迭代器（list迭代器的实现）

其实一种循环结构就是一种迭代操作。
在STL中，容器的迭代器被作为容器元素对象或者I/O流中的对象的位置指示器，因此可以把它理解为面向对象的指针–一种泛型指针或通用指针。不依赖于元素的真实类型。迭代器是为了降低容器和泛型算法至今联系而设计的，泛型算法的参数不是容器。是迭代器。

容器的迭代器的作用就类似一个游标，它屏蔽了底层存储空间的不连续性，在上层使容器元素有一种连续的假象。
接下来简单实现下list迭代器

#pragma once
#include 
#include 
using namespace std;

template<class T>
struct _ListNode
{
    _ListNode* _prev;
    _ListNode* _next;
    T _data;
    _ListNode(const T& data)
        :_data(data)
        , _prev(NULL)
        , _next(NULL)
    {}
};
template<class T,class Ref,class Ptr>
//ref为引用，ptr为指针类型
struct _ListIterator
{
    typedef _ListNode Node;
    typedef _ListIterator self;
    typedef _ListIterator Iterator;
    typedef Ref Reference;
    typedef Ptr Pointer;
    Node* _node;
    _ListIterator(Node* node)
        :_node(node)
    {}
    Ref operator*()
    {
        return _node->_data;
    }
    Ptr operator->()
    {
        return &(_node->_data);
    }
    self& operator++()
    {
        _node = _node->_next;
        return *this;
    }
    self operator++(int)//后置++
    {
        Node* temp = _node;
        _node = _node->_next;
        return temp;
    }
    self& operator--()
    {
        _node= _node->_prev;
        return *this;

    }
    self& operator--(int)//后置--
    {
        Node* temp = _node;
        _node = _node->_prev;
        return temp;
    }
    bool operator==(const self& s)
    {
        return _node == s._node;

    }
    bool `
perator!=(const self& s)
    {
        return _node != s._node;
    }
};

//ReserveIterator迭代器就可以移正向迭代器为模板。再次基础上是反向迭代器。
template <class Iterator>
struct ReserveIterator
{
    ReserveIterator(Iterator it)
    :_it(it)
    {}
   typename Iterator::Reference operator*()
    {
        return *_it;
    }
    typename Iterator::Pointer operator->()
    {
        return operator->();
    }
    ReserveIterator operator++()
    {
        --_it;
        return *this;
    }
    ReserveIterator operator++(int)
    {
        ReserveIterator temp = _it;
        --_it;
        return temp;
    }
    ReserveIterator operator--()
    {
        ++it;
        return *this;
    }
    ReserveIterator operator--(int)
    {
        ReserveIterator temp = _it;
        ++_it;
        return temp;
    }
    bool operator!=(ReserveIterator& rit)
    {
        return _it != rit._it;
    }
    Iterator _it;
};

//list迭代器的使用实现常规操作
template<class T>
class List
{
    typedef _ListNode<T> Node;
public:
    typedef _ListIterator<T, T&, T*> Iterator;
    typedef _ListIterator<T, const T&, const T*> ConstIterator;
    typedef ReserveIterator<Iterator> ReserveIterator;
    //typedef ReserveIterator<ConstIterator> ConstReserveIterator;


    List()
    {
        _head = new Node(T());
        _head->_next = _head;
        _head->_prev = _head;
    }
    void clear()
    {
        Iterator it = Begin();
        while (it!= End())
        {
            Node* del = it._node;
            it++;
            delete del;
        }
    }
    ~List()
    {
        clear();
        delete _head;
        _head = NULL;
    }
    Iterator End()
    {
        return Iterator(_head);
    }
    ConstIterator End() const
    {
        return ConstIterator(_head);
    }
    ReserveIterator REnd()
    {
        return ReserveIterator(End());
    }
    /*ConstReserveIterator REnd() const
    {
        return ConstReserveIterator(End());
    }*/
    ReserveIterator RBegin()
    {
        return ReserveIterator(--End());
    }
    /*ConstReserveIterator RBegin() const
    {
        return ConstReserveIterator(End());
    }*/
    Iterator Begin()
    {
        return Iterator(_head->_next);
    }
    ConstIterator Begin() const
    {
        return ConstIterator(_head->_next);
    }
    Node* Buynode(const T& x)
    { 
        Node* newnode = new Node(x);
        return newnode;
    }
    void Insert(Iterator pos, const T& x)//在pos位置前插入
    {
        Node* cur = pos._node;
        Node* prev = cur->_prev;
        Node* temp = Buynode(x);
        temp->_prev = prev;
        prev->_next = temp;
        temp->_next = cur;
        cur->_prev = temp;
    }
    void Erase(Iterator& pos)
    {
        assert(pos != _head);
        Node* cur = pos._node;
        Node* prev = cur->_prev;
        Node* next = cur->_next;
        prev->_next = next;
        next->_prev = prev;
        delete cur;
    }
    void PushBack(const T& data)
    {/*
        Node* temp = Buynode(data);
        Node* tail = _head->_prev;
        temp->_prev = tail;
        tail->_next = temp;
        temp->_next = _head;
        _head->_prev = temp;*/
        Insert(End(), data);
    }
    void PushFront(const T& data)
    {
        Insert(Begin(), data);

    }
    void PopBack()
    {
        Erase(--End());
    }
    void PopFront()
    {
        Erase(Begin());
    }
    void PrintList()
    {
        Iterator it = Begin();
        while (it != End())
        {
            cout << *it << " ";
            it++;
        }
        cout << endl;
    }
protected:
    Node* _head;
};
void TestList()
{
    List l;
    l.PushBack(1);
    l.PushBack(2);
    l.PushBack(3);
    l.PushBack(4);
    //l.PrintList();
    ///*l.PopBack();
    //l.PopBack();
    //l.PopBack();
    //l.PopBack();*/
    //l.PushFront(8);
    //l.PushFront(0);
    //l.PrintList();
    //l.PopFront();
    //l.PopFront();
    //l.PopFront();
    //l.PrintList();
    List::ReserveIterator it = l.RBegin();
    while (it != l.REnd())
    {
        cout << *it << " ";
        it++;
    }
    cout << endl;
}