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125 lines
5.7 KiB
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<meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" />
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<title>Iterator Traits</title>
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<meta name="author" content="David Abrahams" />
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<meta name="organization" content="Boost Consulting" />
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<meta name="date" content="2006-09-11" />
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<meta name="copyright" content="Copyright David Abrahams 2004." />
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<div class="document" id="iterator-traits">
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<h1 class="title">Iterator Traits</h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>David Abrahams</td></tr>
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<tr><th class="docinfo-name">Contact:</th>
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<td><a class="first last reference external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a></td></tr>
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<tr><th class="docinfo-name">Organization:</th>
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<td><a class="first last reference external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
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<tr><th class="docinfo-name">Date:</th>
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<td>2006-09-11</td></tr>
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<tr><th class="docinfo-name">Copyright:</th>
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<td>Copyright David Abrahams 2004.</td></tr>
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</tbody>
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</table>
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<!-- Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">abstract:</th><td class="field-body">Header <tt class="docutils literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt> provides
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the ability to access an iterator's associated types using
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MPL-compatible <a class="reference external" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</td>
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</tr>
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</tbody>
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</table>
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<div class="section" id="overview">
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<h1>Overview</h1>
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<p><tt class="docutils literal"><span class="pre">std::iterator_traits</span></tt> provides access to five associated types
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of any iterator: its <tt class="docutils literal"><span class="pre">value_type</span></tt>, <tt class="docutils literal"><span class="pre">reference</span></tt>, <tt class="docutils literal"><span class="pre">pointer</span></tt>,
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<tt class="docutils literal"><span class="pre">iterator_category</span></tt>, and <tt class="docutils literal"><span class="pre">difference_type</span></tt>. Unfortunately,
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such a "multi-valued" traits template can be difficult to use in a
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metaprogramming context. <tt class="docutils literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt>
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provides access to these types using a standard <a class="reference external" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</p>
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</div>
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<div class="section" id="summary">
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<h1>Summary</h1>
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<p>Header <tt class="docutils literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt>:</p>
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<pre class="literal-block">
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template <class Iterator>
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struct iterator_value
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{
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typedef typename
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std::iterator_traits<Iterator>::value_type
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type;
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};
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template <class Iterator>
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struct iterator_reference
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{
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typedef typename
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std::iterator_traits<Iterator>::reference
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type;
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};
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template <class Iterator>
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struct iterator_pointer
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{
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typedef typename
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std::iterator_traits<Iterator>::pointer
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type;
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};
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template <class Iterator>
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struct iterator_difference
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{
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typedef typename
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detail::iterator_traits<Iterator>::difference_type
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type;
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};
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template <class Iterator>
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struct iterator_category
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{
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typedef typename
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detail::iterator_traits<Iterator>::iterator_category
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type;
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};
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</pre>
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</div>
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<div class="section" id="broken-compiler-notes">
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<h1>Broken Compiler Notes</h1>
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<p>Because of workarounds in Boost, you may find that these
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<a class="reference external" href="../../mpl/doc/index.html#metafunctions">metafunctions</a> actually work better than the facilities provided by
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your compiler's standard library.</p>
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<p>On compilers that don't support partial specialization, such as
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Microsoft Visual C++ 6.0 or 7.0, you may need to manually invoke
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<a class="reference external" href="../../type_traits/index.html#transformations">BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION</a> on the
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<tt class="docutils literal"><span class="pre">value_type</span></tt> of pointers that are passed to these metafunctions.</p>
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<p>Because of bugs in the implementation of GCC-2.9x, the name of
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<tt class="docutils literal"><span class="pre">iterator_category</span></tt> is changed to <tt class="docutils literal"><span class="pre">iterator_category_</span></tt> on that
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compiler. A macro, <tt class="docutils literal"><span class="pre">BOOST_ITERATOR_CATEGORY</span></tt>, that expands to
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either <tt class="docutils literal"><span class="pre">iterator_category</span></tt> or <tt class="docutils literal"><span class="pre">iterator_category_</span></tt>, as
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appropriate to the platform, is provided for portability.</p>
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