--The boost::iterator_traits class provides access to the -associated types of iterators that model the Boost Iterator Concepts, which are a -replacement for the iterator requirements in the C++ standard. -The main difference between std::iterator_traits and -boost::iterator_traits is that the iterator_category -type has been removed, and replaced with two new types: - -
-An important feature of the boost::iterator_traits is that it -is backwards compatible, i.e., it will automatically work for -iterators for which there are valid definitions of -std::iterator_traits. The old iterator_category is -mapped to the appropriate traversal and return categories. - -
-When creating a new iterator type that is meant to work with -boost::iterator_traits, you can either create a -specialization of boost::iterator_traits for your iterator -type, or you can provide all the necessary associated types as nested -typedefs. In this case, your iterator class will need to inherit from -new_iterator_base to let boost::iterator_traits know -that it will be able to find typedefs for traversal_category -and return_category in you iterator class. - - -
-namespace boost {
-
- // Inherit from iterator_base if your iterator defines its own
- // return_category and traversal_category. Otherwise, the "old style"
- // iterator category will be mapped to the return_category and
- // traversal_category.
- struct new_iterator_base { };
-
- template <typename Iterator>
- struct iterator_traits
- {
- if (Iterator inherits from new_iterator_base) {
- typedef typename Iterator::value_type value_type;
- typedef typename Iterator::reference reference;
- typedef typename Iterator::pointer pointer;
- typedef typename Iterator::difference_type difference_type;
- typedef typename Iterator::return_category return_category;
- typedef typename Iterator::traversal_category traversal_category;
- } else {
- typedef std::iterator_traits<Iterator> OldTraits;
- typedef typename OldTraits::value_type value_type;
- typedef typename OldTraits::reference reference;
- typedef typename OldTraits::pointer pointer;
- typedef typename OldTraits::difference_type difference_type;
-
- typedef typename OldTraits::iterator_category Cat;
-
- // Determine the traversal_category based on the old iterator_category
- if (Cat inherits from std::random_access_iterator_tag)
- typedef boost::random_access_iterator_tag traversal_category;
- else if (Cat inherits from std::bidirectional_iterator_tag)
- typedef boost::bidirectional_iterator_tag traversal_category;
- else if (Cat inherits from std::forward_iterator_tag)
- typedef boost::forward_iterator_tag traversal_category;
- else
- typedef boost::single_pass_iterator_tag traversal_category;
-
- // Determine the return_category based on the old iterator_category and value_type
- if (Cat inherits from std::forward_iterator_tag)
- if (is-const(T))
- typedef boost::constant_lvalue_iterator_tag return_category;
- else
- typedef boost::mutable_lvalue_iterator_tag return_category;
- else if (Cat inherits from std::input_iterator_tag)
- typedef boost::readable_iterator_tag return_category;
- else if (Cat inherits from std::output_iterator_tag)
- typedef boost::writable_iterator_tag return_category;
- else
- typedef boost::error_iterator_tag return_category;
- }
- };
-
- template <typename T>
- struct iterator_traits<T*>
- {
- typedef T value_type;
- typedef T& reference;
- typedef T* pointer;
- typedef std::ptrdiff_t difference_type;
- if (is-const(T))
- typedef boost::constant_lvalue_iterator_tag return_category;
- else
- typedef boost::mutable_lvalue_iterator_tag return_category;
- typedef boost::random_access_iterator_tag traversal_category;
- };
-
-}
-
-
-