_reverse_iterator const_reverse_iterator;
 
#if __cplusplus > 201402L
      using node_type = typename _Rep_type::node_type;
      using insert_return_type = typename _Rep_type::insert_return_type;
#endif
 
      // [23.3.1.1] construct/copy/destroy
      // (get_allocator() is also listed in this section)
 
      /**
       *  @brief  Default constructor creates no elements.
       */
#if __cplusplus < 201103L
      map() : _M_t() { }
#else
      map() = default;
#endif
 
      /**
       *  @brief  Creates a %map with no elements.
       *  @param  __comp  A comparison object.
       *  @param  __a  An allocator object.
       */
      explicit
      map(const _Compare& __comp,
          const allocator_type& __a = allocator_type())
      : _M_t(__comp, _Pair_alloc_type(__a)) { }
 
      /**
       *  @brief  %Map copy constructor.
       *
       *  Whether the allocator is copied depends on the allocator traits.
       */
#if __cplusplus < 201103L
      map(const map& __x)
      : _M_t(__x._M_t) { }
#else
      map(const map&) = default;
 
      /**
       *  @brief  %Map move constructor.
       *
       *  The newly-created %map contains the exact contents of the moved
       *  instance. The moved instance is a valid, but unspecified, %map.
       */
      map(map&&) = default;
 
      /**
       *  @brief  Builds a %map from an initializer_list.
       *  @param  __l  An initializer_list.
       *  @param  __comp  A comparison object.
       *  @param  __a  An allocator object.
       *
       *  Create a %map consisting of copies of the elements in the
       *  initializer_list @a __l.
       *  This is linear in N if the range is already sorted, and NlogN
       *  otherwise (where N is @a __l.size()).
       */
      map(initializer_list<value_type> __l,
          const _Compare& __comp = _Compare(),
          const allocator_type& __a = allocator_type())
      : _M_t(__comp, _Pair_alloc_type(__a))
      { _M_t._M_insert_range_unique(__l.begin(), __l.end()); }
 
      /// Allocator-extended default constructor.
      explicit
      map(const allocator_type& __a)
      : _M_t(_Pair_alloc_type(__a)) { }
 
      /// Allocator-extended copy constructor.
      map(const map& __m, const allocator_type& __a)
      : _M_t(__m._M_t, _Pair_alloc_type(__a)) { }
 
      /// Allocator-extended move constructor.
      map(map&& __m, const allocator_type& __a)
      noexcept(is_nothrow_copy_constructible<_Compare>::value
               && _Alloc_traits::_S_always_equal())
      : _M_t(std::move(__m._M_t), _Pair_alloc_type(__a)) { }
 
      /// Allocator-extended initialier-list constructor.
      map(initializer_list<value_type> __l, const allocator_type& __a)
      : _M_t(_Pair_alloc_type(__a))
      { _M_t._M_insert_range_unique(__l.begin(), __l.end()); }
 
      /// Allocator-extended range constructor.
      template<typename _InputIterator>
        map(_InputIterator __first, _InputIterator __last,
            const allocator_type& __a)
        : _M_t(_Pair_alloc_type(__a))
        { _M_t._M_insert_range_unique(__first, __last); }
#endif
 
      /**
       *  @brief  Builds a %map from a range.
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *
       *  Create a %map consisting of copies of the elements from
       *  [__first,__last).  This is linear in N if the range is
       *  already sorted, and NlogN otherwise (where N is
       *  distance(__first,__last)).
       */
      template<typename _InputIterator>
        map(_InputIterator __first, _InputIterator __last)
        : _M_t()
        { _M_t._M_insert_range_unique(__first, __last); }
 
      /**
       *  @brief  Builds a %map from a range.
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *  @param  __comp  A comparison functor.
       *  @param  __a  An allocator object.
       *
       *  Create a %map consisting of copies of the elements from
       *  [__first,__last).  This is linear in N if the range is
       *  already sorted, and NlogN otherwise (where N is
       *  distance(__first,__last)).
       */
      template<typename _InputIterator>
        map(_InputIterator __first, _InputIterator __last,
            const _Compare& __comp,
            const allocator_type& __a = allocator_type())
        : _M_t(__comp, _Pair_alloc_type(__a))
        { _M_t._M_insert_range_unique(__first, __last); }
 
#if __cplusplus >= 201103L
      /**
       *  The dtor only erases the elements, and note 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.
       */
      ~map() = default;
#endif
 
      /**
       *  @brief  %Map assignment operator.
       *
       *  Whether the allocator is copied depends on the allocator traits.
       */
#if __cplusplus < 201103L
      map&
      operator=(const map& __x)
      {
        _M_t = __x._M_t;
        return *this;
      }
#else
      map&
      operator=(const map&) = default;
 
      /// Move assignment operator.
      map&
      operator=(map&&) = default;
 
      /**
       *  @brief  %Map list assignment operator.
       *  @param  __l  An initializer_list.
       *
       *  This function fills a %map with copies of the elements in the
       *  initializer list @a __l.
       *
       *  Note that the assignment completely changes the %map and
       *  that the resulting %map's size is the same as the number
       *  of elements assigned.
       */
      map&
      operator=(initializer_list<value_type> __l)
      {
        _M_t._M_assign_unique(__l.begin(), __l.end());
        return *this;
      }
#endif
 
      /// Get a copy of the memory allocation object.
      allocator_type
      get_allocator() const _GLIBCXX_NOEXCEPT
      { return allocator_type(_M_t.get_allocator()); }
 
      // iterators
      /**
       *  Returns a read/write iterator that points to the first pair in the
       *  %map.
       *  Iteration is done in ascending order according to the keys.
       */
      iterator
      begin() _GLIBCXX_NOEXCEPT
      { return _M_t.begin(); }
 
      /**
       *  Returns a read-only (constant) iterator that points to the first pair
       *  in the %map.  Iteration is done in ascending order according to the
       *  keys.
       */
      const_iterator
      begin() const _GLIBCXX_NOEXCEPT
      { return _M_t.begin(); }
 
      /**
       *  Returns a read/write iterator that points one past the last
       *  pair in the %map.  Iteration is done in ascending order
       *  according to the keys.
       */
      iterator
      end() _GLIBCXX_NOEXCEPT
      { return _M_t.end(); }
 
      /**
       *  Returns a read-only (constant) iterator that points one past the last
       *  pair in the %map.  Iteration is done in ascending order according to
       *  the keys.
       */
      const_iterator
      end() const _GLIBCXX_NOEXCEPT
      { return _M_t.end(); }
 
      /**
       *  Returns a read/write reverse iterator that points to the last pair in
       *  the %map.  Iteration is done in descending order according to the
       *  keys.
       */
      reverse_iterator
      rbegin() _GLIBCXX_NOEXCEPT
      { return _M_t.rbegin(); }
 
      /**
       *  Returns a read-only (constant) reverse iterator that points to the
       *  last pair in the %map.  Iteration is done in descending order
       *  according to the keys.
       */
      const_reverse_iterator
      rbegin() const _GLIBCXX_NOEXCEPT
      { return _M_t.rbegin(); }
 
      /**
       *  Returns a read/write reverse iterator that points to one before the
       *  first pair in the %map.  Iteration is done in descending order
       *  according to the keys.
       */
      reverse_iterator
      rend() _GLIBCXX_NOEXCEPT
      { return _M_t.rend(); }
 
      /**
       *  Returns a read-only (constant) reverse iterator that points to one
       *  before the first pair in the %map.  Iteration is done in descending
       *  order according to the keys.
       */
      const_reverse_iterator
      rend() const _GLIBCXX_NOEXCEPT
      { return _M_t.rend(); }
 
#if __cplusplus >= 201103L
      /**
       *  Returns a read-only (constant) iterator that points to the first pair
       *  in the %map.  Iteration is done in ascending order according to the
       *  keys.
       */
      const_iterator
      cbegin() const noexcept
      { return _M_t.begin(); }
 
      /**
       *  Returns a read-only (constant) iterator that points one past the last
       *  pair in the %map.  Iteration is done in ascending order according to
       *  the keys.
       */
      const_iterator
      cend() const noexcept
      { return _M_t.end(); }
 
      /**
       *  Returns a read-only (constant) reverse iterator that points to the
       *  last pair in the %map.  Iteration is done in descending order
       *  according to the keys.
       */
      const_reverse_iterator
      crbegin() const noexcept
      { return _M_t.rbegin(); }
 
      /**
       *  Returns a read-only (constant) reverse iterator that points to one
       *  before the first pair in the %map.  Iteration is done in descending
       *  order according to the keys.
       */
      const_reverse_iterator
      crend() const noexcept
      { return _M_t.rend(); }
#endif
 
      // capacity
      /** Returns true if the %map is empty.  (Thus begin() would equal
       *  end().)
      */
      _GLIBCXX_NODISCARD bool
      empty() const _GLIBCXX_NOEXCEPT
      { return _M_t.empty(); }
 
      /** Returns the size of the %map.  */
      size_type
      size() const _GLIBCXX_NOEXCEPT
      { return _M_t.size(); }
 
      /** Returns the maximum size of the %map.  */
      size_type
      max_size() const _GLIBCXX_NOEXCEPT
      { return _M_t.max_size(); }
 
      // [23.3.1.2] element access
      /**
       *  @brief  Subscript ( @c [] ) access to %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 logarithmic time.
       */
      mapped_type&
      operator[](const key_type& __k)
      {
        // concept requirements
        __glibcxx_function_requires(_DefaultConstructibleConcept<mapped_type>)
 
        iterator __i = lower_bound(__k);
        // __i->first is greater than or equivalent to __k.
        if (__i == end() || key_comp()(__k, (*__i).first))
#if __cplusplus >= 201103L
          __i = _M_t._M_emplace_hint_unique(__i, std::piecewise_construct,
                                            std::tuple<const key_type&>(__k),
                                            std::tuple<>());
#else
          __i = insert(__i, value_type(__k, mapped_type()));
#endif
        return (*__i).second;
      }
 
#if __cplusplus >= 201103L
      mapped_type&
      operator[](key_type&& __k)
      {
        // concept requirements
        __glibcxx_function_requires(_DefaultConstructibleConcept<mapped_type>)
 
        iterator __i = lower_bound(__k);
        // __i->first is greater than or equivalent to __k.
        if (__i == end() || key_comp()(__k, (*__i).first))
          __i = _M_t._M_emplace_hint_unique(__i, std::piecewise_construct,
                                        std::forward_as_tuple(std::move(__k)),
                                        std::tuple<>());
        return (*__i).second;
      }
#endif
 
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // DR 464. Suggestion for new member functions in standard containers.
      /**
       *  @brief  Access to %map data.
       *  @param  __k  The key for which data should be retrieved.
       *  @return  A reference to the data whose key is equivalent to @a __k, if
       *           such a data is present in the %map.
       *  @throw  std::out_of_range  If no such data is present.
       */
      mapped_type&
      at(const key_type& __k)
      {
        iterator __i = lower_bound(__k);
        if (__i == end() || key_comp()(__k, (*__i).first))
          __throw_out_of_range(__N("map::at"));
        return (*__i).second;
      }
 
      const mapped_type&
      at(const key_type& __k) const
      {
        const_iterator __i = lower_bound(__k);
        if (__i == end() || key_comp()(__k, (*__i).first))
          __throw_out_of_range(__N("map::at"));
        return (*__i).second;
      }
 
      // modifiers
#if __cplusplus >= 201103L
      /**
       *  @brief Attempts to build and insert a std::pair into the %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 %map.
       *  A %map relies on unique keys and thus a %pair is only inserted if its
       *  first element (the key) is not already present in the %map.
       *
       *  Insertion requires logarithmic time.
       */
      template<typename... _Args>
        std::pair<iterator, bool>
        emplace(_Args&&... __args)
        { return _M_t._M_emplace_unique(std::forward<_Args>(__args)...); }
 
      /**
       *  @brief Attempts to build and insert a std::pair into the %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 logarithmic time (if the hint is not taken).
       */
      template<typename... _Args>
        iterator
        emplace_hint(const_iterator __pos, _Args&&... __args)
        {
          return _M_t._M_emplace_hint_unique(__pos,
                                             std::forward<_Args>(__args)...);
        }
#endif
 
#if __cplusplus > 201402L
      /// Extract a node.
      node_type
      extract(const_iterator __pos)
      {
        __glibcxx_assert(__pos != end());
        return _M_t.extract(__pos);
      }
 
      /// Extract a node.
      node_type
      extract(const key_type& __x)
      { return _M_t.extract(__x); }
 
      /// Re-insert an extracted node.
      insert_return_type
      insert(node_type&& __nh)
      { return _M_t._M_reinsert_node_unique(std::move(__nh)); }
 
      /// Re-insert an extracted node.
      iterator
      insert(const_iterator __hint, node_type&& __nh)
      { return _M_t._M_reinsert_node_hint_unique(__hint, std::move(__nh)); }
 
      template<typename, typename>
        friend class std::_Rb_tree_merge_helper;
 
      template<typename _Cmp2>
        void
        merge(map<_Key, _Tp, _Cmp2, _Alloc>& __source)
        {
          using _Merge_helper = _Rb_tree_merge_helper<map, _Cmp2>;
          _M_t._M_merge_unique(_Merge_helper::_S_get_tree(__source));
        }
 
      template<typename _Cmp2>
        void
        merge(map<_Key, _Tp, _Cmp2, _Alloc>&& __source)
        { merge(__source); }
 
      template<typename _Cmp2>
        void
        merge(multimap<_Key, _Tp, _Cmp2, _Alloc>& __source)
        {
          using _Merge_helper = _Rb_tree_merge_helper<map, _Cmp2>;
          _M_t._M_merge_unique(_Merge_helper::_S_get_tree(__source));
        }
 
      template<typename _Cmp2>
        void
        merge(multimap<_Key, _Tp, _Cmp2, _Alloc>&& __source)
        { merge(__source); }
#endif // C++17
 
#if __cplusplus > 201402L
#define __cpp_lib_map_try_emplace 201411
      /**
       *  @brief Attempts to build and insert a std::pair into the %map.
       *
       *  @param __k    Key to use for finding a possibly existing pair in
       *                the 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 %map.
       *  A %map relies on unique keys and thus a %pair is only inserted if its
       *  first element (the key) is not already present in the %map.
       *  If a %pair is not inserted, this function has no effect.
       *
       *  Insertion requires logarithmic time.
       */
      template <typename... _Args>
        pair<iterator, bool>
        try_emplace(const key_type& __k, _Args&&... __args)
        {
          iterator __i = lower_bound(__k);
          if (__i == end() || key_comp()(__k, (*__i).first))
            {
              __i = emplace_hint(__i, std::piecewise_construct,
                                 std::forward_as_tuple(__k),
                                 std::forward_as_tuple(
                                   std::forward<_Args>(__args)...));
              return {__i, true};
            }
          return {__i, false};
        }
 
      // move-capable overload
      template <typename... _Args>
        pair<iterator, bool>
        try_emplace(key_type&& __k, _Args&&... __args)
        {
          iterator __i = lower_bound(__k);
          if (__i == end() || key_comp()(__k, (*__i).first))
            {
              __i = emplace_hint(__i, std::piecewise_construct,
                                 std::forward_as_tuple(std::move(__k)),
                                 std::forward_as_tuple(
                                   std::forward<_Args>(__args)...));
              return {__i, true};
            }
          return {__i, false};
        }
 
      /**
       *  @brief Attempts to build and insert a std::pair into the %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 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
       *  try_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 logarithmic time (if the hint is not taken).
       */
      template <typename... _Args>
        iterator
        try_emplace(const_iterator __hint, const key_type& __k,
                    _Args&&... __args)
        {
          iterator __i;
          auto __true_hint = _M_t._M_get_insert_hint_unique_pos(__hint, __k);
          if (__true_hint.second)
            __i = emplace_hint(iterator(__true_hint.second),
                               std::piecewise_construct,
                               std::forward_as_tuple(__k),
                               std::forward_as_tuple(
                                 std::forward<_Args>(__args)...));
          else
            __i = iterator(__true_hint.first);
          return __i;
        }
 
      // move-capable overload
      template <typename... _Args>
        iterator
        try_emplace(const_iterator __hint, key_type&& __k, _Args&&... __args)
        {
          iterator __i;
          auto __true_hint = _M_t._M_get_insert_hint_unique_pos(__hint, __k);
          if (__true_hint.second)
            __i = emplace_hint(iterator(__true_hint.second),
                               std::piecewise_construct,
                               std::forward_as_tuple(std::move(__k)),
                               std::forward_as_tuple(
                                 std::forward<_Args>(__args)...));
          else
            __i = iterator(__true_hint.first);
          return __i;
        }
#endif
 
      /**
       *  @brief Attempts to insert a std::pair into the %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 %map.
       *  A %map relies on unique keys and thus a %pair is only inserted if its
       *  first element (the key) is not already present in the %map.
       *
       *  Insertion requires logarithmic time.
       *  @{
       */
      std::pair<iterator, bool>
      insert(const value_type& __x)
      { return _M_t._M_insert_unique(__x); }
 
#if __cplusplus >= 201103L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2354. Unnecessary copying when inserting into maps with braced-init
      std::pair<iterator, bool>
      insert(value_type&& __x)
      { return _M_t._M_insert_unique(std::move(__x)); }
 
      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair>::value,
                      pair<iterator, bool>>
        insert(_Pair&& __x)
        { return _M_t._M_emplace_unique(std::forward<_Pair>(__x)); }
#endif
      /// @}
 
#if __cplusplus >= 201103L
      /**
       *  @brief Attempts to insert a list of std::pairs into the %map.
       *  @param  __list  A std::initializer_list<value_type> of pairs to be
       *                  inserted.
       *
       *  Complexity similar to that of the range constructor.
       */
      void
      insert(std::initializer_list<value_type> __list)
      { insert(__list.begin(), __list.end()); }
#endif
 
      /**
       *  @brief Attempts to insert a std::pair into the %map.
       *  @param  __position  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 logarithmic time (if the hint is not taken).
       *  @{
       */
      iterator
#if __cplusplus >= 201103L
      insert(const_iterator __position, const value_type& __x)
#else
      insert(iterator __position, const value_type& __x)
#endif
      { return _M_t._M_insert_unique_(__position, __x); }
 
#if __cplusplus >= 201103L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 2354. Unnecessary copying when inserting into maps with braced-init
      iterator
      insert(const_iterator __position, value_type&& __x)
      { return _M_t._M_insert_unique_(__position, std::move(__x)); }
 
      template<typename _Pair>
        __enable_if_t<is_constructible<value_type, _Pair>::value, iterator>
        insert(const_iterator __position, _Pair&& __x)
        {
          return _M_t._M_emplace_hint_unique(__position,
                                             std::forward<_Pair>(__x));
        }
#endif
      /// @}
 
      /**
       *  @brief 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_t._M_insert_range_unique(__first, __last); }
 
#if __cplusplus > 201402L
      /**
       *  @brief Attempts to insert or assign a std::pair into the %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 %map.
       *  A %map relies on unique keys and thus a %pair is only inserted if its
       *  first element (the key) is not already present in the %map.
       *  If the %pair was already in the %map, the .second of the %pair
       *  is assigned from __obj.
       *
       *  Insertion requires logarithmic time.
       */
      template <typename _Obj>
        pair<iterator, bool>
        insert_or_assign(const key_type& __k, _Obj&& __obj)
        {
          iterator __i = lower_bound(__k);
          if (__i == end() || key_comp()(__k, (*__i).first))
            {
              __i = emplace_hint(__i, std::piecewise_construct,
                                 std::forward_as_tuple(__k),
                                 std::forward_as_tuple(
                                   std::forward<_Obj>(__obj)));
              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 = lower_bound(__k);
          if (__i == end() || key_comp()(__k, (*__i).first))
            {
              __i = emplace_hint(__i, std::piecewise_construct,
                                 std::forward_as_tuple(std::move(__k)),
                                 std::forward_as_tuple(
                                   std::forward<_Obj>(__obj)));
              return {__i, true};
            }
          (*__i).second = std::forward<_Obj>(__obj);
          return {__i, false};
        }
 
      /**
       *  @brief Attempts to insert or assign a std::pair into the %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 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 attempts to insert a (key, value) %pair into the %map.
       *  A %map relies on unique keys and thus a %pair is only inserted if its
       *  first element (the key) is not already present in the %map.
       *  If the %pair was already in the %map, the .second of the %pair
       *  is assigned from __obj.
       *
       *  Insertion requires logarithmic time.
       */
      template <typename _Obj>
        iterator
        insert_or_assign(const_iterator __hint,
                         const key_type& __k, _Obj&& __obj)
        {
          iterator __i;
          auto __true_hint = _M_t._M_get_insert_hint_unique_pos(__hint, __k);
          if (__true_hint.second)
            {
              return emplace_hint(iterator(__true_hint.second),
                                  std::piecewise_construct,
                                  std::forward_as_tuple(__k),
                                  std::forward_as_tuple(
                                    std::forward<_Obj>(__obj)));
            }
          __i = iterator(__true_hint.first);
          (*__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;
          auto __true_hint = _M_t._M_get_insert_hint_unique_pos(__hint, __k);
          if (__true_hint.second)
            {
              return emplace_hint(iterator(__true_hint.second),
                                  std::piecewise_construct,
                                  std::forward_as_tuple(std::move(__k)),
                                  std::forward_as_tuple(
                                    std::forward<_Obj>(__obj)));
            }
          __i = iterator(__true_hint.first);
          (*__i).second = std::forward<_Obj>(__obj);
          return __i;
        }
#endif
 
#if __cplusplus >= 201103L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // DR 130. Associative erase should return an iterator.
      /**
       *  @brief Erases an element from a %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 a %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_t.erase(__position); }
 
      // LWG 2059
      _GLIBCXX_ABI_TAG_CXX11
      iterator
      erase(iterator __position)
      { return _M_t.erase(__position); }
      /// @}
#else
      /**
       *  @brief Erases an element from a %map.
       *  @param  __position  An iterator pointing to the element to be erased.
       *
       *  This function erases an element, pointed to by the given
       *  iterator, from a %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.
       */
      void
      erase(iterator __position)
      { _M_t.erase(__position); }
#endif
 
      /**
       *  @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
       *  a %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.
       */
      size_type
      erase(const key_type& __x)
      { return _M_t.erase(__x); }
 
#if __cplusplus >= 201103L
      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // DR 130. Associative erase should return an iterator.
      /**
       *  @brief Erases a [first,last) range of elements from a %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 a %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 __first, const_iterator __last)
      { return _M_t.erase(__first, __last); }
#else
      /**
       *  @brief Erases a [__first,__last) range of elements from a %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.
       *
       *  This function erases a sequence of elements from a %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.
       */
      void
      erase(iterator __first, iterator __last)
      { _M_t.erase(__first, __last); }
#endif
 
      /**
       *  @brief  Swaps data with another %map.
       *  @param  __x  A %map of the same element and allocator types.
       *
       *  This exchanges the elements between two maps in constant
       *  time.  (It is only swapping a pointer, an integer, and an
       *  instance of the @c Compare type (which itself is often
       *  stateless and empty), so it should be quite fast.)  Note
       *  that the global std::swap() function is specialized such
       *  that std::swap(m1,m2) will feed to this function.
       *
       *  Whether the allocators are swapped depends on the allocator traits.
       */
      void
      swap(map& __x)
      _GLIBCXX_NOEXCEPT_IF(__is_nothrow_swappable<_Compare>::value)
      { _M_t.swap(__x._M_t); }
 
      /**
       *  Erases all elements in a %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() _GLIBCXX_NOEXCEPT
      { _M_t.clear(); }
 
      // observers
      /**
       *  Returns the key comparison object out of which the %map was
       *  constructed.
       */
      key_compare
      key_comp() const
      { return _M_t.key_comp(); }
 
      /**
       *  Returns a value comparison object, built from the key comparison
       *  object out of which the %map was constructed.
       */
      value_compare
      value_comp() const
      { return value_compare(_M_t.key_comp()); }
 
      // [23.3.1.3] map operations
 
      ///@{
      /**
       *  @brief Tries to locate an element in a %map.
       *  @param  __x  Key of (key, value) %pair 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 %pair.  If unsuccessful it returns the
       *  past-the-end ( @c end() ) iterator.
       */
 
      iterator
      find(const key_type& __x)
      { return _M_t.find(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        find(const _Kt& __x) -> decltype(_M_t._M_find_tr(__x))
        { return _M_t._M_find_tr(__x); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Tries to locate an element in a %map.
       *  @param  __x  Key of (key, value) %pair to be located.
       *  @return  Read-only (constant) 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 a constant
       *  iterator pointing to the sought after %pair. If unsuccessful it
       *  returns the past-the-end ( @c end() ) iterator.
       */
 
      const_iterator
      find(const key_type& __x) const
      { return _M_t.find(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        find(const _Kt& __x) const -> decltype(_M_t._M_find_tr(__x))
        { return _M_t._M_find_tr(__x); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief  Finds the number of elements with given key.
       *  @param  __x  Key of (key, value) pairs to be located.
       *  @return  Number of elements with specified key.
       *
       *  This function only makes sense for multimaps; for map the result will
       *  either be 0 (not present) or 1 (present).
       */
      size_type
      count(const key_type& __x) const
      { return _M_t.find(__x) == _M_t.end() ? 0 : 1; }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        count(const _Kt& __x) const -> decltype(_M_t._M_count_tr(__x))
        { return _M_t._M_count_tr(__x); }
#endif
      ///@}
 
#if __cplusplus > 201703L
      ///@{
      /**
       *  @brief  Finds whether an element with the given key exists.
       *  @param  __x  Key of (key, value) pairs to be located.
       *  @return  True if there is an element with the specified key.
       */
      bool
      contains(const key_type& __x) const
      { return _M_t.find(__x) != _M_t.end(); }
 
      template<typename _Kt>
        auto
        contains(const _Kt& __x) const
        -> decltype(_M_t._M_find_tr(__x), void(), true)
        { return _M_t._M_find_tr(__x) != _M_t.end(); }
      ///@}
#endif
 
      ///@{
      /**
       *  @brief Finds the beginning of a subsequence matching given key.
       *  @param  __x  Key of (key, value) pair to be located.
       *  @return  Iterator pointing to first element equal to or greater
       *           than key, or end().
       *
       *  This function returns the first element of a subsequence of elements
       *  that matches the given key.  If unsuccessful it returns an iterator
       *  pointing to the first element that has a greater value than given key
       *  or end() if no such element exists.
       */
      iterator
      lower_bound(const key_type& __x)
      { return _M_t.lower_bound(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        lower_bound(const _Kt& __x)
        -> decltype(iterator(_M_t._M_lower_bound_tr(__x)))
        { return iterator(_M_t._M_lower_bound_tr(__x)); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Finds the beginning of a subsequence matching given key.
       *  @param  __x  Key of (key, value) pair to be located.
       *  @return  Read-only (constant) iterator pointing to first element
       *           equal to or greater than key, or end().
       *
       *  This function returns the first element of a subsequence of elements
       *  that matches the given key.  If unsuccessful it returns an iterator
       *  pointing to the first element that has a greater value than given key
       *  or end() if no such element exists.
       */
      const_iterator
      lower_bound(const key_type& __x) const
      { return _M_t.lower_bound(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        lower_bound(const _Kt& __x) const
        -> decltype(const_iterator(_M_t._M_lower_bound_tr(__x)))
        { return const_iterator(_M_t._M_lower_bound_tr(__x)); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Finds the end of a subsequence matching given key.
       *  @param  __x  Key of (key, value) pair to be located.
       *  @return Iterator pointing to the first element
       *          greater than key, or end().
       */
      iterator
      upper_bound(const key_type& __x)
      { return _M_t.upper_bound(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        upper_bound(const _Kt& __x)
        -> decltype(iterator(_M_t._M_upper_bound_tr(__x)))
        { return iterator(_M_t._M_upper_bound_tr(__x)); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Finds the end of a subsequence matching given key.
       *  @param  __x  Key of (key, value) pair to be located.
       *  @return  Read-only (constant) iterator pointing to first iterator
       *           greater than key, or end().
       */
      const_iterator
      upper_bound(const key_type& __x) const
      { return _M_t.upper_bound(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        upper_bound(const _Kt& __x) const
        -> decltype(const_iterator(_M_t._M_upper_bound_tr(__x)))
        { return const_iterator(_M_t._M_upper_bound_tr(__x)); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Finds a subsequence matching given key.
       *  @param  __x  Key of (key, value) pairs to be located.
       *  @return  Pair of iterators that possibly points to the subsequence
       *           matching given key.
       *
       *  This function is equivalent to
       *  @code
       *    std::make_pair(c.lower_bound(val),
       *                   c.upper_bound(val))
       *  @endcode
       *  (but is faster than making the calls separately).
       *
       *  This function probably only makes sense for multimaps.
       */
      std::pair<iterator, iterator>
      equal_range(const key_type& __x)
      { return _M_t.equal_range(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        equal_range(const _Kt& __x)
        -> decltype(pair<iterator, iterator>(_M_t._M_equal_range_tr(__x)))
        { return pair<iterator, iterator>(_M_t._M_equal_range_tr(__x)); }
#endif
      ///@}
 
      ///@{
      /**
       *  @brief Finds a subsequence matching given key.
       *  @param  __x  Key of (key, value) pairs to be located.
       *  @return  Pair of read-only (constant) iterators that possibly points
       *           to the subsequence matching given key.
       *
       *  This function is equivalent to
       *  @code
       *    std::make_pair(c.lower_bound(val),
       *                   c.upper_bound(val))
       *  @endcode
       *  (but is faster than making the calls separately).
       *
       *  This function probably only makes sense for multimaps.
       */
      std::pair<const_iterator, const_iterator>
      equal_range(const key_type& __x) const
      { return _M_t.equal_range(__x); }
 
#if __cplusplus > 201103L
      template<typename _Kt>
        auto
        equal_range(const _Kt& __x) const
        -> decltype(pair<const_iterator, const_iterator>(
              _M_t._M_equal_range_tr(__x)))
        {
          return pair<const_iterator, const_iterator>(
              _M_t._M_equal_range_tr(__x));
        }
#endif
      ///@}
 
      template<typename _K1, typename _T1, typename _C1, typename _A1>
        friend bool
        operator==(const map<_K1, _T1, _C1, _A1>&,
                   const map<_K1, _T1, _C1, _A1>&);
 
#if __cpp_lib_three_way_comparison
      template<typename _K1, typename _T1, typename _C1, typename _A1>
        friend __detail::__synth3way_t<pair<const _K1, _T1>>
        operator<=>(const map<_K1, _T1, _C1, _A1>&,
                    const map<_K1, _T1, _C1, _A1>&);
#else
      template<typename _K1, typename _T1, typename _C1, typename _A1>
        friend bool
        operator<(const map<_K1, _T1, _C1, _A1>&,
                  const map<_K1, _T1, _C1, _A1>&);
#endif
    };
 
 
#if __cpp_deduction_guides >= 201606
 
  template<typename _InputIterator,
           typename _Compare = less<__iter_key_t<_InputIterator>>,
           typename _Allocator = allocator<__iter_to_alloc_t<_InputIterator>>,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireNotAllocator<_Compare>,
           typename = _RequireAllocator<_Allocator>>
    map(_InputIterator, _InputIterator,
        _Compare = _Compare(), _Allocator = _Allocator())
    -> map<__iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>,
           _Compare, _Allocator>;
 
  template<typename _Key, typename _Tp, typename _Compare = less<_Key>,
           typename _Allocator = allocator<pair<const _Key, _Tp>>,
           typename = _RequireNotAllocator<_Compare>,
           typename = _RequireAllocator<_Allocator>>
    map(initializer_list<pair<_Key, _Tp>>,
        _Compare = _Compare(), _Allocator = _Allocator())
    -> map<_Key, _Tp, _Compare, _Allocator>;
 
  template <typename _InputIterator, typename _Allocator,
            typename = _RequireInputIter<_InputIterator>,
            typename = _RequireAllocator<_Allocator>>
    map(_InputIterator, _InputIterator, _Allocator)
    -> map<__iter_key_t<_InputIterator>, __iter_val_t<_InputIterator>,
           less<__iter_key_t<_InputIterator>>, _Allocator>;
 
  template<typename _Key, typename _Tp, typename _Allocator,
           typename = _RequireAllocator<_Allocator>>
    map(initializer_list<pair<_Key, _Tp>>, _Allocator)
    -> map<_Key, _Tp, less<_Key>, _Allocator>;
 
#endif // deduction guides
 
  /**
   *  @brief  Map equality comparison.
   *  @param  __x  A %map.
   *  @param  __y  A %map of the same type as @a x.
   *  @return  True iff the size and elements of the maps are equal.
   *
   *  This is an equivalence relation.  It is linear in the size of the
   *  maps.  Maps are considered equivalent if their sizes are equal,
   *  and if corresponding elements compare equal.
  */
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator==(const map<_Key, _Tp, _Compare, _Alloc>& __x,
               const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return __x._M_t == __y._M_t; }
 
#if __cpp_lib_three_way_comparison
  /**
   *  @brief  Map ordering relation.
   *  @param  __x  A `map`.
   *  @param  __y  A `map` of the same type as `x`.
   *  @return  A value indicating whether `__x` is less than, equal to,
   *           greater than, or incomparable with `__y`.
   *
   *  This is a total ordering relation.  It is linear in the size of the
   *  maps.  The elements must be comparable with @c <.
   *
   *  See `std::lexicographical_compare_three_way()` for how the determination
   *  is made. This operator is used to synthesize relational operators like
   *  `<` and `>=` etc.
  */
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline __detail::__synth3way_t<pair<const _Key, _Tp>>
    operator<=>(const map<_Key, _Tp, _Compare, _Alloc>& __x,
                const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return __x._M_t <=> __y._M_t; }
#else
  /**
   *  @brief  Map ordering relation.
   *  @param  __x  A %map.
   *  @param  __y  A %map of the same type as @a x.
   *  @return  True iff @a x is lexicographically less than @a y.
   *
   *  This is a total ordering relation.  It is linear in the size of the
   *  maps.  The elements must be comparable with @c <.
   *
   *  See std::lexicographical_compare() for how the determination is made.
  */
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator<(const map<_Key, _Tp, _Compare, _Alloc>& __x,
              const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return __x._M_t < __y._M_t; }
 
  /// Based on operator==
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator!=(const map<_Key, _Tp, _Compare, _Alloc>& __x,
               const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return !(__x == __y); }
 
  /// Based on operator<
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator>(const map<_Key, _Tp, _Compare, _Alloc>& __x,
              const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return __y < __x; }
 
  /// Based on operator<
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator<=(const map<_Key, _Tp, _Compare, _Alloc>& __x,
               const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return !(__y < __x); }
 
  /// Based on operator<
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline bool
    operator>=(const map<_Key, _Tp, _Compare, _Alloc>& __x,
               const map<_Key, _Tp, _Compare, _Alloc>& __y)
    { return !(__x < __y); }
#endif // three-way comparison
 
  /// See std::map::swap().
  template<typename _Key, typename _Tp, typename _Compare, typename _Alloc>
    inline void
    swap(map<_Key, _Tp, _Compare, _Alloc>& __x,
         map<_Key, _Tp, _Compare, _Alloc>& __y)
    _GLIBCXX_NOEXCEPT_IF(noexcept(__x.swap(__y)))
    { __x.swap(__y); }
 
_GLIBCXX_END_NAMESPACE_CONTAINER
 
#if __cplusplus > 201402L
  // Allow std::map access to internals of compatible maps.
  template<typename _Key, typename _Val, typename _Cmp1, typename _Alloc,
           typename _Cmp2>
    struct
    _Rb_tree_merge_helper<_GLIBCXX_STD_C::map<_Key, _Val, _Cmp1, _Alloc>,
                          _Cmp2>
    {
    private:
      friend class _GLIBCXX_STD_C::map<_Key, _Val, _Cmp1, _Alloc>;
 
      static auto&
      _S_get_tree(_GLIBCXX_STD_C::map<_Key, _Val, _Cmp2, _Alloc>& __map)
      { return __map._M_t; }
 
      static auto&
      _S_get_tree(_GLIBCXX_STD_C::multimap<_Key, _Val, _Cmp2, _Alloc>& __map)
      { return __map._M_t; }
    };
#endif // C++17
 
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std
 
#endif /* _STL_MAP_H */