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7<title>The MPL Reference Manual: Tag Dispatched Metafunction</title>
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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
20infrastructure for easy overriding/extenstion of the metafunction's
21behavior.</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
44returned 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">
52template&lt; typename Tag &gt; struct <em>name</em>_impl;
53
54template&lt;
55 typename X
56 <em>[, ...]</em>
57 &gt;
58struct <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
64template&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
72template&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
84template specialization — given a library-defined metafunction <tt class="literal"><span class="pre">f</span></tt>,
85it'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
88convenient one, especially if it is desirable to specialize a
89metafunction's behavior for a <em>family</em> of related types. A typical
90example 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
93the metafunction implementation infrastructure being employed by the
94library to make it easier for users and implementors to override the
95behavior of library's metafunctions operating on families of specific
96types.</p>
97<p>The infrastructure is built on a variation of the technique commonly
98known as <em>tag dispatching</em> (hence the concept name),
99and involves three entities: a metafunction itself, an associated
100tag-producing <a class="reference internal" href="./tag-dispatched-metafunction.html#tag-metafunction">tag metafunctions</a>, and the metafunction's
101implementation, in the form of a <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a> template
102parametrized by a <tt class="literal"><span class="pre">Tag</span></tt> type parameter. The metafunction redirects
103to its implementation class template by invoking its specialization
104on a tag type produced by the tag metafunction with the original
105metafunction'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
110to 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
119namespace user {
120
121struct bitset_tag;
122
123struct bitset0
124{
125 typedef bitset_tag tag;
126 // ...
127};
128
129template< typename B0 > struct bitset1
130{
131 typedef bitset_tag tag;
132 // ...
133};
134
135template< 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
143namespace boost { namespace mpl {
144template&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
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169<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div>
170Distributed under the Boost Software License, Version 1.0. (See accompanying
171file 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>
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