ceph/src/boost/libs/mpl/doc/tutorial/the-apply-metafunction.html

   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>&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>
  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
  20 that MPL provides an <tt class="literal"><span class="pre">apply</span></tt> metafunction to do just
  21 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>
  22 becomes:</p>
  23 <pre class="literal-block">
  24 #include &lt;boost/mpl/apply.hpp&gt;
  25
  26 template &lt;class F, class X&gt;
  27 struct twice
  28    : mpl::apply&lt;F, typename mpl::apply&lt;F,X&gt;::type&gt;
  29 {};
  30 </pre>
  31 <!-- @ example.append(twice_test + '''
  32 BOOST_MPL_ASSERT((boost::is_same<twice<boost::add_pointer<_1>,int>::type,int**>));
  33 ''')
  34 compile()
  35 prefix.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>
  37 template 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
  40 argument 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
  44 the metafunction class will be applied, <tt class="literal"><span class="pre">mpl::apply</span></tt> can
  45 invoke its first argument on any number from zero to five
  46 additional 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
  49 mpl::apply&lt;
  50     mpl::plus&lt;_1,_2&gt;
  51   , <strong>mpl::int_&lt;6&gt;</strong>
  52   , <strong>mpl::int_&lt;7&gt;</strong>
  53 &gt;::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>
  61 for a description of how to change the maximum number of
  62 arguments 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>']
  66 example.wrap('enum { is13 = ','''};
  67 BOOST_STATIC_ASSERT(is13 == 13);''')
  68 compile() -->
  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,
  72 use <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>&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>
  78 </tr></table></body>
  79 </html>