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| <title>Iterator Traits</title> |
| <meta name="author" content="David Abrahams" /> |
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| <meta name="date" content="2006-09-11" /> |
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| <div class="document" id="iterator-traits"> |
| <h1 class="title">Iterator Traits</h1> |
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| <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 external" 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 external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr> |
| <tr><th class="docinfo-name">Date:</th> |
| <td>2006-09-11</td></tr> |
| <tr><th class="docinfo-name">Copyright:</th> |
| <td>Copyright David Abrahams 2004.</td></tr> |
| </tbody> |
| </table> |
| <!-- Distributed under the Boost --> |
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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 |
| the ability to access an iterator's associated types using |
| MPL-compatible <a class="reference external" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</td> |
| </tr> |
| </tbody> |
| </table> |
| <div class="section" id="overview"> |
| <h1>Overview</h1> |
| <p><tt class="docutils literal"><span class="pre">std::iterator_traits</span></tt> provides access to five associated types |
| 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>, |
| <tt class="docutils literal"><span class="pre">iterator_category</span></tt>, and <tt class="docutils 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="docutils literal"><span class="pre"><boost/iterator/iterator_traits.hpp></span></tt> |
| provides access to these types using a standard <a class="reference external" href="../../mpl/doc/index.html#metafunctions">metafunctions</a>.</p> |
| </div> |
| <div class="section" id="summary"> |
| <h1>Summary</h1> |
| <p>Header <tt class="docutils 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>Broken Compiler Notes</h1> |
| <p>Because of workarounds in Boost, you may find that these |
| <a class="reference external" 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 external" href="../../type_traits/index.html#transformations">BOOST_BROKEN_COMPILER_TYPE_TRAITS_SPECIALIZATION</a> on the |
| <tt class="docutils 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="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 |
| compiler. A macro, <tt class="docutils literal"><span class="pre">BOOST_ITERATOR_CATEGORY</span></tt>, that expands to |
| 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 |
| appropriate to the platform, is provided for portability.</p> |
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