10單向連結串列（slist）

1、slist概述

slist並不在標準規格之內，sllist和list的主要差別在於，slist的迭代器屬於單向的Forward Iterator，而list的迭代器屬於雙向的Bidirectional Iterator。由於slist沒有任何方便的辦法可以回頭定出前一個位置，必須從頭找起，所以對於slist除了起點外其他位置採用insert和erase操作都屬於不智之舉，這是slist相較於list的大缺點。為此，slist特別提供了insert_after()和erase_after()供靈活運用。同樣slist不提供push_back()只提供push_front()，因此slist的元素次序會和元素插入進來的次序相反。

2、slist的節點

slist節點和其迭代器的設計，架構上比list複雜許多，運用了繼承關係，因此在性別轉換上有複雜的表現。__slist_node繼承自__slist_node_base，__slist_iterator繼承自__slist_iterator_base。

struct __slist_node_base
{
  __slist_node_base* next;
};
template <class T>
struct __slist_node : public __slist_node_base
{
  T data;
};

2、slist迭代器

struct __slist_iterator_base
{
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;
  typedef forward_iterator_tag iterator_category;

  __slist_node_base* node;

  __slist_iterator_base(__slist_node_base* x) : node(x) {}
  void incr() { node = node->next; }

  bool operator==(const __slist_iterator_base& x) const {
    return node == x.node;
  }
  bool operator!=(const __slist_iterator_base& x) const {
    return node != x.node;
  }
};

template <class T, class Ref, class Ptr>
struct __slist_iterator : public __slist_iterator_base
{
  typedef __slist_iterator<T, T&, T*>             iterator;
  typedef __slist_iterator<T, const T&, const T*> const_iterator;
  typedef __slist_iterator<T, Ref, Ptr>           self;

  typedef T value_type;
  typedef Ptr pointer;
  typedef Ref reference;
  typedef __slist_node<T> list_node;

  __slist_iterator(list_node* x) : __slist_iterator_base(x) {}
  __slist_iterator() : __slist_iterator_base(0) {}
  __slist_iterator(const iterator& x) : __slist_iterator_base(x.node) {}

  reference operator*() const { return ((list_node*) node)->data; }
  pointer operator->() const { return &(operator*()); }

  self& operator++()
  {
    incr();
    return *this;
  }
  self operator++(int)
  {
    self tmp = *this;
    incr();
    return tmp;
  }
};

3、slist資料結構

初始化的slist就一個head節點物件，head沒有data資料成員只有一個next成員，head.next=0。

template <class T, class Alloc = alloc>
class slist
{
public:
  typedef T value_type;
  typedef value_type* pointer;
  typedef const value_type* const_pointer;
  typedef value_type& reference;
  typedef const value_type& const_reference;
  typedef size_t size_type;
  typedef ptrdiff_t difference_type;

  typedef __slist_iterator<T, T&, T*>             iterator;
  typedef __slist_iterator<T, const T&, const T*> const_iterator;

private:
  typedef __slist_node<T> list_node;
  typedef __slist_node_base list_node_base;
  typedef __slist_iterator_base iterator_base;
  typedef simple_alloc<list_node, Alloc> list_node_allocator;

  static list_node* create_node(const value_type& x) {
    list_node* node = list_node_allocator::allocate();
    __STL_TRY {
      construct(&node->data, x);
      node->next = 0;
    }
    __STL_UNWIND(list_node_allocator::deallocate(node));
    return node;
  }

  static void destroy_node(list_node* node) {
    destroy(&node->data);
    list_node_allocator::deallocate(node);
  }

private:
  list_node_base head;

public:
  slist() { head.next = 0; }
  slist(size_type n, const value_type& x) { fill_initialize(n, x); }
  slist(int n, const value_type& x) { fill_initialize(n, x); }
  slist(long n, const value_type& x) { fill_initialize(n, x); }
  explicit slist(size_type n) { fill_initialize(n, value_type()); }
  template <class InputIterator>
  slist(InputIterator first, InputIterator last) {
    range_initialize(first, last);
  }
  slist(const_iterator first, const_iterator last) {
    range_initialize(first, last);
  }
  slist(const value_type* first, const value_type* last) {
    range_initialize(first, last);
  }
  slist(const slist& L) { range_initialize(L.begin(), L.end()); }
  slist& operator= (const slist& L);
  ~slist() { clear(); }

public:
  iterator begin() { return iterator((list_node*)head.next); }
  const_iterator begin() const { return const_iterator((list_node*)head.next);}
  iterator end() { return iterator(0); }
  const_iterator end() const { return const_iterator(0); }
  size_type size() const { return __slist_size(head.next); }
  size_type max_size() const { return size_type(-1); }
  bool empty() const { return head.next == 0; }
  void swap(slist& L)
  {
    list_node_base* tmp = head.next;
    head.next = L.head.next;
    L.head.next = tmp;
  }

public:
  reference front() { return ((list_node*) head.next)->data; }
  const_reference front() const { return ((list_node*) head.next)->data; }
  void push_front(const value_type& x)   {
    __slist_make_link(&head, create_node(x));
  }
  void pop_front() {
    list_node* node = (list_node*) head.next;
    head.next = node->next;
    destroy_node(node);
  }
}

4、slist元素操作

（1）push_front

inline __slist_node_base* __slist_make_link(__slist_node_base* prev_node,
                                            __slist_node_base* new_node)
{
  new_node->next = prev_node->next;
  prev_node->next = new_node;
  return new_node;
}
void push_front(const value_type& x)   {
    __slist_make_link(&head, create_node(x));
}

（2）pop_front

void pop_front() {
    list_node* node = (list_node*) head.next;
    head.next = node->next;
    destroy_node(node);
}

（3）erase_after

list_node_base* erase_after(list_node_base* pos) {
    list_node* next = (list_node*) (pos->next);
    list_node_base* next_next = next->next;
    pos->next = next_next;
    destroy_node(next);
    return next_next;
}

（4）insert_after

iterator insert_after(iterator pos, const value_type& x) {
    return iterator(_insert_after(pos.node, x));
}
list_node* _insert_after(list_node_base* pos, const value_type& x) {
    return (list_node*) (__slist_make_link(pos, create_node(x)));
}
inline __slist_node_base* __slist_make_link(__slist_node_base* prev_node,
                                            __slist_node_base* new_node)
{
  new_node->next = prev_node->next;
  prev_node->next = new_node;
  return new_node;
}