2001-03-19 17:41:49 +00:00
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<html>
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<!--
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-- Copyright (c) Jeremy Siek 2000
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--
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-- Permission to use, copy, modify, distribute and sell this software
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-- and its documentation for any purpose is hereby granted without fee,
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-- provided that the above copyright notice appears in all copies and
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-- that both that copyright notice and this permission notice appear
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-- in supporting documentation. I make no representations about the
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-- suitability of this software for any purpose. It is provided "as is"
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-- without express or implied warranty.
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-->
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<head>
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<title>Boost Iterator Traits</title>
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</head>
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2001-03-19 18:01:41 +00:00
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<BODY BGCOLOR="#ffffff" LINK="#0000ee" TEXT="#000000" VLINK="#551a8b"
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ALINK="#ff0000">
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<IMG SRC="../../c++boost.gif"
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ALT="C++ Boost" width="277" height="86">
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<BR Clear>
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2001-03-19 17:41:49 +00:00
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<h1>Boost Iterator Traits</h1>
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Header <tt><a href="../../boost/iterator_traits.hpp">boost/iterator_traits.hpp</a></tt>
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2001-03-19 18:01:41 +00:00
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<p>
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The <tt>boost::iterator_traits</tt> class provides access to the
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associated types of iterators that model the Boost <a
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href="./iterator_concepts.htm">Iterator Concepts</a>, which are a
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replacement for the iterator requirements in the C++ standard.
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The main difference between <tt>std::iterator_traits</tt> and
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<tt>boost::iterator_traits</tt> is that the <tt>iterator_category</tt>
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type has been removed, and replaced with two new types:
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<ul>
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<li><tt>traversal_category</tt>: Can the iterator go forward, backward, etc.?
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<li><tt>return_category</tt>: Is the iterator read or write only?
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Is the dereferenced type an lvalue?
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</ul>
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<p>
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An important feature of the <tt>boost::iterator_traits</tt> is that it
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is <b>backwards compatible</b>, i.e., it will automatically work for
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iterators for which there are valid definitions of
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<tt>std::iterator_traits</tt>. The old <tt>iterator_category</tt> is
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mapped to the appropriate traversal and return categories.
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<p>
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When creating a new iterator type that is meant to work with
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<tt>boost::iterator_traits</tt>, you can either create a
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specialization of <tt>boost::iterator_traits</tt> for your iterator
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type, or you can provide all the necessary associated types as nested
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typedefs. In this case, your iterator class will need to inherit from
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<tt>new_iterator_base</tt> to let <tt>boost::iterator_traits</tt> know
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that it will be able to find typedefs for <tt>traversal_category</tt>
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and <tt>return_category</tt> in you iterator class.
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2001-03-19 17:41:49 +00:00
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<pre>
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namespace boost {
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2001-03-19 18:01:41 +00:00
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<i>// Inherit from iterator_base if your iterator defines its own
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// return_category and traversal_category. Otherwise, the "old style"
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// iterator category will be mapped to the return_category and
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// traversal_category.</i>
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struct new_iterator_base { };
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2001-03-19 17:41:49 +00:00
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template <typename Iterator>
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struct iterator_traits
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{
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if (Iterator inherits from new_iterator_base) {
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typedef typename Iterator::value_type value_type;
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typedef typename Iterator::reference reference;
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typedef typename Iterator::pointer pointer;
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typedef typename Iterator::difference_type difference_type;
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typedef typename Iterator::return_category return_category;
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typedef typename Iterator::traversal_category traversal_category;
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} else {
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typedef std::iterator_traits<Iterator> OldTraits;
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typedef typename OldTraits::value_type value_type;
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typedef typename OldTraits::reference reference;
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typedef typename OldTraits::pointer pointer;
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typedef typename OldTraits::difference_type difference_type;
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typedef typename OldTraits::iterator_category Cat;
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<i>// Determine the traversal_category based on the old iterator_category</i>
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if (Cat inherits from std::random_access_iterator_tag)
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typedef boost::random_access_iterator_tag traversal_category;
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else if (Cat inherits from std::bidirectional_iterator_tag)
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typedef boost::bidirectional_iterator_tag traversal_category;
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else if (Cat inherits from std::forward_iterator_tag)
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typedef boost::forward_iterator_tag traversal_category;
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else
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typedef boost::single_pass_iterator_tag traversal_category;
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<i>// Determine the return_category based on the old iterator_category and value_type</i>
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if (Cat inherits from std::forward_iterator_tag)
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if (is-const(T))
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typedef boost::constant_lvalue_iterator_tag return_category;
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else
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typedef boost::mutable_lvalue_iterator_tag return_category;
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else if (Cat inherits from std::input_iterator_tag)
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typedef boost::readable_iterator_tag return_category;
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else if (Cat inherits from std::output_iterator_tag)
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typedef boost::writable_iterator_tag return_category;
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else
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typedef boost::error_iterator_tag return_category;
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}
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};
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template <typename T>
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struct iterator_traits<T*>
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{
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typedef T value_type;
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typedef T& reference;
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typedef T* pointer;
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typedef std::ptrdiff_t difference_type;
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if (is-const(T))
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typedef boost::constant_lvalue_iterator_tag return_category;
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else
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typedef boost::mutable_lvalue_iterator_tag return_category;
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typedef boost::random_access_iterator_tag traversal_category;
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};
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}
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</pre>
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<hr>
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<address><a href="mailto:jsiek@lsc.nd.edu">jeremy siek</a></address>
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<!-- Created: Sun Mar 18 14:06:57 EST 2001 -->
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<!-- hhmts start -->
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2001-03-19 18:01:41 +00:00
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Last modified: Mon Mar 19 12:59:30 EST 2001
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2001-03-19 17:41:49 +00:00
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<!-- hhmts end -->
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</body>
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</html>
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