TODO: Note change in clause 27 docs.

[official-gcc.git] / libstdc++-v3 / include / bits / stl_deque.h

// Deque implementation -*- C++ -*-

// Copyright (C) 2001, 2002 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 2, 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.

// You should have received a copy of the GNU General Public License along
// with this library; see the file COPYING.  If not, write to the Free
// Software Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307,
// USA.

// As a special exception, you may use this file as part of a free software
// library without restriction.  Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compile
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License.  This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.

/*
 *
 * 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) 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 stl_deque.h
 *  This is an internal header file, included by other library headers.
 *  You should not attempt to use it directly.
 */

#ifndef __GLIBCPP_INTERNAL_DEQUE_H
#define __GLIBCPP_INTERNAL_DEQUE_H

#include <bits/concept_check.h>
#include <bits/stl_iterator_base_types.h>
#include <bits/stl_iterator_base_funcs.h>

namespace std
{ 
  /**
   *  @if maint
   *  @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 '512' is
   *  tuneable (and no other code needs to change), but no investigation has
   *  been done since inheriting the SGI code.
   *  @endif
  */
  inline size_t 
  __deque_buf_size(size_t __size) 
  { return __size < 512 ? size_t(512 / __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.
   *
   *  @if maint
   *  All the functions are op overloads except for _M_set_node.
   *  @endif
  */
  template <typename _Tp, typename _Ref, typename _Ptr>
    struct _Deque_iterator
  {
    typedef _Deque_iterator<_Tp, _Tp&, _Tp*>             iterator;
    typedef _Deque_iterator<_Tp, const _Tp&, const _Tp*> const_iterator;
    static size_t _S_buffer_size() { return __deque_buf_size(sizeof(_Tp)); }
  
    typedef 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 _Tp**                      _Map_pointer;
    typedef _Deque_iterator            _Self;
  
    _Tp* _M_cur;
    _Tp* _M_first;
    _Tp* _M_last;
    _Map_pointer _M_node;
  
    _Deque_iterator(_Tp* __x, _Map_pointer __y) 
      : _M_cur(__x), _M_first(*__y),
        _M_last(*__y + _S_buffer_size()), _M_node(__y) {}
    _Deque_iterator() : _M_cur(0), _M_first(0), _M_last(0), _M_node(0) {}
    _Deque_iterator(const iterator& __x)
      : _M_cur(__x._M_cur), _M_first(__x._M_first), 
        _M_last(__x._M_last), _M_node(__x._M_node) {}
  
    reference operator*() const { return *_M_cur; }
    pointer operator->() const { return _M_cur; }
  
    _Self& operator++() {
      ++_M_cur;
      if (_M_cur == _M_last) {
        _M_set_node(_M_node + 1);
        _M_cur = _M_first;
      }
      return *this; 
    }
    _Self operator++(int)  {
      _Self __tmp = *this;
      ++*this;
      return __tmp;
    }
  
    _Self& operator--() {
      if (_M_cur == _M_first) {
        _M_set_node(_M_node - 1);
        _M_cur = _M_last;
      }
      --_M_cur;
      return *this;
    }
    _Self operator--(int) {
      _Self __tmp = *this;
      --*this;
      return __tmp;
    }
  
    _Self& operator+=(difference_type __n)
    {
      difference_type __offset = __n + (_M_cur - _M_first);
      if (__offset >= 0 && __offset < difference_type(_S_buffer_size()))
        _M_cur += __n;
      else {
        difference_type __node_offset =
          __offset > 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
    {
      _Self __tmp = *this;
      return __tmp += __n;
    }
  
    _Self& operator-=(difference_type __n) { return *this += -__n; }
   
    _Self operator-(difference_type __n) const {
      _Self __tmp = *this;
      return __tmp -= __n;
    }
  
    reference operator[](difference_type __n) const { return *(*this + __n); }
  
    /** @if maint
     *  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.
     *  @endif
    */
    void
    _M_set_node(_Map_pointer __new_node)
    {
      _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 <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator==(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return __x._M_cur == __y._M_cur;
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator==(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return __x._M_cur == __y._M_cur;
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator!=(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return !(__x == __y);
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator!=(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return !(__x == __y);
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator<(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return (__x._M_node == __y._M_node) ? 
      (__x._M_cur < __y._M_cur) : (__x._M_node < __y._M_node);
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator<(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return (__x._M_node == __y._M_node) ? 
      (__x._M_cur < __y._M_cur) : (__x._M_node < __y._M_node);
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator>(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return __y < __x;
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator>(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return __y < __x;
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator<=(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return !(__y < __x);
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator<=(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return !(__y < __x);
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline bool
  operator>=(const _Deque_iterator<_Tp, _Ref, _Ptr>& __x,
           const _Deque_iterator<_Tp, _Ref, _Ptr>& __y)
  {
    return !(__x < __y);
  }
  
  template <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline bool
  operator>=(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
           const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return !(__x < __y);
  }
  
  // _GLIBCPP_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 <typename _Tp, typename _RefL, typename _PtrL,
                          typename _RefR, typename _PtrR>
  inline typename _Deque_iterator<_Tp, _RefL, _PtrL>::difference_type
  operator-(const _Deque_iterator<_Tp, _RefL, _PtrL>& __x,
          const _Deque_iterator<_Tp, _RefR, _PtrR>& __y)
  {
    return _Deque_iterator<_Tp, _RefL, _PtrL>::difference_type
      (_Deque_iterator<_Tp, _RefL, _PtrL>::_S_buffer_size()) *
      (__x._M_node - __y._M_node - 1) + (__x._M_cur - __x._M_first) +
      (__y._M_last - __y._M_cur);
  }
  
  template <typename _Tp, typename _Ref, typename _Ptr>
  inline _Deque_iterator<_Tp, _Ref, _Ptr>
  operator+(ptrdiff_t __n, const _Deque_iterator<_Tp, _Ref, _Ptr>& __x)
  {
    return __x + __n;
  }
  
  
  /// @if maint Primary default version.  @endif
  /**
   *  @if maint
   *  Deque base class.  It has two purposes.  First, its constructor
   *  and destructor allocate (but don't initialize) storage.  This makes
   *  %exception safety easier.  Second, the base class encapsulates all of
   *  the differences between SGI-style allocators and standard-conforming
   *  allocators.  (See stl_alloc.h for more on this topic.)  There are two
   *  versions:  this ordinary one, and the space-saving specialization for
   *  instanceless allocators.
   *  @endif
  */
  template <typename _Tp, typename _Alloc, bool __is_static>
    class _Deque_alloc_base
  {
  public:
    typedef typename _Alloc_traits<_Tp,_Alloc>::allocator_type allocator_type;
    allocator_type get_allocator() const { return _M_node_allocator; }
  
    _Deque_alloc_base(const allocator_type& __a)
      : _M_node_allocator(__a), _M_map_allocator(__a),
        _M_map(0), _M_map_size(0)
    {}
    
  protected:
    typedef typename _Alloc_traits<_Tp*, _Alloc>::allocator_type
            _Map_allocator_type;
  
    _Tp*
    _M_allocate_node()
    {
      return _M_node_allocator.allocate(__deque_buf_size(sizeof(_Tp)));
    }
  
    void
    _M_deallocate_node(_Tp* __p)
    {
      _M_node_allocator.deallocate(__p, __deque_buf_size(sizeof(_Tp)));
    }
  
    _Tp**
    _M_allocate_map(size_t __n) 
      { return _M_map_allocator.allocate(__n); }
  
    void
    _M_deallocate_map(_Tp** __p, size_t __n) 
      { _M_map_allocator.deallocate(__p, __n); }
  
    allocator_type       _M_node_allocator;
    _Map_allocator_type  _M_map_allocator;
    _Tp**                _M_map;
    size_t               _M_map_size;
  };
  
  /// @if maint Specialization for instanceless allocators.  @endif
  template <typename _Tp, typename _Alloc>
    class _Deque_alloc_base<_Tp, _Alloc, true>
  {
  public:
    typedef typename _Alloc_traits<_Tp,_Alloc>::allocator_type allocator_type;
    allocator_type get_allocator() const { return allocator_type(); }
  
    _Deque_alloc_base(const allocator_type&)
      : _M_map(0), _M_map_size(0)
    {}
    
  protected:
    typedef typename _Alloc_traits<_Tp,_Alloc>::_Alloc_type  _Node_alloc_type;
    typedef typename _Alloc_traits<_Tp*,_Alloc>::_Alloc_type _Map_alloc_type;
  
    _Tp*
    _M_allocate_node()
    {
      return _Node_alloc_type::allocate(__deque_buf_size(sizeof(_Tp)));
    }
  
    void
    _M_deallocate_node(_Tp* __p)
    {
      _Node_alloc_type::deallocate(__p, __deque_buf_size(sizeof(_Tp)));
    }
  
    _Tp**
    _M_allocate_map(size_t __n) 
      { return _Map_alloc_type::allocate(__n); }
  
    void
    _M_deallocate_map(_Tp** __p, size_t __n) 
      { _Map_alloc_type::deallocate(__p, __n); }
  
    _Tp**   _M_map;
    size_t  _M_map_size;
  };
  
  
  /**
   *  @if maint
   *  Deque base class.  Using _Alloc_traits in the instantiation of the parent
   *  class provides the compile-time dispatching mentioned in the parent's
   *  docs.  This class provides the unified face for %deque's allocation.
   *
   *  Nothing in this class ever constructs or destroys an actual Tp element.
   *  (Deque handles that itself.)  Only/All memory management is performed
   *  here.
   *  @endif
  */
  template <typename _Tp, typename _Alloc>
    class _Deque_base
    : public _Deque_alloc_base<_Tp,_Alloc,
                                _Alloc_traits<_Tp, _Alloc>::_S_instanceless>
  {
  public:
    typedef _Deque_alloc_base<_Tp,_Alloc,
                               _Alloc_traits<_Tp, _Alloc>::_S_instanceless>
            _Base;
    typedef typename _Base::allocator_type             allocator_type;
    typedef _Deque_iterator<_Tp,_Tp&,_Tp*>             iterator;
    typedef _Deque_iterator<_Tp,const _Tp&,const _Tp*> const_iterator;
  
    _Deque_base(const allocator_type& __a, size_t __num_elements)
      : _Base(__a), _M_start(), _M_finish()
      { _M_initialize_map(__num_elements); }
    _Deque_base(const allocator_type& __a) 
      : _Base(__a), _M_start(), _M_finish() {}
    ~_Deque_base();    
  
  protected:
    void _M_initialize_map(size_t);
    void _M_create_nodes(_Tp** __nstart, _Tp** __nfinish);
    void _M_destroy_nodes(_Tp** __nstart, _Tp** __nfinish);
    enum { _S_initial_map_size = 8 };
  
    iterator _M_start;
    iterator _M_finish;
  };
  
  
  template <typename _Tp, typename _Alloc>
  _Deque_base<_Tp,_Alloc>::~_Deque_base()
  {
    if (_M_map)
    {
      _M_destroy_nodes(_M_start._M_node, _M_finish._M_node + 1);
      _M_deallocate_map(_M_map, _M_map_size);
    }
  }
  
  /**
   *  @if maint
   *  @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...
   *  @endif
  */
  template <typename _Tp, typename _Alloc>
  void
  _Deque_base<_Tp,_Alloc>::_M_initialize_map(size_t __num_elements)
  {
    size_t __num_nodes = 
      __num_elements / __deque_buf_size(sizeof(_Tp)) + 1;
  
    _M_map_size = max((size_t) _S_initial_map_size, __num_nodes + 2);
    _M_map = _M_allocate_map(_M_map_size);
  
    // For "small" 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.
  
    _Tp** __nstart = _M_map + (_M_map_size - __num_nodes) / 2;
    _Tp** __nfinish = __nstart + __num_nodes;
      
    try 
      { _M_create_nodes(__nstart, __nfinish); }
    catch(...)
      {
        _M_deallocate_map(_M_map, _M_map_size);
        _M_map = 0;
        _M_map_size = 0;
        __throw_exception_again;
      }
    
    _M_start._M_set_node(__nstart);
    _M_finish._M_set_node(__nfinish - 1);
    _M_start._M_cur = _M_start._M_first;
    _M_finish._M_cur = _M_finish._M_first +
                       __num_elements % __deque_buf_size(sizeof(_Tp));
  }
  
  template <typename _Tp, typename _Alloc>
  void _Deque_base<_Tp,_Alloc>::_M_create_nodes(_Tp** __nstart, _Tp** __nfinish)
  {
    _Tp** __cur;
    try
      {
        for (__cur = __nstart; __cur < __nfinish; ++__cur)
          *__cur = _M_allocate_node();
      }
    catch(...)
      { 
        _M_destroy_nodes(__nstart, __cur);
        __throw_exception_again; 
      }
  }
  
  template <typename _Tp, typename _Alloc>
  void
  _Deque_base<_Tp,_Alloc>::_M_destroy_nodes(_Tp** __nstart, _Tp** __nfinish)
  {
    for (_Tp** __n = __nstart; __n < __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 Containers
   *  @ingroup Sequences
   *
   *  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>.
   *
   *  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.
   *
   *  @if maint
   *  Here's how a deque<Tp> 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-"nodes".
   *  (The name %map has nothing to do with the std::map class, and "nodes"
   *  should not be confused with std::list's usage of "node".)
   *  
   *  A "node" has no specific type name as such, but it is referred to as
   *  "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 "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.
   *  
   *  **** As I write this, the nodes are /not/ allocated using the high-speed
   *  memory pool.  There are 20 hours left in the year; perhaps I can fix
   *  this before 2002.
   *  
   *  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 "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 <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 "aware" of the discontiguous
   *  storage!
   *
   *  The memory setup and layout occurs in the parent, _Base, and the iterator
   *  class is entirely responsible for "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.
   *  @endif
  */
  template <typename _Tp, typename _Alloc = allocator<_Tp> >
    class deque : protected _Deque_base<_Tp, _Alloc>
  {
    // concept requirements
    __glibcpp_class_requires(_Tp, _SGIAssignableConcept)
  
    typedef _Deque_base<_Tp, _Alloc>           _Base;
  
  public:
    typedef _Tp                                value_type;
    typedef value_type*                        pointer;
    typedef const value_type*                  const_pointer;
    typedef typename _Base::iterator           iterator;
    typedef typename _Base::const_iterator     const_iterator;
    typedef std::reverse_iterator<const_iterator>   const_reverse_iterator;
    typedef std::reverse_iterator<iterator>         reverse_iterator;
    typedef value_type&                        reference;
    typedef const value_type&                  const_reference;
    typedef size_t                             size_type;
    typedef ptrdiff_t                          difference_type;
    typedef typename _Base::allocator_type     allocator_type;
  
  protected:
    typedef pointer*                           _Map_pointer;
    static size_t _S_buffer_size() { 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;
  
    /** @if maint
     *  A total of four data members accumulated down the heirarchy.  If the
     *  _Alloc type requires separate instances, then two of them will also be
     *  included in each deque.
     *  @endif
    */
    using _Base::_M_map;
    using _Base::_M_map_size;
    using _Base::_M_start;
    using _Base::_M_finish;
  
  public:
    // [23.2.1.1] construct/copy/destroy
    // (assign() and get_allocator() are also listed in this section)
    /**
     *  @brief  Default constructor creates no elements.
    */
    explicit
    deque(const allocator_type& __a = allocator_type()) 
      : _Base(__a, 0) {}
  
    /**
     *  @brief  Create a %deque with copies of an exemplar element.
     *  @param  n  The number of elements to initially create.
     *  @param  value  An element to copy.
     * 
     *  This constructor fills the %deque with @a n copies of @a value.
    */
    deque(size_type __n, const value_type& __value,
          const allocator_type& __a = allocator_type())
      : _Base(__a, __n)
      { _M_fill_initialize(__value); }
  
    /**
     *  @brief  Create a %deque with default elements.
     *  @param  n  The number of elements to initially create.
     * 
     *  This constructor fills the %deque with @a n copies of a
     *  default-constructed element.
    */
    explicit
    deque(size_type __n)
      : _Base(allocator_type(), __n)
      { _M_fill_initialize(value_type()); }
  
    /**
     *  @brief  %Deque copy constructor.
     *  @param  x  A %deque of identical element and allocator types.
     * 
     *  The newly-created %deque uses a copy of the allocation object used
     *  by @a x.
    */
    deque(const deque& __x)
      : _Base(__x.get_allocator(), __x.size()) 
      { uninitialized_copy(__x.begin(), __x.end(), _M_start); }
  
    /**
     *  @brief  Builds a %deque from a range.
     *  @param  first  An input iterator.
     *  @param  last  An input iterator.
     * 
     *  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.
    */
    template<typename _InputIterator>
      deque(_InputIterator __first, _InputIterator __last,
            const allocator_type& __a = allocator_type())
        : _Base(__a)
      {
        // Check whether it's an integral type.  If so, it's not an iterator.
        typedef typename _Is_integer<_InputIterator>::_Integral _Integral;
        _M_initialize_dispatch(__first, __last, _Integral());
      }
  
    /**
     *  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 responsibilty.
    */
    ~deque() { _Destroy(_M_start, _M_finish); }
  
    /**
     *  @brief  %Deque assignment operator.
     *  @param  x  A %deque of identical element and allocator types.
     * 
     *  All the elements of @a x are copied, but unlike the copy constructor,
     *  the allocator object is not copied.
    */
    deque&
    operator=(const deque& __x);
  
    /**
     *  @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.
     *  Old data may be lost.
    */
    void
    assign(size_type __n, const value_type& __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.
     *  Old data may be lost.
    */
    template<typename _InputIterator>
      void
      assign(_InputIterator __first, _InputIterator __last)
      {
        typedef typename _Is_integer<_InputIterator>::_Integral _Integral;
        _M_assign_dispatch(__first, __last, _Integral());
      }
  
    /// Get a copy of the memory allocation object.
    allocator_type
    get_allocator() const { 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() { return _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 { return _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() { return _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 { return _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() { return reverse_iterator(_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 { return const_reverse_iterator(_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() { return reverse_iterator(_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 { return const_reverse_iterator(_M_start); }
  
    // [23.2.1.2] capacity
    /**  Returns the number of elements in the %deque.  */
    size_type
    size() const { return _M_finish - _M_start; }
  
    /**  Returns the size() of the largest possible %deque.  */
    size_type
    max_size() const { return size_type(-1); }
  
    /**
     *  @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& __x)
    {
      const size_type __len = size();
      if (__new_size < __len) 
        erase(_M_start + __new_size, _M_finish);
      else
        insert(_M_finish, __new_size - __len, __x);
    }
  
    /**
     *  @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 the %deque is extended and new elements
     *  are default-constructed.
    */
    void
    resize(size_type new_size) { resize(new_size, value_type()); }
  
    /**
     *  Returns true if the %deque is empty.  (Thus begin() would equal end().)
    */
    bool empty() const { return _M_finish == _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) { return _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 { return _M_start[difference_type(__n)]; }
  
  protected:
    /// @if maint Safety check used only from at().  @endif
    void
    _M_range_check(size_type __n) const
    {
      if (__n >= this->size())
        __throw_out_of_range("deque [] access out of range");
    }
  
  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() { return *_M_start; }
  
    /**
     *  Returns a read-only (constant) reference to the data at the first
     *  element of the %deque.
    */
    const_reference
    front() const { return *_M_start; }
  
    /**
     *  Returns a read/write reference to the data at the last element of the
     *  %deque.
    */
    reference
    back()
    {
      iterator __tmp = _M_finish;
      --__tmp;
      return *__tmp;
    }
  
    /**
     *  Returns a read-only (constant) reference to the data at the last
     *  element of the %deque.
    */
    const_reference
    back() const
    {
      const_iterator __tmp = _M_finish;
      --__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& __x) 
    {
      if (_M_start._M_cur != _M_start._M_first) {
        _Construct(_M_start._M_cur - 1, __x);
        --_M_start._M_cur;
      }
      else
        _M_push_front_aux(__x);
    }
  
  #ifdef _GLIBCPP_DEPRECATED
    /**
     *  @brief  Add data to the front of the %deque.
     *
     *  This is a typical stack operation.  The function creates a
     *  default-constructed element at the front of the %deque.  Due to the
     *  nature of a %deque this operation can be done in constant time.  You
     *  should consider using push_front(value_type()) instead.
     *
     *  @note This was deprecated in 3.2 and will be removed in 3.4.  You must
     *        define @c _GLIBCPP_DEPRECATED to make this visible in 3.2; see
     *        c++config.h.
    */
    void
    push_front()
    {
      if (_M_start._M_cur != _M_start._M_first) {
        _Construct(_M_start._M_cur - 1);
        --_M_start._M_cur;
      }
      else
        _M_push_front_aux();
    }
  #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& __x)
    {
      if (_M_finish._M_cur != _M_finish._M_last - 1) {
        _Construct(_M_finish._M_cur, __x);
        ++_M_finish._M_cur;
      }
      else
        _M_push_back_aux(__x);
    }
  
  #ifdef _GLIBCPP_DEPRECATED
    /**
     *  @brief  Add data to the end of the %deque.
     *
     *  This is a typical stack operation.  The function creates a
     *  default-constructed element at the end of the %deque.  Due to the nature
     *  of a %deque this operation can be done in constant time.  You should
     *  consider using push_back(value_type()) instead.
     *
     *  @note This was deprecated in 3.2 and will be removed in 3.4.  You must
     *        define @c _GLIBCPP_DEPRECATED to make this visible in 3.2; see
     *        c++config.h.
    */
    void
    push_back()
    {
      if (_M_finish._M_cur != _M_finish._M_last - 1) {
        _Construct(_M_finish._M_cur);
        ++_M_finish._M_cur;
      }
      else
        _M_push_back_aux();
    }
  #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()
    {
      if (_M_start._M_cur != _M_start._M_last - 1) {
        _Destroy(_M_start._M_cur);
        ++_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()
    {
      if (_M_finish._M_cur != _M_finish._M_first) {
        --_M_finish._M_cur;
        _Destroy(_M_finish._M_cur);
      }
      else
        _M_pop_back_aux();
    }
  
    /**
     *  @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& __x);
  
  #ifdef _GLIBCPP_DEPRECATED
    /**
     *  @brief  Inserts an element into the %deque.
     *  @param  position  An iterator into the %deque.
     *  @return  An iterator that points to the inserted element.
     *
     *  This function will insert a default-constructed element before the
     *  specified location.  You should consider using
     *  insert(position,value_type()) instead.
     *
     *  @note This was deprecated in 3.2 and will be removed in 3.4.  You must
     *        define @c _GLIBCPP_DEPRECATED to make this visible in 3.2; see
     *        c++config.h.
    */
    iterator
    insert(iterator __position)
    { return insert(__position, value_type()); }
  #endif
  
    /**
     *  @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& __x)
    { _M_fill_insert(__position, __n, __x); }
  
    /**
     *  @brief  Inserts a range into the %deque.
     *  @param  pos  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 pos.  This is
     *  known as "range insert."
    */
    template<typename _InputIterator>
      void
      insert(iterator __pos, _InputIterator __first, _InputIterator __last)
      {
        // Check whether it's an integral type.  If so, it's not an iterator.
        typedef typename _Is_integer<_InputIterator>::_Integral _Integral;
        _M_insert_dispatch(__pos, __first, __last, _Integral());
      }
  
    /**
     *  @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 responsibilty.
    */
    iterator
    erase(iterator __position);
  
    /**
     *  @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 responsibilty.
    */
    iterator
    erase(iterator __first, iterator __last);
  
    /**
     *  @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.
    */
    void
    swap(deque& __x)
    {
      std::swap(_M_start, __x._M_start);
      std::swap(_M_finish, __x._M_finish);
      std::swap(_M_map, __x._M_map);
      std::swap(_M_map_size, __x._M_map_size);
    }
  
    /**
     *  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 responsibilty.
    */
    void clear(); 
  
  protected:
    // Internal constructor functions follow.
  
    // called by the range constructor to implement [23.1.1]/9
    template<typename _Integer>
      void
      _M_initialize_dispatch(_Integer __n, _Integer __x, __true_type)
      {
        _M_initialize_map(__n);
        _M_fill_initialize(__x);
      }
  
    // called by the range constructor to implement [23.1.1]/9
    template<typename _InputIter>
      void
      _M_initialize_dispatch(_InputIter __first, _InputIter __last,
                             __false_type)
      {
        typedef typename iterator_traits<_InputIter>::iterator_category
                         _IterCategory;
        _M_range_initialize(__first, __last, _IterCategory());
      }
  
    // called by the second initialize_dispatch above
    //@{
    /**
     *  @if maint
     *  @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.
     *  @endif
    */
    template <typename _InputIterator>
      void
      _M_range_initialize(_InputIterator __first, _InputIterator __last,
                          input_iterator_tag);
  
    // called by the second initialize_dispatch above
    template <typename _ForwardIterator>
      void
      _M_range_initialize(_ForwardIterator __first, _ForwardIterator __last,
                          forward_iterator_tag);
    //@}
  
    /**
     *  @if maint
     *  @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).
     *  @endif
    */
    void
    _M_fill_initialize(const value_type& __value);
  
  
    // 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
    template<typename _Integer>
      void
      _M_assign_dispatch(_Integer __n, _Integer __val, __true_type)
      {
        _M_fill_assign(static_cast<size_type>(__n),
                       static_cast<value_type>(__val));
      }
  
    // called by the range assign to implement [23.1.1]/9
    template<typename _InputIter>
      void
      _M_assign_dispatch(_InputIter __first, _InputIter __last, __false_type)
      {
        typedef typename iterator_traits<_InputIter>::iterator_category
                         _IterCategory;
        _M_assign_aux(__first, __last, _IterCategory());
      }
  
    // called by the second assign_dispatch above
    template <typename _InputIterator>
      void
      _M_assign_aux(_InputIterator __first, _InputIterator __last,
                    input_iterator_tag);
  
    // called by the second assign_dispatch above
    template <typename _ForwardIterator>
      void
      _M_assign_aux(_ForwardIterator __first, _ForwardIterator __last,
                    forward_iterator_tag)
      {
        size_type __len = distance(__first, __last);
        if (__len > size()) {
          _ForwardIterator __mid = __first;
          advance(__mid, size());
          copy(__first, __mid, begin());
          insert(end(), __mid, __last);
        }
        else
          erase(copy(__first, __last, begin()), end());
      }
  
    // 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)
    {
      if (__n > size())
      {
        fill(begin(), end(), __val);
        insert(end(), __n - size(), __val);
      }
      else
      {
        erase(begin() + __n, end());
        fill(begin(), end(), __val);
      }
    }
  
  
    //@{
    /**
     *  @if maint
     *  @brief Helper functions for push_* and pop_*.
     *  @endif
    */
    void _M_push_back_aux(const value_type&);
    void _M_push_front_aux(const value_type&);
  #ifdef _GLIBCPP_DEPRECATED
    void _M_push_back_aux();
    void _M_push_front_aux();
  #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
    template<typename _Integer>
      void
      _M_insert_dispatch(iterator __pos,
                         _Integer __n, _Integer __x, __true_type)
      {
        _M_fill_insert(__pos, static_cast<size_type>(__n),
                       static_cast<value_type>(__x));
      }
  
    // called by the range insert to implement [23.1.1]/9
    template<typename _InputIterator>
      void
      _M_insert_dispatch(iterator __pos,
                         _InputIterator __first, _InputIterator __last,
                         __false_type)
      {
        typedef typename iterator_traits<_InputIterator>::iterator_category
                         _IterCategory;
        _M_range_insert_aux(__pos, __first, __last, _IterCategory());
      }
  
    // called by the second insert_dispatch above
    template <typename _InputIterator>
      void
      _M_range_insert_aux(iterator __pos, _InputIterator __first,
                          _InputIterator __last, input_iterator_tag);
  
    // called by the second insert_dispatch above
    template <typename _ForwardIterator>
      void
      _M_range_insert_aux(iterator __pos, _ForwardIterator __first,
                          _ForwardIterator __last, 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& __x); 
  
    // called by insert(p,x)
    iterator
    _M_insert_aux(iterator __pos, const value_type& __x);
  
    // called by insert(p,n,x) via fill_insert
    void
    _M_insert_aux(iterator __pos, size_type __n, const value_type& __x);
  
    // called by range_insert_aux for forward iterators
    template <typename _ForwardIterator>
      void
      _M_insert_aux(iterator __pos, 
                    _ForwardIterator __first, _ForwardIterator __last,
                    size_type __n);
  
  #ifdef _GLIBCPP_DEPRECATED
    // unused, see comment in implementation
    iterator _M_insert_aux(iterator __pos);
  #endif
  
    //@{
    /**
     *  @if maint
     *  @brief Memory-handling helpers for the previous internal insert
     *         functions.
     *  @endif
    */
    iterator
    _M_reserve_elements_at_front(size_type __n)
    {
      size_type __vacancies = _M_start._M_cur - _M_start._M_first;
      if (__n > __vacancies) 
        _M_new_elements_at_front(__n - __vacancies);
      return _M_start - difference_type(__n);
    }
  
    iterator
    _M_reserve_elements_at_back(size_type __n)
    {
      size_type __vacancies = (_M_finish._M_last - _M_finish._M_cur) - 1;
      if (__n > __vacancies)
        _M_new_elements_at_back(__n - __vacancies);
      return _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);
    //@}
  
  
    //@{
    /**
     *  @if maint
     *  @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.)
     *  @endif
    */
    void
    _M_reserve_map_at_back (size_type __nodes_to_add = 1)
    {
      if (__nodes_to_add + 1 > _M_map_size - (_M_finish._M_node - _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 > size_type(_M_start._M_node - _M_map))
        _M_reallocate_map(__nodes_to_add, true);
    }
  
    void
    _M_reallocate_map(size_type __nodes_to_add, bool __add_at_front);
    //@}
  };
  
  
  /**
   *  @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 <typename _Tp, typename _Alloc>
  inline bool operator==(const deque<_Tp, _Alloc>& __x,
                         const deque<_Tp, _Alloc>& __y)
  {
    return __x.size() == __y.size() &&
           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 lexographically 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 <.
   *
   *  See std::lexographical_compare() for how the determination is made.
  */
  template <typename _Tp, typename _Alloc>
  inline bool operator<(const deque<_Tp, _Alloc>& __x,
                        const deque<_Tp, _Alloc>& __y)
  {
    return lexicographical_compare(__x.begin(), __x.end(), 
                                   __y.begin(), __y.end());
  }
  
  /// Based on operator==
  template <typename _Tp, typename _Alloc>
  inline bool operator!=(const deque<_Tp, _Alloc>& __x,
                         const deque<_Tp, _Alloc>& __y) {
    return !(__x == __y);
  }
  
  /// Based on operator<
  template <typename _Tp, typename _Alloc>
  inline bool operator>(const deque<_Tp, _Alloc>& __x,
                        const deque<_Tp, _Alloc>& __y) {
    return __y < __x;
  }
  
  /// Based on operator<
  template <typename _Tp, typename _Alloc>
  inline bool operator<=(const deque<_Tp, _Alloc>& __x,
                         const deque<_Tp, _Alloc>& __y) {
    return !(__y < __x);
  }
  
  /// Based on operator<
  template <typename _Tp, typename _Alloc>
  inline bool operator>=(const deque<_Tp, _Alloc>& __x,
                         const deque<_Tp, _Alloc>& __y) {
    return !(__x < __y);
  }
  
  /// See std::deque::swap().
  template <typename _Tp, typename _Alloc>
  inline void swap(deque<_Tp,_Alloc>& __x, deque<_Tp,_Alloc>& __y)
  {
    __x.swap(__y);
  }
} // namespace std 
  
#endif /* __GLIBCPP_INTERNAL_DEQUE_H */