我读STL

1. 通过迭代器萃取类型：

/** * This class does nothing but define nested typedefs. The general * version simply "forwards" the nested typedefs from the Iterator * argument. Specialized versions for pointers and pointers-to-const * provide tighter, more correct semantics. */ 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; }; 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; }; template<typename _Tp> struct iterator_traits<const _Tp*> { typedef random_access_iterator_tag iterator_category; typedef _Tp value_type; typedef ptrdiff_t difference_type; typedef const _Tp* pointer; typedef const _Tp& reference; };

通过访问iterator_traits<itr>::value_type可以返回itr所指向的类型。

不过要在自己的itr中定义：

 _Iterator::iterator_category
 _Iterator::value_type
 _Iterator::difference_type
 _Iterator::pointer
 _Iterator::reference

后面两个是特化版本。

下面是迭代器的定义，内含需自己定义的类型：

/** * @brief Common %iterator class. * * 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. * * In particular, there are no default implementations of requirements * such as @c operator++ and the like. (How could there be?) */ 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; };

 

2. 迭代器的类型

/** * @defgroup iterators Iterators * 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. */ //@{ /// 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 { };