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> <a href="./lambda.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./lambda.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </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"> 19namespace <a href="./placeholders.html" class="identifier">placeholders</a> { 20typedef <em>unspecified</em> _; 21typedef <a href="./arg.html" class="identifier">arg</a><1> <a href="./placeholders.html" class="identifier">_1</a>; 22typedef <a href="./arg.html" class="identifier">arg</a><2> <a href="./placeholders.html" class="identifier">_2</a>; 23<em>...</em> 24typedef <a href="./arg.html" class="identifier">arg</a><<em>n</em>> _<em>n</em>; 25} 26 27using <a href="./placeholders.html" class="identifier">placeholders</a>::_; 28using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_1</a>; 29using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_2</a>; 30<em>...</em> 31using <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><n></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 39defined 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 <<a href="../../../../boost/mpl/placeholders.hpp" class="header">boost/mpl/placeholders.hpp</a>> 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 49component that is documented to accept a lambda expression makes the placeholders 50implicitly 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 59arbitrary 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"> 61typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em><_<em>n</em>,a1,<em>...</em>a<em>n</em>>::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"> 72typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><em>n</em>< <a href="./arg.html" class="identifier">arg</a><<em>n</em>>,a1,<em>...</em>a<em>n</em> >::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"> 82typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>< <a href="./placeholders.html" class="identifier">_1</a>,bool,char,short,int,long >::type t1; 83typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>< <a href="./placeholders.html" class="identifier">_3</a>,bool,char,short,int,long >::type t3; 84 85<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t1, bool > )); 86<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< 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> <a href="./lambda.html" class="navigation-link">Next</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group">Back <a href="./lambda.html" class="navigation-link">Along</a></span><span class="navigation-group-separator"> | </span><span class="navigation-group"><a href="./composition-and-argument-binding.html" class="navigation-link">Up</a> <a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator"> | </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> 99Distributed under the Boost Software License, Version 1.0. (See accompanying 100file 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> 102