C++ In a Nutshell

Name

iterator_traits class template — Iterator traits

Synopsis

template<typename Iterator>
struct iterator_traits{
  typedef typename Iterator::difference_type difference_type;
  typedef typename Iterator::value_type value_type;
  typedef typename Iterator::pointer pointer;
  typedef typename Iterator::reference reference;
  typedef typename Iterator::iterator_category iterator_category;
};

The iterator_traits class template declares traits for an iterator. If you use the iterator class template as the base for your custom iterator, you don’t need to specialize iterator_traits. If you are writing a custom container or algorithm, you should always use iterator_traits to obtain the traits of an iterator. If you use a plain pointer as an iterator, the standard library specializes iterator_traits for you. See the next subsection.

If you write your own specialization, the iterator_category type must be one of the five iterator tag classes. (See the iterator class template.) For an output iterator, difference_type and value_type are void.

When writing a generic algorithm or other function that uses iterators, you can use iterator_traits to specialize the behavior for certain kinds of iterators. See Example 13-19 (under distance), which shows how iterator traits can be used to improve the performance of a function.

See Also

iterator class template