///STL list为双向循环链表

struct _List_node_base {
  _List_node_base* _M_next;
  _List_node_base* _M_prev;
};

template <class _Tp>
struct _List_node : public _List_node_base {
  _Tp _M_data;
};

struct _List_iterator_base {
  typedef size_t                     size_type;
  typedef ptrdiff_t                  difference_type;
  typedef bidirectional_iterator_tag iterator_category;   ///迭代器为双向迭代器

  _List_node_base* _M_node;   ///迭代器使用_List_node_base*标志其指向

  _List_iterator_base(_List_node_base* __x) : _M_node(__x) {}
  _List_iterator_base() {}

  void _M_incr() { _M_node = _M_node->_M_next; }
  void _M_decr() { _M_node = _M_node->_M_prev; }

  bool operator==(const _List_iterator_base& __x) const {
    return _M_node == __x._M_node;
  }
  bool operator!=(const _List_iterator_base& __x) const {
    return _M_node != __x._M_node;
  }
};

template<class _Tp, class _Ref, class _Ptr>
struct _List_iterator : public _List_iterator_base {
  typedef _List_iterator<_Tp,_Tp&,_Tp*>             iterator;
  typedef _List_iterator<_Tp,const _Tp&,const _Tp*> const_iterator;
  typedef _List_iterator<_Tp,_Ref,_Ptr>             _Self;

  typedef _Tp value_type;
  typedef _Ptr pointer;
  typedef _Ref reference;
  typedef _List_node<_Tp> _Node;   ///实际指向的类型

  _List_iterator(_Node* __x) : _List_iterator_base(__x) {}
  _List_iterator() {}
  _List_iterator(const iterator& __x) : _List_iterator_base(__x._M_node) {}


  reference operator*() const { ///该函数虽然可能修改结点的值,但因迭代器对象只保存
                                                     ///指向结点的指针,因此仍然声明为const
      return ((_Node*) _M_node)->_M_data;
  }

  pointer operator->() const { return &(operator*()); }

  _Self& operator++() {
    this->_M_incr();
    return *this;
  }
  _Self operator++(int) {
    _Self __tmp = *this;
    this->_M_incr();
    return __tmp;
  }
  _Self& operator--() {
    this->_M_decr();
    return *this;
  }
  _Self operator--(int) {
    _Self __tmp = *this;
    this->_M_decr();
    return __tmp;
  }
};

inline bidirectional_iterator_tag
iterator_category(const _List_iterator_base&)
{
  return bidirectional_iterator_tag();
}

template <class _Tp, class _Ref, class _Ptr>
inline _Tp*
value_type(const _List_iterator<_Tp, _Ref, _Ptr>&)
{
  return 0;
}

inline ptrdiff_t*
distance_type(const _List_iterator_base&)
{
  return 0;
}


template <class _Tp, class _Alloc>
class _List_base
{
public:
  typedef _Alloc allocator_type;
  allocator_type get_allocator() const { return allocator_type(); }

  _List_base(const allocator_type&) {
      ///唯一的构造函数,规定了list为空时的合法状态:头结点的前后指针均指向其自身
    _M_node = _M_get_node();
    _M_node->_M_next = _M_node;
    _M_node->_M_prev = _M_node;
  }
  ~_List_base() {
    clear();    ///将每个结点清楚
    _M_put_node(_M_node);  ///将头结点归还
  }

  void clear();

protected:
  typedef simple_alloc<_List_node<_Tp>, _Alloc> _Alloc_type;

  _List_node<_Tp>* _M_get_node() { return _Alloc_type::allocate(1); }
  void _M_put_node(_List_node<_Tp>* __p) { _Alloc_type::deallocate(__p, 1); }

protected:
  _List_node<_Tp>* _M_node;   ///头结点指针,为实指节点类型
};

template <class _Tp, class _Alloc>
void
_List_base<_Tp,_Alloc>::clear()
{
    ///由于结点的_M_next均为基类指针,而基类指针不能直接初始化或者赋值给
    ///派生类指针,因此需要强制类型转化,已将_M_node->_M_next强制转化为其
    ///实质类型的指针.
  _List_node<_Tp>* __cur = (_List_node<_Tp>*) (_M_node->_M_next);
  while (__cur != _M_node) {
    _List_node<_Tp>* __tmp = __cur;
    __cur = (_List_node<_Tp>*) (__cur->_M_next);
    _Destroy(&__tmp->_M_data);    ///析构结点数据元素
    _M_put_node(__tmp);         ///归还结点内存
  }

  ///使链表恢复合法状态
  _M_node->_M_next = _M_node;
  _M_node->_M_prev = _M_node;
}

template <class _Tp, class _Alloc = __STL_DEFAULT_ALLOCATOR(_Tp) >
class list : protected _List_base<_Tp, _Alloc> {

  __STL_CLASS_REQUIRES(_Tp, _Assignable);

  typedef _List_base<_Tp, _Alloc> _Base;
protected:
  typedef void* _Void_pointer;

public:
  typedef _Tp value_type;
  typedef value_type* pointer;
  typedef const value_type* const_pointer;
  typedef value_type& reference;
  typedef const value_type& const_reference;
  typedef _List_node<_Tp> _Node;
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;

  typedef typename _Base::allocator_type allocator_type;
  allocator_type get_allocator() const { return _Base::get_allocator(); }

public:
  typedef _List_iterator<_Tp,_Tp&,_Tp*>             iterator;
  typedef _List_iterator<_Tp,const _Tp&,const _Tp*> const_iterator;

  typedef reverse_bidirectional_iterator<const_iterator,value_type,
                                         const_reference,difference_type>
          const_reverse_iterator;
  typedef reverse_bidirectional_iterator<iterator,value_type,reference,
                                         difference_type>
          reverse_iterator;

protected:
  using _Base::_M_node;
  using _Base::_M_put_node;
  using _Base::_M_get_node;

protected:
  _Node* _M_create_node(const _Tp& __x)  ///用特定数据构造结点
  {
    _Node* __p = _M_get_node();
    try {
      _Construct(&__p->_M_data, __x);
    }catch(...){
        _M_put_node(__p);
    }
    return __p;
  }

  _Node* _M_create_node()   ///构造含默认值的结点
  {
    _Node* __p = _M_get_node();
    try {
      _Construct(&__p->_M_data);
    }catch(...){
        _M_put_node(__p);
    }

    return __p;
  }

public:
  explicit list(const allocator_type& __a = allocator_type()) : _Base(__a) {}

  iterator begin()   {
      ///此处亦必须强制进行指针类型转化,因为iterator类构造函数只接受
      ///派生类指针,而_M_node->_M_next为基类指针,不能自动转化
      return (_Node*)(_M_node->_M_next);
    }
  const_iterator begin() const { return (_Node*)(_M_node->_M_next); }

  iterator end()             { return _M_node; }
  const_iterator end() const { return _M_node; }

  reverse_iterator rbegin()
    { return reverse_iterator(end()); }
  const_reverse_iterator rbegin() const
    { return const_reverse_iterator(end()); }

  reverse_iterator rend()
    { return reverse_iterator(begin()); }
  const_reverse_iterator rend() const
    { return const_reverse_iterator(begin()); }

  bool empty() const { return _M_node->_M_next == _M_node; }
  size_type size() const {
      ///list的size函数需要遍历整个list
    size_type __result = 0;
    distance(begin(), end(), __result);
    return __result;
  }
  size_type max_size() const { return size_type(-1); }

  reference front() { return *begin(); }
  const_reference front() const { return *begin(); }
  reference back() { return *(--end()); }
  const_reference back() const { return *(--end()); }

  ///list的swap函数只需交换各自的头指针
  void swap(list<_Tp, _Alloc>& __x) { __STD::swap(_M_node, __x._M_node); }

  iterator insert(iterator __position, const _Tp& __x) {
      ///该函数只需生成一个新节点,然后修改相关指针将该节点“链”到合适位置即可
    _Node* __tmp = _M_create_node(__x);
    __tmp->_M_next = __position._M_node;
    __tmp->_M_prev = __position._M_node->_M_prev;
    __position._M_node->_M_prev->_M_next = __tmp;
    __position._M_node->_M_prev = __tmp;
    return __tmp;
  }
  iterator insert(iterator __position) { return insert(__position, _Tp()); }

  /// Check whether it's an integral type.  If so, it's not an iterator.
  template<class _Integer>
  void _M_insert_dispatch(iterator __pos, _Integer __n, _Integer __x,
                          __true_type) {
    _M_fill_insert(__pos, (size_type) __n, (_Tp) __x);
  }

  template <class _InputIterator>
  void _M_insert_dispatch(iterator __pos,
                          _InputIterator __first, _InputIterator __last,
                          __false_type);

  template <class _InputIterator>
  void insert(iterator __pos, _InputIterator __first, _InputIterator __last) {
    typedef typename _Is_integer<_InputIterator>::_Integral _Integral;
    _M_insert_dispatch(__pos, __first, __last, _Integral());
  }


  void insert(iterator __pos, size_type __n, const _Tp& __x)
    { _M_fill_insert(__pos, __n, __x); }

  void _M_fill_insert(iterator __pos, size_type __n, const _Tp& __x); ///在x前插入n个x

  void push_front(const _Tp& __x) { insert(begin(), __x); }
  void push_front() {insert(begin());}
  void push_back(const _Tp& __x) { insert(end(), __x); }
  void push_back() {insert(end());}

  iterator erase(iterator __position) {
    _List_node_base* __next_node = __position._M_node->_M_next;
    _List_node_base* __prev_node = __position._M_node->_M_prev;
    _Node* __n = (_Node*) __position._M_node;
    __prev_node->_M_next = __next_node;
    __next_node->_M_prev = __prev_node;
    _Destroy(&__n->_M_data);
    _M_put_node(__n);
    return iterator((_Node*) __next_node);
  }
  iterator erase(iterator __first, iterator __last);
  void clear() { _Base::clear(); }

  void resize(size_type __new_size, const _Tp& __x);
  void resize(size_type __new_size) { this->resize(__new_size, _Tp()); }

  void pop_front() { erase(begin()); }
  void pop_back() {
    iterator __tmp = end();
    erase(--__tmp);
  }

  list(size_type __n, const _Tp& __value,
       const allocator_type& __a = allocator_type()): _Base(__a)
    { insert(begin(), __n, __value); }

  explicit list(size_type __n): _Base(allocator_type())
    { insert(begin(), __n, _Tp()); }


  /// We don't need any dispatching tricks here, because insert does all of
  /// that anyway.
  template <class _InputIterator>
  list(_InputIterator __first, _InputIterator __last,
       const allocator_type& __a = allocator_type())
    : _Base(__a)
    { insert(begin(), __first, __last); }

  list(const list<_Tp, _Alloc>& __x) : _Base(__x.get_allocator())
    { insert(begin(), __x.begin(), __x.end()); }

  ~list() { }  ///善后留给基类中的析构函数

  list<_Tp, _Alloc>& operator=(const list<_Tp, _Alloc>& __x);

public:
  /// assign(), a generalized assignment member function.  Two
  /// versions: one that takes a count, and one that takes a range.
  /// The range version is a member template, so we dispatch on whether
  /// or not the type is an integer.

  void assign(size_type __n, const _Tp& __val) { _M_fill_assign(__n, __val); }

  void _M_fill_assign(size_type __n, const _Tp& __val);


  template <class _InputIterator>
  void assign(_InputIterator __first, _InputIterator __last) {
    typedef typename _Is_integer<_InputIterator>::_Integral _Integral;
    _M_assign_dispatch(__first, __last, _Integral());
  }

  template <class _Integer>
  void _M_assign_dispatch(_Integer __n, _Integer __val, __true_type)
    { _M_fill_assign((size_type) __n, (_Tp) __val); }

  template <class _InputIterator>
  void _M_assign_dispatch(_InputIterator __first, _InputIterator __last,
                          __false_type);


protected:
    ///将[first,last)从原位置中摘下来,插入到position之前
    ///这个函数主要通过指针的修改来完成
  void transfer(iterator __position, iterator __first, iterator __last) {
    if (__position != __last) {
      /// Remove [first, last) from its old position.
      __last._M_node->_M_prev->_M_next     = __position._M_node;
      __first._M_node->_M_prev->_M_next    = __last._M_node;
      __position._M_node->_M_prev->_M_next = __first._M_node;

      /// Splice [first, last) into its new position.
      _List_node_base* __tmp      = __position._M_node->_M_prev;
      __position._M_node->_M_prev = __last._M_node->_M_prev;
      __last._M_node->_M_prev     = __first._M_node->_M_prev;
      __first._M_node->_M_prev    = __tmp;
    }
  }

public:
    ///将x链入本链表position之前
  void splice(iterator __position, list& __x) {
    if (!__x.empty())
      this->transfer(__position, __x.begin(), __x.end());
  }

  ///将i所指结点摘下来,插入到position之前
  void splice(iterator __position, list&, iterator __i) {
    iterator __j = __i;
    ++__j;
    if (__position == __i || __position == __j) return;
    this->transfer(__position, __i, __j);
  }

  void splice(iterator __position, list&, iterator __first, iterator __last) {
    if (__first != __last)
      this->transfer(__position, __first, __last);
  }
  void remove(const _Tp& __value);
  void unique();
  void merge(list& __x);
  void reverse();
  void sort();

  template <class _Predicate> void remove_if(_Predicate);
  template <class _BinaryPredicate> void unique(_BinaryPredicate);
  template <class _StrictWeakOrdering> void merge(list&, _StrictWeakOrdering);
  template <class _StrictWeakOrdering> void sort(_StrictWeakOrdering);
};

template <class _Tp, class _Alloc>
inline bool
operator==(const list<_Tp,_Alloc>& __x, const list<_Tp,_Alloc>& __y)
{
  ///此处必须使用const_iterator,因const list所得到的迭代器均为const_iterator
  typedef typename list<_Tp,_Alloc>::const_iterator const_iterator;
  const_iterator __end1 = __x.end();
  const_iterator __end2 = __y.end();

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

template <class _Tp, class _Alloc>
inline bool operator<(const list<_Tp,_Alloc>& __x,
                      const list<_Tp,_Alloc>& __y)
{
  return lexicographical_compare(__x.begin(), __x.end(),
                                 __y.begin(), __y.end());
}


template <class _Tp, class _Alloc>
template <class _InputIter>
void
list<_Tp, _Alloc>::_M_insert_dispatch(iterator __position,
                                      _InputIter __first, _InputIter __last,
                                      __false_type)
{
  for ( ; __first != __last; ++__first)
    insert(__position, *__first);
}

template <class _Tp, class _Alloc>
void
list<_Tp, _Alloc>::_M_fill_insert(iterator __position,
                                  size_type __n, const _Tp& __x)
{
  for ( ; __n > 0; --__n)
    insert(__position, __x);
}

template <class _Tp, class _Alloc>
typename list<_Tp,_Alloc>::iterator list<_Tp, _Alloc>::erase(iterator __first,
                                                             iterator __last)
{
  while (__first != __last)
    erase(__first++);
  return __last;
}

template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::resize(size_type __new_size, const _Tp& __x)
{
  iterator __i = begin();
  size_type __len = 0;
  for ( ; __i != end() && __len < __new_size; ++__i, ++__len);

  if (__len == __new_size)        ///__new_size <= this->size()
    erase(__i, end());
  else                          /// __new_size > this->size()
    insert(end(), __new_size - __len, __x);
}

template <class _Tp, class _Alloc>
list<_Tp, _Alloc>& list<_Tp, _Alloc>::operator=(const list<_Tp, _Alloc>& __x)
{
  if (this != &__x) {
    iterator __first1 = begin();
    iterator __last1 = end();
    const_iterator __first2 = __x.begin();
    const_iterator __last2 = __x.end();

    ///先挨个赋值
    while (__first1 != __last1 && __first2 != __last2)
      *__first1++ = *__first2++;

    if (__first2 == __last2)    ///x.size() <= this->size()
      erase(__first1, __last1);
    else                                        ///x.size() > this->size()
      insert(__last1, __first2, __last2);
  }
  return *this;
}

template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::_M_fill_assign(size_type __n, const _Tp& __val) {
  iterator __i = begin();
  for ( ; __i != end() && __n > 0; ++__i, --__n)
    *__i = __val;
  if (__n > 0)
    insert(end(), __n, __val);
  else
    erase(__i, end());
}

template <class _Tp, class _Alloc>
template <class _InputIter>
void
list<_Tp, _Alloc>::_M_assign_dispatch(_InputIter __first2, _InputIter __last2,
                                      __false_type)
{
  iterator __first1 = begin();
  iterator __last1 = end();
  for ( ; __first1 != __last1 && __first2 != __last2; ++__first1, ++__first2)
    *__first1 = *__first2;
  if (__first2 == __last2)
    erase(__first1, __last1);
  else
    insert(__last1, __first2, __last2);
}


template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::remove(const _Tp& __value)
{
  iterator __first = begin();
  iterator __last = end();
  while (__first != __last) {
    iterator __next = __first;
    ++__next;
    if (*__first == __value) erase(__first);
    __first = __next;
  }
}

template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::unique()
{
  iterator __first = begin();
  iterator __last = end();
  if (__first == __last) return;
  iterator __next = __first;
  while (++__next != __last) {
    if (*__first == *__next)
      erase(__next);
    else
      __first = __next;

    __next = __first;
  }
}

///将两个非递增排序的链表合并为一个非递增排序的链表
///合并后x链表为空
template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::merge(list<_Tp, _Alloc>& __x)
{
  iterator __first1 = begin();
  iterator __last1 = end();
  iterator __first2 = __x.begin();
  iterator __last2 = __x.end();
  while (__first1 != __last1 && __first2 != __last2)
    if (*__first2 < *__first1) {
      iterator __next = __first2;
      transfer(__first1, __first2, ++__next);
      __first2 = __next;
    }
    else
      ++__first1;
  if (__first2 != __last2) transfer(__last1, __first2, __last2);
}

///从指针p开始,将list翻转
inline void __List_base_reverse(_List_node_base* __p)
{
  _List_node_base* __tmp = __p;
  do {
    __STD::swap(__tmp->_M_next, __tmp->_M_prev);
    __tmp = __tmp->_M_prev;     /// Old next node is now prev.

  } while (__tmp != __p);     ///由于是循环链表,故指针再次指向p时,说明list翻转完毕
}

template <class _Tp, class _Alloc>
inline void list<_Tp, _Alloc>::reverse()
{
  __List_base_reverse(this->_M_node);
}

///由于STL sort算法要求必须为随机迭代器,因此list实现了自己的专用sort算法
///该算法采用的是归并排序的思想
template <class _Tp, class _Alloc>
void list<_Tp, _Alloc>::sort()
{
  /// Do nothing if the list has length 0 or 1.
  if (_M_node->_M_next != _M_node && _M_node->_M_next->_M_next != _M_node) {
    list<_Tp, _Alloc> __carry;
    list<_Tp, _Alloc> __counter[64];
    int __fill = 0;
    while (!empty()) {

      __carry.splice(__carry.begin(), *this, begin());  ///__carry得到list第一个元素
      int __i = 0;

      ///此循环将counter[__fill]之前所有非空链表合并为一个链表
      while(__i < __fill && !__counter[__i].empty()) {
        __counter[__i].merge(__carry);     ///此时__carry为空
        __carry.swap(__counter[__i++]);    ///此时__counter[i]为空,i变为i+1
      }
      __carry.swap(__counter[__i]);   ///至此处i之前的所有链表均被合并至__counter[i]
      if (__i == __fill) ++__fill;
    }

    for (int __i = 1; __i < __fill; ++__i)
      __counter[__i].merge(__counter[__i-1]);

    swap(__counter[__fill-1]);
  }
}


template <class _Tp, class _Alloc> template <class _Predicate>
void list<_Tp, _Alloc>::remove_if(_Predicate __pred)
{
  iterator __first = begin();
  iterator __last = end();
  while (__first != __last) {
    iterator __next = __first;
    ++__next;      ///必须先得到下一个节点位置,再删除当前结点,否则将无法找到下一个结点
    if (__pred(*__first)) erase(__first);
    __first = __next;
  }
}

template <class _Tp, class _Alloc> template <class _BinaryPredicate>
void list<_Tp, _Alloc>::unique(_BinaryPredicate __binary_pred)
{
  iterator __first = begin();
  iterator __last = end();
  if (__first == __last) return;
  iterator __next = __first;
  while (++__next != __last) {
    if (__binary_pred(*__first, *__next))
      erase(__next);
    else
      __first = __next;
    __next = __first;
  }
}

template <class _Tp, class _Alloc> template <class _StrictWeakOrdering>
void list<_Tp, _Alloc>::merge(list<_Tp, _Alloc>& __x,
                              _StrictWeakOrdering __comp)
{
  iterator __first1 = begin();
  iterator __last1 = end();
  iterator __first2 = __x.begin();
  iterator __last2 = __x.end();
  while (__first1 != __last1 && __first2 != __last2)
    if (__comp(*__first2, *__first1)) {
      iterator __next = __first2;
      transfer(__first1, __first2, ++__next);
      __first2 = __next;
    }
    else
      ++__first1;
  if (__first2 != __last2) transfer(__last1, __first2, __last2);
}

template <class _Tp, class _Alloc> template <class _StrictWeakOrdering>
void list<_Tp, _Alloc>::sort(_StrictWeakOrdering __comp)
{
  /// Do nothing if the list has length 0 or 1.
  if (_M_node->_M_next != _M_node && _M_node->_M_next->_M_next != _M_node) {
    list<_Tp, _Alloc> __carry;
    list<_Tp, _Alloc> __counter[64];
    int __fill = 0;
    while (!empty()) {
      __carry.splice(__carry.begin(), *this, begin());
      int __i = 0;
      while(__i < __fill && !__counter[__i].empty()) {
        __counter[__i].merge(__carry, __comp);
        __carry.swap(__counter[__i++]);
      }
      __carry.swap(__counter[__i]);
      if (__i == __fill) ++__fill;
    }

    for (int __i = 1; __i < __fill; ++__i)
      __counter[__i].merge(__counter[__i-1], __comp);
    swap(__counter[__fill-1]);
  }
}

C++ STL源码学习(list篇）