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184 lines
9.0 KiB
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<?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>pointee and indirect_reference</title>
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<meta name="author" content="David Abrahams" />
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<meta name="organization" content="Boost Consulting" />
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<meta name="date" content="2006-09-11" />
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<meta name="copyright" content="Copyright David Abrahams 2004." />
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<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="pointee-and-indirect-reference">
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<h1 class="title"><tt class="docutils literal"><span class="pre">pointee</span></tt> and <tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h1>
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<table class="docinfo" frame="void" rules="none">
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<col class="docinfo-name" />
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<col class="docinfo-content" />
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<tbody valign="top">
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<tr><th class="docinfo-name">Author:</th>
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<td>David Abrahams</td></tr>
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<tr><th class="docinfo-name">Contact:</th>
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<td><a class="first last reference external" href="mailto:dave@boost-consulting.com">dave@boost-consulting.com</a></td></tr>
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<tr><th class="docinfo-name">Organization:</th>
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<td><a class="first last reference external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
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<tr><th class="docinfo-name">Date:</th>
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<td>2006-09-11</td></tr>
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<tr><th class="docinfo-name">Copyright:</th>
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<td>Copyright David Abrahams 2004.</td></tr>
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</tbody>
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</table>
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<!-- Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">abstract:</th><td class="field-body">Provides the capability to deduce the referent types of
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pointers, smart pointers and iterators in generic code.</td>
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</tr>
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</tbody>
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</table>
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<div class="section" id="overview">
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<h1>Overview</h1>
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<p>Have you ever wanted to write a generic function that can operate
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on any kind of dereferenceable object? If you have, you've
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probably run into the problem of how to determine the type that the
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object "points at":</p>
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<pre class="literal-block">
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template <class Dereferenceable>
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void f(Dereferenceable p)
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{
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<em>what-goes-here?</em> value = *p;
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...
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}
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</pre>
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<div class="section" id="pointee">
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<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
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<p>It turns out to be impossible to come up with a fully-general
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algorithm to do determine <em>what-goes-here</em> directly, but it is
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possible to require that <tt class="docutils literal"><span class="pre">pointee<Dereferenceable>::type</span></tt> is
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correct. Naturally, <tt class="docutils literal"><span class="pre">pointee</span></tt> has the same difficulty: it can't
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determine the appropriate <tt class="docutils literal"><span class="pre">::type</span></tt> reliably for all
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<tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>s, but it makes very good guesses (it works
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for all pointers, standard and boost smart pointers, and
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iterators), and when it guesses wrongly, it can be specialized as
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necessary:</p>
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<pre class="literal-block">
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namespace boost
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{
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template <class T>
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struct pointee<third_party_lib::smart_pointer<T> >
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{
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typedef T type;
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};
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}
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</pre>
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</div>
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<div class="section" id="indirect-reference">
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<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
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<p><tt class="docutils literal"><span class="pre">indirect_reference<T>::type</span></tt> is rather more specialized than
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<tt class="docutils literal"><span class="pre">pointee</span></tt>, and is meant to be used to forward the result of
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dereferencing an object of its argument type. Most dereferenceable
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types just return a reference to their pointee, but some return
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proxy references or return the pointee by value. When that
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information is needed, call on <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>.</p>
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<p>Both of these templates are essential to the correct functioning of
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<a class="reference external" href="indirect_iterator.html"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a>.</p>
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</div>
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</div>
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<div class="section" id="reference">
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<h1>Reference</h1>
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<div class="section" id="id1">
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<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
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<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
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<!-- subject to the Boost 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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<pre class="literal-block">
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template <class Dereferenceable>
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struct pointee
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{
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typedef /* see below */ type;
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};
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</pre>
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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">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
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is well-formed. If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
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ambiguous nor shall it violate access control, and
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<tt class="docutils literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
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Otherwise <tt class="docutils literal"><span class="pre">iterator_traits<Dereferenceable>::value_type</span></tt> shall
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be well formed. [Note: These requirements need not apply to
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explicit or partial specializations of <tt class="docutils literal"><span class="pre">pointee</span></tt>]</td>
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</tr>
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</tbody>
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</table>
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<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
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<tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
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<pre class="literal-block">
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if ( ++x is ill-formed )
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{
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return ``Dereferenceable::element_type``
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}
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else if (``*x`` is a mutable reference to
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std::iterator_traits<Dereferenceable>::value_type)
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{
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return iterator_traits<Dereferenceable>::value_type
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}
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else
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{
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return iterator_traits<Dereferenceable>::value_type const
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}
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</pre>
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</div>
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<div class="section" id="id2">
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<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
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<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
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<!-- subject to the Boost 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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<pre class="literal-block">
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template <class Dereferenceable>
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struct indirect_reference
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{
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typedef /* see below */ type;
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};
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</pre>
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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">Requires:</th><td class="field-body">For an object <tt class="docutils literal"><span class="pre">x</span></tt> of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>, <tt class="docutils literal"><span class="pre">*x</span></tt>
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is well-formed. If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
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ambiguous nor shall it violate access control, and
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<tt class="docutils literal"><span class="pre">pointee<Dereferenceable>::type&</span></tt> shall be well-formed.
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Otherwise <tt class="docutils literal"><span class="pre">iterator_traits<Dereferenceable>::reference</span></tt> shall
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be well formed. [Note: These requirements need not apply to
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explicit or partial specializations of <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>]</td>
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</tr>
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</tbody>
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</table>
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<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
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<tt class="docutils literal"><span class="pre">x</span></tt> is an object of type <tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>:</p>
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<pre class="literal-block">
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if ( ++x is ill-formed )
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return ``pointee<Dereferenceable>::type&``
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else
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std::iterator_traits<Dereferenceable>::reference
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</pre>
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</div>
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</div>
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</div>
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<div class="footer">
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<hr class="footer" />
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<a class="reference external" href="pointee.rst">View document source</a>.
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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.
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</div>
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</body>
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</html>
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