stl_list.h

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// List implementation -*- C++ -*-

// Copyright (C) 2001-2024 Free Software Foundation, Inc.
// Copyright The GNU Toolchain Authors.
//
// 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_list.h
 *  This is an internal header file, included by other library headers.
 *  Do not attempt to use it directly. @headername{list}
 */

#ifndef _STL_LIST_H
#define _STL_LIST_H 1

#include <bits/concept_check.h>
#include <ext/alloc_traits.h>
#include <debug/assertions.h>
#if __cplusplus >= 201103L
#include <initializer_list>
#include <bits/allocated_ptr.h>
#include <ext/aligned_buffer.h>
#endif

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION

  namespace __detail
  {
    // Supporting structures are split into common and templated
    // types; the latter publicly inherits from the former in an
    // effort to reduce code duplication.  This results in some
    // "needless" static_cast'ing later on, but it's all safe
    // downcasting.

    /// Common part of a node in the %list.
    struct _List_node_base
    {
      _List_node_base* _M_next;
      _List_node_base* _M_prev;

      static void
      swap(_List_node_base& __x, _List_node_base& __y) _GLIBCXX_USE_NOEXCEPT;

      void
      _M_transfer(_List_node_base* const __first,
                  _List_node_base* const __last) _GLIBCXX_USE_NOEXCEPT;

      void
      _M_reverse() _GLIBCXX_USE_NOEXCEPT;

      void
      _M_hook(_List_node_base* const __position) _GLIBCXX_USE_NOEXCEPT;

      void
      _M_unhook() _GLIBCXX_USE_NOEXCEPT;
    };

    /// The %list node header.
    struct _List_node_header : public _List_node_base
    {
#if _GLIBCXX_USE_CXX11_ABI
      std::size_t _M_size;
#endif

      _List_node_header() _GLIBCXX_NOEXCEPT
      { _M_init(); }

#if __cplusplus >= 201103L
      _List_node_header(_List_node_header&& __x) noexcept
      : _List_node_base{ __x._M_next, __x._M_prev }
# if _GLIBCXX_USE_CXX11_ABI
      , _M_size(__x._M_size)
# endif
      {
        if (__x._M_base()->_M_next == __x._M_base())
          this->_M_next = this->_M_prev = this;
        else
          {
            this->_M_next->_M_prev = this->_M_prev->_M_next = this->_M_base();
            __x._M_init();
          }
      }

      void
      _M_move_nodes(_List_node_header&& __x)
      {
        _List_node_base* const __xnode = __x._M_base();
        if (__xnode->_M_next == __xnode)
          _M_init();
        else
          {
            _List_node_base* const __node = this->_M_base();
            __node->_M_next = __xnode->_M_next;
            __node->_M_prev = __xnode->_M_prev;
            __node->_M_next->_M_prev = __node->_M_prev->_M_next = __node;
# if _GLIBCXX_USE_CXX11_ABI
            _M_size = __x._M_size;
# endif
            __x._M_init();
          }
      }
#endif

      void
      _M_init() _GLIBCXX_NOEXCEPT
      {
        this->_M_next = this->_M_prev = this;
#if _GLIBCXX_USE_CXX11_ABI
        this->_M_size = 0;
#endif
      }

    private:
      _List_node_base* _M_base() { return this; }
    };

    // Used by list::sort to hold nodes being sorted.
    struct _Scratch_list : _List_node_base
    {
      _Scratch_list() { _M_next = _M_prev = this; }

      bool empty() const { return _M_next == this; }

      void swap(_List_node_base& __l) { _List_node_base::swap(*this, __l); }

      template<typename _Iter, typename _Cmp>
        struct _Ptr_cmp
        {
          _Cmp _M_cmp;

          bool
          operator()(__detail::_List_node_base* __lhs,
                     __detail::_List_node_base* __rhs) /* not const */
          { return _M_cmp(*_Iter(__lhs), *_Iter(__rhs)); }
        };

      template<typename _Iter>
        struct _Ptr_cmp<_Iter, void>
        {
          bool
          operator()(__detail::_List_node_base* __lhs,
                     __detail::_List_node_base* __rhs) const
          { return *_Iter(__lhs) < *_Iter(__rhs); }
        };

      // Merge nodes from __x into *this. Both lists must be sorted wrt _Cmp.
      template<typename _Cmp>
        void
        merge(_List_node_base& __x, _Cmp __comp)
        {
          _List_node_base* __first1 = _M_next;
          _List_node_base* const __last1 = this;
          _List_node_base* __first2 = __x._M_next;
          _List_node_base* const __last2 = std::__addressof(__x);

          while (__first1 != __last1 && __first2 != __last2)
            {
              if (__comp(__first2, __first1))
                {
                  _List_node_base* __next = __first2->_M_next;
                  __first1->_M_transfer(__first2, __next);
                  __first2 = __next;
                }
              else
                __first1 = __first1->_M_next;
            }
          if (__first2 != __last2)
            this->_M_transfer(__first2, __last2);
        }

      // Splice the node at __i into *this.
      void _M_take_one(_List_node_base* __i)
      { this->_M_transfer(__i, __i->_M_next); }

      // Splice all nodes from *this after __i.
      void _M_put_all(_List_node_base* __i)
      {
        if (!empty())
          __i->_M_transfer(_M_next, this);
      }
    };

  } // namespace detail

_GLIBCXX_BEGIN_NAMESPACE_CONTAINER

  /// An actual node in the %list.
  template<typename _Tp>
    struct _List_node : public __detail::_List_node_base
    {
#if __cplusplus >= 201103L
      __gnu_cxx::__aligned_membuf<_Tp> _M_storage;
      _Tp*       _M_valptr()       { return _M_storage._M_ptr(); }
      _Tp const* _M_valptr() const { return _M_storage._M_ptr(); }
#else
      _Tp _M_data;
      _Tp*       _M_valptr()       { return std::__addressof(_M_data); }
      _Tp const* _M_valptr() const { return std::__addressof(_M_data); }
#endif
    };

  /**
   *  @brief A list::iterator.
   *
   *  All the functions are op overloads.
  */
  template<typename _Tp>
    struct _List_iterator
    {
      typedef _List_iterator<_Tp>               _Self;
      typedef _List_node<_Tp>                   _Node;

      typedef ptrdiff_t                         difference_type;
      typedef std::bidirectional_iterator_tag   iterator_category;
      typedef _Tp                               value_type;
      typedef _Tp*                              pointer;
      typedef _Tp&                              reference;

      _List_iterator() _GLIBCXX_NOEXCEPT
      : _M_node() { }

      explicit
      _List_iterator(__detail::_List_node_base* __x) _GLIBCXX_NOEXCEPT
      : _M_node(__x) { }

      _Self
      _M_const_cast() const _GLIBCXX_NOEXCEPT
      { return *this; }

      // Must downcast from _List_node_base to _List_node to get to value.
      _GLIBCXX_NODISCARD
      reference
      operator*() const _GLIBCXX_NOEXCEPT
      { return *static_cast<_Node*>(_M_node)->_M_valptr(); }

      _GLIBCXX_NODISCARD
      pointer
      operator->() const _GLIBCXX_NOEXCEPT
      { return static_cast<_Node*>(_M_node)->_M_valptr(); }

      _Self&
      operator++() _GLIBCXX_NOEXCEPT
      {
        _M_node = _M_node->_M_next;
        return *this;
      }

      _Self
      operator++(int) _GLIBCXX_NOEXCEPT
      {
        _Self __tmp = *this;
        _M_node = _M_node->_M_next;
        return __tmp;
      }

      _Self&
      operator--() _GLIBCXX_NOEXCEPT
      {
        _M_node = _M_node->_M_prev;
        return *this;
      }

      _Self
      operator--(int) _GLIBCXX_NOEXCEPT
      {
        _Self __tmp = *this;
        _M_node = _M_node->_M_prev;
        return __tmp;
      }

      _GLIBCXX_NODISCARD
      friend bool
      operator==(const _Self& __x, const _Self& __y) _GLIBCXX_NOEXCEPT
      { return __x._M_node == __y._M_node; }

#if __cpp_impl_three_way_comparison < 201907L
      _GLIBCXX_NODISCARD
      friend bool
      operator!=(const _Self& __x, const _Self& __y) _GLIBCXX_NOEXCEPT
      { return __x._M_node != __y._M_node; }
#endif

      // The only member points to the %list element.
      __detail::_List_node_base* _M_node;
    };

  /**
   *  @brief A list::const_iterator.
   *
   *  All the functions are op overloads.
  */
  template<typename _Tp>
    struct _List_const_iterator
    {
      typedef _List_const_iterator<_Tp>         _Self;
      typedef const _List_node<_Tp>             _Node;
      typedef _List_iterator<_Tp>               iterator;

      typedef ptrdiff_t                         difference_type;
      typedef std::bidirectional_iterator_tag   iterator_category;
      typedef _Tp                               value_type;
      typedef const _Tp*                        pointer;
      typedef const _Tp&                        reference;

      _List_const_iterator() _GLIBCXX_NOEXCEPT
      : _M_node() { }

      explicit
      _List_const_iterator(const __detail::_List_node_base* __x)
      _GLIBCXX_NOEXCEPT
      : _M_node(__x) { }

      _List_const_iterator(const iterator& __x) _GLIBCXX_NOEXCEPT
      : _M_node(__x._M_node) { }

      iterator
      _M_const_cast() const _GLIBCXX_NOEXCEPT
      { return iterator(const_cast<__detail::_List_node_base*>(_M_node)); }

      // Must downcast from List_node_base to _List_node to get to value.
      _GLIBCXX_NODISCARD
      reference
      operator*() const _GLIBCXX_NOEXCEPT
      { return *static_cast<_Node*>(_M_node)->_M_valptr(); }

      _GLIBCXX_NODISCARD
      pointer
      operator->() const _GLIBCXX_NOEXCEPT
      { return static_cast<_Node*>(_M_node)->_M_valptr(); }

      _Self&
      operator++() _GLIBCXX_NOEXCEPT
      {
        _M_node = _M_node->_M_next;
        return *this;
      }

      _Self
      operator++(int) _GLIBCXX_NOEXCEPT
      {
        _Self __tmp = *this;
        _M_node = _M_node->_M_next;
        return __tmp;
      }

      _Self&
      operator--() _GLIBCXX_NOEXCEPT
      {
        _M_node = _M_node->_M_prev;
        return *this;
      }

      _Self
      operator--(int) _GLIBCXX_NOEXCEPT
      {
        _Self __tmp = *this;
        _M_node = _M_node->_M_prev;
        return __tmp;
      }

      _GLIBCXX_NODISCARD
      friend bool
      operator==(const _Self& __x, const _Self& __y) _GLIBCXX_NOEXCEPT
      { return __x._M_node == __y._M_node; }

#if __cpp_impl_three_way_comparison < 201907L
      _GLIBCXX_NODISCARD
      friend bool
      operator!=(const _Self& __x, const _Self& __y) _GLIBCXX_NOEXCEPT
      { return __x._M_node != __y._M_node; }
#endif

      // The only member points to the %list element.
      const __detail::_List_node_base* _M_node;
    };

_GLIBCXX_BEGIN_NAMESPACE_CXX11
  /// See bits/stl_deque.h's _Deque_base for an explanation.
  template<typename _Tp, typename _Alloc>
    class _List_base
    {
    protected:
      typedef typename __gnu_cxx::__alloc_traits<_Alloc>::template
        rebind<_Tp>::other                              _Tp_alloc_type;
      typedef __gnu_cxx::__alloc_traits<_Tp_alloc_type> _Tp_alloc_traits;
      typedef typename _Tp_alloc_traits::template
        rebind<_List_node<_Tp> >::other _Node_alloc_type;
      typedef __gnu_cxx::__alloc_traits<_Node_alloc_type> _Node_alloc_traits;

#if !_GLIBCXX_INLINE_VERSION
      static size_t
      _S_distance(const __detail::_List_node_base* __first,
                  const __detail::_List_node_base* __last)
      {
        size_t __n = 0;
        while (__first != __last)
          {
            __first = __first->_M_next;
            ++__n;
          }
        return __n;
      }
#endif

      struct _List_impl
      : public _Node_alloc_type
      {
        __detail::_List_node_header _M_node;

        _List_impl() _GLIBCXX_NOEXCEPT_IF(
            is_nothrow_default_constructible<_Node_alloc_type>::value)
        : _Node_alloc_type()
        { }

        _List_impl(const _Node_alloc_type& __a) _GLIBCXX_NOEXCEPT
        : _Node_alloc_type(__a)
        { }

#if __cplusplus >= 201103L
        _List_impl(_List_impl&&) = default;

        _List_impl(_Node_alloc_type&& __a, _List_impl&& __x)
        : _Node_alloc_type(std::move(__a)), _M_node(std::move(__x._M_node))
        { }

        _List_impl(_Node_alloc_type&& __a) noexcept
        : _Node_alloc_type(std::move(__a))
        { }
#endif
      };

      _List_impl _M_impl;

#if _GLIBCXX_USE_CXX11_ABI
      size_t _M_get_size() const { return _M_impl._M_node._M_size; }

      void _M_set_size(size_t __n) { _M_impl._M_node._M_size = __n; }

      void _M_inc_size(size_t __n) { _M_impl._M_node._M_size += __n; }

      void _M_dec_size(size_t __n) { _M_impl._M_node._M_size -= __n; }

# if !_GLIBCXX_INLINE_VERSION
      size_t
      _M_distance(const __detail::_List_node_base* __first,
                  const __detail::_List_node_base* __last) const
      { return _S_distance(__first, __last); }

      // return the stored size
      size_t _M_node_count() const { return _M_get_size(); }
# endif
#else
      // dummy implementations used when the size is not stored
      size_t _M_get_size() const { return 0; }
      void _M_set_size(size_t) { }
      void _M_inc_size(size_t) { }
      void _M_dec_size(size_t) { }

# if !_GLIBCXX_INLINE_VERSION
      size_t _M_distance(const void*, const void*) const { return 0; }

      // count the number of nodes
      size_t _M_node_count() const
      {
        return _S_distance(_M_impl._M_node._M_next,
                           std::__addressof(_M_impl._M_node));
      }
# endif
#endif

      typename _Node_alloc_traits::pointer
      _M_get_node()
      { return _Node_alloc_traits::allocate(_M_impl, 1); }

      void
      _M_put_node(typename _Node_alloc_traits::pointer __p) _GLIBCXX_NOEXCEPT
      { _Node_alloc_traits::deallocate(_M_impl, __p, 1); }

  public:
      typedef _Alloc allocator_type;

      _Node_alloc_type&
      _M_get_Node_allocator() _GLIBCXX_NOEXCEPT
      { return _M_impl; }

      const _Node_alloc_type&
      _M_get_Node_allocator() const _GLIBCXX_NOEXCEPT
      { return _M_impl; }

#if __cplusplus >= 201103L
      _List_base() = default;
#else
      _List_base() { }
#endif

      _List_base(const _Node_alloc_type& __a) _GLIBCXX_NOEXCEPT
      : _M_impl(__a)
      { }

#if __cplusplus >= 201103L
      _List_base(_List_base&&) = default;

# if !_GLIBCXX_INLINE_VERSION
      _List_base(_List_base&& __x, _Node_alloc_type&& __a)
      : _M_impl(std::move(__a))
      {
        if (__x._M_get_Node_allocator() == _M_get_Node_allocator())
          _M_move_nodes(std::move(__x));
        // else caller must move individual elements.
      }
# endif

      // Used when allocator is_always_equal.
      _List_base(_Node_alloc_type&& __a, _List_base&& __x)
      : _M_impl(std::move(__a), std::move(__x._M_impl))
      { }

      // Used when allocator !is_always_equal.
      _List_base(_Node_alloc_type&& __a)
      : _M_impl(std::move(__a))
      { }

      void
      _M_move_nodes(_List_base&& __x)
      { _M_impl._M_node._M_move_nodes(std::move(__x._M_impl._M_node)); }
#endif

      // This is what actually destroys the list.
      ~_List_base() _GLIBCXX_NOEXCEPT
      { _M_clear(); }

      void
      _M_clear() _GLIBCXX_NOEXCEPT;

      void
      _M_init() _GLIBCXX_NOEXCEPT
      { this->_M_impl._M_node._M_init(); }
    };

  /**
   *  @brief A standard container with linear time access to elements,
   *  and fixed time insertion/deletion at any point in the sequence.
   *
   *  @ingroup sequences
   *
   *  @tparam _Tp  Type of element.
   *  @tparam _Alloc  Allocator type, defaults to allocator<_Tp>.
   *
   *  Meets the requirements of a <a href="tables.html#65">container</a>, a
   *  <a href="tables.html#66">reversible container</a>, and a
   *  <a href="tables.html#67">sequence</a>, including the
   *  <a href="tables.html#68">optional sequence requirements</a> with the
   *  %exception of @c at and @c operator[].
   *
   *  This is a @e doubly @e linked %list.  Traversal up and down the
   *  %list requires linear time, but adding and removing elements (or
   *  @e nodes) is done in constant time, regardless of where the
   *  change takes place.  Unlike std::vector and std::deque,
   *  random-access iterators are not provided, so subscripting ( @c
   *  [] ) access is not allowed.  For algorithms which only need
   *  sequential access, this lack makes no difference.
   *
   *  Also unlike the other standard containers, std::list provides
   *  specialized algorithms %unique to linked lists, such as
   *  splicing, sorting, and in-place reversal.
   *
   *  A couple points on memory allocation for list<Tp>:
   *
   *  First, we never actually allocate a Tp, we allocate
   *  List_node<Tp>'s and trust [20.1.5]/4 to DTRT.  This is to ensure
   *  that after elements from %list<X,Alloc1> are spliced into
   *  %list<X,Alloc2>, destroying the memory of the second %list is a
   *  valid operation, i.e., Alloc1 giveth and Alloc2 taketh away.
   *
   *  Second, a %list conceptually represented as
   *  @code
   *    A <---> B <---> C <---> D
   *  @endcode
   *  is actually circular; a link exists between A and D.  The %list
   *  class holds (as its only data member) a private list::iterator
   *  pointing to @e D, not to @e A!  To get to the head of the %list,
   *  we start at the tail and move forward by one.  When this member
   *  iterator's next/previous pointers refer to itself, the %list is
   *  %empty.
  */
  template<typename _Tp, typename _Alloc = std::allocator<_Tp> >
    class list : protected _List_base<_Tp, _Alloc>
    {
#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_requires2(_Tp, _Alloc_value_type, _SameTypeConcept)
#endif

#if __cplusplus >= 201103L
      static_assert(is_same<typename remove_cv<_Tp>::type, _Tp>::value,
          "std::list must have a non-const, non-volatile value_type");
# if __cplusplus > 201703L || defined __STRICT_ANSI__
      static_assert(is_same<typename _Alloc::value_type, _Tp>::value,
          "std::list must have the same value_type as its allocator");
# endif
#endif

      typedef _List_base<_Tp, _Alloc>                   _Base;
      typedef typename _Base::_Tp_alloc_type            _Tp_alloc_type;
      typedef typename _Base::_Tp_alloc_traits          _Tp_alloc_traits;
      typedef typename _Base::_Node_alloc_type          _Node_alloc_type;
      typedef typename _Base::_Node_alloc_traits        _Node_alloc_traits;

    public:
      typedef _Tp                                        value_type;
      typedef typename _Tp_alloc_traits::pointer         pointer;
      typedef typename _Tp_alloc_traits::const_pointer   const_pointer;
      typedef typename _Tp_alloc_traits::reference       reference;
      typedef typename _Tp_alloc_traits::const_reference const_reference;
      typedef _List_iterator<_Tp>                        iterator;
      typedef _List_const_iterator<_Tp>                  const_iterator;
      typedef std::reverse_iterator<const_iterator>      const_reverse_iterator;
      typedef std::reverse_iterator<iterator>            reverse_iterator;
      typedef size_t                                     size_type;
      typedef ptrdiff_t                                  difference_type;
      typedef _Alloc                                     allocator_type;

    protected:
      // Note that pointers-to-_Node's can be ctor-converted to
      // iterator types.
      typedef _List_node<_Tp>                            _Node;

      using _Base::_M_impl;
      using _Base::_M_put_node;
      using _Base::_M_get_node;
      using _Base::_M_get_Node_allocator;

      /**
       *  @param  __args  An instance of user data.
       *
       *  Allocates space for a new node and constructs a copy of
       *  @a __args in it.
       */
#if __cplusplus < 201103L
      _Node*
      _M_create_node(const value_type& __x)
      {
        _Node* __p = this->_M_get_node();
        __try
          {
            _Tp_alloc_type __alloc(_M_get_Node_allocator());
            __alloc.construct(__p->_M_valptr(), __x);
          }
        __catch(...)
          {
            _M_put_node(__p);
            __throw_exception_again;
          }
        return __p;
      }
#else
      template<typename... _Args>
        _Node*
        _M_create_node(_Args&&... __args)
        {
          auto __p = this->_M_get_node();
          auto& __alloc = _M_get_Node_allocator();
          __allocated_ptr<_Node_alloc_type> __guard{__alloc, __p};
          _Node_alloc_traits::construct(__alloc, __p->_M_valptr(),
                                        std::forward<_Args>(__args)...);
          __guard = nullptr;
          return __p;
        }
#endif

#if _GLIBCXX_USE_CXX11_ABI
      static size_t
      _S_distance(const_iterator __first, const_iterator __last)
      { return std::distance(__first, __last); }

      // return the stored size
      size_t
      _M_node_count() const
      { return this->_M_get_size(); }
#else
      // dummy implementations used when the size is not stored
      static size_t
      _S_distance(const_iterator, const_iterator)
      { return 0; }

      // count the number of nodes
      size_t
      _M_node_count() const
      { return std::distance(begin(), end()); }
#endif

    public:
      // [23.2.2.1] construct/copy/destroy
      // (assign() and get_allocator() are also listed in this section)

      /**
       *  @brief  Creates a %list with no elements.
       */
#if __cplusplus >= 201103L
      list() = default;
#else
      list() { }
#endif

      /**
       *  @brief  Creates a %list with no elements.
       *  @param  __a  An allocator object.
       */
      explicit
      list(const allocator_type& __a) _GLIBCXX_NOEXCEPT
      : _Base(_Node_alloc_type(__a)) { }

#if __cplusplus >= 201103L
      /**
       *  @brief  Creates a %list with default constructed elements.
       *  @param  __n  The number of elements to initially create.
       *  @param  __a  An allocator object.
       *
       *  This constructor fills the %list with @a __n default
       *  constructed elements.
       */
      explicit
      list(size_type __n, const allocator_type& __a = allocator_type())
      : _Base(_Node_alloc_type(__a))
      { _M_default_initialize(__n); }

      /**
       *  @brief  Creates a %list with copies of an exemplar element.
       *  @param  __n  The number of elements to initially create.
       *  @param  __value  An element to copy.
       *  @param  __a  An allocator object.
       *
       *  This constructor fills the %list with @a __n copies of @a __value.
       */
      list(size_type __n, const value_type& __value,
           const allocator_type& __a = allocator_type())
      : _Base(_Node_alloc_type(__a))
      { _M_fill_initialize(__n, __value); }
#else
      /**
       *  @brief  Creates a %list with copies of an exemplar element.
       *  @param  __n  The number of elements to initially create.
       *  @param  __value  An element to copy.
       *  @param  __a  An allocator object.
       *
       *  This constructor fills the %list with @a __n copies of @a __value.
       */
      explicit
      list(size_type __n, const value_type& __value = value_type(),
           const allocator_type& __a = allocator_type())
      : _Base(_Node_alloc_type(__a))
      { _M_fill_initialize(__n, __value); }
#endif

      /**
       *  @brief  %List copy constructor.
       *  @param  __x  A %list of identical element and allocator types.
       *
       *  The newly-created %list uses a copy of the allocation object used
       *  by @a __x (unless the allocator traits dictate a different object).
       */
      list(const list& __x)
      : _Base(_Node_alloc_traits::
              _S_select_on_copy(__x._M_get_Node_allocator()))
      { _M_initialize_dispatch(__x.begin(), __x.end(), __false_type()); }

#if __cplusplus >= 201103L
      /**
       *  @brief  %List move constructor.
       *
       *  The newly-created %list contains the exact contents of the moved
       *  instance. The contents of the moved instance are a valid, but
       *  unspecified %list.
       */
      list(list&&) = default;

      /**
       *  @brief  Builds a %list from an initializer_list
       *  @param  __l  An initializer_list of value_type.
       *  @param  __a  An allocator object.
       *
       *  Create a %list consisting of copies of the elements in the
       *  initializer_list @a __l.  This is linear in __l.size().
       */
      list(initializer_list<value_type> __l,
           const allocator_type& __a = allocator_type())
      : _Base(_Node_alloc_type(__a))
      { _M_initialize_dispatch(__l.begin(), __l.end(), __false_type()); }

      list(const list& __x, const __type_identity_t<allocator_type>& __a)
      : _Base(_Node_alloc_type(__a))
      { _M_initialize_dispatch(__x.begin(), __x.end(), __false_type()); }

    private:
      list(list&& __x, const allocator_type& __a, true_type) noexcept
      : _Base(_Node_alloc_type(__a), std::move(__x))
      { }

      list(list&& __x, const allocator_type& __a, false_type)
      : _Base(_Node_alloc_type(__a))
      {
        if (__x._M_get_Node_allocator() == this->_M_get_Node_allocator())
          this->_M_move_nodes(std::move(__x));
        else
          insert(begin(), std::__make_move_if_noexcept_iterator(__x.begin()),
                          std::__make_move_if_noexcept_iterator(__x.end()));
      }

    public:
      list(list&& __x, const __type_identity_t<allocator_type>& __a)
      noexcept(_Node_alloc_traits::_S_always_equal())
      : list(std::move(__x), __a,
             typename _Node_alloc_traits::is_always_equal{})
      { }
#endif

      /**
       *  @brief  Builds a %list from a range.
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *  @param  __a  An allocator object.
       *
       *  Create a %list consisting of copies of the elements from
       *  [@a __first,@a __last).  This is linear in N (where N is
       *  distance(@a __first,@a __last)).
       */
#if __cplusplus >= 201103L
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        list(_InputIterator __first, _InputIterator __last,
             const allocator_type& __a = allocator_type())
        : _Base(_Node_alloc_type(__a))
        { _M_initialize_dispatch(__first, __last, __false_type()); }
#else
      template<typename _InputIterator>
        list(_InputIterator __first, _InputIterator __last,
             const allocator_type& __a = allocator_type())
        : _Base(_Node_alloc_type(__a))
        {
          // Check whether it's an integral type.  If so, it's not an iterator.
          typedef typename std::__is_integer<_InputIterator>::__type _Integral;
          _M_initialize_dispatch(__first, __last, _Integral());
        }
#endif

#if __cplusplus >= 201103L
      /**
       *  No explicit dtor needed as the _Base dtor takes care of
       *  things.  The _Base 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.
       */
      ~list() = default;
#endif

      /**
       *  @brief  %List assignment operator.
       *  @param  __x  A %list of identical element and allocator types.
       *
       *  All the elements of @a __x are copied.
       *
       *  Whether the allocator is copied depends on the allocator traits.
       */
      list&
      operator=(const list& __x);

#if __cplusplus >= 201103L
      /**
       *  @brief  %List move assignment operator.
       *  @param  __x  A %list of identical element and allocator types.
       *
       *  The contents of @a __x are moved into this %list (without copying).
       *
       *  Afterwards @a __x is a valid, but unspecified %list
       *
       *  Whether the allocator is moved depends on the allocator traits.
       */
      list&
      operator=(list&& __x)
      noexcept(_Node_alloc_traits::_S_nothrow_move())
      {
        constexpr bool __move_storage =
          _Node_alloc_traits::_S_propagate_on_move_assign()
          || _Node_alloc_traits::_S_always_equal();
        _M_move_assign(std::move(__x), __bool_constant<__move_storage>());
        return *this;
      }

      /**
       *  @brief  %List initializer list assignment operator.
       *  @param  __l  An initializer_list of value_type.
       *
       *  Replace the contents of the %list with copies of the elements
       *  in the initializer_list @a __l.  This is linear in l.size().
       */
      list&
      operator=(initializer_list<value_type> __l)
      {
        this->assign(__l.begin(), __l.end());
        return *this;
      }
#endif

      /**
       *  @brief  Assigns a given value to a %list.
       *  @param  __n  Number of elements to be assigned.
       *  @param  __val  Value to be assigned.
       *
       *  This function fills a %list with @a __n copies of the given
       *  value.  Note that the assignment completely changes the %list
       *  and that the resulting %list's size is the same as the number
       *  of elements assigned.
       */
      void
      assign(size_type __n, const value_type& __val)
      { _M_fill_assign(__n, __val); }

      /**
       *  @brief  Assigns a range to a %list.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *
       *  This function fills a %list with copies of the elements in the
       *  range [@a __first,@a __last).
       *
       *  Note that the assignment completely changes the %list and
       *  that the resulting %list's size is the same as the number of
       *  elements assigned.
       */
#if __cplusplus >= 201103L
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        void
        assign(_InputIterator __first, _InputIterator __last)
        { _M_assign_dispatch(__first, __last, __false_type()); }
#else
      template<typename _InputIterator>
        void
        assign(_InputIterator __first, _InputIterator __last)
        {
          // Check whether it's an integral type.  If so, it's not an iterator.
          typedef typename std::__is_integer<_InputIterator>::__type _Integral;
          _M_assign_dispatch(__first, __last, _Integral());
        }
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Assigns an initializer_list to a %list.
       *  @param  __l  An initializer_list of value_type.
       *
       *  Replace the contents of the %list with copies of the elements
       *  in the initializer_list @a __l.  This is linear in __l.size().
       */
      void
      assign(initializer_list<value_type> __l)
      { this->_M_assign_dispatch(__l.begin(), __l.end(), __false_type()); }
#endif

      /// Get a copy of the memory allocation object.
      allocator_type
      get_allocator() const _GLIBCXX_NOEXCEPT
      { return allocator_type(_Base::_M_get_Node_allocator()); }

      // iterators
      /**
       *  Returns a read/write iterator that points to the first element in the
       *  %list.  Iteration is done in ordinary element order.
       */
      _GLIBCXX_NODISCARD
      iterator
      begin() _GLIBCXX_NOEXCEPT
      { return iterator(this->_M_impl._M_node._M_next); }

      /**
       *  Returns a read-only (constant) iterator that points to the
       *  first element in the %list.  Iteration is done in ordinary
       *  element order.
       */
      _GLIBCXX_NODISCARD
      const_iterator
      begin() const _GLIBCXX_NOEXCEPT
      { return const_iterator(this->_M_impl._M_node._M_next); }

      /**
       *  Returns a read/write iterator that points one past the last
       *  element in the %list.  Iteration is done in ordinary element
       *  order.
       */
      _GLIBCXX_NODISCARD
      iterator
      end() _GLIBCXX_NOEXCEPT
      { return iterator(&this->_M_impl._M_node); }

      /**
       *  Returns a read-only (constant) iterator that points one past
       *  the last element in the %list.  Iteration is done in ordinary
       *  element order.
       */
      _GLIBCXX_NODISCARD
      const_iterator
      end() const _GLIBCXX_NOEXCEPT
      { return const_iterator(&this->_M_impl._M_node); }

      /**
       *  Returns a read/write reverse iterator that points to the last
       *  element in the %list.  Iteration is done in reverse element
       *  order.
       */
      _GLIBCXX_NODISCARD
      reverse_iterator
      rbegin() _GLIBCXX_NOEXCEPT
      { return reverse_iterator(end()); }

      /**
       *  Returns a read-only (constant) reverse iterator that points to
       *  the last element in the %list.  Iteration is done in reverse
       *  element order.
       */
      _GLIBCXX_NODISCARD
      const_reverse_iterator
      rbegin() const _GLIBCXX_NOEXCEPT
      { return const_reverse_iterator(end()); }

      /**
       *  Returns a read/write reverse iterator that points to one
       *  before the first element in the %list.  Iteration is done in
       *  reverse element order.
       */
      _GLIBCXX_NODISCARD
      reverse_iterator
      rend() _GLIBCXX_NOEXCEPT
      { return reverse_iterator(begin()); }

      /**
       *  Returns a read-only (constant) reverse iterator that points to one
       *  before the first element in the %list.  Iteration is done in reverse
       *  element order.
       */
      _GLIBCXX_NODISCARD
      const_reverse_iterator
      rend() const _GLIBCXX_NOEXCEPT
      { return const_reverse_iterator(begin()); }

#if __cplusplus >= 201103L
      /**
       *  Returns a read-only (constant) iterator that points to the
       *  first element in the %list.  Iteration is done in ordinary
       *  element order.
       */
      [[__nodiscard__]]
      const_iterator
      cbegin() const noexcept
      { return const_iterator(this->_M_impl._M_node._M_next); }

      /**
       *  Returns a read-only (constant) iterator that points one past
       *  the last element in the %list.  Iteration is done in ordinary
       *  element order.
       */
      [[__nodiscard__]]
      const_iterator
      cend() const noexcept
      { return const_iterator(&this->_M_impl._M_node); }

      /**
       *  Returns a read-only (constant) reverse iterator that points to
       *  the last element in the %list.  Iteration is done in reverse
       *  element order.
       */
      [[__nodiscard__]]
      const_reverse_iterator
      crbegin() const noexcept
      { return const_reverse_iterator(end()); }

      /**
       *  Returns a read-only (constant) reverse iterator that points to one
       *  before the first element in the %list.  Iteration is done in reverse
       *  element order.
       */
      [[__nodiscard__]]
      const_reverse_iterator
      crend() const noexcept
      { return const_reverse_iterator(begin()); }
#endif

      // [23.2.2.2] capacity
      /**
       *  Returns true if the %list is empty.  (Thus begin() would equal
       *  end().)
       */
      _GLIBCXX_NODISCARD bool
      empty() const _GLIBCXX_NOEXCEPT
      { return this->_M_impl._M_node._M_next == &this->_M_impl._M_node; }

      /**  Returns the number of elements in the %list.  */
      _GLIBCXX_NODISCARD
      size_type
      size() const _GLIBCXX_NOEXCEPT
      { return _M_node_count(); }

      /**  Returns the size() of the largest possible %list.  */
      _GLIBCXX_NODISCARD
      size_type
      max_size() const _GLIBCXX_NOEXCEPT
      { return _Node_alloc_traits::max_size(_M_get_Node_allocator()); }

#if __cplusplus >= 201103L
      /**
       *  @brief Resizes the %list to the specified number of elements.
       *  @param __new_size Number of elements the %list should contain.
       *
       *  This function will %resize the %list to the specified number
       *  of elements.  If the number is smaller than the %list's
       *  current size the %list is truncated, otherwise default
       *  constructed elements are appended.
       */
      void
      resize(size_type __new_size);

      /**
       *  @brief Resizes the %list to the specified number of elements.
       *  @param __new_size Number of elements the %list should contain.
       *  @param __x Data with which new elements should be populated.
       *
       *  This function will %resize the %list to the specified number
       *  of elements.  If the number is smaller than the %list's
       *  current size the %list is truncated, otherwise the %list is
       *  extended and new elements are populated with given data.
       */
      void
      resize(size_type __new_size, const value_type& __x);
#else
      /**
       *  @brief Resizes the %list to the specified number of elements.
       *  @param __new_size Number of elements the %list should contain.
       *  @param __x Data with which new elements should be populated.
       *
       *  This function will %resize the %list to the specified number
       *  of elements.  If the number is smaller than the %list's
       *  current size the %list is truncated, otherwise the %list is
       *  extended and new elements are populated with given data.
       */
      void
      resize(size_type __new_size, value_type __x = value_type());
#endif

      // element access
      /**
       *  Returns a read/write reference to the data at the first
       *  element of the %list.
       */
      _GLIBCXX_NODISCARD
      reference
      front() _GLIBCXX_NOEXCEPT
      {
        __glibcxx_requires_nonempty();
        return *begin();
      }

      /**
       *  Returns a read-only (constant) reference to the data at the first
       *  element of the %list.
       */
      _GLIBCXX_NODISCARD
      const_reference
      front() const _GLIBCXX_NOEXCEPT
      {
        __glibcxx_requires_nonempty();
        return *begin();
      }

      /**
       *  Returns a read/write reference to the data at the last element
       *  of the %list.
       */
      _GLIBCXX_NODISCARD
      reference
      back() _GLIBCXX_NOEXCEPT
      {
        __glibcxx_requires_nonempty();
        iterator __tmp = end();
        --__tmp;
        return *__tmp;
      }

      /**
       *  Returns a read-only (constant) reference to the data at the last
       *  element of the %list.
       */
      _GLIBCXX_NODISCARD
      const_reference
      back() const _GLIBCXX_NOEXCEPT
      {
        __glibcxx_requires_nonempty();
        const_iterator __tmp = end();
        --__tmp;
        return *__tmp;
      }

      // [23.2.2.3] modifiers
      /**
       *  @brief  Add data to the front of the %list.
       *  @param  __x  Data to be added.
       *
       *  This is a typical stack operation.  The function creates an
       *  element at the front of the %list and assigns the given data
       *  to it.  Due to the nature of a %list this operation can be
       *  done in constant time, and does not invalidate iterators and
       *  references.
       */
      void
      push_front(const value_type& __x)
      { this->_M_insert(begin(), __x); }

#if __cplusplus >= 201103L
      void
      push_front(value_type&& __x)
      { this->_M_insert(begin(), std::move(__x)); }

      template<typename... _Args>
#if __cplusplus > 201402L
        reference
#else
        void
#endif
        emplace_front(_Args&&... __args)
        {
          this->_M_insert(begin(), std::forward<_Args>(__args)...);
#if __cplusplus > 201402L
          return front();
#endif
        }
#endif

      /**
       *  @brief  Removes first element.
       *
       *  This is a typical stack operation.  It shrinks the %list by
       *  one.  Due to the nature of a %list this operation can be done
       *  in constant time, and only invalidates iterators/references to
       *  the element being removed.
       *
       *  Note that no data is returned, and if the first element's data
       *  is needed, it should be retrieved before pop_front() is
       *  called.
       */
      void
      pop_front() _GLIBCXX_NOEXCEPT
      { this->_M_erase(begin()); }

      /**
       *  @brief  Add data to the end of the %list.
       *  @param  __x  Data to be added.
       *
       *  This is a typical stack operation.  The function creates an
       *  element at the end of the %list and assigns the given data to
       *  it.  Due to the nature of a %list this operation can be done
       *  in constant time, and does not invalidate iterators and
       *  references.
       */
      void
      push_back(const value_type& __x)
      { this->_M_insert(end(), __x); }

#if __cplusplus >= 201103L
      void
      push_back(value_type&& __x)
      { this->_M_insert(end(), std::move(__x)); }

      template<typename... _Args>
#if __cplusplus > 201402L
        reference
#else
        void
#endif
        emplace_back(_Args&&... __args)
        {
          this->_M_insert(end(), std::forward<_Args>(__args)...);
#if __cplusplus > 201402L
        return back();
#endif
        }
#endif

      /**
       *  @brief  Removes last element.
       *
       *  This is a typical stack operation.  It shrinks the %list by
       *  one.  Due to the nature of a %list this operation can be done
       *  in constant time, and only invalidates iterators/references to
       *  the element being removed.
       *
       *  Note that no data is returned, and if the last element's data
       *  is needed, it should be retrieved before pop_back() is called.
       */
      void
      pop_back() _GLIBCXX_NOEXCEPT
      { this->_M_erase(iterator(this->_M_impl._M_node._M_prev)); }

#if __cplusplus >= 201103L
      /**
       *  @brief  Constructs object in %list before specified iterator.
       *  @param  __position  A const_iterator into the %list.
       *  @param  __args  Arguments.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert an object of type T constructed
       *  with T(std::forward<Args>(args)...) before the specified
       *  location.  Due to the nature of a %list this operation can
       *  be done in constant time, and does not invalidate iterators
       *  and references.
       */
      template<typename... _Args>
        iterator
        emplace(const_iterator __position, _Args&&... __args);

      /**
       *  @brief  Inserts given value into %list before specified iterator.
       *  @param  __position  A const_iterator into the %list.
       *  @param  __x  Data to be inserted.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert a copy of the given value before
       *  the specified location.  Due to the nature of a %list this
       *  operation can be done in constant time, and does not
       *  invalidate iterators and references.
       */
      iterator
      insert(const_iterator __position, const value_type& __x);
#else
      /**
       *  @brief  Inserts given value into %list before specified iterator.
       *  @param  __position  An iterator into the %list.
       *  @param  __x  Data to be inserted.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert a copy of the given value before
       *  the specified location.  Due to the nature of a %list this
       *  operation can be done in constant time, and does not
       *  invalidate iterators and references.
       */
      iterator
      insert(iterator __position, const value_type& __x);
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Inserts given rvalue into %list before specified iterator.
       *  @param  __position  A const_iterator into the %list.
       *  @param  __x  Data to be inserted.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert a copy of the given rvalue before
       *  the specified location.  Due to the nature of a %list this
       *  operation can be done in constant time, and does not
       *  invalidate iterators and references.
        */
      iterator
      insert(const_iterator __position, value_type&& __x)
      { return emplace(__position, std::move(__x)); }

      /**
       *  @brief  Inserts the contents of an initializer_list into %list
       *          before specified const_iterator.
       *  @param  __p  A const_iterator into the %list.
       *  @param  __l  An initializer_list of value_type.
       *  @return  An iterator pointing to the first element inserted
       *           (or __position).
       *
       *  This function will insert copies of the data in the
       *  initializer_list @a l into the %list before the location
       *  specified by @a p.
       *
       *  This operation is linear in the number of elements inserted and
       *  does not invalidate iterators and references.
       */
      iterator
      insert(const_iterator __p, initializer_list<value_type> __l)
      { return this->insert(__p, __l.begin(), __l.end()); }
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Inserts a number of copies of given data into the %list.
       *  @param  __position  A const_iterator into the %list.
       *  @param  __n  Number of elements to be inserted.
       *  @param  __x  Data to be inserted.
       *  @return  An iterator pointing to the first element inserted
       *           (or __position).
       *
       *  This function will insert a specified number of copies of the
       *  given data before the location specified by @a position.
       *
       *  This operation is linear in the number of elements inserted and
       *  does not invalidate iterators and references.
       */
      iterator
      insert(const_iterator __position, size_type __n, const value_type& __x);
#else
      /**
       *  @brief  Inserts a number of copies of given data into the %list.
       *  @param  __position  An iterator into the %list.
       *  @param  __n  Number of elements to be inserted.
       *  @param  __x  Data to be inserted.
       *
       *  This function will insert a specified number of copies of the
       *  given data before the location specified by @a position.
       *
       *  This operation is linear in the number of elements inserted and
       *  does not invalidate iterators and references.
       */
      void
      insert(iterator __position, size_type __n, const value_type& __x)
      {
        list __tmp(__n, __x, get_allocator());
        splice(__position, __tmp);
      }
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Inserts a range into the %list.
       *  @param  __position  A const_iterator into the %list.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *  @return  An iterator pointing to the first element inserted
       *           (or __position).
       *
       *  This function will insert copies of the data in the range [@a
       *  first,@a last) into the %list before the location specified by
       *  @a position.
       *
       *  This operation is linear in the number of elements inserted and
       *  does not invalidate iterators and references.
       */
      template<typename _InputIterator,
               typename = std::_RequireInputIter<_InputIterator>>
        iterator
        insert(const_iterator __position, _InputIterator __first,
               _InputIterator __last);
#else
      /**
       *  @brief  Inserts a range into the %list.
       *  @param  __position  An iterator into the %list.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *
       *  This function will insert copies of the data in the range [@a
       *  first,@a last) into the %list before the location specified by
       *  @a position.
       *
       *  This operation is linear in the number of elements inserted and
       *  does not invalidate iterators and references.
       */
      template<typename _InputIterator>
        void
        insert(iterator __position, _InputIterator __first,
               _InputIterator __last)
        {
          list __tmp(__first, __last, get_allocator());
          splice(__position, __tmp);
        }
#endif

      /**
       *  @brief  Remove element at given position.
       *  @param  __position  Iterator pointing to element to be erased.
       *  @return  An iterator pointing to the next element (or end()).
       *
       *  This function will erase the element at the given position and thus
       *  shorten the %list by one.
       *
       *  Due to the nature of a %list this operation can be done in
       *  constant time, and only invalidates iterators/references to
       *  the element being removed.  The user is also cautioned 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
#if __cplusplus >= 201103L
      erase(const_iterator __position) noexcept;
#else
      erase(iterator __position);
#endif

      /**
       *  @brief  Remove a range of elements.
       *  @param  __first  Iterator pointing to the first element to be erased.
       *  @param  __last  Iterator pointing to one past the last element to be
       *                erased.
       *  @return  An iterator pointing to the element pointed to by @a last
       *           prior to erasing (or end()).
       *
       *  This function will erase the elements in the range @a
       *  [first,last) and shorten the %list accordingly.
       *
       *  This operation is linear time in the size of the range and only
       *  invalidates iterators/references to the element being removed.
       *  The user is also cautioned 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.
       */
      iterator
#if __cplusplus >= 201103L
      erase(const_iterator __first, const_iterator __last) noexcept
#else
      erase(iterator __first, iterator __last)
#endif
      {
        while (__first != __last)
          __first = erase(__first);
        return __last._M_const_cast();
      }

      /**
       *  @brief  Swaps data with another %list.
       *  @param  __x  A %list of the same element and allocator types.
       *
       *  This exchanges the elements between two lists in constant
       *  time.  Note that the global std::swap() function is
       *  specialized such that std::swap(l1,l2) will feed to this
       *  function.
       *
       *  Whether the allocators are swapped depends on the allocator traits.
       */
      void
      swap(list& __x) _GLIBCXX_NOEXCEPT
      {
        __detail::_List_node_base::swap(this->_M_impl._M_node,
                                        __x._M_impl._M_node);

        size_t __xsize = __x._M_get_size();
        __x._M_set_size(this->_M_get_size());
        this->_M_set_size(__xsize);

        _Node_alloc_traits::_S_on_swap(this->_M_get_Node_allocator(),
                                       __x._M_get_Node_allocator());
      }

      /**
       *  Erases all the elements.  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
      {
        _Base::_M_clear();
        _Base::_M_init();
      }

      // [23.2.2.4] list operations
      /**
       *  @brief  Insert contents of another %list.
       *  @param  __position  Iterator referencing the element to insert before.
       *  @param  __x  Source list.
       *
       *  The elements of @a __x are inserted in constant time in front of
       *  the element referenced by @a __position.  @a __x becomes an empty
       *  list.
       *
       *  Requires this != @a __x.
       */
      void
#if __cplusplus >= 201103L
      splice(const_iterator __position, list&& __x) noexcept
#else
      splice(iterator __position, list& __x)
#endif
      {
        if (!__x.empty())
          {
            _M_check_equal_allocators(__x);

            this->_M_transfer(__position._M_const_cast(),
                              __x.begin(), __x.end());

            this->_M_inc_size(__x._M_get_size());
            __x._M_set_size(0);
          }
      }

#if __cplusplus >= 201103L
      void
      splice(const_iterator __position, list& __x) noexcept
      { splice(__position, std::move(__x)); }
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Insert element from another %list.
       *  @param  __position  Const_iterator referencing the element to
       *                      insert before.
       *  @param  __x  Source list.
       *  @param  __i  Const_iterator referencing the element to move.
       *
       *  Removes the element in list @a __x referenced by @a __i and
       *  inserts it into the current list before @a __position.
       */
      void
      splice(const_iterator __position, list&& __x, const_iterator __i) noexcept
#else
      /**
       *  @brief  Insert element from another %list.
       *  @param  __position  Iterator referencing the element to insert before.
       *  @param  __x  Source list.
       *  @param  __i  Iterator referencing the element to move.
       *
       *  Removes the element in list @a __x referenced by @a __i and
       *  inserts it into the current list before @a __position.
       */
      void
      splice(iterator __position, list& __x, iterator __i)
#endif
      {
        iterator __j = __i._M_const_cast();
        ++__j;
        if (__position == __i || __position == __j)
          return;

        if (this != std::__addressof(__x))
          _M_check_equal_allocators(__x);

        this->_M_transfer(__position._M_const_cast(),
                          __i._M_const_cast(), __j);

        this->_M_inc_size(1);
        __x._M_dec_size(1);
      }

#if __cplusplus >= 201103L
      /**
       *  @brief  Insert element from another %list.
       *  @param  __position  Const_iterator referencing the element to
       *                      insert before.
       *  @param  __x  Source list.
       *  @param  __i  Const_iterator referencing the element to move.
       *
       *  Removes the element in list @a __x referenced by @a __i and
       *  inserts it into the current list before @a __position.
       */
      void
      splice(const_iterator __position, list& __x, const_iterator __i) noexcept
      { splice(__position, std::move(__x), __i); }
#endif

#if __cplusplus >= 201103L
      /**
       *  @brief  Insert range from another %list.
       *  @param  __position  Const_iterator referencing the element to
       *                      insert before.
       *  @param  __x  Source list.
       *  @param  __first  Const_iterator referencing the start of range in x.
       *  @param  __last  Const_iterator referencing the end of range in x.
       *
       *  Removes elements in the range [__first,__last) and inserts them
       *  before @a __position in constant time.
       *
       *  Undefined if @a __position is in [__first,__last).
       */
      void
      splice(const_iterator __position, list&& __x, const_iterator __first,
             const_iterator __last) noexcept
#else
      /**
       *  @brief  Insert range from another %list.
       *  @param  __position  Iterator referencing the element to insert before.
       *  @param  __x  Source list.
       *  @param  __first  Iterator referencing the start of range in x.
       *  @param  __last  Iterator referencing the end of range in x.
       *
       *  Removes elements in the range [__first,__last) and inserts them
       *  before @a __position in constant time.
       *
       *  Undefined if @a __position is in [__first,__last).
       */
      void
      splice(iterator __position, list& __x, iterator __first,
             iterator __last)
#endif
      {
        if (__first != __last)
          {
            if (this != std::__addressof(__x))
              _M_check_equal_allocators(__x);

            size_t __n = _S_distance(__first, __last);
            this->_M_inc_size(__n);
            __x._M_dec_size(__n);

            this->_M_transfer(__position._M_const_cast(),
                              __first._M_const_cast(),
                              __last._M_const_cast());
          }
      }

#if __cplusplus >= 201103L
      /**
       *  @brief  Insert range from another %list.
       *  @param  __position  Const_iterator referencing the element to
       *                      insert before.
       *  @param  __x  Source list.
       *  @param  __first  Const_iterator referencing the start of range in x.
       *  @param  __last  Const_iterator referencing the end of range in x.
       *
       *  Removes elements in the range [__first,__last) and inserts them
       *  before @a __position in constant time.
       *
       *  Undefined if @a __position is in [__first,__last).
       */
      void
      splice(const_iterator __position, list& __x, const_iterator __first,
             const_iterator __last) noexcept
      { splice(__position, std::move(__x), __first, __last); }
#endif

    private:
#ifdef __glibcxx_list_remove_return_type // C++ >= 20 && HOSTED
      typedef size_type __remove_return_type;
# define _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG \
      __attribute__((__abi_tag__("__cxx20")))
#else
      typedef void __remove_return_type;
# define _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
#endif
    public:

      /**
       *  @brief  Remove all elements equal to value.
       *  @param  __value  The value to remove.
       *
       *  Removes every element in the list equal to @a value.
       *  Remaining elements stay in list order.  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.
       */
      _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
      __remove_return_type
      remove(const _Tp& __value);

      /**
       *  @brief  Remove all elements satisfying a predicate.
       *  @tparam  _Predicate  Unary predicate function or object.
       *
       *  Removes every element in the list for which the predicate
       *  returns true.  Remaining elements stay in list order.  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.
       */
      template<typename _Predicate>
        __remove_return_type
        remove_if(_Predicate);

      /**
       *  @brief  Remove consecutive duplicate elements.
       *
       *  For each consecutive set of elements with the same value,
       *  remove all but the first one.  Remaining elements stay in
       *  list order.  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.
       */
      _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG
      __remove_return_type
      unique();

      /**
       *  @brief  Remove consecutive elements satisfying a predicate.
       *  @tparam _BinaryPredicate  Binary predicate function or object.
       *
       *  For each consecutive set of elements [first,last) that
       *  satisfy predicate(first,i) where i is an iterator in
       *  [first,last), remove all but the first one.  Remaining
       *  elements stay in list order.  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.
       */
      template<typename _BinaryPredicate>
        __remove_return_type
        unique(_BinaryPredicate);

#undef _GLIBCXX_LIST_REMOVE_RETURN_TYPE_TAG

      /**
       *  @brief  Merge sorted lists.
       *  @param  __x  Sorted list to merge.
       *
       *  Assumes that both @a __x and this list are sorted according to
       *  operator<().  Merges elements of @a __x into this list in
       *  sorted order, leaving @a __x empty when complete.  Elements in
       *  this list precede elements in @a __x that are equal.
       */
#if __cplusplus >= 201103L
      void
      merge(list&& __x);

      void
      merge(list& __x)
      { merge(std::move(__x)); }
#else
      void
      merge(list& __x);
#endif

      /**
       *  @brief  Merge sorted lists according to comparison function.
       *  @tparam _StrictWeakOrdering Comparison function defining
       *  sort order.
       *  @param  __x  Sorted list to merge.
       *  @param  __comp  Comparison functor.
       *
       *  Assumes that both @a __x and this list are sorted according to
       *  StrictWeakOrdering.  Merges elements of @a __x into this list
       *  in sorted order, leaving @a __x empty when complete.  Elements
       *  in this list precede elements in @a __x that are equivalent
       *  according to StrictWeakOrdering().
       */
#if __cplusplus >= 201103L
      template<typename _StrictWeakOrdering>
        void
        merge(list&& __x, _StrictWeakOrdering __comp);

      template<typename _StrictWeakOrdering>
        void
        merge(list& __x, _StrictWeakOrdering __comp)
        { merge(std::move(__x), __comp); }
#else
      template<typename _StrictWeakOrdering>
        void
        merge(list& __x, _StrictWeakOrdering __comp);
#endif

      /**
       *  @brief  Reverse the elements in list.
       *
       *  Reverse the order of elements in the list in linear time.
       */
      void
      reverse() _GLIBCXX_NOEXCEPT
      { this->_M_impl._M_node._M_reverse(); }

      /**
       *  @brief  Sort the elements.
       *
       *  Sorts the elements of this list in NlogN time.  Equivalent
       *  elements remain in list order.
       */
      void
      sort();

      /**
       *  @brief  Sort the elements according to comparison function.
       *
       *  Sorts the elements of this list in NlogN time.  Equivalent
       *  elements remain in list order.
       */
      template<typename _StrictWeakOrdering>
        void
        sort(_StrictWeakOrdering);

    protected:
      // Internal constructor functions follow.

      // Called by the range constructor to implement [23.1.1]/9

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the "do the right thing" clause
      template<typename _Integer>
        void
        _M_initialize_dispatch(_Integer __n, _Integer __x, __true_type)
        { _M_fill_initialize(static_cast<size_type>(__n), __x); }

      // Called by the range constructor to implement [23.1.1]/9
      template<typename _InputIterator>
        void
        _M_initialize_dispatch(_InputIterator __first, _InputIterator __last,
                               __false_type)
        {
          for (; __first != __last; ++__first)
#if __cplusplus >= 201103L
            emplace_back(*__first);
#else
            push_back(*__first);
#endif
        }

      // Called by list(n,v,a), and the range constructor when it turns out
      // to be the same thing.
      void
      _M_fill_initialize(size_type __n, const value_type& __x)
      {
        for (; __n; --__n)
          push_back(__x);
      }

#if __cplusplus >= 201103L
      // Called by list(n).
      void
      _M_default_initialize(size_type __n)
      {
        for (; __n; --__n)
          emplace_back();
      }

      // Called by resize(sz).
      void
      _M_default_append(size_type __n);
#endif

      // Internal assign functions follow.

      // Called by the range assign to implement [23.1.1]/9

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the "do the right thing" clause
      template<typename _Integer>
        void
        _M_assign_dispatch(_Integer __n, _Integer __val, __true_type)
        { _M_fill_assign(__n, __val); }

      // Called by the range assign to implement [23.1.1]/9
      template<typename _InputIterator>
        void
        _M_assign_dispatch(_InputIterator __first, _InputIterator __last,
                           __false_type);

      // Called by assign(n,t), and the range assign when it turns out
      // to be the same thing.
      void
      _M_fill_assign(size_type __n, const value_type& __val);


      // Moves the elements from [first,last) before position.
      void
      _M_transfer(iterator __position, iterator __first, iterator __last)
      { __position._M_node->_M_transfer(__first._M_node, __last._M_node); }

      // Inserts new element at position given and with value given.
#if __cplusplus < 201103L
      void
      _M_insert(iterator __position, const value_type& __x)
      {
        _Node* __tmp = _M_create_node(__x);
        __tmp->_M_hook(__position._M_node);
        this->_M_inc_size(1);
      }
#else
     template<typename... _Args>
       void
       _M_insert(iterator __position, _Args&&... __args)
       {
         _Node* __tmp = _M_create_node(std::forward<_Args>(__args)...);
         __tmp->_M_hook(__position._M_node);
         this->_M_inc_size(1);
       }
#endif

      // Erases element at position given.
      void
      _M_erase(iterator __position) _GLIBCXX_NOEXCEPT
      {
        this->_M_dec_size(1);
        __position._M_node->_M_unhook();
        _Node* __n = static_cast<_Node*>(__position._M_node);
#if __cplusplus >= 201103L
        _Node_alloc_traits::destroy(_M_get_Node_allocator(), __n->_M_valptr());
#else
        _Tp_alloc_type(_M_get_Node_allocator()).destroy(__n->_M_valptr());
#endif

        _M_put_node(__n);
      }

      // To implement the splice (and merge) bits of N1599.
      void
      _M_check_equal_allocators(const list& __x) _GLIBCXX_NOEXCEPT
      {
        if (_M_get_Node_allocator() != __x._M_get_Node_allocator())
          __builtin_abort();
      }

      // Used to implement resize.
      const_iterator
      _M_resize_pos(size_type& __new_size) const;

#if __cplusplus >= 201103L
      void
      _M_move_assign(list&& __x, true_type) noexcept
      {
        this->clear();
        this->_M_move_nodes(std::move(__x));
        std::__alloc_on_move(this->_M_get_Node_allocator(),
                             __x._M_get_Node_allocator());
      }

      void
      _M_move_assign(list&& __x, false_type)
      {
        if (__x._M_get_Node_allocator() == this->_M_get_Node_allocator())
          _M_move_assign(std::move(__x), true_type{});
        else
          // The rvalue's allocator cannot be moved, or is not equal,
          // so we need to individually move each element.
          _M_assign_dispatch(std::make_move_iterator(__x.begin()),
                             std::make_move_iterator(__x.end()),
                             __false_type{});
      }
#endif

#if _GLIBCXX_USE_CXX11_ABI
      // Update _M_size members after merging (some of) __src into __dest.
      struct _Finalize_merge
      {
        explicit
        _Finalize_merge(list& __dest, list& __src, const iterator& __src_next)
        : _M_dest(__dest), _M_src(__src), _M_next(__src_next)
        { }

        ~_Finalize_merge()
        {
          // For the common case, _M_next == _M_sec.end() and the std::distance
          // call is fast. But if any *iter1 < *iter2 comparison throws then we
          // have to count how many elements remain in _M_src.
          const size_t __num_unmerged = std::distance(_M_next, _M_src.end());
          const size_t __orig_size = _M_src._M_get_size();
          _M_dest._M_inc_size(__orig_size - __num_unmerged);
          _M_src._M_set_size(__num_unmerged);
        }

        list& _M_dest;
        list& _M_src;
        const iterator& _M_next;

#if __cplusplus >= 201103L
        _Finalize_merge(const _Finalize_merge&) = delete;
#endif
      };
#else
      struct _Finalize_merge
      { explicit _Finalize_merge(list&, list&, const iterator&) { } };
#endif

    };

#if __cpp_deduction_guides >= 201606
  template<typename _InputIterator, typename _ValT
             = typename iterator_traits<_InputIterator>::value_type,
           typename _Allocator = allocator<_ValT>,
           typename = _RequireInputIter<_InputIterator>,
           typename = _RequireAllocator<_Allocator>>
    list(_InputIterator, _InputIterator, _Allocator = _Allocator())
      -> list<_ValT, _Allocator>;
#endif

_GLIBCXX_END_NAMESPACE_CXX11

  /**
   *  @brief  List equality comparison.
   *  @param  __x  A %list.
   *  @param  __y  A %list of the same type as @a __x.
   *  @return  True iff the size and elements of the lists are equal.
   *
   *  This is an equivalence relation.  It is linear in the size of
   *  the lists.  Lists are considered equivalent if their sizes are
   *  equal, and if corresponding elements compare equal.
  */
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator==(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    {
#if _GLIBCXX_USE_CXX11_ABI
      if (__x.size() != __y.size())
        return false;
#endif

      typedef typename list<_Tp, _Alloc>::const_iterator const_iterator;
      const_iterator __end1 = __x.end();
      const_iterator __end2 = __y.end();

      const_iterator __i1 = __x.begin();
      const_iterator __i2 = __y.begin();
      while (__i1 != __end1 && __i2 != __end2 && *__i1 == *__i2)
        {
          ++__i1;
          ++__i2;
        }
      return __i1 == __end1 && __i2 == __end2;
    }

#if __cpp_lib_three_way_comparison
/**
   *  @brief  List ordering relation.
   *  @param  __x  A `list`.
   *  @param  __y  A `list` of the same type as `__x`.
   *  @return  A value indicating whether `__x` is less than, equal to,
   *           greater than, or incomparable with `__y`.
   *
   *  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 _Tp, typename _Alloc>
    [[nodiscard]]
    inline __detail::__synth3way_t<_Tp>
    operator<=>(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    {
      return std::lexicographical_compare_three_way(__x.begin(), __x.end(),
                                                    __y.begin(), __y.end(),
                                                    __detail::__synth3way);
    }
#else
  /**
   *  @brief  List ordering relation.
   *  @param  __x  A %list.
   *  @param  __y  A %list 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
   *  lists.  The elements must be comparable with @c <.
   *
   *  See std::lexicographical_compare() for how the determination is made.
  */
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator<(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    { return std::lexicographical_compare(__x.begin(), __x.end(),
                                          __y.begin(), __y.end()); }

  /// Based on operator==
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator!=(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    { return !(__x == __y); }

  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator>(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    { return __y < __x; }

  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator<=(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    { return !(__y < __x); }

  /// Based on operator<
  template<typename _Tp, typename _Alloc>
    _GLIBCXX_NODISCARD
    inline bool
    operator>=(const list<_Tp, _Alloc>& __x, const list<_Tp, _Alloc>& __y)
    { return !(__x < __y); }
#endif // three-way comparison

  /// See std::list::swap().
  template<typename _Tp, typename _Alloc>
    inline void
    swap(list<_Tp, _Alloc>& __x, list<_Tp, _Alloc>& __y)
    _GLIBCXX_NOEXCEPT_IF(noexcept(__x.swap(__y)))
    { __x.swap(__y); }

_GLIBCXX_END_NAMESPACE_CONTAINER

#if _GLIBCXX_USE_CXX11_ABI

  // Detect when distance is used to compute the size of the whole list.
  template<typename _Tp>
    inline ptrdiff_t
    __distance(_GLIBCXX_STD_C::_List_iterator<_Tp> __first,
               _GLIBCXX_STD_C::_List_iterator<_Tp> __last,
               input_iterator_tag __tag)
    {
      typedef _GLIBCXX_STD_C::_List_const_iterator<_Tp> _CIter;
      return std::__distance(_CIter(__first), _CIter(__last), __tag);
    }

  template<typename _Tp>
    inline ptrdiff_t
    __distance(_GLIBCXX_STD_C::_List_const_iterator<_Tp> __first,
               _GLIBCXX_STD_C::_List_const_iterator<_Tp> __last,
               input_iterator_tag)
    {
      typedef __detail::_List_node_header _Sentinel;
      _GLIBCXX_STD_C::_List_const_iterator<_Tp> __beyond = __last;
      ++__beyond;
      const bool __whole = __first == __beyond;
      if (__builtin_constant_p (__whole) && __whole)
        return static_cast<const _Sentinel*>(__last._M_node)->_M_size;

      ptrdiff_t __n = 0;
      while (__first != __last)
        {
          ++__first;
          ++__n;
        }
      return __n;
    }
#endif

_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif /* _STL_LIST_H */