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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> |
| </div> |
| </div> |
| </div> |
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