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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"> | |
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| 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"> | |
| 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><1> <a href="./placeholders.html" class="identifier">_1</a>; | |
| 22 | typedef <a href="./arg.html" class="identifier">arg</a><2> <a href="./placeholders.html" class="identifier">_2</a>; | |
| 23 | <em>...</em> | |
| 24 | typedef <a href="./arg.html" class="identifier">arg</a><<em>n</em>> _<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><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 | |
| 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 <<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 | |
| 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><_<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"> | |
| 72 | typedef <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"> | |
| 82 | typedef <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; | |
| 83 | typedef <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> | |
| 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> |