Planet
navi homePPSaboutscreenshotsdownloaddevelopmentforum

source: downloads/boost_1_33_1/libs/mpl/doc/tutorial/the-lambda-metafunction.html @ 12

Last change on this file since 12 was 12, checked in by landauf, 18 years ago

added boost
File size: 6.1 KB
Line 
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<head>
5<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
6<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
7<title>THE BOOST MPL LIBRARY: The lambda Metafunction</title>
8<link rel="stylesheet" href="../style.css" type="text/css" />
9</head>
10<body class="docframe">
11<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>&nbsp;<a href="./the-apply-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./the-apply-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
12<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>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="the-lambda-metafunction">
15<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>
16<p>We can <em>generate</em> a metafunction class from
17<tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt>, using MPL's <tt class="literal"><span class="pre">lambda</span></tt> metafunction:</p>
18<pre class="literal-block">
19template &lt;class X&gt;
20struct two_pointers
21  : twice&lt;<strong>typename mpl::lambda&lt;boost::add_pointer&lt;_1&gt; &gt;::type</strong>, X&gt;
22{};
23
24BOOST_STATIC_ASSERT((
25    boost::is_same&lt;
26        two_pointers&lt;int&gt;::type
27      , int**
28    &gt;::value
29));
30</pre>
31<!-- @ prefix.append('#include <boost/mpl/lambda.hpp>')
32compile('all') -->
33<p>We'll refer to metafunction classes like <tt class="literal"><span class="pre">add_pointer_f</span></tt> and
34placeholder expressions like <tt class="literal"><span class="pre">boost::add_pointer&lt;_1&gt;</span></tt>
35as <strong>lambda expressions</strong>.  The term, meaning &quot;unnamed function
36object,&quot; was introduced in the 1930s by the logician Alonzo Church
37as part of a fundamental theory of computation he called the
38<em>lambda-calculus</em>. <a class="footnote-reference" href="#lambda" id="id10" name="id10">[4]</a>  MPL uses the somewhat obscure word
39<tt class="literal"><span class="pre">lambda</span></tt> because of its well-established precedent in functional
40programming languages.</p>
41<table class="footnote" frame="void" id="lambda" rules="none">
42<colgroup><col class="label" /><col /></colgroup>
43<tbody valign="top">
44<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
45an in-depth treatment, including a reference to Church's paper
46proving that the equivalence of lambda expressions is in general
47not decidable.</td></tr>
48</tbody>
49</table>
50<p>Although its primary purpose is to turn placeholder expressions
51into metafunction classes, <tt class="literal"><span class="pre">mpl::lambda</span></tt> can accept any lambda
52expression, even if it's already a metafunction class.  In that
53case, <tt class="literal"><span class="pre">lambda</span></tt> returns its argument unchanged.  MPL algorithms
54like <tt class="literal"><span class="pre">transform</span></tt> call <tt class="literal"><span class="pre">lambda</span></tt> internally, before invoking the
55resulting metafunction class, so that they work equally well with
56either kind of lambda expression.  We can apply the same strategy
57to <tt class="literal"><span class="pre">twice</span></tt>:</p>
58<pre class="literal-block">
59template &lt;class F, class X&gt;
60struct twice
61   : apply1&lt;
62         typename mpl::lambda&lt;F&gt;::type
63       , typename apply1&lt;
64             typename mpl::lambda&lt;F&gt;::type
65           , X
66         &gt;::type
67     &gt;
68{};
69</pre>
70<p>Now we can use <tt class="literal"><span class="pre">twice</span></tt> with metafunction classes <em>and</em>
71placeholder expressions:</p>
72<pre class="literal-block">
73int* x;
74
75twice&lt;<strong>add_pointer_f</strong>, int&gt;::type           p = &amp;x;
76twice&lt;<strong>boost::add_pointer&lt;_1&gt;</strong>, int&gt;::type q = &amp;x;
77</pre>
78<!-- @ stack[-2:] = [ apply1, stack[-2], add_pointer_f, stack[-1]]
79compile('all') -->
80</div>
81
82<div class="footer-separator"></div>
83<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>&nbsp;<a href="./the-apply-metafunction.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./the-apply-metafunction.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./handling-placeholders.html" class="navigation-link">Up</a>&nbsp;<a href="../index.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./tutorial_toc.html" class="navigation-link">Full TOC</a></span></td>
84</tr></table></body>
85</html>
Note: See TracBrowser for help on using the repository browser.