ceph/src/boost/libs/mpl/doc/refmanual/placeholders.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 <head>
   5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
   6 <meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" />
   7 <title>The MPL Reference Manual: Placeholders</title>
   8 <link rel="stylesheet" href="../style.css" type="text/css" />
   9 </head>
  10 <body class="docframe refmanual">
  11 <table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Prev</a>&nbsp;<a href="./lambda.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./lambda.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
  12 <td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./composition-and-argument-binding.html" class="navigation-link">Composition and Argument Binding</a> / <a href="./placeholders.html" class="navigation-link">Placeholders</a></td>
  13 </tr></table><div class="header-separator"></div>
  14 <div class="section" id="placeholders">
  15 <h1><a class="toc-backref" href="./composition-and-argument-binding.html#id1529">Placeholders</a></h1>
  16 <div class="section" id="id909">
  17 <span id="placeholder"></span><h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
  18 <pre class="literal-block">
  19 namespace <a href="./placeholders.html" class="identifier">placeholders</a> {
  20 typedef <em>unspecified</em> _;
  21 typedef <a href="./arg.html" class="identifier">arg</a>&lt;1&gt;      <a href="./placeholders.html" class="identifier">_1</a>;
  22 typedef <a href="./arg.html" class="identifier">arg</a>&lt;2&gt;      <a href="./placeholders.html" class="identifier">_2</a>;
  23 <em>...</em>
  24 typedef <a href="./arg.html" class="identifier">arg</a>&lt;<em>n</em>&gt;      _<em>n</em>;
  25 }
  26
  27 using <a href="./placeholders.html" class="identifier">placeholders</a>::_;
  28 using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_1</a>;
  29 using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_2</a>;
  30 <em>...</em>
  31 using <a href="./placeholders.html" class="identifier">placeholders</a>::_<em>n</em>;
  32 </pre>
  33 </div>
  34 <div class="section" id="id910">
  35 <h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
  36 <p>A placeholder in a form <tt class="literal"><span class="pre">_</span></tt><em>n</em> is simply a synonym for the corresponding
  37 <tt class="literal"><span class="pre"><a href="./arg.html" class="identifier">arg</a>&lt;n&gt;</span></tt> specialization. The unnamed placeholder <tt class="literal"><span class="pre">_</span></tt> (underscore) carries
  38 <a class="reference internal" href="./bind.html#bind-semantics">special meaning</a> in bind and lambda expressions, and does not have
  39 defined semantics outside of these contexts.</p>
  40 <p>Placeholder names can be made available in the user namespace through
  41 <tt class="literal"><span class="pre">using</span> <span class="pre">namespace</span> <span class="pre">mpl::<a href="./placeholders.html" class="identifier">placeholders</a>;</span></tt> directive.</p>
  42 </div>
  43 <div class="section" id="id912">
  44 <h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
  45 <pre class="literal-block">
  46 #include &lt;<a href="../../../../boost/mpl/placeholders.hpp" class="header">boost/mpl/placeholders.hpp</a>&gt;
  47 </pre>
  48 <p>[<em>Note:</em> The include might be omitted when using placeholders to construct a <a class="reference internal" href="./lambda-expression.html">Lambda Expression</a> for passing it to MPL's own algorithm or metafunction: any library
  49 component that is documented to accept a lambda expression makes the placeholders
  50 implicitly available for the user code — <em>end note</em>]</p>
  51 </div>
  52 <div class="section" id="id913">
  53 <h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
  54 <p>None.</p>
  55 </div>
  56 <div class="section" id="id914">
  57 <h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
  58 <p>For any integral constant <tt class="literal"><span class="pre">n</span></tt> in the range [1, <a class="reference internal" href="./limit-metafunction-arity.html">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a>] and
  59 arbitrary types <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>:</p>
  60 <pre class="literal-block">
  61 typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em>&lt;_<em>n</em>,a1,<em>...</em>a<em>n</em>&gt;::type x;
  62 </pre>
  63 <table class="docutils field-list" frame="void" rules="none">
  64 <col class="field-name" />
  65 <col class="field-body" />
  66 <tbody valign="top">
  67 <tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
  68 </td>
  69 </tr>
  70 <tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
  71 <pre class="last literal-block">
  72 typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em>&lt; <a href="./arg.html" class="identifier">arg</a>&lt;<em>n</em>&gt;,a1,<em>...</em>a<em>n</em> &gt;::type x;
  73 </pre>
  74 </td>
  75 </tr>
  76 </tbody>
  77 </table>
  78 </div>
  79 <div class="section" id="id915">
  80 <h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
  81 <pre class="literal-block">
  82 typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>&lt; <a href="./placeholders.html" class="identifier">_1</a>,bool,char,short,int,long &gt;::type t1;
  83 typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>&lt; <a href="./placeholders.html" class="identifier">_3</a>,bool,char,short,int,long &gt;::type t3;
  84
  85 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, bool &gt; ));
  86 <a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t3, short > ));
  87 </pre>
  88 </div>
  89 <div class="section" id="id916">
  90 <h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
  91 <p><a class="reference internal" href="./composition-and-argument-binding.html">Composition and Argument Binding</a>, <a class="reference internal" href="./arg.html">arg</a>, <a class="reference internal" href="./lambda.html">lambda</a>, <a class="reference internal" href="./bind.html">bind</a>, <a class="reference internal" href="./apply.html">apply</a>, <a class="reference internal" href="./apply-wrap.html">apply_wrap</a></p>
  92 <!-- Metafunctions/Composition and Argument Binding//lambda |20 -->
  93 </div>
  94 </div>
  95
  96 <div class="footer-separator"></div>
  97 <table class="footer"><tr class="footer"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Prev</a>&nbsp;<a href="./lambda.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./lambda.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
  98 <td><div class="copyright-footer"><div class="copyright">Copyright ©  2001-2009 Aleksey Gurtovoy and David Abrahams</div>
  99 Distributed under the Boost Software License, Version 1.0. (See accompanying
 100 file LICENSE_1_0.txt or copy at <a class="reference external" href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)</div></td></tr></table></body>
 101 </html>