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 apply 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="./the-lambda-metafunction.html" class="navigation-link">Prev</a> <a href="./more-lambda-capabilities.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./the-lambda-metafunction.html" class="navigation-link">Back</a> Along</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-apply-metafunction.html" class="navigation-link">The apply Metafunction</a></td> 16</tr></table><div class="header-separator"></div> 17<div class="section" id="the-apply-metafunction"> 18<h1><a class="toc-backref" href="./handling-placeholders.html#id50" name="the-apply-metafunction">The <tt class="literal"><span class="pre">apply</span></tt> Metafunction</a></h1> 19<p>Invoking the result of <tt class="literal"><span class="pre">lambda</span></tt> is such a common pattern 20that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just 21that. Using <tt class="literal"><span class="pre">mpl::apply</span></tt>, our flexible version of <tt class="literal"><span class="pre">twice</span></tt> 22becomes:</p> 23<pre class="literal-block"> 24#include <boost/mpl/apply.hpp> 25 26template <class F, class X> 27struct twice 28 : mpl::apply<F, typename mpl::apply<F,X>::type> 29{}; 30</pre> 31<!-- @ example.append(twice_test + ''' 32BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>)); 33''') 34compile() 35prefix.append('#include <boost/mpl/apply.hpp>') --> 36<p>You can think of <tt class="literal"><span class="pre">mpl::apply</span></tt> as being just like the <tt class="literal"><span class="pre">apply1</span></tt> 37template that we wrote, with two additional features:</p> 38<ol class="arabic"> 39<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> operates only on metafunction classes, the first 40argument to <tt class="literal"><span class="pre">mpl::apply</span></tt> can be any lambda expression 41(including those built with placeholders).</p> 42</li> 43<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> accepts only one additional argument to which 44the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can 45invoke its first argument on any number from zero to five 46additional arguments. <a class="footnote-reference" href="#arity" id="id11" name="id11">[5]</a> For example:</p> 47<pre class="literal-block"> 48// binary lambda expression applied to 2 additional arguments 49mpl::apply< 50 mpl::plus<_1,_2> 51 , <strong>mpl::int_<6></strong> 52 , <strong>mpl::int_<7></strong> 53>::type::value // == 13 54</pre> 55</li> 56</ol> 57<table class="footnote" frame="void" id="arity" rules="none"> 58<colgroup><col class="label" /><col /></colgroup> 59<tbody valign="top"> 60<tr><td class="label"><a class="fn-backref" href="#id11" name="arity">[5]</a></td><td>See the Configuration Macros section of the <a class="reference" href="./reference-manual.html">the MPL reference manual</a> 61for a description of how to change the maximum number of 62arguments handled by <tt class="literal"><span class="pre">mpl::apply</span></tt>.</td></tr> 63</tbody> 64</table> 65<!-- @ prefix+=['#include <boost/mpl/plus.hpp>'] 66example.wrap('enum { is13 = ','''}; 67BOOST_STATIC_ASSERT(is13 == 13);''') 68compile() --> 69<div class="admonition-guideline admonition"> 70<p class="admonition-title first">Guideline</p> 71<p>When writing a metafunction that invokes one of its arguments, 72use <tt class="literal"><span class="pre">mpl::apply</span></tt> so that it works with lambda expressions.</p> 73</div> 74</div> 75 76<div class="footer-separator"></div> 77<table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./the-lambda-metafunction.html" class="navigation-link">Prev</a> <a href="./more-lambda-capabilities.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./the-lambda-metafunction.html" class="navigation-link">Back</a> Along</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> 78</tr></table></body> 79</html> 80