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| <title>The MPL Reference Manual: Tag Dispatched Metafunction</title> |
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| <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> |
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| <div class="section" id="tag-dispatched-metafunction"> |
| <h1><a class="toc-backref" href="./metafunctions-concepts.html#id1516">Tag Dispatched Metafunction</a></h1> |
| <div class="section" id="id840"> |
| <h3><a class="subsection-title" href="#summary" name="summary">Summary</a></h3> |
| <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 |
| <em>tag dispatching</em> technique in its implementation to build an |
| infrastructure for easy overriding/extenstion of the metafunction's |
| behavior.</p> |
| </div> |
| <div class="section" id="notation"> |
| <h3>Notation</h3> |
| <table border="1" class="docutils table" id="tag-metafunction"> |
| <colgroup> |
| <col width="31%" /> |
| <col width="69%" /> |
| </colgroup> |
| <thead valign="bottom"> |
| <tr><th class="head">Symbol</th> |
| <th class="head">Legend</th> |
| </tr> |
| </thead> |
| <tbody valign="top"> |
| <tr><td><em>name</em></td> |
| <td>A placeholder token for the specific metafunction's name.</td> |
| </tr> |
| <tr><td><em>tag-metafunction</em></td> |
| <td>A placeholder token for the tag metafunction's name.</td> |
| </tr> |
| <tr><td><em>tag</em></td> |
| <td>A placeholder token for one of possible tag types |
| returned by the tag metafunction.</td> |
| </tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id841"> |
| <h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3> |
| <pre class="literal-block"> |
| template< typename Tag > struct <em>name</em>_impl; |
| |
| template< |
| typename X |
| <em>[, ...]</em> |
| > |
| struct <em>name</em> |
| : <em>name</em>_impl< typename <em>tag-<a href="./metafunction.html" class="identifier">metafunction</a></em><X>::type > |
| ::template <a href="./apply.html" class="identifier">apply</a><X <em>[, ...]</em>> |
| { |
| }; |
| |
| template< typename Tag > struct <em>name</em>_impl |
| { |
| template< typename X <em>[, ...]</em> > struct <a href="./apply.html" class="identifier">apply</a> |
| { |
| // <em>default implementation</em> |
| }; |
| }; |
| |
| template<> struct <em>name</em>_impl<<em>tag</em>> |
| { |
| template< typename X <em>[, ...]</em> > struct <a href="./apply.html" class="identifier">apply</a> |
| { |
| // <em>tag-specific implementation</em> |
| }; |
| }; |
| </pre> |
| </div> |
| <div class="section" id="id842"> |
| <h3><a class="subsection-title" href="#description" name="description">Description</a></h3> |
| <p>The usual mechanism for overriding a metafunction's behavior is class |
| template specialization — given a library-defined metafunction <tt class="literal"><span class="pre">f</span></tt>, |
| it's possible to write a specialization of <tt class="literal"><span class="pre">f</span></tt> for a specific type |
| <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> |
| <p>While this mechanism is always available, it's not always the most |
| convenient one, especially if it is desirable to specialize a |
| metafunction's behavior for a <em>family</em> of related types. A typical |
| example of it is numbered forms of sequence classes in MPL itself |
| (<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> |
| <p>A <a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a> is a concept name for an instance of |
| the metafunction implementation infrastructure being employed by the |
| library to make it easier for users and implementors to override the |
| behavior of library's metafunctions operating on families of specific |
| types.</p> |
| <p>The infrastructure is built on a variation of the technique commonly |
| known as <em>tag dispatching</em> (hence the concept name), |
| and involves three entities: a metafunction itself, an associated |
| tag-producing <a class="reference internal" href="./tag-dispatched-metafunction.html#tag-metafunction">tag metafunctions</a>, and the metafunction's |
| implementation, in the form of a <a class="reference internal" href="./metafunction-class.html">Metafunction Class</a> template |
| parametrized by a <tt class="literal"><span class="pre">Tag</span></tt> type parameter. The metafunction redirects |
| to its implementation class template by invoking its specialization |
| on a tag type produced by the tag metafunction with the original |
| metafunction's parameters.</p> |
| <table class="docutils footnote" frame="void" id="spec" rules="none"> |
| <colgroup><col class="label" /><col /></colgroup> |
| <tbody valign="top"> |
| <tr><td class="label"><a class="fn-backref" href="#id843">[4]</a></td><td>Usually such user-defined specialization is still required |
| to preserve the <tt class="literal"><span class="pre">f</span></tt>'s original invariants and complexity requirements.</td></tr> |
| </tbody> |
| </table> |
| </div> |
| <div class="section" id="id844"> |
| <h3><a class="subsection-title" href="#example" name="example">Example</a></h3> |
| <pre class="literal-block"> |
| #include <<a href="../../../../boost/mpl/size.hpp" class="header">boost/mpl/size.hpp</a>> |
| |
| namespace user { |
| |
| struct bitset_tag; |
| |
| struct bitset0 |
| { |
| typedef bitset_tag tag; |
| // ... |
| }; |
| |
| template< typename B0 > struct bitset1 |
| { |
| typedef bitset_tag tag; |
| // ... |
| }; |
| |
| template< typename B0, <em>...,</em> typename B<em>n</em> > struct bitset<em>n</em> |
| { |
| typedef bitset_tag tag; |
| // ... |
| }; |
| |
| } // namespace user |
| |
| namespace boost { namespace mpl { |
| template<> struct size_impl<user::bitset_tag> |
| { |
| template< typename Bitset > struct <a href="./apply.html" class="identifier">apply</a> |
| { |
| typedef typename Bitset::<a href="./size.html" class="identifier">size</a> type; |
| }; |
| }; |
| }} |
| </pre> |
| </div> |
| <div class="section" id="id845"> |
| <h3><a class="subsection-title" href="#models" name="models">Models</a></h3> |
| <ul class="simple"> |
| <li><a class="reference internal" href="./sequence-tag.html">sequence_tag</a></li> |
| </ul> |
| </div> |
| <div class="section" id="id846"> |
| <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3> |
| <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> |
| <!-- Metafunctions/Concepts//Numeric Metafunction |60 --> |
| </div> |
| </div> |
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| Distributed under the Boost Software License, Version 1.0. (See accompanying |
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