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