51 lines
4.6 KiB
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51 lines
4.6 KiB
HTML
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<title>THE BOOST MPL LIBRARY: Lambda and Non-Metafunction Templates</title>
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<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> <a href="./the-importance-of-being.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./placeholder-expression.html" class="navigation-link">Back</a> <a href="./the-importance-of-being.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./lambda-details.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
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<td class="header-group page-location"><a href="../index.html" class="navigation-link">Front Page</a> / <a href="./tutorial-metafunctions.html" class="navigation-link">Tutorial: Metafunctions and Higher-Order Metaprogramming</a> / <a href="./lambda-details.html" class="navigation-link">Lambda Details</a> / <a href="./lambda-and-non.html" class="navigation-link">Lambda and Non-Metafunction Templates</a></td>
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<div class="section" id="lambda-and-non">
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<h1><a class="toc-backref" href="./lambda-details.html#id57" name="lambda-and-non">Lambda and Non-Metafunction Templates</a></h1>
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<p>There is just one detail of placeholder expressions that we haven't
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discussed yet. MPL uses a special rule to make it easier to
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integrate ordinary templates into metaprograms: After all of the
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placeholders have been replaced with actual
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arguments, if the resulting template specialization <em>X</em> doesn't
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have a nested <tt class="literal"><span class="pre">::type</span></tt>, the result is just
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<em>X</em> itself.</p>
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<p>For example, <tt class="literal"><span class="pre">mpl::apply<std::vector<_>,</span> <span class="pre">T></span></tt> is always just
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<tt class="literal"><span class="pre">std::vector<T></span></tt>. If it weren't for this behavior, we would
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have to build trivial metafunctions to create ordinary template
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specializations in lambda expressions:</p>
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<pre class="literal-block">
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// trivial std::vector generator
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template<class U>
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struct make_vector { typedef std::vector<U> type; };
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typedef mpl::apply<<strong>make_vector<_></strong>, T>::type vector_of_t;
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</pre>
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<p>Instead, we can simply write:</p>
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<pre class="literal-block">
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typedef mpl::apply<<strong>std::vector<_></strong>, T>::type vector_of_t;
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</pre>
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