<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" /> <title>Iterator Traits</title> <meta name="author" content="David Abrahams" /> <meta name="organization" content="Boost Consulting" /> <meta name="date" content="2006-09-11" /> <meta name="copyright" content="Copyright David Abrahams 2004." /> <link rel="stylesheet" href="../../../rst.css" type="text/css" /> </head> <body> <div class="document" id="iterator-traits"> <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 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 --> <!-- Software License, Version 1.0. (See accompanying --> <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <table class="docutils field-list" frame="void" rules="none"> <col class="field-name" /> <col class="field-body" /> <tbody valign="top"> <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> </div> </div> <div class="footer"> <hr class="footer" /> <a class="reference external" href="iterator_traits.rst">View document source</a>. Generated by <a class="reference external" href="http://docutils.sourceforge.net/">Docutils</a> from <a class="reference external" href="http://docutils.sourceforge.net/rst.html">reStructuredText</a> source. </div> </body> </html>