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| 4 | <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> | |
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| 6 | <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> | |
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| 9 | <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> | |
| 10 | <title>THE BOOST MPL LIBRARY: The lambda Metafunction</title> | |
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| 14 | <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Prev</a> <a href="./the-apply-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./the-apply-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> | |
| 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> | |
| 16 | </tr></table><div class="header-separator"></div> | |
| 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<_1></span></tt>, using MPL's <tt class="literal"><span class="pre">lambda</span></tt> metafunction:</p> | |
| 21 | <pre class="literal-block"> | |
| 22 | template <class X> | |
| 23 | struct two_pointers | |
| 24 | : twice<<strong>typename mpl::lambda<boost::add_pointer<_1> >::type</strong>, X> | |
| 25 | {}; | |
| 26 | ||
| 27 | BOOST_STATIC_ASSERT(( | |
| 28 | boost::is_same< | |
| 29 | two_pointers<int>::type | |
| 30 | , int** | |
| 31 | >::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<_1></span></tt> | |
| 38 | as <strong>lambda expressions</strong>. The term, meaning "unnamed function | |
| 39 | object," 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 <class F, class X> | |
| 63 | struct twice | |
| 64 | : apply1< | |
| 65 | typename mpl::lambda<F>::type | |
| 66 | , typename apply1< | |
| 67 | typename mpl::lambda<F>::type | |
| 68 | , X | |
| 69 | >::type | |
| 70 | > | |
| 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<<strong>add_pointer_f</strong>, int>::type p = &x; | |
| 79 | twice<<strong>boost::add_pointer<_1></strong>, int>::type q = &x; | |
| 80 | </pre> | |
| 81 | <!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]] | |
| 82 | compile('all') --> | |
| 83 | </div> | |
| 84 | ||
| 85 | <div class="footer-separator"></div> | |
| 86 | <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Prev</a> <a href="./the-apply-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./the-apply-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Up</a> <a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td> | |
| 87 | </tr></table></body> | |
| 88 | </html> |