ceph/src/boost/libs/mpl/doc/tutorial/the-lambda-metafunction.html

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  10 <title>THE BOOST MPL LIBRARY: The lambda Metafunction</title>
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  17 <div class="section" id="the-lambda-metafunction">
  18 <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>
  19 <p>We can <em>generate</em> a metafunction class from
  20 <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt>, using MPL's <tt class="literal"><span class="pre">lambda</span></tt> metafunction:</p>
  21 <pre class="literal-block">
  22 template &lt;class X&gt;
  23 struct two_pointers
  24   : twice&lt;<strong>typename mpl::lambda&lt;boost::add_pointer&lt;_1&gt; &gt;::type</strong>, X&gt;
  25 {};
  26
  27 BOOST_STATIC_ASSERT((
  28     boost::is_same&lt;
  29         two_pointers&lt;int&gt;::type
  30       , int**
  31     &gt;::value
  32 ));
  33 </pre>
  34 <!-- @ prefix.append('#include <boost/mpl/lambda.hpp>')
  35 compile('all') -->
  36 <p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and
  37 placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt>
  38 as <strong>lambda expressions</strong>.  The term, meaning &quot;unnamed function
  39 object,&quot; was introduced in the 1930s by the logician Alonzo Church
  40 as part of a fundamental theory of computation he called the
  41 <em>lambda-calculus</em>. <a class="footnote-reference" href="#lambda" id="id10" name="id10">[4]</a>  MPL uses the somewhat obscure word
  42 <tt class="literal"><span class="pre">lambda</span></tt> because of its well-established precedent in functional
  43 programming languages.</p>
  44 <table class="footnote" frame="void" id="lambda" rules="none">
  45 <colgroup><col class="label" /><col /></colgroup>
  46 <tbody valign="top">
  47 <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
  48 an in-depth treatment, including a reference to Church's paper
  49 proving that the equivalence of lambda expressions is in general
  50 not decidable.</td></tr>
  51 </tbody>
  52 </table>
  53 <p>Although its primary purpose is to turn placeholder expressions
  54 into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda
  55 expression, even if it's already a metafunction class.  In that
  56 case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged.  MPL algorithms
  57 like <tt class="literal"><span class="pre">transform</span></tt> call <tt class="literal"><span class="pre">lambda</span></tt> internally, before invoking the
  58 resulting metafunction class, so that they work equally well with
  59 either kind of lambda expression.  We can apply the same strategy
  60 to <tt class="literal"><span class="pre">twice</span></tt>:</p>
  61 <pre class="literal-block">
  62 template &lt;class F, class X&gt;
  63 struct twice
  64    : apply1&lt;
  65          typename mpl::lambda&lt;F&gt;::type
  66        , typename apply1&lt;
  67              typename mpl::lambda&lt;F&gt;::type
  68            , X
  69          &gt;::type
  70      &gt;
  71 {};
  72 </pre>
  73 <p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with metafunction classes <em>and</em>
  74 placeholder expressions:</p>
  75 <pre class="literal-block">
  76 int* x;
  77
  78 twice&lt;<strong>add_pointer_f</strong>, int&gt;::type           p = &amp;x;
  79 twice&lt;<strong>boost::add_pointer&lt;_1&gt;</strong>, int&gt;::type q = &amp;x;
  80 </pre>
  81 <!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]]
  82 compile('all') -->
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