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7 <title>The MPL Reference Manual: Placeholders
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14 <div class=
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"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>;
24 typedef
<a href=
"./arg.html" class=
"identifier">arg
</a><<em>n
</em>> _
<em>n
</em>;
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>;
31 using
<a href=
"./placeholders.html" class=
"identifier">placeholders
</a>::_
<em>n
</em>;
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>
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>>
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>
52 <div class=
"section" id=
"id913">
53 <h3><a class=
"subsection-title" href=
"#parameters" name=
"parameters">Parameters
</a></h3>
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;
63 <table class=
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"void" rules=
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64 <col class=
"field-name" />
65 <col class=
"field-body" />
67 <tr class=
"field"><th class=
"field-name">Return type:
</th><td class=
"field-body"><p class=
"first">A type.
</p>
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;
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;
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
> ));
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 -->
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98 <td><div class=
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2001-
2009 Aleksey Gurtovoy and David Abrahams
</div>
99 Distributed under the Boost Software License, Version
1.0. (See accompanying
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<a class=
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