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<title>THE BOOST MPL LIBRARY: Metafunction Composition</title>
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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="./more-lambda-capabilities.html" class="navigation-link">More Lambda Capabilities</a> / <a href="./metafunction-composition.html" class="navigation-link">Metafunction Composition</a></td>
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<div class="section" id="metafunction-composition">
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<h1><a class="toc-backref" href="./more-lambda-capabilities.html#id53" name="metafunction-composition">Metafunction Composition</a></h1>
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<p>Lambda expressions can also be used to assemble more interesting
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computations from simple metafunctions. For example, the following
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expression, which multiplies the sum of two numbers by their
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difference, is a <strong>composition</strong> of the three metafunctions <tt class="literal"><span class="pre">multiplies</span></tt>,
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<tt class="literal"><span class="pre">plus</span></tt>, and <tt class="literal"><span class="pre">minus</span></tt>:</p>
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<pre class="literal-block">
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mpl::multiplies<mpl::plus<_1,_2>, mpl::minus<_1,_2> >
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</pre>
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<!-- @ example.wrap(apply_test[0],
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', mpl::int_<5>,mpl::int_<3>' + apply_test[1] % 16)
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# Can't exactly justify this yet, but there's no way to get
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# it into the text
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prefix += ['#include <boost/mpl/multiplies.hpp>']
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compile() -->
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<p>When evaluating a lambda expression, MPL checks to see if any of its
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arguments are themselves lambda expressions, and evaluates each one
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that it finds. The results of these inner evaluations are substituted
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into the outer expression before it is evaluated.</p>
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