1<?xml version="1.0" encoding="utf-8" ?> 2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> 3<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> 4<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost --> 5<!-- Software License, Version 1.0. (See accompanying --> 6<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) --> 7<head> 8<meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> 9<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" /> 10<title>THE BOOST MPL LIBRARY: The lambda Metafunction</title> 11<link rel="stylesheet" href="../style.css" type="text/css" /> 12</head> 13<body class="docframe"> 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"> 22template <class X> 23struct two_pointers 24 : twice<<strong>typename mpl::lambda<boost::add_pointer<_1> >::type</strong>, X> 25{}; 26 27BOOST_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>') 35compile('all') --> 36<p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and 37placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer<_1></span></tt> 38as <strong>lambda expressions</strong>. The term, meaning "unnamed function 39object," was introduced in the 1930s by the logician Alonzo Church 40as 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 43programming 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 48an in-depth treatment, including a reference to Church's paper 49proving that the equivalence of lambda expressions is in general 50not decidable.</td></tr> 51</tbody> 52</table> 53<p>Although its primary purpose is to turn placeholder expressions 54into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda 55expression, even if it's already a metafunction class. In that 56case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged. MPL algorithms 57like <tt class="literal"><span class="pre">transform</span></tt> call <tt class="literal"><span class="pre">lambda</span></tt> internally, before invoking the 58resulting metafunction class, so that they work equally well with 59either kind of lambda expression. We can apply the same strategy 60to <tt class="literal"><span class="pre">twice</span></tt>:</p> 61<pre class="literal-block"> 62template <class F, class X> 63struct 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> 74placeholder expressions:</p> 75<pre class="literal-block"> 76int* x; 77 78twice<<strong>add_pointer_f</strong>, int>::type p = &x; 79twice<<strong>boost::add_pointer<_1></strong>, int>::type q = &x; 80</pre> 81<!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]] 82compile('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> 89