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- <title>pointee and indirect_reference</title>
- <meta name="author" content="David Abrahams" />
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- <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>
- </div>
- </div>
- </div>
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