| <?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> |
