89 lines
6.3 KiB
HTML
89 lines
6.3 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: The lambda Metafunction</title>
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</head>
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<body class="docframe">
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<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>
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<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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</tr></table><div class="header-separator"></div>
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<div class="section" id="the-lambda-metafunction">
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<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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<p>We can <em>generate</em> a metafunction class from
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<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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<pre class="literal-block">
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template <class X>
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struct two_pointers
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: twice<<strong>typename mpl::lambda<boost::add_pointer<_1> >::type</strong>, X>
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{};
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BOOST_STATIC_ASSERT((
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boost::is_same<
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two_pointers<int>::type
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, int**
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>::value
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));
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</pre>
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<!-- @ prefix.append('#include <boost/mpl/lambda.hpp>')
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compile('all') -->
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<p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and
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placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt>
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as <strong>lambda expressions</strong>. The term, meaning "unnamed function
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object," was introduced in the 1930s by the logician Alonzo Church
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as part of a fundamental theory of computation he called the
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<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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<tt class="literal"><span class="pre">lambda</span></tt> because of its well-established precedent in functional
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programming languages.</p>
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<table class="footnote" frame="void" id="lambda" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<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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an in-depth treatment, including a reference to Church's paper
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proving that the equivalence of lambda expressions is in general
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not decidable.</td></tr>
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</tbody>
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</table>
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<p>Although its primary purpose is to turn placeholder expressions
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into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda
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expression, even if it's already a metafunction class. In that
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case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged. MPL algorithms
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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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resulting metafunction class, so that they work equally well with
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either kind of lambda expression. We can apply the same strategy
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to <tt class="literal"><span class="pre">twice</span></tt>:</p>
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<pre class="literal-block">
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template <class F, class X>
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struct twice
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: apply1<
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typename mpl::lambda<F>::type
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, typename apply1<
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typename mpl::lambda<F>::type
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, X
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>::type
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>
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{};
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</pre>
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<p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with metafunction classes <em>and</em>
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placeholder expressions:</p>
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<pre class="literal-block">
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int* x;
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twice<<strong>add_pointer_f</strong>, int>::type p = &x;
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twice<<strong>boost::add_pointer<_1></strong>, int>::type q = &x;
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</pre>
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<!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]]
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compile('all') -->
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</div>
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<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>
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