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<title>Iterator Traits</title>
<meta name="author" content="David Abrahams" />
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<h1 class="title">Iterator Traits</h1>
<table class="docinfo" frame="void" rules="none">
<col class="docinfo-name" />
<col class="docinfo-content" />
<tbody valign="top">
<tr><th class="docinfo-name">Author:</th>
<td>David Abrahams</td></tr>
<tr><th class="docinfo-name">Contact:</th>
<td><a class="first last reference" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a></td></tr>
<tr><th class="docinfo-name">Organization:</th>
<td><a class="first last reference" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
<tr><th class="docinfo-name">Date:</th>
<td>2004-11-01</td></tr>
<tr><th class="docinfo-name">Copyright:</th>
<td>Copyright David Abrahams 2004.</td></tr>
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</table>
<div class="document" id="iterator-traits">
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<tbody valign="top">
<tr class="field"><th class="field-name">abstract:</th><td class="field-body">Header <tt class="literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt> provides
the ability to access an iterator's associated types using
MPL-compatible <a class="reference" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</td>
</tr>
</tbody>
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<div class="section" id="overview">
<h1><a name="overview">Overview</a></h1>
<p><tt class="literal"><span class="pre">std::iterator_traits</span></tt> provides access to five associated types
of any iterator: its <tt class="literal"><span class="pre">value_type</span></tt>, <tt class="literal"><span class="pre">reference</span></tt>, <tt class="literal"><span class="pre">pointer</span></tt>,
<tt class="literal"><span class="pre">iterator_category</span></tt>, and <tt class="literal"><span class="pre">difference_type</span></tt>. Unfortunately,
such a "multi-valued" traits template can be difficult to use in a
metaprogramming context. <tt class="literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt>
provides access to these types using a standard <a class="reference" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</p>
</div>
<div class="section" id="summary">
<h1><a name="summary">Summary</a></h1>
<p>Header <tt class="literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt>:</p>
<pre class="literal-block">
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;
};
</pre>
</div>
<div class="section" id="broken-compiler-notes">
<h1><a name="broken-compiler-notes">Broken Compiler Notes</a></h1>
<p>Because of workarounds in Boost, you may find that these
<a class="reference" href="../../mpl/doc/index.html#metafunctions">metafunctions</a> actually work better than the facilities provided by
your compiler's standard library.</p>
<p>On compilers that don't support partial specialization, such as
Microsoft Visual C++ 6.0 or 7.0, you may need to manually invoke
<a class="reference" href="../../type_traits/index.html#transformations">BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION</a> on the
<tt class="literal"><span class="pre">value_type</span></tt> of pointers that are passed to these metafunctions.</p>
<p>Because of bugs in the implementation of GCC-2.9x, the name of
<tt class="literal"><span class="pre">iterator_category</span></tt> is changed to <tt class="literal"><span class="pre">iterator_category_</span></tt> on that
compiler. A macro, <tt class="literal"><span class="pre">BOOST_ITERATOR_CATEGORY</span></tt>, that expands to
either <tt class="literal"><span class="pre">iterator_category</span></tt> or <tt class="literal"><span class="pre">iterator_category_</span></tt>, as
appropriate to the platform, is provided for portability.</p>
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