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> <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>
- <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 <boost/mpl/apply.hpp>
- template <class F, class X>
- struct twice
- : mpl::apply<F, typename mpl::apply<F,X>::type>
- {};
- </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<
- mpl::plus<_1,_2>
- , <strong>mpl::int_<6></strong>
- , <strong>mpl::int_<7></strong>
- >::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> <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>
- </tr></table></body>
- </html>
|