+++++++++++++++++
 Iterator Traits
+++++++++++++++++

:Author: David Abrahams
:Contact: dave@boost-consulting.com
:organization: `Boost Consulting`_
:date: $Date: 2004/11/02 14:39:04 $
:copyright: Copyright David Abrahams 2004. 

.. _`Boost Consulting`: http://www.boost-consulting.com

:abstract: Header ``<boost/iterator/iterator_traits.hpp>`` provides
  the ability to access an iterator's associated types using
  MPL-compatible metafunctions_.

.. _metafunctions: ../../mpl/doc/index.html#metafunctions

Overview
========

``std::iterator_traits`` provides access to five associated types
of any iterator: its ``value_type``, ``reference``, ``pointer``,
``iterator_category``, and ``difference_type``.  Unfortunately,
such a "multi-valued" traits template can be difficult to use in a
metaprogramming context.  ``<boost/iterator/iterator_traits.hpp>``
provides access to these types using a standard metafunctions_.

Summary
=======

Header ``<boost/iterator/iterator_traits.hpp>``::

  template <class Iterator>
  struct iterator_value
  {
      typedef typename 
        std::iterator_traits<Iterator>::value_type 
      type;
  };

  template <class Iterator>
  struct iterator_reference
  {
      typedef typename 
        std::iterator_traits<Iterator>::reference
      type;
  };


  template <class Iterator>
  struct iterator_pointer
  {
      typedef typename 
        std::iterator_traits<Iterator>::pointer 
      type;
  };

  template <class Iterator>
  struct iterator_difference
  {
      typedef typename
        detail::iterator_traits<Iterator>::difference_type
      type;
  };

  template <class Iterator>
  struct iterator_category
  {
      typedef typename
        detail::iterator_traits<Iterator>::iterator_category
      type;
  };

Broken Compiler Notes
=====================

Because of workarounds in Boost, you may find that these
metafunctions_ actually work better than the facilities provided by
your compiler's standard library.

On compilers that don't support partial specialization, such as
Microsoft Visual C++ 6.0 or 7.0, you may need to manually invoke
BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION_ on the
``value_type`` of pointers that are passed to these metafunctions.

Because of bugs in the implementation of GCC-2.9x, the name of
``iterator_category`` is changed to ``iterator_category_`` on that
compiler.  A macro, ``BOOST_ITERATOR_CATEGORY``, that expands to
either ``iterator_category`` or ``iterator_category_``, as
appropriate to the platform, is provided for portability.

.. _BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION: ../../type_traits/index.html#transformations