STL(8)之五种迭代器的类型

1.五种迭代器类型的定义

 ///  Marking input iterators.
  struct input_iterator_tag { };

  ///  Marking output iterators.
  struct output_iterator_tag { };

  /// Forward iterators support a superset of input iterator operations.
  struct forward_iterator_tag : public input_iterator_tag { };

  /// Bidirectional iterators support a superset of forward iterator
  /// operations.
  struct bidirectional_iterator_tag : public forward_iterator_tag { };

  /// Random-access iterators support a superset of bidirectional
  /// iterator operations.
  struct random_access_iterator_tag : public bidirectional_iterator_tag { };

2.  These are empty types, used to distinguish different iterators.  The distinction is not made by what they contain, but simply by what they  are.  Different underlying algorithms can then be used based on the different operations supported by different iterator types.（运用此类型来激活模板的重载机制，编译期即可完成）

3.自己设计的迭代器应该继承下面这个，以保证STL所需的规范：（每个迭代器都应该提供五个内嵌的类型）

<span style="font-size:18px;">This class does nothing but define nested typedefs.  %Iterator classes
   *  can inherit from this class to save some work.  The typedefs are then
   *  used in specializations and overloading.
  */
  template<typename _Category, typename _Tp, typename _Distance = ptrdiff_t,
           typename _Pointer = _Tp*, typename _Reference = _Tp&>
    struct iterator
    {
      /// One of the @link iterator_tags tag types@endlink.
      typedef _Category  iterator_category;
      /// The type "pointed to" by the iterator.
      typedef _Tp        value_type;
      /// Distance between iterators is represented as this type.
      typedef _Distance  difference_type;
      /// This type represents a pointer-to-value_type.
      typedef _Pointer   pointer;
      /// This type represents a reference-to-value_type.
      typedef _Reference reference;
    };</span>

4.特性萃取机traits:

<span style="font-size:18px;">@brief  Traits class for iterators.
   *
   *  This class does nothing but define nested typedefs.  The general
   *  version simply @a forwards the nested typedefs from the Iterator
   *  argument.  Specialized versions for pointers and pointers-to-const
   *  provide tighter, more correct semantics.</span>

<span style="font-size:18px;">template<typename _Iterator>
    struct iterator_traits
    {
      typedef typename _Iterator::iterator_category iterator_category;
      typedef typename _Iterator::value_type        value_type;
      typedef typename _Iterator::difference_type   difference_type;
      typedef typename _Iterator::pointer           pointer;
      typedef typename _Iterator::reference         reference;

    }</span>

特化版本：一个是为一般的指针，一个是为常量指针（指向常量的指针）而设计出的iterator_traits偏特化版本

<span style="font-size:18px;">  /// Partial specialization for pointer types.
  template<typename _Tp>
    struct iterator_traits<_Tp*>
    {
      typedef random_access_iterator_tag iterator_category;
      typedef _Tp                         value_type;
      typedef ptrdiff_t                   difference_type;
      typedef _Tp*                        pointer;
      typedef _Tp&                        reference;
    };

  /// Partial specialization for const pointer types.
  template<typename _Tp>
    struct iterator_traits<const _Tp*>
    {
      typedef random_access_iterator_tag iterator_category;
      typedef _Tp                         value_type;
      typedef ptrdiff_t                   difference_type; //内建的原生指针的距离
      typedef const _Tp*                  pointer;
      typedef const _Tp&                  reference;
    };</span>