102 lines
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102 lines
8.6 KiB
HTML
<?xml version="1.0" encoding="utf-8" ?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="generator" content="Docutils 0.7: http://docutils.sourceforge.net/" />
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<title>The MPL Reference Manual: Placeholders</title>
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</head>
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<body class="docframe refmanual">
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<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>
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<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>
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</tr></table><div class="header-separator"></div>
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<div class="section" id="placeholders">
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<h1><a class="toc-backref" href="./composition-and-argument-binding.html#id1529">Placeholders</a></h1>
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<div class="section" id="id909">
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<span id="placeholder"></span><h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
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<pre class="literal-block">
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namespace <a href="./placeholders.html" class="identifier">placeholders</a> {
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typedef <em>unspecified</em> _;
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typedef <a href="./arg.html" class="identifier">arg</a><1> <a href="./placeholders.html" class="identifier">_1</a>;
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typedef <a href="./arg.html" class="identifier">arg</a><2> <a href="./placeholders.html" class="identifier">_2</a>;
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<em>...</em>
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typedef <a href="./arg.html" class="identifier">arg</a><<em>n</em>> _<em>n</em>;
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}
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using <a href="./placeholders.html" class="identifier">placeholders</a>::_;
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using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_1</a>;
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using <a href="./placeholders.html" class="identifier">placeholders</a>::<a href="./placeholders.html" class="identifier">_2</a>;
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<em>...</em>
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using <a href="./placeholders.html" class="identifier">placeholders</a>::_<em>n</em>;
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</pre>
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</div>
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<div class="section" id="id910">
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<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
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<p>A placeholder in a form <tt class="literal"><span class="pre">_</span></tt><em>n</em> is simply a synonym for the corresponding
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<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
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<a class="reference internal" href="./bind.html#bind-semantics">special meaning</a> in bind and lambda expressions, and does not have
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defined semantics outside of these contexts.</p>
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<p>Placeholder names can be made available in the user namespace through
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<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>
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</div>
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<div class="section" id="id912">
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<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
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<pre class="literal-block">
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#include <<a href="../../../../boost/mpl/placeholders.hpp" class="header">boost/mpl/placeholders.hpp</a>>
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</pre>
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<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
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component that is documented to accept a lambda expression makes the placeholders
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implicitly available for the user code — <em>end note</em>]</p>
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</div>
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<div class="section" id="id913">
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<p>None.</p>
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</div>
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<div class="section" id="id914">
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<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
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<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
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arbitrary types <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>:</p>
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<pre class="literal-block">
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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;
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</pre>
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<table class="docutils field-list" frame="void" rules="none">
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<col class="field-name" />
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<col class="field-body" />
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<tbody valign="top">
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
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</td>
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</tr>
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<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
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<pre class="last literal-block">
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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;
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</pre>
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</td>
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</tr>
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</tbody>
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</table>
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</div>
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<div class="section" id="id915">
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<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
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<pre class="literal-block">
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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;
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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;
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<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t1, bool > ));
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<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same< t3, short > ));
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</pre>
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</div>
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<div class="section" id="id916">
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<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
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<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>
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<!-- Metafunctions/Composition and Argument Binding//lambda |20 -->
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<td><div class="copyright-footer"><div class="copyright">Copyright © 2001-2009 Aleksey Gurtovoy and David Abrahams</div>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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>
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