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| 10 | <title>THE BOOST MPL LIBRARY: The lambda Metafunction</title> |
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| 15 | <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="./handling-placeholders.html" class="navigation-link">Handling Placeholders</a> / <a href="./the-lambda-metafunction.html" class="navigation-link">The lambda Metafunction</a></td> |
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| 16 | </tr></table><div class="header-separator"></div> |
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| 17 | <div class="section" id="the-lambda-metafunction"> |
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| 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> |
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| 19 | <p>We can <em>generate</em> a metafunction class from |
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| 20 | <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> |
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| 21 | <pre class="literal-block"> |
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| 22 | template <class X> |
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| 23 | struct two_pointers |
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| 24 | : twice<<strong>typename mpl::lambda<boost::add_pointer<_1> >::type</strong>, X> |
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| 25 | {}; |
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| 26 | |
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| 27 | BOOST_STATIC_ASSERT(( |
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| 28 | boost::is_same< |
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| 29 | two_pointers<int>::type |
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| 30 | , int** |
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| 31 | >::value |
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| 32 | )); |
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| 33 | </pre> |
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| 34 | <!-- @ prefix.append('#include <boost/mpl/lambda.hpp>') |
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| 35 | compile('all') --> |
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| 36 | <p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and |
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| 37 | placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> |
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| 38 | as <strong>lambda expressions</strong>. The term, meaning "unnamed function |
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| 39 | object," was introduced in the 1930s by the logician Alonzo Church |
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| 40 | as part of a fundamental theory of computation he called the |
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| 41 | <em>lambda-calculus</em>. <a class="footnote-reference" href="#lambda" id="id10" name="id10">[4]</a> MPL uses the somewhat obscure word |
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| 42 | <tt class="literal"><span class="pre">lambda</span></tt> because of its well-established precedent in functional |
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| 43 | programming languages.</p> |
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| 44 | <table class="footnote" frame="void" id="lambda" rules="none"> |
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| 45 | <colgroup><col class="label" /><col /></colgroup> |
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| 46 | <tbody valign="top"> |
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| 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 |
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| 48 | an in-depth treatment, including a reference to Church's paper |
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| 49 | proving that the equivalence of lambda expressions is in general |
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| 50 | not decidable.</td></tr> |
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| 51 | </tbody> |
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| 52 | </table> |
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| 53 | <p>Although its primary purpose is to turn placeholder expressions |
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| 54 | into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda |
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| 55 | expression, even if it's already a metafunction class. In that |
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| 56 | case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged. MPL algorithms |
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| 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 |
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| 58 | resulting metafunction class, so that they work equally well with |
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| 59 | either kind of lambda expression. We can apply the same strategy |
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| 60 | to <tt class="literal"><span class="pre">twice</span></tt>:</p> |
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| 61 | <pre class="literal-block"> |
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| 62 | template <class F, class X> |
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| 63 | struct twice |
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| 64 | : apply1< |
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| 65 | typename mpl::lambda<F>::type |
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| 66 | , typename apply1< |
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| 67 | typename mpl::lambda<F>::type |
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| 68 | , X |
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| 69 | >::type |
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| 70 | > |
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| 71 | {}; |
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| 72 | </pre> |
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| 73 | <p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with metafunction classes <em>and</em> |
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| 74 | placeholder expressions:</p> |
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| 75 | <pre class="literal-block"> |
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| 76 | int* x; |
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| 77 | |
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| 78 | twice<<strong>add_pointer_f</strong>, int>::type p = &x; |
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| 79 | twice<<strong>boost::add_pointer<_1></strong>, int>::type q = &x; |
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| 80 | </pre> |
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| 81 | <!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]] |
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| 82 | compile('all') --> |
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| 83 | </div> |
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