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1349 lines
74 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" />
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<title>Iterator Facade</title>
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<meta name="author" content="David Abrahams, Jeremy Siek, Thomas Witt" />
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<meta name="organization" content="Boost Consulting, Indiana University Open Systems Lab, University of Hanover Institute for Transport Railway Operation and Construction" />
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<meta name="date" content="2006-09-11" />
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<meta name="copyright" content="Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003." />
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<link rel="stylesheet" href="../../../rst.css" type="text/css" />
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</head>
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<body>
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<div class="document" id="iterator-facade">
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<h1 class="title">Iterator Facade</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, Jeremy Siek, Thomas Witt</td></tr>
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<tr><th class="docinfo-name">Contact:</th>
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<td><a class="first reference external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a>, <a class="reference external" href="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</a>, <a class="last reference external" href="mailto:witt@ive.uni-hannover.de">witt@ive.uni-hannover.de</a></td></tr>
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<tr><th class="docinfo-name">Organization:</th>
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<td><a class="first reference external" href="http://www.boost-consulting.com">Boost Consulting</a>, Indiana University <a class="reference external" href="http://www.osl.iu.edu">Open Systems
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Lab</a>, University of Hanover <a class="last reference external" href="http://www.ive.uni-hannover.de">Institute for Transport
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Railway Operation and Construction</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, Jeremy Siek, and Thomas Witt 2003.</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"><!-- Copyright David Abrahams 2006. 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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<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is a base class template that implements the
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interface of standard iterators in terms of a few core functions
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and associated types, to be supplied by a derived iterator class.</td>
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</tr>
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</tbody>
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</table>
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<div class="contents topic" id="table-of-contents">
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<p class="topic-title first">Table of Contents</p>
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<ul class="simple">
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<li><a class="reference internal" href="#overview" id="id23">Overview</a><ul>
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<li><a class="reference internal" href="#usage" id="id24">Usage</a></li>
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<li><a class="reference internal" href="#iterator-core-access" id="id25">Iterator Core Access</a></li>
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<li><a class="reference internal" href="#operator" id="id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></li>
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<li><a class="reference internal" href="#id2" id="id27"><tt class="docutils literal"><span class="pre">operator-></span></tt></a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#reference" id="id28">Reference</a><ul>
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<li><a class="reference internal" href="#iterator-facade-requirements" id="id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></li>
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<li><a class="reference internal" href="#iterator-facade-operations" id="id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#tutorial-example" id="id31">Tutorial Example</a><ul>
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<li><a class="reference internal" href="#the-problem" id="id32">The Problem</a></li>
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<li><a class="reference internal" href="#a-basic-iterator-using-iterator-facade" id="id33">A Basic Iterator Using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul>
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<li><a class="reference internal" href="#template-arguments-for-iterator-facade" id="id34">Template Arguments for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a><ul>
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<li><a class="reference internal" href="#derived" id="id35"><tt class="docutils literal"><span class="pre">Derived</span></tt></a></li>
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<li><a class="reference internal" href="#value" id="id36"><tt class="docutils literal"><span class="pre">Value</span></tt></a></li>
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<li><a class="reference internal" href="#categoryortraversal" id="id37"><tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt></a></li>
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<li><a class="reference internal" href="#id12" id="id38"><tt class="docutils literal"><span class="pre">Reference</span></tt></a></li>
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<li><a class="reference internal" href="#difference" id="id39"><tt class="docutils literal"><span class="pre">Difference</span></tt></a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#constructors-and-data-members" id="id40">Constructors and Data Members</a></li>
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<li><a class="reference internal" href="#implementing-the-core-operations" id="id41">Implementing the Core Operations</a></li>
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</ul>
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</li>
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<li><a class="reference internal" href="#a-constant-node-iterator" id="id42">A constant <tt class="docutils literal"><span class="pre">node_iterator</span></tt></a></li>
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<li><a class="reference internal" href="#interoperability" id="id43">Interoperability</a></li>
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<li><a class="reference internal" href="#telling-the-truth" id="id44">Telling the Truth</a></li>
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<li><a class="reference internal" href="#wrap-up" id="id45">Wrap Up</a></li>
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</ul>
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</li>
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</ul>
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</div>
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<div class="section" id="overview">
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<h1><a class="toc-backref" href="#id23">Overview</a></h1>
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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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<!-- Version 1.1 of this ReStructuredText document corresponds to
|
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n1530_, the paper accepted by the LWG for TR1. -->
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<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
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<p>While the iterator interface is rich, there is a core subset of the
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interface that is necessary for all the functionality. We have
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identified the following core behaviors for iterators:</p>
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<ul class="simple">
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<li>dereferencing</li>
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<li>incrementing</li>
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<li>decrementing</li>
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<li>equality comparison</li>
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<li>random-access motion</li>
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<li>distance measurement</li>
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</ul>
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<p>In addition to the behaviors listed above, the core interface elements
|
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include the associated types exposed through iterator traits:
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<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">difference_type</span></tt>, and
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<tt class="docutils literal"><span class="pre">iterator_category</span></tt>.</p>
|
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<p>Iterator facade uses the Curiously Recurring Template
|
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Pattern (CRTP) <a class="citation-reference" href="#cop95" id="id1">[Cop95]</a> so that the user can specify the behavior
|
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of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> in a derived class. Former designs used
|
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policy objects to specify the behavior, but that approach was
|
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discarded for several reasons:</p>
|
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<blockquote>
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<ol class="arabic simple">
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<li>the creation and eventual copying of the policy object may create
|
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overhead that can be avoided with the current approach.</li>
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<li>The policy object approach does not allow for custom constructors
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on the created iterator types, an essential feature if
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<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> should be used in other library
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implementations.</li>
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<li>Without the use of CRTP, the standard requirement that an
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iterator's <tt class="docutils literal"><span class="pre">operator++</span></tt> returns the iterator type itself
|
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would mean that all iterators built with the library would
|
|
have to be specializations of <tt class="docutils literal"><span class="pre">iterator_facade<...></span></tt>, rather
|
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than something more descriptive like
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<tt class="docutils literal"><span class="pre">indirect_iterator<T*></span></tt>. Cumbersome type generator
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metafunctions would be needed to build new parameterized
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iterators, and a separate <tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt> layer would be
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impossible.</li>
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</ol>
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</blockquote>
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<div class="section" id="usage">
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<h2><a class="toc-backref" href="#id24">Usage</a></h2>
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<p>The user of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> derives his iterator class from a
|
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specialization of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and passes the derived
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iterator class as <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s first template parameter.
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The order of the other template parameters have been carefully
|
|
chosen to take advantage of useful defaults. For example, when
|
|
defining a constant lvalue iterator, the user can pass a
|
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const-qualified version of the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> as
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<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Value</span></tt> parameter and omit the
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<tt class="docutils literal"><span class="pre">Reference</span></tt> parameter which follows.</p>
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<p>The derived iterator class must define member functions implementing
|
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the iterator's core behaviors. The following table describes
|
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expressions which are required to be valid depending on the category
|
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of the derived iterator type. These member functions are described
|
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briefly below and in more detail in the iterator facade
|
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requirements.</p>
|
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<blockquote>
|
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<table border="1" class="docutils">
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<colgroup>
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<col width="44%" />
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<col width="56%" />
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</colgroup>
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<thead valign="bottom">
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<tr><th class="head">Expression</th>
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<th class="head">Effects</th>
|
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</tr>
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</thead>
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<tbody valign="top">
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<tr><td><tt class="docutils literal"><span class="pre">i.dereference()</span></tt></td>
|
|
<td>Access the value referred to</td>
|
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</tr>
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<tr><td><tt class="docutils literal"><span class="pre">i.equal(j)</span></tt></td>
|
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<td>Compare for equality with <tt class="docutils literal"><span class="pre">j</span></tt></td>
|
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</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">i.increment()</span></tt></td>
|
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<td>Advance by one position</td>
|
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</tr>
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<tr><td><tt class="docutils literal"><span class="pre">i.decrement()</span></tt></td>
|
|
<td>Retreat by one position</td>
|
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</tr>
|
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<tr><td><tt class="docutils literal"><span class="pre">i.advance(n)</span></tt></td>
|
|
<td>Advance by <tt class="docutils literal"><span class="pre">n</span></tt> positions</td>
|
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</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">i.distance_to(j)</span></tt></td>
|
|
<td>Measure the distance to <tt class="docutils literal"><span class="pre">j</span></tt></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</blockquote>
|
|
<!-- Should we add a comment that a zero overhead implementation of iterator_facade
|
|
is possible with proper inlining? -->
|
|
<p>In addition to implementing the core interface functions, an iterator
|
|
derived from <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> typically defines several
|
|
constructors. To model any of the standard iterator concepts, the
|
|
iterator must at least have a copy constructor. Also, if the iterator
|
|
type <tt class="docutils literal"><span class="pre">X</span></tt> is meant to be automatically interoperate with another
|
|
iterator type <tt class="docutils literal"><span class="pre">Y</span></tt> (as with constant and mutable iterators) then
|
|
there must be an implicit conversion from <tt class="docutils literal"><span class="pre">X</span></tt> to <tt class="docutils literal"><span class="pre">Y</span></tt> or from <tt class="docutils literal"><span class="pre">Y</span></tt>
|
|
to <tt class="docutils literal"><span class="pre">X</span></tt> (but not both), typically implemented as a conversion
|
|
constructor. Finally, if the iterator is to model Forward Traversal
|
|
Iterator or a more-refined iterator concept, a default constructor is
|
|
required.</p>
|
|
</div>
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<div class="section" id="iterator-core-access">
|
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<h2><a class="toc-backref" href="#id25">Iterator Core Access</a></h2>
|
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<p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and the operator implementations need to be able
|
|
to access the core member functions in the derived class. Making the
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core member functions public would expose an implementation detail to
|
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the user. The design used here ensures that implementation details do
|
|
not appear in the public interface of the derived iterator type.</p>
|
|
<p>Preventing direct access to the core member functions has two
|
|
advantages. First, there is no possibility for the user to accidently
|
|
use a member function of the iterator when a member of the value_type
|
|
was intended. This has been an issue with smart pointer
|
|
implementations in the past. The second and main advantage is that
|
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library implementers can freely exchange a hand-rolled iterator
|
|
implementation for one based on <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> without fear of
|
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breaking code that was accessing the public core member functions
|
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directly.</p>
|
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<p>In a naive implementation, keeping the derived class' core member
|
|
functions private would require it to grant friendship to
|
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<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> and each of the seven operators. In order to
|
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reduce the burden of limiting access, <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> is
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provided, a class that acts as a gateway to the core member functions
|
|
in the derived iterator class. The author of the derived class only
|
|
needs to grant friendship to <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> to make his core
|
|
member functions available to the library.</p>
|
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<!-- This is no long uptodate -thw -->
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<!-- Yes it is; I made sure of it! -DWA -->
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<p><tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> will be typically implemented as an empty
|
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class containing only private static member functions which invoke the
|
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iterator core member functions. There is, however, no need to
|
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standardize the gateway protocol. Note that even if
|
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<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt> used public member functions it would not
|
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open a safety loophole, as every core member function preserves the
|
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invariants of the iterator.</p>
|
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</div>
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<div class="section" id="operator">
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<h2><a class="toc-backref" href="#id26"><tt class="docutils literal"><span class="pre">operator[]</span></tt></a></h2>
|
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<p>The indexing operator for a generalized iterator presents special
|
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challenges. A random access iterator's <tt class="docutils literal"><span class="pre">operator[]</span></tt> is only
|
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required to return something convertible to its <tt class="docutils literal"><span class="pre">value_type</span></tt>.
|
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Requiring that it return an lvalue would rule out currently-legal
|
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random-access iterators which hold the referenced value in a data
|
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member (e.g. <a class="reference external" href="counting_iterator.html"><tt class="docutils literal"><span class="pre">counting_iterator</span></tt></a>), because <tt class="docutils literal"><span class="pre">*(p+n)</span></tt> is a reference
|
|
into the temporary iterator <tt class="docutils literal"><span class="pre">p+n</span></tt>, which is destroyed when
|
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<tt class="docutils literal"><span class="pre">operator[]</span></tt> returns.</p>
|
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<p>Writable iterators built with <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implement the
|
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semantics required by the preferred resolution to <a class="reference external" href="http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-active.html#299">issue 299</a> and
|
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adopted by proposal <a class="reference external" href="http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2003/n1550.htm">n1550</a>: the result of <tt class="docutils literal"><span class="pre">p[n]</span></tt> is an object
|
|
convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">p[n]</span> <span class="pre">=</span> <span class="pre">x</span></tt> is
|
|
equivalent to <tt class="docutils literal"><span class="pre">*(p</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">x</span></tt> (Note: This result object may be
|
|
implemented as a proxy containing a copy of <tt class="docutils literal"><span class="pre">p+n</span></tt>). This approach
|
|
will work properly for any random-access iterator regardless of the
|
|
other details of its implementation. A user who knows more about
|
|
the implementation of her iterator is free to implement an
|
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<tt class="docutils literal"><span class="pre">operator[]</span></tt> that returns an lvalue in the derived iterator
|
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class; it will hide the one supplied by <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> from
|
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clients of her iterator.</p>
|
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</div>
|
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<div class="section" id="id2">
|
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<span id="operator-arrow"></span><h2><a class="toc-backref" href="#id27"><tt class="docutils literal"><span class="pre">operator-></span></tt></a></h2>
|
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<p>The <tt class="docutils literal"><span class="pre">reference</span></tt> type of a readable iterator (and today's input
|
|
iterator) need not in fact be a reference, so long as it is
|
|
convertible to the iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>. When the <tt class="docutils literal"><span class="pre">value_type</span></tt>
|
|
is a class, however, it must still be possible to access members
|
|
through <tt class="docutils literal"><span class="pre">operator-></span></tt>. Therefore, an iterator whose <tt class="docutils literal"><span class="pre">reference</span></tt>
|
|
type is not in fact a reference must return a proxy containing a copy
|
|
of the referenced value from its <tt class="docutils literal"><span class="pre">operator-></span></tt>.</p>
|
|
<p>The return types for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">operator-></span></tt> and
|
|
<tt class="docutils literal"><span class="pre">operator[]</span></tt> are not explicitly specified. Instead, those types
|
|
are described in terms of a set of requirements, which must be
|
|
satisfied by the <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> implementation.</p>
|
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<table class="docutils citation" frame="void" id="cop95" rules="none">
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|
<tbody valign="top">
|
|
<tr><td class="label">[Cop95]</td><td><em>(<a class="fn-backref" href="#id1">1</a>, <a class="fn-backref" href="#id10">2</a>)</em> [Coplien, 1995] Coplien, J., Curiously Recurring Template
|
|
Patterns, C++ Report, February 1995, pp. 24-27.</td></tr>
|
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</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="reference">
|
|
<h1><a class="toc-backref" href="#id28">Reference</a></h1>
|
|
<!-- 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) -->
|
|
<!-- Version 1.3 of this ReStructuredText document corresponds to
|
|
n1530_, the paper accepted by the LWG for TR1. -->
|
|
<!-- Copyright David Abrahams, Jeremy Siek, and Thomas Witt 2003. -->
|
|
<pre class="literal-block">
|
|
template <
|
|
class Derived
|
|
, class Value
|
|
, class CategoryOrTraversal
|
|
, class Reference = Value&
|
|
, class Difference = ptrdiff_t
|
|
>
|
|
class iterator_facade {
|
|
public:
|
|
typedef remove_const<Value>::type value_type;
|
|
typedef Reference reference;
|
|
typedef Value* pointer;
|
|
typedef Difference difference_type;
|
|
typedef /* see <a class="reference internal" href="#iterator-category">below</a> */ iterator_category;
|
|
|
|
reference operator*() const;
|
|
/* see <a class="reference internal" href="#operator-arrow">below</a> */ operator->() const;
|
|
/* see <a class="reference internal" href="#brackets">below</a> */ operator[](difference_type n) const;
|
|
Derived& operator++();
|
|
Derived operator++(int);
|
|
Derived& operator--();
|
|
Derived operator--(int);
|
|
Derived& operator+=(difference_type n);
|
|
Derived& operator-=(difference_type n);
|
|
Derived operator-(difference_type n) const;
|
|
protected:
|
|
typedef iterator_facade iterator_facade_;
|
|
};
|
|
|
|
// Comparison operators
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type // exposition
|
|
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
// Iterator difference
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
/* see <a class="reference internal" href="#minus">below</a> */
|
|
operator-(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
|
|
// Iterator addition
|
|
template <class Dr, class V, class TC, class R, class D>
|
|
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
|
|
typename Derived::difference_type n);
|
|
|
|
template <class Dr, class V, class TC, class R, class D>
|
|
Derived operator+ (typename Derived::difference_type n,
|
|
iterator_facade<Dr,V,TC,R,D> const&);
|
|
</pre>
|
|
<p id="iterator-category">The <tt class="docutils literal"><span class="pre">iterator_category</span></tt> member of <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is</p>
|
|
<pre class="literal-block">
|
|
<em>iterator-category</em>(CategoryOrTraversal, value_type, reference)
|
|
</pre>
|
|
<p>where <em>iterator-category</em> is defined as follows:</p>
|
|
<pre class="literal-block" id="id7">
|
|
<em>iterator-category</em>(C,R,V) :=
|
|
if (C is convertible to std::input_iterator_tag
|
|
|| C is convertible to std::output_iterator_tag
|
|
)
|
|
return C
|
|
|
|
else if (C is not convertible to incrementable_traversal_tag)
|
|
<em>the program is ill-formed</em>
|
|
|
|
else return a type X satisfying the following two constraints:
|
|
|
|
1. X is convertible to X1, and not to any more-derived
|
|
type, where X1 is defined by:
|
|
|
|
if (R is a reference type
|
|
&& C is convertible to forward_traversal_tag)
|
|
{
|
|
if (C is convertible to random_access_traversal_tag)
|
|
X1 = random_access_iterator_tag
|
|
else if (C is convertible to bidirectional_traversal_tag)
|
|
X1 = bidirectional_iterator_tag
|
|
else
|
|
X1 = forward_iterator_tag
|
|
}
|
|
else
|
|
{
|
|
if (C is convertible to single_pass_traversal_tag
|
|
&& R is convertible to V)
|
|
X1 = input_iterator_tag
|
|
else
|
|
X1 = C
|
|
}
|
|
|
|
2. <a class="reference external" href="new-iter-concepts.html#category-to-traversal"><em>category-to-traversal</em></a>(X) is convertible to the most
|
|
derived traversal tag type to which X is also
|
|
convertible, and not to any more-derived traversal tag
|
|
type.
|
|
</pre>
|
|
<p>[Note: the intention is to allow <tt class="docutils literal"><span class="pre">iterator_category</span></tt> to be one of
|
|
the five original category tags when convertibility to one of the
|
|
traversal tags would add no information]</p>
|
|
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
|
|
<!-- subject to 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) -->
|
|
<p>The <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> template used above is for exposition
|
|
purposes. The member operators should only be in an overload set
|
|
provided the derived types <tt class="docutils literal"><span class="pre">Dr1</span></tt> and <tt class="docutils literal"><span class="pre">Dr2</span></tt> are interoperable,
|
|
meaning that at least one of the types is convertible to the other. The
|
|
<tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt> approach uses SFINAE to take the operators
|
|
out of the overload set when the types are not interoperable.
|
|
The operators should behave <em>as-if</em> <tt class="docutils literal"><span class="pre">enable_if_interoperable</span></tt>
|
|
were defined to be:</p>
|
|
<pre class="literal-block">
|
|
template <bool, typename> enable_if_interoperable_impl
|
|
{};
|
|
|
|
template <typename T> enable_if_interoperable_impl<true,T>
|
|
{ typedef T type; };
|
|
|
|
template<typename Dr1, typename Dr2, typename T>
|
|
struct enable_if_interoperable
|
|
: enable_if_interoperable_impl<
|
|
is_convertible<Dr1,Dr2>::value || is_convertible<Dr2,Dr1>::value
|
|
, T
|
|
>
|
|
{};
|
|
</pre>
|
|
<div class="section" id="iterator-facade-requirements">
|
|
<h2><a class="toc-backref" href="#id29"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Requirements</a></h2>
|
|
<p>The following table describes the typical valid expressions on
|
|
<tt class="docutils literal"><span class="pre">iterator_facade</span></tt>'s <tt class="docutils literal"><span class="pre">Derived</span></tt> parameter, depending on the
|
|
iterator concept(s) it will model. The operations in the first
|
|
column must be made accessible to member functions of class
|
|
<tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>. In addition,
|
|
<tt class="docutils literal"><span class="pre">static_cast<Derived*>(iterator_facade*)</span></tt> shall be well-formed.</p>
|
|
<p>In the table below, <tt class="docutils literal"><span class="pre">F</span></tt> is <tt class="docutils literal"><span class="pre">iterator_facade<X,V,C,R,D></span></tt>, <tt class="docutils literal"><span class="pre">a</span></tt> is an
|
|
object of type <tt class="docutils literal"><span class="pre">X</span></tt>, <tt class="docutils literal"><span class="pre">b</span></tt> and <tt class="docutils literal"><span class="pre">c</span></tt> are objects of type <tt class="docutils literal"><span class="pre">const</span> <span class="pre">X</span></tt>,
|
|
<tt class="docutils literal"><span class="pre">n</span></tt> is an object of <tt class="docutils literal"><span class="pre">F::difference_type</span></tt>, <tt class="docutils literal"><span class="pre">y</span></tt> is a constant
|
|
object of a single pass iterator type interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>, and <tt class="docutils literal"><span class="pre">z</span></tt>
|
|
is a constant object of a random access traversal iterator type
|
|
interoperable with <tt class="docutils literal"><span class="pre">X</span></tt>.</p>
|
|
<div class="topic" id="core-operations">
|
|
<p class="topic-title first"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> Core Operations</p>
|
|
<table border="1" class="docutils">
|
|
<colgroup>
|
|
<col width="21%" />
|
|
<col width="23%" />
|
|
<col width="27%" />
|
|
<col width="29%" />
|
|
</colgroup>
|
|
<thead valign="bottom">
|
|
<tr><th class="head">Expression</th>
|
|
<th class="head">Return Type</th>
|
|
<th class="head">Assertion/Note</th>
|
|
<th class="head">Used to implement Iterator
|
|
Concept(s)</th>
|
|
</tr>
|
|
</thead>
|
|
<tbody valign="top">
|
|
<tr><td><tt class="docutils literal"><span class="pre">c.dereference()</span></tt></td>
|
|
<td><tt class="docutils literal"><span class="pre">F::reference</span></tt></td>
|
|
<td> </td>
|
|
<td>Readable Iterator, Writable
|
|
Iterator</td>
|
|
</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">c.equal(y)</span></tt></td>
|
|
<td>convertible to bool</td>
|
|
<td>true iff <tt class="docutils literal"><span class="pre">c</span></tt> and <tt class="docutils literal"><span class="pre">y</span></tt>
|
|
refer to the same
|
|
position.</td>
|
|
<td>Single Pass Iterator</td>
|
|
</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">a.increment()</span></tt></td>
|
|
<td>unused</td>
|
|
<td> </td>
|
|
<td>Incrementable Iterator</td>
|
|
</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">a.decrement()</span></tt></td>
|
|
<td>unused</td>
|
|
<td> </td>
|
|
<td>Bidirectional Traversal
|
|
Iterator</td>
|
|
</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">a.advance(n)</span></tt></td>
|
|
<td>unused</td>
|
|
<td> </td>
|
|
<td>Random Access Traversal
|
|
Iterator</td>
|
|
</tr>
|
|
<tr><td><tt class="docutils literal"><span class="pre">c.distance_to(z)</span></tt></td>
|
|
<td>convertible to
|
|
<tt class="docutils literal"><span class="pre">F::difference_type</span></tt></td>
|
|
<td>equivalent to
|
|
<tt class="docutils literal"><span class="pre">distance(c,</span> <span class="pre">X(z))</span></tt>.</td>
|
|
<td>Random Access Traversal
|
|
Iterator</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="iterator-facade-operations">
|
|
<h2><a class="toc-backref" href="#id30"><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> operations</a></h2>
|
|
<p>The operations in this section are described in terms of operations on
|
|
the core interface of <tt class="docutils literal"><span class="pre">Derived</span></tt> which may be inaccessible
|
|
(i.e. private). The implementation should access these operations
|
|
through member functions of class <tt class="docutils literal"><span class="pre">iterator_core_access</span></tt>.</p>
|
|
<p><tt class="docutils literal"><span class="pre">reference</span> <span class="pre">operator*()</span> <span class="pre">const;</span></tt></p>
|
|
<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">Returns:</th><td class="field-body"><tt class="docutils literal"><span class="pre">static_cast<Derived</span> <span class="pre">const*>(this)->dereference()</span></tt></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">operator->()</span> <span class="pre">const;</span></tt> (see <a class="reference internal" href="#operator-arrow">below</a>)</p>
|
|
<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">Returns:</th><td class="field-body"><p class="first">If <tt class="docutils literal"><span class="pre">reference</span></tt> is a reference type, an object
|
|
of type <tt class="docutils literal"><span class="pre">pointer</span></tt> equal to:</p>
|
|
<pre class="literal-block">
|
|
&static_cast<Derived const*>(this)->dereference()
|
|
</pre>
|
|
<p class="last">Otherwise returns an object of unspecified type such that,
|
|
<tt class="docutils literal"><span class="pre">(*static_cast<Derived</span> <span class="pre">const*>(this))->m</span></tt> is equivalent to <tt class="docutils literal"><span class="pre">(w</span> <span class="pre">=</span> <span class="pre">**static_cast<Derived</span> <span class="pre">const*>(this),</span>
|
|
<span class="pre">w.m)</span></tt> for some temporary object <tt class="docutils literal"><span class="pre">w</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>.</p>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p id="brackets"><em>unspecified</em> <tt class="docutils literal"><span class="pre">operator[](difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
|
|
<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">Returns:</th><td class="field-body">an object convertible to <tt class="docutils literal"><span class="pre">value_type</span></tt>. For constant
|
|
objects <tt class="docutils literal"><span class="pre">v</span></tt> of type <tt class="docutils literal"><span class="pre">value_type</span></tt>, and <tt class="docutils literal"><span class="pre">n</span></tt> of type
|
|
<tt class="docutils literal"><span class="pre">difference_type</span></tt>, <tt class="docutils literal"><span class="pre">(*this)[n]</span> <span class="pre">=</span> <span class="pre">v</span></tt> is equivalent to
|
|
<tt class="docutils literal"><span class="pre">*(*this</span> <span class="pre">+</span> <span class="pre">n)</span> <span class="pre">=</span> <span class="pre">v</span></tt>, and <tt class="docutils literal"><span class="pre">static_cast<value_type</span>
|
|
<span class="pre">const&>((*this)[n])</span></tt> is equivalent to
|
|
<tt class="docutils literal"><span class="pre">static_cast<value_type</span> <span class="pre">const&>(*(*this</span> <span class="pre">+</span> <span class="pre">n))</span></tt></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator++();</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
static_cast<Derived*>(this)->increment();
|
|
return *static_cast<Derived*>(this);
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator++(int);</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
Derived tmp(static_cast<Derived const*>(this));
|
|
++*this;
|
|
return tmp;
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator--();</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
static_cast<Derived*>(this)->decrement();
|
|
return *static_cast<Derived*>(this);
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator--(int);</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
Derived tmp(static_cast<Derived const*>(this));
|
|
--*this;
|
|
return tmp;
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator+=(difference_type</span> <span class="pre">n);</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
static_cast<Derived*>(this)->advance(n);
|
|
return *static_cast<Derived*>(this);
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived&</span> <span class="pre">operator-=(difference_type</span> <span class="pre">n);</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
static_cast<Derived*>(this)->advance(-n);
|
|
return *static_cast<Derived*>(this);
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p><tt class="docutils literal"><span class="pre">Derived</span> <span class="pre">operator-(difference_type</span> <span class="pre">n)</span> <span class="pre">const;</span></tt></p>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
Derived tmp(static_cast<Derived const*>(this));
|
|
return tmp -= n;
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr, class V, class TC, class R, class D>
|
|
Derived operator+ (iterator_facade<Dr,V,TC,R,D> const&,
|
|
typename Derived::difference_type n);
|
|
|
|
template <class Dr, class V, class TC, class R, class D>
|
|
Derived operator+ (typename Derived::difference_type n,
|
|
iterator_facade<Dr,V,TC,R,D> const&);
|
|
</pre>
|
|
<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">Effects:</th><td class="field-body"><pre class="first last literal-block">
|
|
Derived tmp(static_cast<Derived const*>(this));
|
|
return tmp += n;
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator ==(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator !=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr1</span> <span class="pre">const&)lhs).equal((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">!((Dr2</span> <span class="pre">const&)rhs).equal((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator <(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator <=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator >(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">></span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,bool>::type
|
|
operator >=(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span> <span class="pre">>=</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span> <span class="pre"><=</span> <span class="pre">0</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<pre class="literal-block" id="minus">
|
|
template <class Dr1, class V1, class TC1, class R1, class D1,
|
|
class Dr2, class V2, class TC2, class R2, class D2>
|
|
typename enable_if_interoperable<Dr1,Dr2,difference>::type
|
|
operator -(iterator_facade<Dr1,V1,TC1,R1,D1> const& lhs,
|
|
iterator_facade<Dr2,V2,TC2,R2,D2> const& rhs);
|
|
</pre>
|
|
<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">Return Type:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<blockquote>
|
|
<dl class="docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be
|
|
<tt class="docutils literal"><span class="pre">iterator_traits<Dr1>::difference_type</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">difference</span></tt> shall be <tt class="docutils literal"><span class="pre">iterator_traits<Dr2>::difference_type</span></tt></p>
|
|
</dd>
|
|
</dl>
|
|
</blockquote>
|
|
</td>
|
|
</tr>
|
|
<tr class="field"><th class="field-name">Returns:</th><td class="field-body"><p class="first">if <tt class="docutils literal"><span class="pre">is_convertible<Dr2,Dr1>::value</span></tt></p>
|
|
<dl class="last docutils">
|
|
<dt>then</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">-((Dr1</span> <span class="pre">const&)lhs).distance_to((Dr2</span> <span class="pre">const&)rhs)</span></tt>.</p>
|
|
</dd>
|
|
<dt>Otherwise,</dt>
|
|
<dd><p class="first last"><tt class="docutils literal"><span class="pre">((Dr2</span> <span class="pre">const&)rhs).distance_to((Dr1</span> <span class="pre">const&)lhs)</span></tt>.</p>
|
|
</dd>
|
|
</dl>
|
|
</td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="tutorial-example">
|
|
<h1><a class="toc-backref" href="#id31">Tutorial Example</a></h1>
|
|
<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
|
|
<!-- subject to 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) -->
|
|
<p>In this section we'll walk through the implementation of a few
|
|
iterators using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt>, based around the simple
|
|
example of a linked list of polymorphic objects. This example was
|
|
inspired by a <a class="reference external" href="http://thread.gmane.org/gmane.comp.lib.boost.user/5100">posting</a> by Keith Macdonald on the <a class="reference external" href="http://www.boost.org/more/mailing_lists.htm#users">Boost-Users</a>
|
|
mailing list.</p>
|
|
<div class="section" id="the-problem">
|
|
<h2><a class="toc-backref" href="#id32">The Problem</a></h2>
|
|
<p>Say we've written a polymorphic linked list node base class:</p>
|
|
<pre class="literal-block">
|
|
# include <iostream>
|
|
|
|
struct node_base
|
|
{
|
|
node_base() : m_next(0) {}
|
|
|
|
// Each node manages all of its tail nodes
|
|
virtual ~node_base() { delete m_next; }
|
|
|
|
// Access the rest of the list
|
|
node_base* next() const { return m_next; }
|
|
|
|
// print to the stream
|
|
virtual void print(std::ostream& s) const = 0;
|
|
|
|
// double the value
|
|
virtual void double_me() = 0;
|
|
|
|
void append(node_base* p)
|
|
{
|
|
if (m_next)
|
|
m_next->append(p);
|
|
else
|
|
m_next = p;
|
|
}
|
|
|
|
private:
|
|
node_base* m_next;
|
|
};
|
|
</pre>
|
|
<p>Lists can hold objects of different types by linking together
|
|
specializations of the following template:</p>
|
|
<pre class="literal-block">
|
|
template <class T>
|
|
struct node : node_base
|
|
{
|
|
node(T x)
|
|
: m_value(x)
|
|
{}
|
|
|
|
void print(std::ostream& s) const { s << this->m_value; }
|
|
void double_me() { m_value += m_value; }
|
|
|
|
private:
|
|
T m_value;
|
|
};
|
|
</pre>
|
|
<p>And we can print any node using the following streaming operator:</p>
|
|
<pre class="literal-block">
|
|
inline std::ostream& operator<<(std::ostream& s, node_base const& n)
|
|
{
|
|
n.print(s);
|
|
return s;
|
|
}
|
|
</pre>
|
|
<p>Our first challenge is to build an appropriate iterator over these
|
|
lists.</p>
|
|
</div>
|
|
<div class="section" id="a-basic-iterator-using-iterator-facade">
|
|
<h2><a class="toc-backref" href="#id33">A Basic Iterator Using <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h2>
|
|
<p>We will construct a <tt class="docutils literal"><span class="pre">node_iterator</span></tt> class using inheritance from
|
|
<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> to implement most of the iterator's operations.</p>
|
|
<pre class="literal-block">
|
|
# include "node.hpp"
|
|
# include <boost/iterator/iterator_facade.hpp>
|
|
|
|
class node_iterator
|
|
: public boost::iterator_facade<...>
|
|
{
|
|
...
|
|
};
|
|
</pre>
|
|
<div class="section" id="template-arguments-for-iterator-facade">
|
|
<h3><a class="toc-backref" href="#id34">Template Arguments for <tt class="docutils literal"><span class="pre">iterator_facade</span></tt></a></h3>
|
|
<p><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> has several template parameters, so we must decide
|
|
what types to use for the arguments. The parameters are <tt class="docutils literal"><span class="pre">Derived</span></tt>,
|
|
<tt class="docutils literal"><span class="pre">Value</span></tt>, <tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt>, <tt class="docutils literal"><span class="pre">Reference</span></tt>, and <tt class="docutils literal"><span class="pre">Difference</span></tt>.</p>
|
|
<div class="section" id="derived">
|
|
<h4><a class="toc-backref" href="#id35"><tt class="docutils literal"><span class="pre">Derived</span></tt></a></h4>
|
|
<p>Because <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> is meant to be used with the CRTP
|
|
<a class="citation-reference" href="#cop95" id="id10">[Cop95]</a> the first parameter is the iterator class name itself,
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt>.</p>
|
|
</div>
|
|
<div class="section" id="value">
|
|
<h4><a class="toc-backref" href="#id36"><tt class="docutils literal"><span class="pre">Value</span></tt></a></h4>
|
|
<p>The <tt class="docutils literal"><span class="pre">Value</span></tt> parameter determines the <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s
|
|
<tt class="docutils literal"><span class="pre">value_type</span></tt>. In this case, we are iterating over <tt class="docutils literal"><span class="pre">node_base</span></tt>
|
|
objects, so <tt class="docutils literal"><span class="pre">Value</span></tt> will be <tt class="docutils literal"><span class="pre">node_base</span></tt>.</p>
|
|
</div>
|
|
<div class="section" id="categoryortraversal">
|
|
<h4><a class="toc-backref" href="#id37"><tt class="docutils literal"><span class="pre">CategoryOrTraversal</span></tt></a></h4>
|
|
<p>Now we have to determine which <a class="reference external" href="new-iter-concepts.html#iterator-traversal-concepts-lib-iterator-traversal">iterator traversal concept</a> our
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt> is going to model. Singly-linked lists only have
|
|
forward links, so our iterator can't can't be a <a class="reference external" href="new-iter-concepts.html#bidirectional-traversal-iterators-lib-bidirectional-traversal-iterators">bidirectional
|
|
traversal iterator</a>. Our iterator should be able to make multiple
|
|
passes over the same linked list (unlike, say, an
|
|
<tt class="docutils literal"><span class="pre">istream_iterator</span></tt> which consumes the stream it traverses), so it
|
|
must be a <a class="reference external" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a>. Therefore, we'll pass
|
|
<tt class="docutils literal"><span class="pre">boost::forward_traversal_tag</span></tt> in this position<a class="footnote-reference" href="#category" id="id11"><sup>1</sup></a>.</p>
|
|
<table class="docutils footnote" frame="void" id="category" rules="none">
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|
<tbody valign="top">
|
|
<tr><td class="label"><a class="fn-backref" href="#id11">[1]</a></td><td><tt class="docutils literal"><span class="pre">iterator_facade</span></tt> also supports old-style category
|
|
tags, so we could have passed <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt> here;
|
|
either way, the resulting iterator's <tt class="docutils literal"><span class="pre">iterator_category</span></tt> will
|
|
end up being <tt class="docutils literal"><span class="pre">std::forward_iterator_tag</span></tt>.</td></tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
<div class="section" id="id12">
|
|
<h4><a class="toc-backref" href="#id38"><tt class="docutils literal"><span class="pre">Reference</span></tt></a></h4>
|
|
<p>The <tt class="docutils literal"><span class="pre">Reference</span></tt> argument becomes the type returned by
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s dereference operation, and will also be the
|
|
same as <tt class="docutils literal"><span class="pre">std::iterator_traits<node_iterator>::reference</span></tt>. The
|
|
library's default for this parameter is <tt class="docutils literal"><span class="pre">Value&</span></tt>; since
|
|
<tt class="docutils literal"><span class="pre">node_base&</span></tt> is a good choice for the iterator's <tt class="docutils literal"><span class="pre">reference</span></tt>
|
|
type, we can omit this argument, or pass <tt class="docutils literal"><span class="pre">use_default</span></tt>.</p>
|
|
</div>
|
|
<div class="section" id="difference">
|
|
<h4><a class="toc-backref" href="#id39"><tt class="docutils literal"><span class="pre">Difference</span></tt></a></h4>
|
|
<p>The <tt class="docutils literal"><span class="pre">Difference</span></tt> argument determines how the distance between
|
|
two <tt class="docutils literal"><span class="pre">node_iterator</span></tt>s will be measured and will also be the
|
|
same as <tt class="docutils literal"><span class="pre">std::iterator_traits<node_iterator>::difference_type</span></tt>.
|
|
The library's default for <tt class="docutils literal"><span class="pre">Difference</span></tt> is <tt class="docutils literal"><span class="pre">std::ptrdiff_t</span></tt>, an
|
|
appropriate type for measuring the distance between any two
|
|
addresses in memory, and one that works for almost any iterator,
|
|
so we can omit this argument, too.</p>
|
|
<p>The declaration of <tt class="docutils literal"><span class="pre">node_iterator</span></tt> will therefore look something
|
|
like:</p>
|
|
<pre class="literal-block">
|
|
# include "node.hpp"
|
|
# include <boost/iterator/iterator_facade.hpp>
|
|
|
|
class node_iterator
|
|
: public boost::iterator_facade<
|
|
node_iterator
|
|
, node_base
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
...
|
|
};
|
|
</pre>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="constructors-and-data-members">
|
|
<h3><a class="toc-backref" href="#id40">Constructors and Data Members</a></h3>
|
|
<p>Next we need to decide how to represent the iterator's position.
|
|
This representation will take the form of data members, so we'll
|
|
also need to write constructors to initialize them. The
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s position is quite naturally represented using
|
|
a pointer to a <tt class="docutils literal"><span class="pre">node_base</span></tt>. We'll need a constructor to build an
|
|
iterator from a <tt class="docutils literal"><span class="pre">node_base*</span></tt>, and a default constructor to
|
|
satisfy the <a class="reference external" href="new-iter-concepts.html#forward-traversal-iterators-lib-forward-traversal-iterators">forward traversal iterator</a> requirements<a class="footnote-reference" href="#default" id="id13"><sup>2</sup></a>.
|
|
Our <tt class="docutils literal"><span class="pre">node_iterator</span></tt> then becomes:</p>
|
|
<pre class="literal-block">
|
|
# include "node.hpp"
|
|
# include <boost/iterator/iterator_facade.hpp>
|
|
|
|
class node_iterator
|
|
: public boost::iterator_facade<
|
|
node_iterator
|
|
, node_base
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
public:
|
|
node_iterator()
|
|
: m_node(0)
|
|
{}
|
|
|
|
explicit node_iterator(node_base* p)
|
|
: m_node(p)
|
|
{}
|
|
|
|
private:
|
|
...
|
|
node_base* m_node;
|
|
};
|
|
</pre>
|
|
<table class="docutils footnote" frame="void" id="default" rules="none">
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|
<tbody valign="top">
|
|
<tr><td class="label"><a class="fn-backref" href="#id13">[2]</a></td><td>Technically, the C++ standard places almost no
|
|
requirements on a default-constructed iterator, so if we were
|
|
really concerned with efficiency, we could've written the
|
|
default constructor to leave <tt class="docutils literal"><span class="pre">m_node</span></tt> uninitialized.</td></tr>
|
|
</tbody>
|
|
</table>
|
|
</div>
|
|
<div class="section" id="implementing-the-core-operations">
|
|
<h3><a class="toc-backref" href="#id41">Implementing the Core Operations</a></h3>
|
|
<p>The last step is to implement the <a class="reference internal" href="#core-operations">core operations</a> required by
|
|
the concepts we want our iterator to model. Referring to the
|
|
<a class="reference internal" href="#core-operations">table</a>, we can see that the first three rows are applicable
|
|
because <tt class="docutils literal"><span class="pre">node_iterator</span></tt> needs to satisfy the requirements for
|
|
<a class="reference external" href="new-iter-concepts.html#readable-iterators-lib-readable-iterators">readable iterator</a>, <a class="reference external" href="new-iter-concepts.html#single-pass-iterators-lib-single-pass-iterators">single pass iterator</a>, and <a class="reference external" href="new-iter-concepts.html#incrementable-iterators-lib-incrementable-iterators">incrementable
|
|
iterator</a>.</p>
|
|
<p>We therefore need to supply <tt class="docutils literal"><span class="pre">dereference</span></tt>,
|
|
<tt class="docutils literal"><span class="pre">equal</span></tt>, and <tt class="docutils literal"><span class="pre">increment</span></tt> members. We don't want these members
|
|
to become part of <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s public interface, so we can
|
|
make them private and grant friendship to
|
|
<tt class="docutils literal"><span class="pre">boost::iterator_core_access</span></tt>, a "back-door" that
|
|
<tt class="docutils literal"><span class="pre">iterator_facade</span></tt> uses to get access to the core operations:</p>
|
|
<pre class="literal-block">
|
|
# include "node.hpp"
|
|
# include <boost/iterator/iterator_facade.hpp>
|
|
|
|
class node_iterator
|
|
: public boost::iterator_facade<
|
|
node_iterator
|
|
, node_base
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
public:
|
|
node_iterator()
|
|
: m_node(0) {}
|
|
|
|
explicit node_iterator(node_base* p)
|
|
: m_node(p) {}
|
|
|
|
private:
|
|
friend class boost::iterator_core_access;
|
|
|
|
void increment() { m_node = m_node->next(); }
|
|
|
|
bool equal(node_iterator const& other) const
|
|
{
|
|
return this->m_node == other.m_node;
|
|
}
|
|
|
|
node_base& dereference() const { return *m_node; }
|
|
|
|
node_base* m_node;
|
|
};
|
|
</pre>
|
|
<p>Voilà; a complete and conforming readable, forward-traversal
|
|
iterator! For a working example of its use, see <a class="reference external" href="../example/node_iterator1.cpp">this program</a>.</p>
|
|
</div>
|
|
</div>
|
|
<div class="section" id="a-constant-node-iterator">
|
|
<h2><a class="toc-backref" href="#id42">A constant <tt class="docutils literal"><span class="pre">node_iterator</span></tt></a></h2>
|
|
<div class="sidebar">
|
|
<p class="first sidebar-title">Constant and Mutable iterators</p>
|
|
<p>The term <strong>mutable iterator</strong> means an iterator through which
|
|
the object it references (its "referent") can be modified. A
|
|
<strong>constant iterator</strong> is one which doesn't allow modification of
|
|
its referent.</p>
|
|
<p>The words <em>constant</em> and <em>mutable</em> don't refer to the ability to
|
|
modify the iterator itself. For example, an <tt class="docutils literal"><span class="pre">int</span> <span class="pre">const*</span></tt> is a
|
|
non-<tt class="docutils literal"><span class="pre">const</span></tt> <em>constant iterator</em>, which can be incremented
|
|
but doesn't allow modification of its referent, and <tt class="docutils literal"><span class="pre">int*</span>
|
|
<span class="pre">const</span></tt> is a <tt class="docutils literal"><span class="pre">const</span></tt> <em>mutable iterator</em>, which cannot be
|
|
modified but which allows modification of its referent.</p>
|
|
<p class="last">Confusing? We agree, but those are the standard terms. It
|
|
probably doesn't help much that a container's constant iterator
|
|
is called <tt class="docutils literal"><span class="pre">const_iterator</span></tt>.</p>
|
|
</div>
|
|
<p>Now, our <tt class="docutils literal"><span class="pre">node_iterator</span></tt> gives clients access to both <tt class="docutils literal"><span class="pre">node</span></tt>'s <tt class="docutils literal"><span class="pre">print(std::ostream&)</span> <span class="pre">const</span></tt> member function, but also its
|
|
mutating <tt class="docutils literal"><span class="pre">double_me()</span></tt> member. If we wanted to build a
|
|
<em>constant</em> <tt class="docutils literal"><span class="pre">node_iterator</span></tt>, we'd only have to make three
|
|
changes:</p>
|
|
<pre class="literal-block">
|
|
class const_node_iterator
|
|
: public boost::iterator_facade<
|
|
const_node_iterator
|
|
, node_base <strong>const</strong>
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
public:
|
|
const_node_iterator()
|
|
: m_node(0) {}
|
|
|
|
explicit const_node_iterator(node_base* p)
|
|
: m_node(p) {}
|
|
|
|
private:
|
|
friend class boost::iterator_core_access;
|
|
|
|
void increment() { m_node = m_node->next(); }
|
|
|
|
bool equal(const_node_iterator const& other) const
|
|
{
|
|
return this->m_node == other.m_node;
|
|
}
|
|
|
|
node_base <strong>const</strong>& dereference() const { return *m_node; }
|
|
|
|
node_base <strong>const</strong>* m_node;
|
|
};
|
|
</pre>
|
|
<div class="sidebar">
|
|
<p class="first sidebar-title"><tt class="docutils literal"><span class="pre">const</span></tt> and an iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt></p>
|
|
<p class="last">The C++ standard requires an iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt> <em>not</em> be
|
|
<tt class="docutils literal"><span class="pre">const</span></tt>-qualified, so <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> strips the
|
|
<tt class="docutils literal"><span class="pre">const</span></tt> from its <tt class="docutils literal"><span class="pre">Value</span></tt> parameter in order to produce the
|
|
iterator's <tt class="docutils literal"><span class="pre">value_type</span></tt>. Making the <tt class="docutils literal"><span class="pre">Value</span></tt> argument
|
|
<tt class="docutils literal"><span class="pre">const</span></tt> provides a useful hint to <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> that the
|
|
iterator is a <em>constant iterator</em>, and the default <tt class="docutils literal"><span class="pre">Reference</span></tt>
|
|
argument will be correct for all lvalue iterators.</p>
|
|
</div>
|
|
<p>As a matter of fact, <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and <tt class="docutils literal"><span class="pre">const_node_iterator</span></tt>
|
|
are so similar that it makes sense to factor the common code out
|
|
into a template as follows:</p>
|
|
<pre class="literal-block">
|
|
template <class Value>
|
|
class node_iter
|
|
: public boost::iterator_facade<
|
|
node_iter<Value>
|
|
, Value
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
public:
|
|
node_iter()
|
|
: m_node(0) {}
|
|
|
|
explicit node_iter(Value* p)
|
|
: m_node(p) {}
|
|
|
|
private:
|
|
friend class boost::iterator_core_access;
|
|
|
|
bool equal(node_iter<Value> const& other) const
|
|
{
|
|
return this->m_node == other.m_node;
|
|
}
|
|
|
|
void increment()
|
|
{ m_node = m_node->next(); }
|
|
|
|
Value& dereference() const
|
|
{ return *m_node; }
|
|
|
|
Value* m_node;
|
|
};
|
|
typedef node_iter<node_base> node_iterator;
|
|
typedef node_iter<node_base const> node_const_iterator;
|
|
</pre>
|
|
</div>
|
|
<div class="section" id="interoperability">
|
|
<h2><a class="toc-backref" href="#id43">Interoperability</a></h2>
|
|
<p>Our <tt class="docutils literal"><span class="pre">const_node_iterator</span></tt> works perfectly well on its own, but
|
|
taken together with <tt class="docutils literal"><span class="pre">node_iterator</span></tt> it doesn't quite meet
|
|
expectations. For example, we'd like to be able to pass a
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt> where a <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> was expected,
|
|
just as you can with <tt class="docutils literal"><span class="pre">std::list<int></span></tt>'s <tt class="docutils literal"><span class="pre">iterator</span></tt> and
|
|
<tt class="docutils literal"><span class="pre">const_iterator</span></tt>. Furthermore, given a <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and a
|
|
<tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> into the same list, we should be able to
|
|
compare them for equality.</p>
|
|
<p>This expected ability to use two different iterator types together
|
|
is known as <a class="reference external" href="new-iter-concepts.html#interoperable-iterators-lib-interoperable-iterators"><strong>interoperability</strong></a>. Achieving interoperability in
|
|
our case is as simple as templatizing the <tt class="docutils literal"><span class="pre">equal</span></tt> function and
|
|
adding a templatized converting constructor<a class="footnote-reference" href="#broken" id="id16"><sup>3</sup></a><a class="footnote-reference" href="#random" id="id17"><sup>4</sup></a>:</p>
|
|
<pre class="literal-block">
|
|
template <class Value>
|
|
class node_iter
|
|
: public boost::iterator_facade<
|
|
node_iter<Value>
|
|
, Value
|
|
, boost::forward_traversal_tag
|
|
>
|
|
{
|
|
public:
|
|
node_iter()
|
|
: m_node(0) {}
|
|
|
|
explicit node_iter(Value* p)
|
|
: m_node(p) {}
|
|
|
|
template <class OtherValue>
|
|
node_iter(node_iter<OtherValue> const& other)
|
|
: m_node(other.m_node) {}
|
|
|
|
private:
|
|
friend class boost::iterator_core_access;
|
|
template <class> friend class node_iter;
|
|
|
|
template <class OtherValue>
|
|
bool equal(node_iter<OtherValue> const& other) const
|
|
{
|
|
return this->m_node == other.m_node;
|
|
}
|
|
|
|
void increment()
|
|
{ m_node = m_node->next(); }
|
|
|
|
Value& dereference() const
|
|
{ return *m_node; }
|
|
|
|
Value* m_node;
|
|
};
|
|
typedef impl::node_iterator<node_base> node_iterator;
|
|
typedef impl::node_iterator<node_base const> node_const_iterator;
|
|
</pre>
|
|
<table class="docutils footnote" frame="void" id="broken" rules="none">
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|
<tbody valign="top">
|
|
<tr><td class="label"><a class="fn-backref" href="#id16">[3]</a></td><td>If you're using an older compiler and it can't handle
|
|
this example, see the <a class="reference external" href="../example/node_iterator2.hpp">example code</a> for workarounds.</td></tr>
|
|
</tbody>
|
|
</table>
|
|
<table class="docutils footnote" frame="void" id="random" rules="none">
|
|
<colgroup><col class="label" /><col /></colgroup>
|
|
<tbody valign="top">
|
|
<tr><td class="label"><a class="fn-backref" href="#id17">[4]</a></td><td>If <tt class="docutils literal"><span class="pre">node_iterator</span></tt> had been a <a class="reference external" href="new-iter-concepts.html#random-access-traversal-iterators-lib-random-access-traversal-iterators">random access
|
|
traversal iterator</a>, we'd have had to templatize its
|
|
<tt class="docutils literal"><span class="pre">distance_to</span></tt> function as well.</td></tr>
|
|
</tbody>
|
|
</table>
|
|
<p>You can see an example program which exercises our interoperable
|
|
iterators <a class="reference external" href="../example/node_iterator2.cpp">here</a>.</p>
|
|
</div>
|
|
<div class="section" id="telling-the-truth">
|
|
<h2><a class="toc-backref" href="#id44">Telling the Truth</a></h2>
|
|
<p>Now <tt class="docutils literal"><span class="pre">node_iterator</span></tt> and <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> behave exactly as
|
|
you'd expect... almost. We can compare them and we can convert in
|
|
one direction: from <tt class="docutils literal"><span class="pre">node_iterator</span></tt> to <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt>.
|
|
If we try to convert from <tt class="docutils literal"><span class="pre">node_const_iterator</span></tt> to
|
|
<tt class="docutils literal"><span class="pre">node_iterator</span></tt>, we'll get an error when the converting
|
|
constructor tries to initialize <tt class="docutils literal"><span class="pre">node_iterator</span></tt>'s <tt class="docutils literal"><span class="pre">m_node</span></tt>, a
|
|
<tt class="docutils literal"><span class="pre">node*</span></tt> with a <tt class="docutils literal"><span class="pre">node</span> <span class="pre">const*</span></tt>. So what's the problem?</p>
|
|
<p>The problem is that
|
|
<tt class="docutils literal"><span class="pre">boost::</span></tt><a class="reference external" href="../../type_traits/index.html#relationships"><tt class="docutils literal"><span class="pre">is_convertible</span></tt></a><tt class="docutils literal"><span class="pre"><node_const_iterator,node_iterator>::value</span></tt>
|
|
will be <tt class="docutils literal"><span class="pre">true</span></tt>, but it should be <tt class="docutils literal"><span class="pre">false</span></tt>. <a class="reference external" href="../../type_traits/index.html#relationships"><tt class="docutils literal"><span class="pre">is_convertible</span></tt></a>
|
|
lies because it can only see as far as the <em>declaration</em> of
|
|
<tt class="docutils literal"><span class="pre">node_iter</span></tt>'s converting constructor, but can't look inside at
|
|
the <em>definition</em> to make sure it will compile. A perfect solution
|
|
would make <tt class="docutils literal"><span class="pre">node_iter</span></tt>'s converting constructor disappear when
|
|
the <tt class="docutils literal"><span class="pre">m_node</span></tt> conversion would fail.</p>
|
|
<p>In fact, that sort of magic is possible using
|
|
<a class="reference external" href="../../utility/enable_if.html"><tt class="docutils literal"><span class="pre">boost::enable_if</span></tt></a>. By rewriting the converting constructor as
|
|
follows, we can remove it from the overload set when it's not
|
|
appropriate:</p>
|
|
<pre class="literal-block">
|
|
#include <boost/type_traits/is_convertible.hpp>
|
|
#include <boost/utility/enable_if.hpp>
|
|
|
|
...
|
|
|
|
private:
|
|
struct enabler {};
|
|
|
|
public:
|
|
template <class OtherValue>
|
|
node_iter(
|
|
node_iter<OtherValue> const& other
|
|
, typename boost::enable_if<
|
|
boost::is_convertible<OtherValue*,Value*>
|
|
, enabler
|
|
>::type = enabler()
|
|
)
|
|
: m_node(other.m_node) {}
|
|
</pre>
|
|
</div>
|
|
<div class="section" id="wrap-up">
|
|
<h2><a class="toc-backref" href="#id45">Wrap Up</a></h2>
|
|
<p>This concludes our <tt class="docutils literal"><span class="pre">iterator_facade</span></tt> tutorial, but before you
|
|
stop reading we urge you to take a look at <a class="reference external" href="iterator_adaptor.html"><tt class="docutils literal"><span class="pre">iterator_adaptor</span></tt></a>.
|
|
There's another way to approach writing these iterators which might
|
|
even be superior.</p>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
<div class="footer">
|
|
<hr class="footer" />
|
|
<a class="reference external" href="iterator_facade.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>
|