80 lines
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80 lines
6.0 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<!-- Copyright Aleksey Gurtovoy 2006. Distributed under the Boost -->
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<!-- Software License, Version 1.0. (See accompanying -->
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<!-- file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) -->
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<meta name="generator" content="Docutils 0.3.6: http://docutils.sourceforge.net/" />
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<title>THE BOOST MPL LIBRARY: The apply Metafunction</title>
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<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>
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<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>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="the-apply-metafunction">
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<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>
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<p>Invoking the result of <tt class="literal"><span class="pre">lambda</span></tt> is such a common pattern
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that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just
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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>
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becomes:</p>
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<pre class="literal-block">
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#include <boost/mpl/apply.hpp>
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template <class F, class X>
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struct twice
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: mpl::apply<F, typename mpl::apply<F,X>::type>
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{};
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</pre>
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<!-- @ example.append(twice_test + '''
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BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>));
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''')
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compile()
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prefix.append('#include <boost/mpl/apply.hpp>') -->
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<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>
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template that we wrote, with two additional features:</p>
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<ol class="arabic">
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<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> operates only on metafunction classes, the first
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argument to <tt class="literal"><span class="pre">mpl::apply</span></tt> can be any lambda expression
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(including those built with placeholders).</p>
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</li>
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<li><p class="first">While <tt class="literal"><span class="pre">apply1</span></tt> accepts only one additional argument to which
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the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can
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invoke its first argument on any number from zero to five
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additional arguments. <a class="footnote-reference" href="#arity" id="id11" name="id11">[5]</a> For example:</p>
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<pre class="literal-block">
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// binary lambda expression applied to 2 additional arguments
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mpl::apply<
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mpl::plus<_1,_2>
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, <strong>mpl::int_<6></strong>
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, <strong>mpl::int_<7></strong>
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>::type::value // == 13
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</pre>
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</li>
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</ol>
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<table class="footnote" frame="void" id="arity" rules="none">
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<colgroup><col class="label" /><col /></colgroup>
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<tbody valign="top">
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<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>
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for a description of how to change the maximum number of
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arguments handled by <tt class="literal"><span class="pre">mpl::apply</span></tt>.</td></tr>
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</tbody>
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</table>
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<!-- @ prefix+=['#include <boost/mpl/plus.hpp>']
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example.wrap('enum { is13 = ','''};
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BOOST_STATIC_ASSERT(is13 == 13);''')
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compile() -->
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<div class="admonition-guideline admonition">
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<p class="admonition-title first">Guideline</p>
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<p>When writing a metafunction that invokes one of its arguments,
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use <tt class="literal"><span class="pre">mpl::apply</span></tt> so that it works with lambda expressions.</p>
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</div>
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</div>
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