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10<title>The MPL Reference Manual: Placeholders</title>
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15<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.html" class="navigation-link">Composition and Argument Binding</a> / <a href="./placeholders.html" class="navigation-link">Placeholders</a></td>
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17<div class="section" id="placeholders">
18<h1><a class="toc-backref" href="./composition-and-argument.html#id455" name="placeholders">Placeholders</a></h1>
19<div class="section" id="placeholders-synopsis">
20<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
21<pre class="literal-block">
22namespace <a href="./placeholders.html" class="identifier">placeholders</a> {
23typedef <em>unspecified</em> _;
24typedef <a href="./arg.html" class="identifier">arg</a>&lt;1&gt;      _1;
25typedef <a href="./arg.html" class="identifier">arg</a>&lt;2&gt;      _2;
26<em>...</em>
27typedef <a href="./arg.html" class="identifier">arg</a>&lt;<em>n</em>&gt;      _<em>n</em>;
28}
29
30using <a href="./placeholders.html" class="identifier">placeholders</a>::_;
31using <a href="./placeholders.html" class="identifier">placeholders</a>::_1;
32using <a href="./placeholders.html" class="identifier">placeholders</a>::_2;
33<em>...</em>
34using <a href="./placeholders.html" class="identifier">placeholders</a>::_<em>n</em>;
35</pre>
36</div>
37<div class="section" id="placeholders-description">
38<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
39<p>A placeholder in a form <tt class="literal"><span class="pre">_</span></tt><em>n</em> is simply a synonym for the corresponding
40<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
41<a class="reference" href="./bind.html#bind-semantics">special meaning</a> in bind and lambda expressions, and does not have
42defined semantics outside of these contexts.</p>
43<p>Placeholder names can be made available in the user namespace through
44<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>
45</div>
46<div class="section" id="placeholders-header">
47<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
48<pre class="literal-block">
49#include &lt;<a href="../../../../boost/mpl/placeholders.hpp" class="header">boost/mpl/placeholders.hpp</a>&gt;
50</pre>
51<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
52component that is documented to accept a lambda expression makes the placeholders
53implicitly available for the user code — <em>end note</em>]</p>
54</div>
55<div class="section" id="placeholders-parameters">
56<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
57<p>None.</p>
58</div>
59<div class="section" id="placeholders-expression-semantics">
60<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
61<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
62arbitrary types <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt>:</p>
63<pre class="literal-block">
64typedef <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;
65</pre>
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67<col class="field-name" />
68<col class="field-body" />
69<tbody valign="top">
70<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
71</tr>
72<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
73<pre class="last literal-block">
74typedef <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;
75</pre>
76</td>
77</tr>
78</tbody>
79</table>
80</div>
81<div class="section" id="placeholders-example">
82<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
83<pre class="literal-block">
84typedef <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;
85typedef <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;
86
87<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t1, bool &gt; ));
88<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; t3, short > ));
89</pre>
90</div>
91<div class="section" id="placeholders-see-also">
92<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
93<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>
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