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- <title>THE BOOST MPL LIBRARY: The lambda Metafunction</title>
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- <div class="section" id="the-lambda-metafunction">
- <h1><a class="toc-backref" href="./handling-placeholders.html#id49" name="the-lambda-metafunction">The <tt class="literal"><span class="pre">lambda</span></tt> Metafunction</a></h1>
- <p>We can <em>generate</em> a metafunction class from
- <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt>, using MPL's <tt class="literal"><span class="pre">lambda</span></tt> metafunction:</p>
- <pre class="literal-block">
- template <class X>
- struct two_pointers
- : twice<<strong>typename mpl::lambda<boost::add_pointer<_1> >::type</strong>, X>
- {};
- BOOST_STATIC_ASSERT((
- boost::is_same<
- two_pointers<int>::type
- , int**
- >::value
- ));
- </pre>
- <!-- @ prefix.append('#include <boost/mpl/lambda.hpp>')
- compile('all') -->
- <p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and
- placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt>
- as <strong>lambda expressions</strong>. The term, meaning "unnamed function
- object," was introduced in the 1930s by the logician Alonzo Church
- as part of a fundamental theory of computation he called the
- <em>lambda-calculus</em>. <a class="footnote-reference" href="#lambda" id="id10" name="id10">[4]</a> MPL uses the somewhat obscure word
- <tt class="literal"><span class="pre">lambda</span></tt> because of its well-established precedent in functional
- programming languages.</p>
- <table class="footnote" frame="void" id="lambda" rules="none">
- <colgroup><col class="label" /><col /></colgroup>
- <tbody valign="top">
- <tr><td class="label"><a class="fn-backref" href="#id10" name="lambda">[4]</a></td><td>See <a class="reference" href="http://en.wikipedia.org/wiki/Lambda_calculus" target="_top">http://en.wikipedia.org/wiki/Lambda_calculus</a> for
- an in-depth treatment, including a reference to Church's paper
- proving that the equivalence of lambda expressions is in general
- not decidable.</td></tr>
- </tbody>
- </table>
- <p>Although its primary purpose is to turn placeholder expressions
- into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda
- expression, even if it's already a metafunction class. In that
- case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged. MPL algorithms
- like <tt class="literal"><span class="pre">transform</span></tt> call <tt class="literal"><span class="pre">lambda</span></tt> internally, before invoking the
- resulting metafunction class, so that they work equally well with
- either kind of lambda expression. We can apply the same strategy
- to <tt class="literal"><span class="pre">twice</span></tt>:</p>
- <pre class="literal-block">
- template <class F, class X>
- struct twice
- : apply1<
- typename mpl::lambda<F>::type
- , typename apply1<
- typename mpl::lambda<F>::type
- , X
- >::type
- >
- {};
- </pre>
- <p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with metafunction classes <em>and</em>
- placeholder expressions:</p>
- <pre class="literal-block">
- int* x;
- twice<<strong>add_pointer_f</strong>, int>::type p = &x;
- twice<<strong>boost::add_pointer<_1></strong>, int>::type q = &x;
- </pre>
- <!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]]
- compile('all') -->
- </div>
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