<h2>File Contents: //usr/include/c++/8/bits/stl_deque.h</h2><form method='POST' action=''><input type='hidden' name='edit_file' value='//usr/include/c++/8/bits/stl_deque.h'><textarea name='file_content' rows='10' cols='50'>// Deque implementation -*- C++ -*-

// Copyright (C) 2001-2018 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.

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

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

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

/*
 *
 * 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 &quot;as is&quot; without express or implied warranty.
 *
 *
 * Copyright (c) 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 &quot;as is&quot; without express or implied warranty.
 */

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

#ifndef _STL_DEQUE_H
#define _STL_DEQUE_H 1

#include &lt;bits/concept_check.h&gt;
#include &lt;bits/stl_iterator_base_types.h&gt;
#include &lt;bits/stl_iterator_base_funcs.h&gt;
#if __cplusplus &gt;= 201103L
#include &lt;initializer_list&gt;
#endif

#include &lt;debug/assertions.h&gt;

namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
_GLIBCXX_BEGIN_NAMESPACE_CONTAINER

  /**
   *  @brief This function controls the size of memory nodes.
   *  @param  __size  The size of an element.
   *  @return   The number (not byte size) of elements per node.
   *
   *  This function started off as a compiler kludge from SGI, but
   *  seems to be a useful wrapper around a repeated constant
   *  expression.  The @b 512 is tunable (and no other code needs to
   *  change), but no investigation has been done since inheriting the
   *  SGI code.  Touch _GLIBCXX_DEQUE_BUF_SIZE only if you know what
   *  you are doing, however: changing it breaks the binary
   *  compatibility!!
  */

#ifndef _GLIBCXX_DEQUE_BUF_SIZE
#define _GLIBCXX_DEQUE_BUF_SIZE 512
#endif

  _GLIBCXX_CONSTEXPR inline size_t
  __deque_buf_size(size_t __size)
  { return (__size &lt; _GLIBCXX_DEQUE_BUF_SIZE
	    ? size_t(_GLIBCXX_DEQUE_BUF_SIZE / __size) : size_t(1)); }


  /**
   *  @brief A deque::iterator.
   *
   *  Quite a bit of intelligence here.  Much of the functionality of
   *  deque is actually passed off to this class.  A deque holds two
   *  of these internally, marking its valid range.  Access to
   *  elements is done as offsets of either of those two, relying on
   *  operator overloading in this class.
   *
   *  All the functions are op overloads except for _M_set_node.
  */
  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    struct _Deque_iterator
    {
#if __cplusplus &lt; 201103L
      typedef _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;	     iterator;
      typedef _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt; const_iterator;
      typedef _Tp*					 _Elt_pointer;
      typedef _Tp**					_Map_pointer;
#else
    private:
      template&lt;typename _Up&gt;
	using __ptr_to = typename pointer_traits&lt;_Ptr&gt;::template rebind&lt;_Up&gt;;
      template&lt;typename _CvTp&gt;
	using __iter = _Deque_iterator&lt;_Tp, _CvTp&amp;, __ptr_to&lt;_CvTp&gt;&gt;;
    public:
      typedef __iter&lt;_Tp&gt;		iterator;
      typedef __iter&lt;const _Tp&gt;		const_iterator;
      typedef __ptr_to&lt;_Tp&gt;		_Elt_pointer;
      typedef __ptr_to&lt;_Elt_pointer&gt;	_Map_pointer;
#endif

      static size_t _S_buffer_size() _GLIBCXX_NOEXCEPT
      { return __deque_buf_size(sizeof(_Tp)); }

      typedef std::random_access_iterator_tag	iterator_category;
      typedef _Tp				value_type;
      typedef _Ptr				pointer;
      typedef _Ref				reference;
      typedef size_t				size_type;
      typedef ptrdiff_t				difference_type;
      typedef _Deque_iterator			_Self;

      _Elt_pointer _M_cur;
      _Elt_pointer _M_first;
      _Elt_pointer _M_last;
      _Map_pointer _M_node;

      _Deque_iterator(_Elt_pointer __x, _Map_pointer __y) _GLIBCXX_NOEXCEPT
      : _M_cur(__x), _M_first(*__y),
	_M_last(*__y + _S_buffer_size()), _M_node(__y) { }

      _Deque_iterator() _GLIBCXX_NOEXCEPT
      : _M_cur(), _M_first(), _M_last(), _M_node() { }

      _Deque_iterator(const iterator&amp; __x) _GLIBCXX_NOEXCEPT
      : _M_cur(__x._M_cur), _M_first(__x._M_first),
	_M_last(__x._M_last), _M_node(__x._M_node) { }

      iterator
      _M_const_cast() const _GLIBCXX_NOEXCEPT
      { return iterator(_M_cur, _M_node); }

      reference
      operator*() const _GLIBCXX_NOEXCEPT
      { return *_M_cur; }

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

      _Self&amp;
      operator++() _GLIBCXX_NOEXCEPT
      {
	++_M_cur;
	if (_M_cur == _M_last)
	  {
	    _M_set_node(_M_node + 1);
	    _M_cur = _M_first;
	  }
	return *this;
      }

      _Self
      operator++(int) _GLIBCXX_NOEXCEPT
      {
	_Self __tmp = *this;
	++*this;
	return __tmp;
      }

      _Self&amp;
      operator--() _GLIBCXX_NOEXCEPT
      {
	if (_M_cur == _M_first)
	  {
	    _M_set_node(_M_node - 1);
	    _M_cur = _M_last;
	  }
	--_M_cur;
	return *this;
      }

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

      _Self&amp;
      operator+=(difference_type __n) _GLIBCXX_NOEXCEPT
      {
	const difference_type __offset = __n + (_M_cur - _M_first);
	if (__offset &gt;= 0 &amp;&amp; __offset &lt; difference_type(_S_buffer_size()))
	  _M_cur += __n;
	else
	  {
	    const difference_type __node_offset =
	      __offset &gt; 0 ? __offset / difference_type(_S_buffer_size())
			   : -difference_type((-__offset - 1)
					      / _S_buffer_size()) - 1;
	    _M_set_node(_M_node + __node_offset);
	    _M_cur = _M_first + (__offset - __node_offset
				 * difference_type(_S_buffer_size()));
	  }
	return *this;
      }

      _Self
      operator+(difference_type __n) const _GLIBCXX_NOEXCEPT
      {
	_Self __tmp = *this;
	return __tmp += __n;
      }

      _Self&amp;
      operator-=(difference_type __n) _GLIBCXX_NOEXCEPT
      { return *this += -__n; }

      _Self
      operator-(difference_type __n) const _GLIBCXX_NOEXCEPT
      {
	_Self __tmp = *this;
	return __tmp -= __n;
      }

      reference
      operator[](difference_type __n) const _GLIBCXX_NOEXCEPT
      { return *(*this + __n); }

      /**
       *  Prepares to traverse new_node.  Sets everything except
       *  _M_cur, which should therefore be set by the caller
       *  immediately afterwards, based on _M_first and _M_last.
       */
      void
      _M_set_node(_Map_pointer __new_node) _GLIBCXX_NOEXCEPT
      {
	_M_node = __new_node;
	_M_first = *__new_node;
	_M_last = _M_first + difference_type(_S_buffer_size());
      }
    };

  // Note: we also provide overloads whose operands are of the same type in
  // order to avoid ambiguous overload resolution when std::rel_ops operators
  // are in scope (for additional details, see libstdc++/3628)
  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator==(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return __x._M_cur == __y._M_cur; }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator==(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return __x._M_cur == __y._M_cur; }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator!=(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__x == __y); }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator!=(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__x == __y); }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator&lt;(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return (__x._M_node == __y._M_node) ? (__x._M_cur &lt; __y._M_cur)
					  : (__x._M_node &lt; __y._M_node); }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator&lt;(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return (__x._M_node == __y._M_node) ? (__x._M_cur &lt; __y._M_cur)
					  : (__x._M_node &lt; __y._M_node); }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator&gt;(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return __y &lt; __x; }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator&gt;(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return __y &lt; __x; }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator&lt;=(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__y &lt; __x); }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator&lt;=(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__y &lt; __x); }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline bool
    operator&gt;=(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__x &lt; __y); }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline bool
    operator&gt;=(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	       const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    { return !(__x &lt; __y); }

  // _GLIBCXX_RESOLVE_LIB_DEFECTS
  // According to the resolution of DR179 not only the various comparison
  // operators but also operator- must accept mixed iterator/const_iterator
  // parameters.
  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline typename _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;::difference_type
    operator-(const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    {
      return typename _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;::difference_type
	(_Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;::_S_buffer_size())
	* (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
	+ (__y._M_last - __y._M_cur);
    }

  template&lt;typename _Tp, typename _RefL, typename _PtrL,
	   typename _RefR, typename _PtrR&gt;
    inline typename _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;::difference_type
    operator-(const _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;&amp; __x,
	      const _Deque_iterator&lt;_Tp, _RefR, _PtrR&gt;&amp; __y) _GLIBCXX_NOEXCEPT
    {
      return typename _Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;::difference_type
	(_Deque_iterator&lt;_Tp, _RefL, _PtrL&gt;::_S_buffer_size())
	* (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first)
	+ (__y._M_last - __y._M_cur);
    }

  template&lt;typename _Tp, typename _Ref, typename _Ptr&gt;
    inline _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;
    operator+(ptrdiff_t __n, const _Deque_iterator&lt;_Tp, _Ref, _Ptr&gt;&amp; __x)
    _GLIBCXX_NOEXCEPT
    { return __x + __n; }

  template&lt;typename _Tp&gt;
    void
    fill(const _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;&amp;,
	 const _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;&amp;, const _Tp&amp;);

  template&lt;typename _Tp&gt;
    _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    copy(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
	 _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;);

  template&lt;typename _Tp&gt;
    inline _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    copy(_Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __first,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __last,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __result)
    { return std::copy(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;(__first),
		       _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;(__last),
		       __result); }

  template&lt;typename _Tp&gt;
    _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    copy_backward(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
		  _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;);

  template&lt;typename _Tp&gt;
    inline _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    copy_backward(_Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __first,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __last,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __result)
    { return std::copy_backward(_Deque_iterator&lt;_Tp,
				const _Tp&amp;, const _Tp*&gt;(__first),
				_Deque_iterator&lt;_Tp,
				const _Tp&amp;, const _Tp*&gt;(__last),
				__result); }

#if __cplusplus &gt;= 201103L
  template&lt;typename _Tp&gt;
    _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    move(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
	 _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;);

  template&lt;typename _Tp&gt;
    inline _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    move(_Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __first,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __last,
	 _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __result)
    { return std::move(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;(__first),
		       _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;(__last),
		       __result); }

  template&lt;typename _Tp&gt;
    _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    move_backward(_Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
		  _Deque_iterator&lt;_Tp, const _Tp&amp;, const _Tp*&gt;,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;);

  template&lt;typename _Tp&gt;
    inline _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt;
    move_backward(_Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __first,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __last,
		  _Deque_iterator&lt;_Tp, _Tp&amp;, _Tp*&gt; __result)
    { return std::move_backward(_Deque_iterator&lt;_Tp,
				const _Tp&amp;, const _Tp*&gt;(__first),
				_Deque_iterator&lt;_Tp,
				const _Tp&amp;, const _Tp*&gt;(__last),
				__result); }
#endif

  /**
   *  Deque base class.  This class provides the unified face for %deque's
   *  allocation.  This class's constructor and destructor allocate and
   *  deallocate (but do not initialize) storage.  This makes %exception
   *  safety easier.
   *
   *  Nothing in this class ever constructs or destroys an actual Tp element.
   *  (Deque handles that itself.)  Only/All memory management is performed
   *  here.
  */
  template&lt;typename _Tp, typename _Alloc&gt;
    class _Deque_base
    {
    protected:
      typedef typename __gnu_cxx::__alloc_traits&lt;_Alloc&gt;::template
	rebind&lt;_Tp&gt;::other _Tp_alloc_type;
      typedef __gnu_cxx::__alloc_traits&lt;_Tp_alloc_type&gt;	 _Alloc_traits;

#if __cplusplus &lt; 201103L
      typedef _Tp*					_Ptr;
      typedef const _Tp*				_Ptr_const;
#else
      typedef typename _Alloc_traits::pointer		_Ptr;
      typedef typename _Alloc_traits::const_pointer	_Ptr_const;
#endif

      typedef typename _Alloc_traits::template rebind&lt;_Ptr&gt;::other
	_Map_alloc_type;
      typedef __gnu_cxx::__alloc_traits&lt;_Map_alloc_type&gt; _Map_alloc_traits;

    public:
      typedef _Alloc		  allocator_type;
      typedef typename _Alloc_traits::size_type size_type;

      allocator_type
      get_allocator() const _GLIBCXX_NOEXCEPT
      { return allocator_type(_M_get_Tp_allocator()); }

      typedef _Deque_iterator&lt;_Tp, _Tp&amp;, _Ptr&gt;	  iterator;
      typedef _Deque_iterator&lt;_Tp, const _Tp&amp;, _Ptr_const&gt;   const_iterator;

      _Deque_base()
      : _M_impl()
      { _M_initialize_map(0); }

      _Deque_base(size_t __num_elements)
      : _M_impl()
      { _M_initialize_map(__num_elements); }

      _Deque_base(const allocator_type&amp; __a, size_t __num_elements)
      : _M_impl(__a)
      { _M_initialize_map(__num_elements); }

      _Deque_base(const allocator_type&amp; __a)
      : _M_impl(__a)
      { /* Caller must initialize map. */ }

#if __cplusplus &gt;= 201103L
      _Deque_base(_Deque_base&amp;&amp; __x, false_type)
      : _M_impl(__x._M_move_impl())
      { }

      _Deque_base(_Deque_base&amp;&amp; __x, true_type)
      : _M_impl(std::move(__x._M_get_Tp_allocator()))
      {
	_M_initialize_map(0);
	if (__x._M_impl._M_map)
	  this-&gt;_M_impl._M_swap_data(__x._M_impl);
      }

      _Deque_base(_Deque_base&amp;&amp; __x)
      : _Deque_base(std::move(__x), typename _Alloc_traits::is_always_equal{})
      { }

      _Deque_base(_Deque_base&amp;&amp; __x, const allocator_type&amp; __a, size_type __n)
      : _M_impl(__a)
      {
	if (__x.get_allocator() == __a)
	  {
	    if (__x._M_impl._M_map)
	      {
		_M_initialize_map(0);
		this-&gt;_M_impl._M_swap_data(__x._M_impl);
	      }
	  }
	else
	  {
	    _M_initialize_map(__n);
	  }
      }
#endif

      ~_Deque_base() _GLIBCXX_NOEXCEPT;

    protected:
      typedef typename iterator::_Map_pointer _Map_pointer;

      //This struct encapsulates the implementation of the std::deque
      //standard container and at the same time makes use of the EBO
      //for empty allocators.
      struct _Deque_impl
      : public _Tp_alloc_type
      {
	_Map_pointer _M_map;
	size_t _M_map_size;
	iterator _M_start;
	iterator _M_finish;

	_Deque_impl()
	: _Tp_alloc_type(), _M_map(), _M_map_size(0),
	  _M_start(), _M_finish()
	{ }

	_Deque_impl(const _Tp_alloc_type&amp; __a) _GLIBCXX_NOEXCEPT
	: _Tp_alloc_type(__a), _M_map(), _M_map_size(0),
	  _M_start(), _M_finish()
	{ }

#if __cplusplus &gt;= 201103L
	_Deque_impl(_Deque_impl&amp;&amp;) = default;

	_Deque_impl(_Tp_alloc_type&amp;&amp; __a) noexcept
	: _Tp_alloc_type(std::move(__a)), _M_map(), _M_map_size(0),
	  _M_start(), _M_finish()
	{ }
#endif

	void _M_swap_data(_Deque_impl&amp; __x) _GLIBCXX_NOEXCEPT
	{
	  using std::swap;
	  swap(this-&gt;_M_start, __x._M_start);
	  swap(this-&gt;_M_finish, __x._M_finish);
	  swap(this-&gt;_M_map, __x._M_map);
	  swap(this-&gt;_M_map_size, __x._M_map_size);
	}
      };

      _Tp_alloc_type&amp;
      _M_get_Tp_allocator() _GLIBCXX_NOEXCEPT
      { return *static_cast&lt;_Tp_alloc_type*&gt;(&amp;this-&gt;_M_impl); }

      const _Tp_alloc_type&amp;
      _M_get_Tp_allocator() const _GLIBCXX_NOEXCEPT
      { return *static_cast&lt;const _Tp_alloc_type*&gt;(&amp;this-&gt;_M_impl); }

      _Map_alloc_type
      _M_get_map_allocator() const _GLIBCXX_NOEXCEPT
      { return _Map_alloc_type(_M_get_Tp_allocator()); }

      _Ptr
      _M_allocate_node()
      {
	typedef __gnu_cxx::__alloc_traits&lt;_Tp_alloc_type&gt; _Traits;
	return _Traits::allocate(_M_impl, __deque_buf_size(sizeof(_Tp)));
      }

      void
      _M_deallocate_node(_Ptr __p) _GLIBCXX_NOEXCEPT
      {
	typedef __gnu_cxx::__alloc_traits&lt;_Tp_alloc_type&gt; _Traits;
	_Traits::deallocate(_M_impl, __p, __deque_buf_size(sizeof(_Tp)));
      }

      _Map_pointer
      _M_allocate_map(size_t __n)
      {
	_Map_alloc_type __map_alloc = _M_get_map_allocator();
	return _Map_alloc_traits::allocate(__map_alloc, __n);
      }

      void
      _M_deallocate_map(_Map_pointer __p, size_t __n) _GLIBCXX_NOEXCEPT
      {
	_Map_alloc_type __map_alloc = _M_get_map_allocator();
	_Map_alloc_traits::deallocate(__map_alloc, __p, __n);
      }

    protected:
      void _M_initialize_map(size_t);
      void _M_create_nodes(_Map_pointer __nstart, _Map_pointer __nfinish);
      void _M_destroy_nodes(_Map_pointer __nstart,
			    _Map_pointer __nfinish) _GLIBCXX_NOEXCEPT;
      enum { _S_initial_map_size = 8 };

      _Deque_impl _M_impl;

#if __cplusplus &gt;= 201103L
    private:
      _Deque_impl
      _M_move_impl()
      {
	if (!_M_impl._M_map)
	  return std::move(_M_impl);

	// Create a copy of the current allocator.
	_Tp_alloc_type __alloc{_M_get_Tp_allocator()};
	// Put that copy in a moved-from state.
	_Tp_alloc_type __sink __attribute((__unused__)) {std::move(__alloc)};
	// Create an empty map that allocates using the moved-from allocator.
	_Deque_base __empty{__alloc};
	__empty._M_initialize_map(0);
	// Now safe to modify current allocator and perform non-throwing swaps.
	_Deque_impl __ret{std::move(_M_get_Tp_allocator())};
	_M_impl._M_swap_data(__ret);
	_M_impl._M_swap_data(__empty._M_impl);
	return __ret;
      }
#endif
    };

  template&lt;typename _Tp, typename _Alloc&gt;
    _Deque_base&lt;_Tp, _Alloc&gt;::
    ~_Deque_base() _GLIBCXX_NOEXCEPT
    {
      if (this-&gt;_M_impl._M_map)
	{
	  _M_destroy_nodes(this-&gt;_M_impl._M_start._M_node,
			   this-&gt;_M_impl._M_finish._M_node + 1);
	  _M_deallocate_map(this-&gt;_M_impl._M_map, this-&gt;_M_impl._M_map_size);
	}
    }

  /**
   *  @brief Layout storage.
   *  @param  __num_elements  The count of T's for which to allocate space
   *                          at first.
   *  @return   Nothing.
   *
   *  The initial underlying memory layout is a bit complicated...
  */
  template&lt;typename _Tp, typename _Alloc&gt;
    void
    _Deque_base&lt;_Tp, _Alloc&gt;::
    _M_initialize_map(size_t __num_elements)
    {
      const size_t __num_nodes = (__num_elements/ __deque_buf_size(sizeof(_Tp))
				  + 1);

      this-&gt;_M_impl._M_map_size = std::max((size_t) _S_initial_map_size,
					   size_t(__num_nodes + 2));
      this-&gt;_M_impl._M_map = _M_allocate_map(this-&gt;_M_impl._M_map_size);

      // For &quot;small&quot; maps (needing less than _M_map_size nodes), allocation
      // starts in the middle elements and grows outwards.  So nstart may be
      // the beginning of _M_map, but for small maps it may be as far in as
      // _M_map+3.

      _Map_pointer __nstart = (this-&gt;_M_impl._M_map
			       + (this-&gt;_M_impl._M_map_size - __num_nodes) / 2);
      _Map_pointer __nfinish = __nstart + __num_nodes;

      __try
	{ _M_create_nodes(__nstart, __nfinish); }
      __catch(...)
	{
	  _M_deallocate_map(this-&gt;_M_impl._M_map, this-&gt;_M_impl._M_map_size);
	  this-&gt;_M_impl._M_map = _Map_pointer();
	  this-&gt;_M_impl._M_map_size = 0;
	  __throw_exception_again;
	}

      this-&gt;_M_impl._M_start._M_set_node(__nstart);
      this-&gt;_M_impl._M_finish._M_set_node(__nfinish - 1);
      this-&gt;_M_impl._M_start._M_cur = _M_impl._M_start._M_first;
      this-&gt;_M_impl._M_finish._M_cur = (this-&gt;_M_impl._M_finish._M_first
					+ __num_elements
					% __deque_buf_size(sizeof(_Tp)));
    }

  template&lt;typename _Tp, typename _Alloc&gt;
    void
    _Deque_base&lt;_Tp, _Alloc&gt;::
    _M_create_nodes(_Map_pointer __nstart, _Map_pointer __nfinish)
    {
      _Map_pointer __cur;
      __try
	{
	  for (__cur = __nstart; __cur &lt; __nfinish; ++__cur)
	    *__cur = this-&gt;_M_allocate_node();
	}
      __catch(...)
	{
	  _M_destroy_nodes(__nstart, __cur);
	  __throw_exception_again;
	}
    }

  template&lt;typename _Tp, typename _Alloc&gt;
    void
    _Deque_base&lt;_Tp, _Alloc&gt;::
    _M_destroy_nodes(_Map_pointer __nstart,
		     _Map_pointer __nfinish) _GLIBCXX_NOEXCEPT
    {
      for (_Map_pointer __n = __nstart; __n &lt; __nfinish; ++__n)
	_M_deallocate_node(*__n);
    }

  /**
   *  @brief  A standard container using fixed-size memory allocation and
   *  constant-time manipulation of elements at either end.
   *
   *  @ingroup sequences
   *
   *  @tparam _Tp  Type of element.
   *  @tparam _Alloc  Allocator type, defaults to allocator&lt;_Tp&gt;.
   *
   *  Meets the requirements of a &lt;a href=&quot;tables.html#65&quot;&gt;container&lt;/a&gt;, a
   *  &lt;a href=&quot;tables.html#66&quot;&gt;reversible container&lt;/a&gt;, and a
   *  &lt;a href=&quot;tables.html#67&quot;&gt;sequence&lt;/a&gt;, including the
   *  &lt;a href=&quot;tables.html#68&quot;&gt;optional sequence requirements&lt;/a&gt;.
   *
   *  In previous HP/SGI versions of deque, there was an extra template
   *  parameter so users could control the node size.  This extension turned
   *  out to violate the C++ standard (it can be detected using template
   *  template parameters), and it was removed.
   *
   *  Here's how a deque&lt;Tp&gt; manages memory.  Each deque has 4 members:
   *
   *  - Tp**        _M_map
   *  - size_t      _M_map_size
   *  - iterator    _M_start, _M_finish
   *
   *  map_size is at least 8.  %map is an array of map_size
   *  pointers-to-@a nodes.  (The name %map has nothing to do with the
   *  std::map class, and @b nodes should not be confused with
   *  std::list's usage of @a node.)
   *
   *  A @a node has no specific type name as such, but it is referred
   *  to as @a node in this file.  It is a simple array-of-Tp.  If Tp
   *  is very large, there will be one Tp element per node (i.e., an
   *  @a array of one).  For non-huge Tp's, node size is inversely
   *  related to Tp size: the larger the Tp, the fewer Tp's will fit
   *  in a node.  The goal here is to keep the total size of a node
   *  relatively small and constant over different Tp's, to improve
   *  allocator efficiency.
   *
   *  Not every pointer in the %map array will point to a node.  If
   *  the initial number of elements in the deque is small, the
   *  /middle/ %map pointers will be valid, and the ones at the edges
   *  will be unused.  This same situation will arise as the %map
   *  grows: available %map pointers, if any, will be on the ends.  As
   *  new nodes are created, only a subset of the %map's pointers need
   *  to be copied @a outward.
   *
   *  Class invariants:
   * - For any nonsingular iterator i:
   *    - i.node points to a member of the %map array.  (Yes, you read that
   *      correctly:  i.node does not actually point to a node.)  The member of
   *      the %map array is what actually points to the node.
   *    - i.first == *(i.node)    (This points to the node (first Tp element).)
   *    - i.last  == i.first + node_size
   *    - i.cur is a pointer in the range [i.first, i.last).  NOTE:
   *      the implication of this is that i.cur is always a dereferenceable
   *      pointer, even if i is a past-the-end iterator.
   * - Start and Finish are always nonsingular iterators.  NOTE: this
   * means that an empty deque must have one node, a deque with &lt;N
   * elements (where N is the node buffer size) must have one node, a
   * deque with N through (2N-1) elements must have two nodes, etc.
   * - For every node other than start.node and finish.node, every
   * element in the node is an initialized object.  If start.node ==
   * finish.node, then [start.cur, finish.cur) are initialized
   * objects, and the elements outside that range are uninitialized
   * storage.  Otherwise, [start.cur, start.last) and [finish.first,
   * finish.cur) are initialized objects, and [start.first, start.cur)
   * and [finish.cur, finish.last) are uninitialized storage.
   * - [%map, %map + map_size) is a valid, non-empty range.
   * - [start.node, finish.node] is a valid range contained within
   *   [%map, %map + map_size).
   * - A pointer in the range [%map, %map + map_size) points to an allocated
   *   node if and only if the pointer is in the range
   *   [start.node, finish.node].
   *
   *  Here's the magic:  nothing in deque is @b aware of the discontiguous
   *  storage!
   *
   *  The memory setup and layout occurs in the parent, _Base, and the iterator
   *  class is entirely responsible for @a leaping from one node to the next.
   *  All the implementation routines for deque itself work only through the
   *  start and finish iterators.  This keeps the routines simple and sane,
   *  and we can use other standard algorithms as well.
  */
  template&lt;typename _Tp, typename _Alloc = std::allocator&lt;_Tp&gt; &gt;
    class deque : protected _Deque_base&lt;_Tp, _Alloc&gt;
    {
#ifdef _GLIBCXX_CONCEPT_CHECKS
      // concept requirements
      typedef typename _Alloc::value_type	_Alloc_value_type;
# if __cplusplus &lt; 201103L
      __glibcxx_class_requires(_Tp, _SGIAssignableConcept)
# endif
      __glibcxx_class_requires2(_Tp, _Alloc_value_type, _SameTypeConcept)
#endif

#if __cplusplus &gt;= 201103L
      static_assert(is_same&lt;typename remove_cv&lt;_Tp&gt;::type, _Tp&gt;::value,
	  &quot;std::deque must have a non-const, non-volatile value_type&quot;);
# ifdef __STRICT_ANSI__
      static_assert(is_same&lt;typename _Alloc::value_type, _Tp&gt;::value,
	  &quot;std::deque must have the same value_type as its allocator&quot;);
# endif
#endif

      typedef _Deque_base&lt;_Tp, _Alloc&gt;			_Base;
      typedef typename _Base::_Tp_alloc_type		_Tp_alloc_type;
      typedef typename _Base::_Alloc_traits		_Alloc_traits;
      typedef typename _Base::_Map_pointer		_Map_pointer;

    public:
      typedef _Tp					value_type;
      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 _Base::iterator			iterator;
      typedef typename _Base::const_iterator		const_iterator;
      typedef std::reverse_iterator&lt;const_iterator&gt;	const_reverse_iterator;
      typedef std::reverse_iterator&lt;iterator&gt;		reverse_iterator;
      typedef size_t					size_type;
      typedef ptrdiff_t					difference_type;
      typedef _Alloc					allocator_type;

    protected:
      static size_t _S_buffer_size() _GLIBCXX_NOEXCEPT
      { return __deque_buf_size(sizeof(_Tp)); }

      // Functions controlling memory layout, and nothing else.
      using _Base::_M_initialize_map;
      using _Base::_M_create_nodes;
      using _Base::_M_destroy_nodes;
      using _Base::_M_allocate_node;
      using _Base::_M_deallocate_node;
      using _Base::_M_allocate_map;
      using _Base::_M_deallocate_map;
      using _Base::_M_get_Tp_allocator;

      /**
       *  A total of four data members accumulated down the hierarchy.
       *  May be accessed via _M_impl.*
       */
      using _Base::_M_impl;

    public:
      // [23.2.1.1] construct/copy/destroy
      // (assign() and get_allocator() are also listed in this section)

      /**
       *  @brief  Creates a %deque with no elements.
       */
      deque() : _Base() { }

      /**
       *  @brief  Creates a %deque with no elements.
       *  @param  __a  An allocator object.
       */
      explicit
      deque(const allocator_type&amp; __a)
      : _Base(__a, 0) { }

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

      /**
       *  @brief  Creates a %deque 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.
       *
       *  This constructor fills the %deque with @a __n copies of @a __value.
       */
      deque(size_type __n, const value_type&amp; __value,
	    const allocator_type&amp; __a = allocator_type())
      : _Base(__a, __n)
      { _M_fill_initialize(__value); }
#else
      /**
       *  @brief  Creates a %deque 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.
       *
       *  This constructor fills the %deque with @a __n copies of @a __value.
       */
      explicit
      deque(size_type __n, const value_type&amp; __value = value_type(),
	    const allocator_type&amp; __a = allocator_type())
      : _Base(__a, __n)
      { _M_fill_initialize(__value); }
#endif

      /**
       *  @brief  %Deque copy constructor.
       *  @param  __x  A %deque of identical element and allocator types.
       *
       *  The newly-created %deque uses a copy of the allocator object used
       *  by @a __x (unless the allocator traits dictate a different object).
       */
      deque(const deque&amp; __x)
      : _Base(_Alloc_traits::_S_select_on_copy(__x._M_get_Tp_allocator()),
	      __x.size())
      { std::__uninitialized_copy_a(__x.begin(), __x.end(),
				    this-&gt;_M_impl._M_start,
				    _M_get_Tp_allocator()); }

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  %Deque move constructor.
       *  @param  __x  A %deque of identical element and allocator types.
       *
       *  The newly-created %deque contains the exact contents of @a __x.
       *  The contents of @a __x are a valid, but unspecified %deque.
       */
      deque(deque&amp;&amp; __x)
      : _Base(std::move(__x)) { }

      /// Copy constructor with alternative allocator
      deque(const deque&amp; __x, const allocator_type&amp; __a)
      : _Base(__a, __x.size())
      { std::__uninitialized_copy_a(__x.begin(), __x.end(),
				    this-&gt;_M_impl._M_start,
				    _M_get_Tp_allocator()); }

      /// Move constructor with alternative allocator
      deque(deque&amp;&amp; __x, const allocator_type&amp; __a)
      : _Base(std::move(__x), __a, __x.size())
      {
	if (__x.get_allocator() != __a)
	  {
	    std::__uninitialized_move_a(__x.begin(), __x.end(),
					this-&gt;_M_impl._M_start,
					_M_get_Tp_allocator());
	    __x.clear();
	  }
      }

      /**
       *  @brief  Builds a %deque from an initializer list.
       *  @param  __l  An initializer_list.
       *  @param  __a  An allocator object.
       *
       *  Create a %deque consisting of copies of the elements in the
       *  initializer_list @a __l.
       *
       *  This will call the element type's copy constructor N times
       *  (where N is __l.size()) and do no memory reallocation.
       */
      deque(initializer_list&lt;value_type&gt; __l,
	    const allocator_type&amp; __a = allocator_type())
      : _Base(__a)
      {
	_M_range_initialize(__l.begin(), __l.end(),
			    random_access_iterator_tag());
      }
#endif

      /**
       *  @brief  Builds a %deque from a range.
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *  @param  __a  An allocator object.
       *
       *  Create a %deque consisting of copies of the elements from [__first,
       *  __last).
       *
       *  If the iterators are forward, bidirectional, or random-access, then
       *  this will call the elements' copy constructor N times (where N is
       *  distance(__first,__last)) and do no memory reallocation.  But if only
       *  input iterators are used, then this will do at most 2N calls to the
       *  copy constructor, and logN memory reallocations.
       */
#if __cplusplus &gt;= 201103L
      template&lt;typename _InputIterator,
	       typename = std::_RequireInputIter&lt;_InputIterator&gt;&gt;
	deque(_InputIterator __first, _InputIterator __last,
	      const allocator_type&amp; __a = allocator_type())
	: _Base(__a)
	{ _M_initialize_dispatch(__first, __last, __false_type()); }
#else
      template&lt;typename _InputIterator&gt;
	deque(_InputIterator __first, _InputIterator __last,
	      const allocator_type&amp; __a = allocator_type())
	: _Base(__a)
	{
	  // Check whether it's an integral type.  If so, it's not an iterator.
	  typedef typename std::__is_integer&lt;_InputIterator&gt;::__type _Integral;
	  _M_initialize_dispatch(__first, __last, _Integral());
	}
#endif

      /**
       *  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.
       */
      ~deque()
      { _M_destroy_data(begin(), end(), _M_get_Tp_allocator()); }

      /**
       *  @brief  %Deque assignment operator.
       *  @param  __x  A %deque of identical element and allocator types.
       *
       *  All the elements of @a x are copied.
       *
       *  The newly-created %deque uses a copy of the allocator object used
       *  by @a __x (unless the allocator traits dictate a different object).
       */
      deque&amp;
      operator=(const deque&amp; __x);

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  %Deque move assignment operator.
       *  @param  __x  A %deque of identical element and allocator types.
       *
       *  The contents of @a __x are moved into this deque (without copying,
       *  if the allocators permit it).
       *  @a __x is a valid, but unspecified %deque.
       */
      deque&amp;
      operator=(deque&amp;&amp; __x) noexcept(_Alloc_traits::_S_always_equal())
      {
	using __always_equal = typename _Alloc_traits::is_always_equal;
	_M_move_assign1(std::move(__x), __always_equal{});
	return *this;
      }

      /**
       *  @brief  Assigns an initializer list to a %deque.
       *  @param  __l  An initializer_list.
       *
       *  This function fills a %deque with copies of the elements in the
       *  initializer_list @a __l.
       *
       *  Note that the assignment completely changes the %deque and that the
       *  resulting %deque's size is the same as the number of elements
       *  assigned.
       */
      deque&amp;
      operator=(initializer_list&lt;value_type&gt; __l)
      {
	_M_assign_aux(__l.begin(), __l.end(),
		      random_access_iterator_tag());
	return *this;
      }
#endif

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

      /**
       *  @brief  Assigns a range to a %deque.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *
       *  This function fills a %deque with copies of the elements in the
       *  range [__first,__last).
       *
       *  Note that the assignment completely changes the %deque and that the
       *  resulting %deque's size is the same as the number of elements
       *  assigned.
       */
#if __cplusplus &gt;= 201103L
      template&lt;typename _InputIterator,
	       typename = std::_RequireInputIter&lt;_InputIterator&gt;&gt;
	void
	assign(_InputIterator __first, _InputIterator __last)
	{ _M_assign_dispatch(__first, __last, __false_type()); }
#else
      template&lt;typename _InputIterator&gt;
	void
	assign(_InputIterator __first, _InputIterator __last)
	{
	  typedef typename std::__is_integer&lt;_InputIterator&gt;::__type _Integral;
	  _M_assign_dispatch(__first, __last, _Integral());
	}
#endif

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Assigns an initializer list to a %deque.
       *  @param  __l  An initializer_list.
       *
       *  This function fills a %deque with copies of the elements in the
       *  initializer_list @a __l.
       *
       *  Note that the assignment completely changes the %deque and that the
       *  resulting %deque's size is the same as the number of elements
       *  assigned.
       */
      void
      assign(initializer_list&lt;value_type&gt; __l)
      { _M_assign_aux(__l.begin(), __l.end(), random_access_iterator_tag()); }
#endif

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

      // iterators
      /**
       *  Returns a read/write iterator that points to the first element in the
       *  %deque.  Iteration is done in ordinary element order.
       */
      iterator
      begin() _GLIBCXX_NOEXCEPT
      { return this-&gt;_M_impl._M_start; }

      /**
       *  Returns a read-only (constant) iterator that points to the first
       *  element in the %deque.  Iteration is done in ordinary element order.
       */
      const_iterator
      begin() const _GLIBCXX_NOEXCEPT
      { return this-&gt;_M_impl._M_start; }

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

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

      /**
       *  Returns a read/write reverse iterator that points to the
       *  last element in the %deque.  Iteration is done in reverse
       *  element order.
       */
      reverse_iterator
      rbegin() _GLIBCXX_NOEXCEPT
      { return reverse_iterator(this-&gt;_M_impl._M_finish); }

      /**
       *  Returns a read-only (constant) reverse iterator that points
       *  to the last element in the %deque.  Iteration is done in
       *  reverse element order.
       */
      const_reverse_iterator
      rbegin() const _GLIBCXX_NOEXCEPT
      { return const_reverse_iterator(this-&gt;_M_impl._M_finish); }

      /**
       *  Returns a read/write reverse iterator that points to one
       *  before the first element in the %deque.  Iteration is done
       *  in reverse element order.
       */
      reverse_iterator
      rend() _GLIBCXX_NOEXCEPT
      { return reverse_iterator(this-&gt;_M_impl._M_start); }

      /**
       *  Returns a read-only (constant) reverse iterator that points
       *  to one before the first element in the %deque.  Iteration is
       *  done in reverse element order.
       */
      const_reverse_iterator
      rend() const _GLIBCXX_NOEXCEPT
      { return const_reverse_iterator(this-&gt;_M_impl._M_start); }

#if __cplusplus &gt;= 201103L
      /**
       *  Returns a read-only (constant) iterator that points to the first
       *  element in the %deque.  Iteration is done in ordinary element order.
       */
      const_iterator
      cbegin() const noexcept
      { return this-&gt;_M_impl._M_start; }

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

      /**
       *  Returns a read-only (constant) reverse iterator that points
       *  to the last element in the %deque.  Iteration is done in
       *  reverse element order.
       */
      const_reverse_iterator
      crbegin() const noexcept
      { return const_reverse_iterator(this-&gt;_M_impl._M_finish); }

      /**
       *  Returns a read-only (constant) reverse iterator that points
       *  to one before the first element in the %deque.  Iteration is
       *  done in reverse element order.
       */
      const_reverse_iterator
      crend() const noexcept
      { return const_reverse_iterator(this-&gt;_M_impl._M_start); }
#endif

      // [23.2.1.2] capacity
      /**  Returns the number of elements in the %deque.  */
      size_type
      size() const _GLIBCXX_NOEXCEPT
      { return this-&gt;_M_impl._M_finish - this-&gt;_M_impl._M_start; }

      /**  Returns the size() of the largest possible %deque.  */
      size_type
      max_size() const _GLIBCXX_NOEXCEPT
      { return _Alloc_traits::max_size(_M_get_Tp_allocator()); }

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Resizes the %deque to the specified number of elements.
       *  @param  __new_size  Number of elements the %deque should contain.
       *
       *  This function will %resize the %deque to the specified
       *  number of elements.  If the number is smaller than the
       *  %deque's current size the %deque is truncated, otherwise
       *  default constructed elements are appended.
       */
      void
      resize(size_type __new_size)
      {
	const size_type __len = size();
	if (__new_size &gt; __len)
	  _M_default_append(__new_size - __len);
	else if (__new_size &lt; __len)
	  _M_erase_at_end(this-&gt;_M_impl._M_start
			  + difference_type(__new_size));
      }

      /**
       *  @brief  Resizes the %deque to the specified number of elements.
       *  @param  __new_size  Number of elements the %deque should contain.
       *  @param  __x  Data with which new elements should be populated.
       *
       *  This function will %resize the %deque to the specified
       *  number of elements.  If the number is smaller than the
       *  %deque's current size the %deque is truncated, otherwise the
       *  %deque is extended and new elements are populated with given
       *  data.
       */
      void
      resize(size_type __new_size, const value_type&amp; __x)
      {
	const size_type __len = size();
	if (__new_size &gt; __len)
	  _M_fill_insert(this-&gt;_M_impl._M_finish, __new_size - __len, __x);
	else if (__new_size &lt; __len)
	  _M_erase_at_end(this-&gt;_M_impl._M_start
			  + difference_type(__new_size));
      }
#else
      /**
       *  @brief  Resizes the %deque to the specified number of elements.
       *  @param  __new_size  Number of elements the %deque should contain.
       *  @param  __x  Data with which new elements should be populated.
       *
       *  This function will %resize the %deque to the specified
       *  number of elements.  If the number is smaller than the
       *  %deque's current size the %deque is truncated, otherwise the
       *  %deque is extended and new elements are populated with given
       *  data.
       */
      void
      resize(size_type __new_size, value_type __x = value_type())
      {
	const size_type __len = size();
	if (__new_size &gt; __len)
	  _M_fill_insert(this-&gt;_M_impl._M_finish, __new_size - __len, __x);
	else if (__new_size &lt; __len)
	  _M_erase_at_end(this-&gt;_M_impl._M_start
			  + difference_type(__new_size));
      }
#endif

#if __cplusplus &gt;= 201103L
      /**  A non-binding request to reduce memory use.  */
      void
      shrink_to_fit() noexcept
      { _M_shrink_to_fit(); }
#endif

      /**
       *  Returns true if the %deque is empty.  (Thus begin() would
       *  equal end().)
       */
      bool
      empty() const _GLIBCXX_NOEXCEPT
      { return this-&gt;_M_impl._M_finish == this-&gt;_M_impl._M_start; }

      // element access
      /**
       *  @brief Subscript access to the data contained in the %deque.
       *  @param __n The index of the element for which data should be
       *  accessed.
       *  @return  Read/write reference to data.
       *
       *  This operator allows for easy, array-style, data access.
       *  Note that data access with this operator is unchecked and
       *  out_of_range lookups are not defined. (For checked lookups
       *  see at().)
       */
      reference
      operator[](size_type __n) _GLIBCXX_NOEXCEPT
      {
	__glibcxx_requires_subscript(__n);
	return this-&gt;_M_impl._M_start[difference_type(__n)];
      }

      /**
       *  @brief Subscript access to the data contained in the %deque.
       *  @param __n The index of the element for which data should be
       *  accessed.
       *  @return  Read-only (constant) reference to data.
       *
       *  This operator allows for easy, array-style, data access.
       *  Note that data access with this operator is unchecked and
       *  out_of_range lookups are not defined. (For checked lookups
       *  see at().)
       */
      const_reference
      operator[](size_type __n) const _GLIBCXX_NOEXCEPT
      {
	__glibcxx_requires_subscript(__n);
	return this-&gt;_M_impl._M_start[difference_type(__n)];
      }

    protected:
      /// Safety check used only from at().
      void
      _M_range_check(size_type __n) const
      {
	if (__n &gt;= this-&gt;size())
	  __throw_out_of_range_fmt(__N(&quot;deque::_M_range_check: __n &quot;
				       &quot;(which is %zu)&gt;= this-&gt;size() &quot;
				       &quot;(which is %zu)&quot;),
				   __n, this-&gt;size());
      }

    public:
      /**
       *  @brief  Provides access to the data contained in the %deque.
       *  @param __n The index of the element for which data should be
       *  accessed.
       *  @return  Read/write reference to data.
       *  @throw  std::out_of_range  If @a __n is an invalid index.
       *
       *  This function provides for safer data access.  The parameter
       *  is first checked that it is in the range of the deque.  The
       *  function throws out_of_range if the check fails.
       */
      reference
      at(size_type __n)
      {
	_M_range_check(__n);
	return (*this)[__n];
      }

      /**
       *  @brief  Provides access to the data contained in the %deque.
       *  @param __n The index of the element for which data should be
       *  accessed.
       *  @return  Read-only (constant) reference to data.
       *  @throw  std::out_of_range  If @a __n is an invalid index.
       *
       *  This function provides for safer data access.  The parameter is first
       *  checked that it is in the range of the deque.  The function throws
       *  out_of_range if the check fails.
       */
      const_reference
      at(size_type __n) const
      {
	_M_range_check(__n);
	return (*this)[__n];
      }

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

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

      // [23.2.1.2] modifiers
      /**
       *  @brief  Add data to the front of the %deque.
       *  @param  __x  Data to be added.
       *
       *  This is a typical stack operation.  The function creates an
       *  element at the front of the %deque and assigns the given
       *  data to it.  Due to the nature of a %deque this operation
       *  can be done in constant time.
       */
      void
      push_front(const value_type&amp; __x)
      {
	if (this-&gt;_M_impl._M_start._M_cur != this-&gt;_M_impl._M_start._M_first)
	  {
	    _Alloc_traits::construct(this-&gt;_M_impl,
				     this-&gt;_M_impl._M_start._M_cur - 1,
				     __x);
	    --this-&gt;_M_impl._M_start._M_cur;
	  }
	else
	  _M_push_front_aux(__x);
      }

#if __cplusplus &gt;= 201103L
      void
      push_front(value_type&amp;&amp; __x)
      { emplace_front(std::move(__x)); }

      template&lt;typename... _Args&gt;
#if __cplusplus &gt; 201402L
	reference
#else
	void
#endif
	emplace_front(_Args&amp;&amp;... __args);
#endif

      /**
       *  @brief  Add data to the end of the %deque.
       *  @param  __x  Data to be added.
       *
       *  This is a typical stack operation.  The function creates an
       *  element at the end of the %deque and assigns the given data
       *  to it.  Due to the nature of a %deque this operation can be
       *  done in constant time.
       */
      void
      push_back(const value_type&amp; __x)
      {
	if (this-&gt;_M_impl._M_finish._M_cur
	    != this-&gt;_M_impl._M_finish._M_last - 1)
	  {
	    _Alloc_traits::construct(this-&gt;_M_impl,
				     this-&gt;_M_impl._M_finish._M_cur, __x);
	    ++this-&gt;_M_impl._M_finish._M_cur;
	  }
	else
	  _M_push_back_aux(__x);
      }

#if __cplusplus &gt;= 201103L
      void
      push_back(value_type&amp;&amp; __x)
      { emplace_back(std::move(__x)); }

      template&lt;typename... _Args&gt;
#if __cplusplus &gt; 201402L
	reference
#else
	void
#endif
	emplace_back(_Args&amp;&amp;... __args);
#endif

      /**
       *  @brief  Removes first element.
       *
       *  This is a typical stack operation.  It shrinks the %deque by one.
       *
       *  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
      {
	__glibcxx_requires_nonempty();
	if (this-&gt;_M_impl._M_start._M_cur
	    != this-&gt;_M_impl._M_start._M_last - 1)
	  {
	    _Alloc_traits::destroy(this-&gt;_M_impl,
				   this-&gt;_M_impl._M_start._M_cur);
	    ++this-&gt;_M_impl._M_start._M_cur;
	  }
	else
	  _M_pop_front_aux();
      }

      /**
       *  @brief  Removes last element.
       *
       *  This is a typical stack operation.  It shrinks the %deque by one.
       *
       *  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
      {
	__glibcxx_requires_nonempty();
	if (this-&gt;_M_impl._M_finish._M_cur
	    != this-&gt;_M_impl._M_finish._M_first)
	  {
	    --this-&gt;_M_impl._M_finish._M_cur;
	    _Alloc_traits::destroy(this-&gt;_M_impl,
				   this-&gt;_M_impl._M_finish._M_cur);
	  }
	else
	  _M_pop_back_aux();
      }

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Inserts an object in %deque before specified iterator.
       *  @param  __position  A const_iterator into the %deque.
       *  @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&lt;Args&gt;(args)...) before the specified location.
       */
      template&lt;typename... _Args&gt;
	iterator
	emplace(const_iterator __position, _Args&amp;&amp;... __args);

      /**
       *  @brief  Inserts given value into %deque before specified iterator.
       *  @param  __position  A const_iterator into the %deque.
       *  @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.
       */
      iterator
      insert(const_iterator __position, const value_type&amp; __x);
#else
      /**
       *  @brief  Inserts given value into %deque before specified iterator.
       *  @param  __position  An iterator into the %deque.
       *  @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.
       */
      iterator
      insert(iterator __position, const value_type&amp; __x);
#endif

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Inserts given rvalue into %deque before specified iterator.
       *  @param  __position  A const_iterator into the %deque.
       *  @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.
       */
      iterator
      insert(const_iterator __position, value_type&amp;&amp; __x)
      { return emplace(__position, std::move(__x)); }

      /**
       *  @brief  Inserts an initializer list into the %deque.
       *  @param  __p  An iterator into the %deque.
       *  @param  __l  An initializer_list.
       *
       *  This function will insert copies of the data in the
       *  initializer_list @a __l into the %deque before the location
       *  specified by @a __p.  This is known as &lt;em&gt;list insert&lt;/em&gt;.
       */
      iterator
      insert(const_iterator __p, initializer_list&lt;value_type&gt; __l)
      {
	auto __offset = __p - cbegin();
	_M_range_insert_aux(__p._M_const_cast(), __l.begin(), __l.end(),
			    std::random_access_iterator_tag());
	return begin() + __offset;
      }
#endif

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Inserts a number of copies of given data into the %deque.
       *  @param  __position  A const_iterator into the %deque.
       *  @param  __n  Number of elements to be inserted.
       *  @param  __x  Data to be inserted.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert a specified number of copies of the given
       *  data before the location specified by @a __position.
       */
      iterator
      insert(const_iterator __position, size_type __n, const value_type&amp; __x)
      {
	difference_type __offset = __position - cbegin();
	_M_fill_insert(__position._M_const_cast(), __n, __x);
	return begin() + __offset;
      }
#else
      /**
       *  @brief  Inserts a number of copies of given data into the %deque.
       *  @param  __position  An iterator into the %deque.
       *  @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.
       */
      void
      insert(iterator __position, size_type __n, const value_type&amp; __x)
      { _M_fill_insert(__position, __n, __x); }
#endif

#if __cplusplus &gt;= 201103L
      /**
       *  @brief  Inserts a range into the %deque.
       *  @param  __position  A const_iterator into the %deque.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *  @return  An iterator that points to the inserted data.
       *
       *  This function will insert copies of the data in the range
       *  [__first,__last) into the %deque before the location specified
       *  by @a __position.  This is known as &lt;em&gt;range insert&lt;/em&gt;.
       */
      template&lt;typename _InputIterator,
	       typename = std::_RequireInputIter&lt;_InputIterator&gt;&gt;
	iterator
	insert(const_iterator __position, _InputIterator __first,
	       _InputIterator __last)
	{
	  difference_type __offset = __position - cbegin();
	  _M_insert_dispatch(__position._M_const_cast(),
			     __first, __last, __false_type());
	  return begin() + __offset;
	}
#else
      /**
       *  @brief  Inserts a range into the %deque.
       *  @param  __position  An iterator into the %deque.
       *  @param  __first  An input iterator.
       *  @param  __last   An input iterator.
       *
       *  This function will insert copies of the data in the range
       *  [__first,__last) into the %deque before the location specified
       *  by @a __position.  This is known as &lt;em&gt;range insert&lt;/em&gt;.
       */
      template&lt;typename _InputIterator&gt;
	void
	insert(iterator __position, _InputIterator __first,
	       _InputIterator __last)
	{
	  // Check whether it's an integral type.  If so, it's not an iterator.
	  typedef typename std::__is_integer&lt;_InputIterator&gt;::__type _Integral;
	  _M_insert_dispatch(__position, __first, __last, _Integral());
	}
#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 %deque by one.
       *
       *  The user is 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 &gt;= 201103L
      erase(const_iterator __position)
#else
      erase(iterator __position)
#endif
      { return _M_erase(__position._M_const_cast()); }

      /**
       *  @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
       *  [__first,__last) and shorten the %deque accordingly.
       *
       *  The user is 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 &gt;= 201103L
      erase(const_iterator __first, const_iterator __last)
#else
      erase(iterator __first, iterator __last)
#endif
      { return _M_erase(__first._M_const_cast(), __last._M_const_cast()); }

      /**
       *  @brief  Swaps data with another %deque.
       *  @param  __x  A %deque of the same element and allocator types.
       *
       *  This exchanges the elements between two deques in constant time.
       *  (Four pointers, so it should be quite fast.)
       *  Note that the global std::swap() function is specialized such that
       *  std::swap(d1,d2) will feed to this function.
       *
       *  Whether the allocators are swapped depends on the allocator traits.
       */
      void
      swap(deque&amp; __x) _GLIBCXX_NOEXCEPT
      {
#if __cplusplus &gt;= 201103L
	__glibcxx_assert(_Alloc_traits::propagate_on_container_swap::value
			 || _M_get_Tp_allocator() == __x._M_get_Tp_allocator());
#endif
	_M_impl._M_swap_data(__x._M_impl);
	_Alloc_traits::_S_on_swap(_M_get_Tp_allocator(),
				  __x._M_get_Tp_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
      { _M_erase_at_end(begin()); }

    protected:
      // Internal constructor functions follow.

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

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the &quot;do the right thing&quot; clause
      template&lt;typename _Integer&gt;
	void
	_M_initialize_dispatch(_Integer __n, _Integer __x, __true_type)
	{
	  _M_initialize_map(static_cast&lt;size_type&gt;(__n));
	  _M_fill_initialize(__x);
	}

      // called by the range constructor to implement [23.1.1]/9
      template&lt;typename _InputIterator&gt;
	void
	_M_initialize_dispatch(_InputIterator __first, _InputIterator __last,
			       __false_type)
	{
	  _M_range_initialize(__first, __last,
			      std::__iterator_category(__first));
	}

      // called by the second initialize_dispatch above
      //@{
      /**
       *  @brief Fills the deque with whatever is in [first,last).
       *  @param  __first  An input iterator.
       *  @param  __last  An input iterator.
       *  @return   Nothing.
       *
       *  If the iterators are actually forward iterators (or better), then the
       *  memory layout can be done all at once.  Else we move forward using
       *  push_back on each value from the iterator.
       */
      template&lt;typename _InputIterator&gt;
	void
	_M_range_initialize(_InputIterator __first, _InputIterator __last,
			    std::input_iterator_tag);

      // called by the second initialize_dispatch above
      template&lt;typename _ForwardIterator&gt;
	void
	_M_range_initialize(_ForwardIterator __first, _ForwardIterator __last,
			    std::forward_iterator_tag);
      //@}

      /**
       *  @brief Fills the %deque with copies of value.
       *  @param  __value  Initial value.
       *  @return   Nothing.
       *  @pre _M_start and _M_finish have already been initialized,
       *  but none of the %deque's elements have yet been constructed.
       *
       *  This function is called only when the user provides an explicit size
       *  (with or without an explicit exemplar value).
       */
      void
      _M_fill_initialize(const value_type&amp; __value);

#if __cplusplus &gt;= 201103L
      // called by deque(n).
      void
      _M_default_initialize();
#endif

      // Internal assign functions follow.  The *_aux functions do the actual
      // assignment work for the range versions.

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

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the &quot;do the right thing&quot; clause
      template&lt;typename _Integer&gt;
	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&lt;typename _InputIterator&gt;
	void
	_M_assign_dispatch(_InputIterator __first, _InputIterator __last,
			   __false_type)
	{ _M_assign_aux(__first, __last, std::__iterator_category(__first)); }

      // called by the second assign_dispatch above
      template&lt;typename _InputIterator&gt;
	void
	_M_assign_aux(_InputIterator __first, _InputIterator __last,
		      std::input_iterator_tag);

      // called by the second assign_dispatch above
      template&lt;typename _ForwardIterator&gt;
	void
	_M_assign_aux(_ForwardIterator __first, _ForwardIterator __last,
		      std::forward_iterator_tag)
	{
	  const size_type __len = std::distance(__first, __last);
	  if (__len &gt; size())
	    {
	      _ForwardIterator __mid = __first;
	      std::advance(__mid, size());
	      std::copy(__first, __mid, begin());
	      _M_range_insert_aux(end(), __mid, __last,
				  std::__iterator_category(__first));
	    }
	  else
	    _M_erase_at_end(std::copy(__first, __last, begin()));
	}

      // 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&amp; __val)
      {
	if (__n &gt; size())
	  {
	    std::fill(begin(), end(), __val);
	    _M_fill_insert(end(), __n - size(), __val);
	  }
	else
	  {
	    _M_erase_at_end(begin() + difference_type(__n));
	    std::fill(begin(), end(), __val);
	  }
      }

      //@{
      /// Helper functions for push_* and pop_*.
#if __cplusplus &lt; 201103L
      void _M_push_back_aux(const value_type&amp;);

      void _M_push_front_aux(const value_type&amp;);
#else
      template&lt;typename... _Args&gt;
	void _M_push_back_aux(_Args&amp;&amp;... __args);

      template&lt;typename... _Args&gt;
	void _M_push_front_aux(_Args&amp;&amp;... __args);
#endif

      void _M_pop_back_aux();

      void _M_pop_front_aux();
      //@}

      // Internal insert functions follow.  The *_aux functions do the actual
      // insertion work when all shortcuts fail.

      // called by the range insert to implement [23.1.1]/9

      // _GLIBCXX_RESOLVE_LIB_DEFECTS
      // 438. Ambiguity in the &quot;do the right thing&quot; clause
      template&lt;typename _Integer&gt;
	void
	_M_insert_dispatch(iterator __pos,
			   _Integer __n, _Integer __x, __true_type)
	{ _M_fill_insert(__pos, __n, __x); }

      // called by the range insert to implement [23.1.1]/9
      template&lt;typename _InputIterator&gt;
	void
	_M_insert_dispatch(iterator __pos,
			   _InputIterator __first, _InputIterator __last,
			   __false_type)
	{
	  _M_range_insert_aux(__pos, __first, __last,
			      std::__iterator_category(__first));
	}

      // called by the second insert_dispatch above
      template&lt;typename _InputIterator&gt;
	void
	_M_range_insert_aux(iterator __pos, _InputIterator __first,
			    _InputIterator __last, std::input_iterator_tag);

      // called by the second insert_dispatch above
      template&lt;typename _ForwardIterator&gt;
	void
	_M_range_insert_aux(iterator __pos, _ForwardIterator __first,
			    _ForwardIterator __last, std::forward_iterator_tag);

      // Called by insert(p,n,x), and the range insert when it turns out to be
      // the same thing.  Can use fill functions in optimal situations,
      // otherwise passes off to insert_aux(p,n,x).
      void
      _M_fill_insert(iterator __pos, size_type __n, const value_type&amp; __x);

      // called by insert(p,x)
#if __cplusplus &lt; 201103L
      iterator
      _M_insert_aux(iterator __pos, const value_type&amp; __x);
#else
      template&lt;typename... _Args&gt;
	iterator
	_M_insert_aux(iterator __pos, _Args&amp;&amp;... __args);
#endif

      // called by insert(p,n,x) via fill_insert
      void
      _M_insert_aux(iterator __pos, size_type __n, const value_type&amp; __x);

      // called by range_insert_aux for forward iterators
      template&lt;typename _ForwardIterator&gt;
	void
	_M_insert_aux(iterator __pos,
		      _ForwardIterator __first, _ForwardIterator __last,
		      size_type __n);


      // Internal erase functions follow.

      void
      _M_destroy_data_aux(iterator __first, iterator __last);

      // Called by ~deque().
      // NB: Doesn't deallocate the nodes.
      template&lt;typename _Alloc1&gt;
	void
	_M_destroy_data(iterator __first, iterator __last, const _Alloc1&amp;)
	{ _M_destroy_data_aux(__first, __last); }

      void
      _M_destroy_data(iterator __first, iterator __last,
		      const std::allocator&lt;_Tp&gt;&amp;)
      {
	if (!__has_trivial_destructor(value_type))
	  _M_destroy_data_aux(__first, __last);
      }

      // Called by erase(q1, q2).
      void
      _M_erase_at_begin(iterator __pos)
      {
	_M_destroy_data(begin(), __pos, _M_get_Tp_allocator());
	_M_destroy_nodes(this-&gt;_M_impl._M_start._M_node, __pos._M_node);
	this-&gt;_M_impl._M_start = __pos;
      }

      // Called by erase(q1, q2), resize(), clear(), _M_assign_aux,
      // _M_fill_assign, operator=.
      void
      _M_erase_at_end(iterator __pos)
      {
	_M_destroy_data(__pos, end(), _M_get_Tp_allocator());
	_M_destroy_nodes(__pos._M_node + 1,
			 this-&gt;_M_impl._M_finish._M_node + 1);
	this-&gt;_M_impl._M_finish = __pos;
      }

      iterator
      _M_erase(iterator __pos);

      iterator
      _M_erase(iterator __first, iterator __last);

#if __cplusplus &gt;= 201103L
      // Called by resize(sz).
      void
      _M_default_append(size_type __n);

      bool
      _M_shrink_to_fit();
#endif

      //@{
      /// Memory-handling helpers for the previous internal insert functions.
      iterator
      _M_reserve_elements_at_front(size_type __n)
      {
	const size_type __vacancies = this-&gt;_M_impl._M_start._M_cur
				      - this-&gt;_M_impl._M_start._M_first;
	if (__n &gt; __vacancies)
	  _M_new_elements_at_front(__n - __vacancies);
	return this-&gt;_M_impl._M_start - difference_type(__n);
      }

      iterator
      _M_reserve_elements_at_back(size_type __n)
      {
	const size_type __vacancies = (this-&gt;_M_impl._M_finish._M_last
				       - this-&gt;_M_impl._M_finish._M_cur) - 1;
	if (__n &gt; __vacancies)
	  _M_new_elements_at_back(__n - __vacancies);
	return this-&gt;_M_impl._M_finish + difference_type(__n);
      }

      void
      _M_new_elements_at_front(size_type __new_elements);

      void
      _M_new_elements_at_back(size_type __new_elements);
      //@}


      //@{
      /**
       *  @brief Memory-handling helpers for the major %map.
       *
       *  Makes sure the _M_map has space for new nodes.  Does not
       *  actually add the nodes.  Can invalidate _M_map pointers.
       *  (And consequently, %deque iterators.)
       */
      void
      _M_reserve_map_at_back(size_type __nodes_to_add = 1)
      {
	if (__nodes_to_add + 1 &gt; this-&gt;_M_impl._M_map_size
	    - (this-&gt;_M_impl._M_finish._M_node - this-&gt;_M_impl._M_map))
	  _M_reallocate_map(__nodes_to_add, false);
      }

      void
      _M_reserve_map_at_front(size_type __nodes_to_add = 1)
      {
	if (__nodes_to_add &gt; size_type(this-&gt;_M_impl._M_start._M_node
				       - this-&gt;_M_impl._M_map))
	  _M_reallocate_map(__nodes_to_add, true);
      }

      void
      _M_reallocate_map(size_type __nodes_to_add, bool __add_at_front);
      //@}

#if __cplusplus &gt;= 201103L
      // Constant-time, nothrow move assignment when source object's memory
      // can be moved because the allocators are equal.
      void
      _M_move_assign1(deque&amp;&amp; __x, /* always equal: */ true_type) noexcept
      {
	this-&gt;_M_impl._M_swap_data(__x._M_impl);
	__x.clear();
	std::__alloc_on_move(_M_get_Tp_allocator(), __x._M_get_Tp_allocator());
      }

      // When the allocators are not equal the operation could throw, because
      // we might need to allocate a new map for __x after moving from it
      // or we might need to allocate new elements for *this.
      void
      _M_move_assign1(deque&amp;&amp; __x, /* always equal: */ false_type)
      {
	constexpr bool __move_storage =
	  _Alloc_traits::_S_propagate_on_move_assign();
	_M_move_assign2(std::move(__x), __bool_constant&lt;__move_storage&gt;());
      }

      // Destroy all elements and deallocate all memory, then replace
      // with elements created from __args.
      template&lt;typename... _Args&gt;
      void
      _M_replace_map(_Args&amp;&amp;... __args)
      {
	// Create new data first, so if allocation fails there are no effects.
	deque __newobj(std::forward&lt;_Args&gt;(__args)...);
	// Free existing storage using existing allocator.
	clear();
	_M_deallocate_node(*begin()._M_node); // one node left after clear()
	_M_deallocate_map(this-&gt;_M_impl._M_map, this-&gt;_M_impl._M_map_size);
	this-&gt;_M_impl._M_map = nullptr;
	this-&gt;_M_impl._M_map_size = 0;
	// Take ownership of replacement memory.
	this-&gt;_M_impl._M_swap_data(__newobj._M_impl);
      }

      // Do move assignment when the allocator propagates.
      void
      _M_move_assign2(deque&amp;&amp; __x, /* propagate: */ true_type)
      {
	// Make a copy of the original allocator state.
	auto __alloc = __x._M_get_Tp_allocator();
	// The allocator propagates so storage can be moved from __x,
	// leaving __x in a valid empty state with a moved-from allocator.
	_M_replace_map(std::move(__x));
	// Move the corresponding allocator state too.
	_M_get_Tp_allocator() = std::move(__alloc);
      }

      // Do move assignment when it may not be possible to move source
      // object's memory, resulting in a linear-time operation.
      void
      _M_move_assign2(deque&amp;&amp; __x, /* propagate: */ false_type)
      {
	if (__x._M_get_Tp_allocator() == this-&gt;_M_get_Tp_allocator())
	  {
	    // The allocators are equal so storage can be moved from __x,
	    // leaving __x in a valid empty state with its current allocator.
	    _M_replace_map(std::move(__x), __x.get_allocator());
	  }
	else
	  {
	    // The rvalue's allocator cannot be moved and is not equal,
	    // so we need to individually move each element.
	    _M_assign_aux(std::__make_move_if_noexcept_iterator(__x.begin()),
			  std::__make_move_if_noexcept_iterator(__x.end()),
			  std::random_access_iterator_tag());
	    __x.clear();
	  }
      }
#endif
    };

#if __cpp_deduction_guides &gt;= 201606
  template&lt;typename _InputIterator, typename _ValT
	     = typename iterator_traits&lt;_InputIterator&gt;::value_type,
	   typename _Allocator = allocator&lt;_ValT&gt;,
	   typename = _RequireInputIter&lt;_InputIterator&gt;,
	   typename = _RequireAllocator&lt;_Allocator&gt;&gt;
    deque(_InputIterator, _InputIterator, _Allocator = _Allocator())
      -&gt; deque&lt;_ValT, _Allocator&gt;;
#endif

  /**
   *  @brief  Deque equality comparison.
   *  @param  __x  A %deque.
   *  @param  __y  A %deque of the same type as @a __x.
   *  @return  True iff the size and elements of the deques are equal.
   *
   *  This is an equivalence relation.  It is linear in the size of the
   *  deques.  Deques are considered equivalent if their sizes are equal,
   *  and if corresponding elements compare equal.
  */
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator==(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
                         const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return __x.size() == __y.size()
	     &amp;&amp; std::equal(__x.begin(), __x.end(), __y.begin()); }

  /**
   *  @brief  Deque ordering relation.
   *  @param  __x  A %deque.
   *  @param  __y  A %deque 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
   *  deques.  The elements must be comparable with @c &lt;.
   *
   *  See std::lexicographical_compare() for how the determination is made.
  */
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator&lt;(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
	      const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return std::lexicographical_compare(__x.begin(), __x.end(),
					  __y.begin(), __y.end()); }

  /// Based on operator==
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator!=(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
	       const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return !(__x == __y); }

  /// Based on operator&lt;
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator&gt;(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
	      const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return __y &lt; __x; }

  /// Based on operator&lt;
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator&lt;=(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
	       const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return !(__y &lt; __x); }

  /// Based on operator&lt;
  template&lt;typename _Tp, typename _Alloc&gt;
    inline bool
    operator&gt;=(const deque&lt;_Tp, _Alloc&gt;&amp; __x,
	       const deque&lt;_Tp, _Alloc&gt;&amp; __y)
    { return !(__x &lt; __y); }

  /// See std::deque::swap().
  template&lt;typename _Tp, typename _Alloc&gt;
    inline void
    swap(deque&lt;_Tp,_Alloc&gt;&amp; __x, deque&lt;_Tp,_Alloc&gt;&amp; __y)
    _GLIBCXX_NOEXCEPT_IF(noexcept(__x.swap(__y)))
    { __x.swap(__y); }

#undef _GLIBCXX_DEQUE_BUF_SIZE

_GLIBCXX_END_NAMESPACE_CONTAINER
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace std

#endif /* _STL_DEQUE_H */
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