<?xml version="1.0" encoding="utf-8" ?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> <!-- Software License, Version 1.0. (See accompanying --> <!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> <head> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> <title>THE BOOST MPL LIBRARY: The lambda Metafunction</title> <link rel="stylesheet" href="../style.css" type="text/css" /> </head> <body class="docframe"> <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> <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> </tr></table><div class="header-separator"></div> <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> <div class="footer-separator"></div> <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> </tr></table></body> </html>