devn00b
/
EQ2EMu


			
				
					
						
						
							12345678910111213141516171819202122232425262728293031323334353637383940414243444546474849505152535455565758596061626364656667686970717273747576777879
							<?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 apply 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="./the-lambda-metafunction.html" class="navigation-link">Prev</a>&nbsp;<a href="./more-lambda-capabilities.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./the-lambda-metafunction.html" class="navigation-link">Back</a>&nbsp;Along</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>
<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>
</tr></table><div class="header-separator"></div>
<div class="section" id="the-apply-metafunction">
<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>
<p>Invoking the result of <tt class="literal"><span class="pre">lambda</span></tt> is such a common pattern
that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just
that.  Using <tt class="literal"><span class="pre">mpl::apply</span></tt>, our flexible version of <tt class="literal"><span class="pre">twice</span></tt>
becomes:</p>
<pre class="literal-block">
#include &lt;boost/mpl/apply.hpp&gt;

template &lt;class F, class X&gt;
struct twice
   : mpl::apply&lt;F, typename mpl::apply&lt;F,X&gt;::type&gt;
{};
</pre>
<!-- @ example.append(twice_test + '''
BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>));
''')
compile()
prefix.append('#include <boost/mpl/apply.hpp>') -->
<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>
template that we wrote, with two additional features:</p>
<ol class="arabic">
<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> operates only on metafunction classes, the first
argument to <tt class="literal"><span class="pre">mpl::apply</span></tt> can be any lambda expression
(including those built with placeholders).</p>
</li>
<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> accepts only one additional argument to which
the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can
invoke its first argument on any number from zero to five
additional arguments. <a class="footnote-reference" href="#arity" id="id11" name="id11">[5]</a>  For example:</p>
<pre class="literal-block">
// binary lambda expression applied to 2 additional arguments
mpl::apply&lt;
    mpl::plus&lt;_1,_2&gt;   
  , <strong>mpl::int_&lt;6&gt;</strong>
  , <strong>mpl::int_&lt;7&gt;</strong>
&gt;::type::value // == 13
</pre>
</li>
</ol>
<table class="footnote" frame="void" id="arity" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<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>
for a description of how to change the maximum number of
arguments handled by <tt class="literal"><span class="pre">mpl::apply</span></tt>.</td></tr>
</tbody>
</table>
<!-- @ prefix+=['#include <boost/mpl/plus.hpp>']
example.wrap('enum { is13 = ','''};
BOOST_STATIC_ASSERT(is13 == 13);''')
compile() -->
<div class="admonition-guideline admonition">
<p class="admonition-title first">Guideline</p>
<p>When writing a metafunction that invokes one of its arguments,
use <tt class="literal"><span class="pre">mpl::apply</span></tt> so that it works with lambda expressions.</p>
</div>
</div>

<div class="footer-separator"></div>
<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>&nbsp;<a href="./more-lambda-capabilities.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./the-lambda-metafunction.html" class="navigation-link">Back</a>&nbsp;Along</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>
</tr></table></body>
</html>