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7<title>The MPL Reference Manual: Placeholders</title>
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14<div class="section" id="placeholders">
15<h1><a class="toc-backref" href="./composition-and-argument.html#id455" name="placeholders">Placeholders</a></h1>
16<div class="section" id="placeholders-synopsis">
17<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>&lt;1&gt;      _1;
22typedef <a href="./arg.html" class="identifier">arg</a>&lt;2&gt;      _2;
23<em>...</em>
24typedef <a href="./arg.html" class="identifier">arg</a>&lt;<em>n</em>&gt;      _<em>n</em>;
25}
26
27using <a href="./placeholders.html" class="identifier">placeholders</a>::_;
28using <a href="./placeholders.html" class="identifier">placeholders</a>::_1;
29using <a href="./placeholders.html" class="identifier">placeholders</a>::_2;
30<em>...</em>
31using <a href="./placeholders.html" class="identifier">placeholders</a>::_<em>n</em>;
32</pre>
33</div>
34<div class="section" id="placeholders-description">
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" 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="placeholders-header">
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" 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="placeholders-parameters">
53<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
54<p>None.</p>
55</div>
56<div class="section" id="placeholders-expression-semantics">
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="refentry reference" href="./limit-metafunction-arity.html"><tt class="refentry literal"><span class="pre">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</span></tt></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>&lt;_<em>n</em>,a1,<em>...</em>a<em>n</em>&gt;::type x;
62</pre>
63<table class="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">A type.</td>
68</tr>
69<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
70<pre class="last literal-block">
71typedef <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;
72</pre>
73</td>
74</tr>
75</tbody>
76</table>
77</div>
78<div class="section" id="placeholders-example">
79<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
80<pre class="literal-block">
81typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>&lt; _1,bool,char,short,int,long &gt;::type t1;
82typedef <a href="./apply-wrap.html" class="identifier">apply_wrap</a><tt class="literal"><span class="pre">5</span></tt>&lt; _3,bool,char,short,int,long &gt;::type t3;
83
84<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, bool &gt; ));
85<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t3, short > ));
86</pre>
87</div>
88<div class="section" id="placeholders-see-also">
89<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
90<p><a class="reference" href="./composition-and-argument.html">Composition and Argument Binding</a>, <a class="refentry reference" href="./arg.html"><tt class="refentry literal"><span class="pre">arg</span></tt></a>, <a class="refentry reference" href="./lambda.html"><tt class="refentry literal"><span class="pre">lambda</span></tt></a>, <a class="refentry reference" href="./bind.html"><tt class="refentry literal"><span class="pre">bind</span></tt></a>, <a class="refentry reference" href="./apply.html"><tt class="refentry literal"><span class="pre">apply</span></tt></a>, <a class="refentry reference" href="./apply-wrap.html"><tt class="refentry literal"><span class="pre">apply_wrap</span></tt></a></p>
91<!-- modtime: November 13, 2004 18:35:13 +0000 -->
92<!-- Metafunctions/Composition and Argument Binding//lambda |20 -->
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