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60 lines
2.7 KiB
HTML
60 lines
2.7 KiB
HTML
<!doctype html public "-//ietf//dtd html//en">
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<html><head><title>boost::mpl::Metafunction</title>
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<link rel="stylesheet" href="./mpl_wiki.css">
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</head><body bgcolor="white">
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<h1><a href="./Table_of_Content.html"><img src="./mpl_logo.jpg" alt="[Home]" border=0 align="right"></a>Metafunction</h1><h3>Description</h3>
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<p>
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A Metafunction is a class template that represents a function invocable at compile-time. A metafunction is invoked by instantiating the class template with particular template parameters (metafunction arguments); the result of metafunction application is accessible through the instantiation's nested <code>type</code> typedef. A metafunction can have a variable number of parameters.
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<p>
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<h3>Example</h3>
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<p>
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<pre>
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<span class="cxx-comment">// binary metafunction</span>
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template< typename T1, typename T2 >
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struct is_same
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{
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typedef false_c type;
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};
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<p>
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template< typename T >
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struct is_same
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{
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typedef true_c type;
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};
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<p>
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<span class="cxx-comment">// metafunction invocation</span>
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typedef is_same<int,char>::type res;
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BOOST_STATIC_ASSERT(!res::value);
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</pre>
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<p>
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<p>
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<h3>Valid expressions</h3>
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<p>
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<table border="1">
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<tr><th> Expression </th><th> Expression type </th></tr>
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<tr><td><code>typename f::type</code></td><td>A type </td></tr>
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<tr><td><code>typename f<t1,..,tn>::type</code></td><td>A type </td></tr>
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</table>
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<p>
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<h3>Expression semantics</h3>
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<p>
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<table border="1">
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<tr><th> Expression </th><th> Complexity </th><th> Precondition </th><th> Semantics </th><th> Postcondition </th></tr>
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<tr><td><code>typename f::type</code></td><td>Metafunction dependent</td><td><code>f</code> is a nullary metafunction; <code>f::type</code> is a <em>type-name</em></td><td><code>f::type</code> is the result of the metafunction invocation</td><td></td></tr>
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<tr><td><code>typename f<t1,..,tn>::type</code></td><td>Metafunction dependent</td><td><code>f</code> is an <code>n</code>-ary metafunction; <code>t1,..,tn</code> are types; <code>f<t1,..,tn>::type</code> is a <em>type-name</em></td><td><code>f<t1,..,tn>::type</code> is the result of the metafunction invocation with the input arguments <code>t1,..,tn</code></td><td></td></tr>
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</table>
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<p>
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<h3>Models</h3>
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<p>
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<ul>
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<li><code><a href="./Reference/plus.html">plus</a></code>
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<li><code><a href="./Reference/logical_not.html">logical_not</a></code>
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<li><code><a href="./Reference/size.html">size</a></code>
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<li><code>max_element</code>
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</ul>
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<p>
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<h3>See also</h3>
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<p>
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<a href="./Metafunctions.html">Metafunctions</a>, [Metafunction Class]
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<p><hr>
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<a href="./Table_of_Content.html">Table of Content</a><br>Last edited July 16, 2002 11:27 pm</body></html> |