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+++++++++++++++++
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
|
