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1<?xml version="1.0" encoding="utf-8" ?>
2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
4<head>
5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
6<meta name="generator" content="Docutils 0.5: http://docutils.sourceforge.net/" />
7<title>pointee and indirect_reference</title>
8<meta name="author" content="David Abrahams" />
9<meta name="organization" content="Boost Consulting" />
10<meta name="date" content="2006-09-11" />
11<meta name="copyright" content="Copyright David Abrahams 2004." />
12<link rel="stylesheet" href="../../../rst.css" type="text/css" />
13</head>
14<body>
15<div class="document" id="pointee-and-indirect-reference">
16<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>
17<table class="docinfo" frame="void" rules="none">
18<col class="docinfo-name" />
19<col class="docinfo-content" />
20<tbody valign="top">
21<tr><th class="docinfo-name">Author:</th>
22<td>David Abrahams</td></tr>
23<tr><th class="docinfo-name">Contact:</th>
24<td><a class="first last reference external" href="mailto:dave&#64;boost-consulting.com">dave&#64;boost-consulting.com</a></td></tr>
25<tr><th class="docinfo-name">Organization:</th>
26<td><a class="first last reference external" href="http://www.boost-consulting.com">Boost Consulting</a></td></tr>
27<tr><th class="docinfo-name">Date:</th>
28<td>2006-09-11</td></tr>
29<tr><th class="docinfo-name">Copyright:</th>
30<td>Copyright David Abrahams 2004.</td></tr>
31</tbody>
32</table>
33<!-- Distributed under the Boost -->
34<!-- Software License, Version 1.0. (See accompanying -->
35<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
36<table class="docutils field-list" frame="void" rules="none">
37<col class="field-name" />
38<col class="field-body" />
39<tbody valign="top">
40<tr class="field"><th class="field-name">abstract:</th><td class="field-body">Provides the capability to deduce the referent types of
41pointers, smart pointers and iterators in generic code.</td>
42</tr>
43</tbody>
44</table>
45<div class="section" id="overview">
46<h1>Overview</h1>
47<p>Have you ever wanted to write a generic function that can operate
48on any kind of dereferenceable object?  If you have, you've
49probably run into the problem of how to determine the type that the
50object &quot;points at&quot;:</p>
51<pre class="literal-block">
52template &lt;class Dereferenceable&gt;
53void f(Dereferenceable p)
54{
55    <em>what-goes-here?</em> value = *p;
56    ...
57}
58</pre>
59<div class="section" id="pointee">
60<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
61<p>It turns out to be impossible to come up with a fully-general
62algorithm to do determine <em>what-goes-here</em> directly, but it is
63possible to require that <tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type</span></tt> is
64correct. Naturally, <tt class="docutils literal"><span class="pre">pointee</span></tt> has the same difficulty: it can't
65determine the appropriate <tt class="docutils literal"><span class="pre">::type</span></tt> reliably for all
66<tt class="docutils literal"><span class="pre">Dereferenceable</span></tt>s, but it makes very good guesses (it works
67for all pointers, standard and boost smart pointers, and
68iterators), and when it guesses wrongly, it can be specialized as
69necessary:</p>
70<pre class="literal-block">
71namespace boost
72{
73  template &lt;class T&gt;
74  struct pointee&lt;third_party_lib::smart_pointer&lt;T&gt; &gt;
75  {
76      typedef T type;
77  };
78}
79</pre>
80</div>
81<div class="section" id="indirect-reference">
82<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
83<p><tt class="docutils literal"><span class="pre">indirect_reference&lt;T&gt;::type</span></tt> is rather more specialized than
84<tt class="docutils literal"><span class="pre">pointee</span></tt>, and is meant to be used to forward the result of
85dereferencing an object of its argument type.  Most dereferenceable
86types just return a reference to their pointee, but some return
87proxy references or return the pointee by value.  When that
88information is needed, call on <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>.</p>
89<p>Both of these templates are essential to the correct functioning of
90<a class="reference external" href="indirect_iterator.html"><tt class="docutils literal"><span class="pre">indirect_iterator</span></tt></a>.</p>
91</div>
92</div>
93<div class="section" id="reference">
94<h1>Reference</h1>
95<div class="section" id="id1">
96<h2><tt class="docutils literal"><span class="pre">pointee</span></tt></h2>
97<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
98<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
99<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
100<pre class="literal-block">
101template &lt;class Dereferenceable&gt;
102struct pointee
103{
104    typedef /* see below */ type;
105};
106</pre>
107<table class="docutils field-list" frame="void" rules="none">
108<col class="field-name" />
109<col class="field-body" />
110<tbody valign="top">
111<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>
112is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
113ambiguous nor shall it violate access control, and
114<tt class="docutils literal"><span class="pre">Dereferenceable::element_type</span></tt> shall be an accessible type.
115Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::value_type</span></tt> shall
116be well formed.  [Note: These requirements need not apply to
117explicit or partial specializations of <tt class="docutils literal"><span class="pre">pointee</span></tt>]</td>
118</tr>
119</tbody>
120</table>
121<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
122<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>
123<pre class="literal-block">
124if ( ++x is ill-formed )
125{
126    return ``Dereferenceable::element_type``
127}
128else if (``*x`` is a mutable reference to
129         std::iterator_traits&lt;Dereferenceable&gt;::value_type)
130{
131    return iterator_traits&lt;Dereferenceable&gt;::value_type
132}
133else
134{
135    return iterator_traits&lt;Dereferenceable&gt;::value_type const
136}
137</pre>
138</div>
139<div class="section" id="id2">
140<h2><tt class="docutils literal"><span class="pre">indirect_reference</span></tt></h2>
141<!-- Copyright David Abrahams 2004. Use, modification and distribution is -->
142<!-- subject to the Boost Software License, Version 1.0. (See accompanying -->
143<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
144<pre class="literal-block">
145template &lt;class Dereferenceable&gt;
146struct indirect_reference
147{
148    typedef /* see below */ type;
149};
150</pre>
151<table class="docutils field-list" frame="void" rules="none">
152<col class="field-name" />
153<col class="field-body" />
154<tbody valign="top">
155<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>
156is well-formed.  If <tt class="docutils literal"><span class="pre">++x</span></tt> is ill-formed it shall neither be
157ambiguous nor shall it violate access control, and
158<tt class="docutils literal"><span class="pre">pointee&lt;Dereferenceable&gt;::type&amp;</span></tt> shall be well-formed.
159Otherwise <tt class="docutils literal"><span class="pre">iterator_traits&lt;Dereferenceable&gt;::reference</span></tt> shall
160be well formed.  [Note: These requirements need not apply to
161explicit or partial specializations of <tt class="docutils literal"><span class="pre">indirect_reference</span></tt>]</td>
162</tr>
163</tbody>
164</table>
165<p><tt class="docutils literal"><span class="pre">type</span></tt> is determined according to the following algorithm, where
166<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>
167<pre class="literal-block">
168if ( ++x is ill-formed )
169    return ``pointee&lt;Dereferenceable&gt;::type&amp;``
170else
171    std::iterator_traits&lt;Dereferenceable&gt;::reference
172</pre>
173</div>
174</div>
175</div>
176<div class="footer">
177<hr class="footer" />
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180
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