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