ceph/src/boost/libs/mpl/doc/refmanual/tag-dispatched-metafunction.html

   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 0.7: http://docutils.sourceforge.net/" />
   7 <title>The MPL Reference Manual: Tag Dispatched Metafunction</title>
   8 <link rel="stylesheet" href="../style.css" type="text/css" />
   9 </head>
  10 <body class="docframe refmanual">
  11 <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Prev</a>&nbsp;<a href="./numeric-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Back</a>&nbsp;<a href="./numeric-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
  12 <td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./metafunctions-concepts.html" class="navigation-link">Concepts</a> / <a href="./tag-dispatched-metafunction.html" class="navigation-link">Tag Dispatched Metafunction</a></td>
  13 </tr></table><div class="header-separator"></div>
  14 <div class="section" id="tag-dispatched-metafunction">
  15 <h1><a class="toc-backref" href="./metafunctions-concepts.html#id1516">Tag Dispatched Metafunction</a></h1>
  16 <div class="section" id="id840">
  17 <h3><a class="subsection-title" href="#summary" name="summary">Summary</a></h3>
  18 <p>A <a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a> is a <a class="reference internal" href="./metafunction.html">Metafunction</a> that employs a
  19 <em>tag dispatching</em> technique in its implementation to build an
  20 infrastructure for easy overriding/extenstion of the metafunction's
  21 behavior.</p>
  22 </div>
  23 <div class="section" id="notation">
  24 <h3>Notation</h3>
  25 <table border="1" class="docutils table" id="tag-metafunction">
  26 <colgroup>
  27 <col width="31%" />
  28 <col width="69%" />
  29 </colgroup>
  30 <thead valign="bottom">
  31 <tr><th class="head">Symbol</th>
  32 <th class="head">Legend</th>
  33 </tr>
  34 </thead>
  35 <tbody valign="top">
  36 <tr><td><em>name</em></td>
  37 <td>A placeholder token for the specific metafunction's name.</td>
  38 </tr>
  39 <tr><td><em>tag-metafunction</em></td>
  40 <td>A placeholder token for the tag metafunction's name.</td>
  41 </tr>
  42 <tr><td><em>tag</em></td>
  43 <td>A placeholder token for one of possible tag types
  44 returned by the tag metafunction.</td>
  45 </tr>
  46 </tbody>
  47 </table>
  48 </div>
  49 <div class="section" id="id841">
  50 <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
  51 <pre class="literal-block">
  52 template&lt; typename Tag &gt; struct <em>name</em>_impl;
  53
  54 template&lt;
  55       typename X
  56     <em>[, ...]</em>
  57     &gt;
  58 struct <em>name</em>
  59     : <em>name</em>_impl&lt; typename <em>tag-<a href="./metafunction.html" class="identifier">metafunction</a></em>&lt;X&gt;::type &gt;
  60         ::template <a href="./apply.html" class="identifier">apply</a>&lt;X <em>[, ...]</em>&gt;
  61 {
  62 };
  63
  64 template&lt; typename Tag &gt; struct <em>name</em>_impl
  65 {
  66     template&lt; typename X <em>[, ...]</em> &gt; struct <a href="./apply.html" class="identifier">apply</a>
  67     {
  68         // <em>default implementation</em>
  69     };
  70 };
  71
  72 template&lt;&gt; struct <em>name</em>_impl&lt;<em>tag</em>&gt;
  73 {
  74     template&lt; typename X <em>[, ...]</em> &gt; struct <a href="./apply.html" class="identifier">apply</a>
  75     {
  76         // <em>tag-specific implementation</em>
  77     };
  78 };
  79 </pre>
  80 </div>
  81 <div class="section" id="id842">
  82 <h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
  83 <p>The usual mechanism for overriding a metafunction's behavior is class
  84 template specialization — given a library-defined metafunction <tt class="literal"><span class="pre">f</span></tt>,
  85 it's possible to write a specialization of <tt class="literal"><span class="pre">f</span></tt> for a specific type
  86 <tt class="literal"><span class="pre">user_type</span></tt> that would have the required semantics <a class="footnote-reference" href="#spec" id="id843">[4]</a>.</p>
  87 <p>While this mechanism is always available, it's not always the most
  88 convenient one, especially if it is desirable to specialize a
  89 metafunction's behavior for a <em>family</em> of related types. A typical
  90 example of it is numbered forms of sequence classes in MPL itself
  91 (<tt class="literal"><span class="pre">list0</span></tt>, ..., <tt class="literal"><span class="pre">list50</span></tt>, et al.), and sequence classes in general.</p>
  92 <p>A <a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a> is a concept name for an instance of
  93 the metafunction implementation infrastructure being employed by the
  94 library to make it easier for users and implementors to override the
  95 behavior of library's metafunctions operating on families of specific
  96 types.</p>
  97 <p>The infrastructure is built on a variation of the technique commonly
  98 known as <em>tag dispatching</em> (hence the concept name),
  99 and involves three entities: a metafunction itself, an associated
 100 tag-producing <a class="reference internal" href="./tag-dispatched-metafunction.html#tag-metafunction">tag metafunctions</a>, and the metafunction's
 101 implementation, in the form of a <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a> template
 102 parametrized by a <tt class="literal"><span class="pre">Tag</span></tt> type parameter. The metafunction redirects
 103 to its implementation class template by invoking its specialization
 104 on a tag type produced by the tag metafunction with the original
 105 metafunction's parameters.</p>
 106 <table class="docutils footnote" frame="void" id="spec" rules="none">
 107 <colgroup><col class="label" /><col /></colgroup>
 108 <tbody valign="top">
 109 <tr><td class="label"><a class="fn-backref" href="#id843">[4]</a></td><td>Usually such user-defined specialization is still required
 110 to preserve the <tt class="literal"><span class="pre">f</span></tt>'s original invariants and complexity requirements.</td></tr>
 111 </tbody>
 112 </table>
 113 </div>
 114 <div class="section" id="id844">
 115 <h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
 116 <pre class="literal-block">
 117 #include &lt;<a href="../../../../boost/mpl/size.hpp" class="header">boost/mpl/size.hpp</a>&gt;
 118
 119 namespace user {
 120
 121 struct bitset_tag;
 122
 123 struct bitset0
 124 {
 125     typedef bitset_tag tag;
 126     // ...
 127 };
 128
 129 template< typename B0 > struct bitset1
 130 {
 131     typedef bitset_tag tag;
 132     // ...
 133 };
 134
 135 template< typename B0, <em>...,</em> typename B<em>n</em> &gt; struct bitset<em>n</em>
 136 {
 137     typedef bitset_tag tag;
 138     // ...
 139 };
 140
 141 } // namespace user
 142
 143 namespace boost { namespace mpl {
 144 template&lt;&gt; struct size_impl&lt;user::bitset_tag&gt;
 145 {
 146     template&lt; typename Bitset &gt; struct <a href="./apply.html" class="identifier">apply</a>
 147     {
 148         typedef typename Bitset::<a href="./size.html" class="identifier">size</a> type;
 149     };
 150 };
 151 }}
 152 </pre>
 153 </div>
 154 <div class="section" id="id845">
 155 <h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
 156 <ul class="simple">
 157 <li><a class="reference internal" href="./sequence-tag.html">sequence_tag</a></li>
 158 </ul>
 159 </div>
 160 <div class="section" id="id846">
 161 <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
 162 <p><a class="reference internal" href="./metafunction.html">Metafunction</a>, <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a>, <a class="reference internal" href="./numeric-metafunction.html">Numeric Metafunction</a></p>
 163 <!-- Metafunctions/Concepts//Numeric Metafunction |60 -->
 164 </div>
 165 </div>
 166
 167 <div class="footer-separator"></div>
 168 <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Prev</a>&nbsp;<a href="./numeric-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Back</a>&nbsp;<a href="./numeric-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./metafunctions-concepts.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
 169 <td><div class="copyright-footer"><div class="copyright">Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams</div>
 170 Distributed under the Boost Software License, Version 1.0. (See accompanying
 171 file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body>
 172 </html>