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7 <title>The MPL Reference Manual: identity
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14 <div class=
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15 <h1><a class=
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"./miscellaneous.html#id1564">identity
</a></h1>
16 <div class=
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"id1146">
17 <h3><a class=
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18 <pre class=
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22 struct
<a href=
"./identity.html" class=
"identifier">identity
</a>
28 <div class=
"section" id=
"id1147">
29 <h3><a class=
"subsection-title" href=
"#description" name=
"description">Description
</a></h3>
30 <p>The
<a class=
"reference external" href=
"http://mathworld.wolfram.com/IdentityFunction.html" target=
"_top">identity
</a> metafunction. Returns
<tt class=
"literal"><span class=
"pre">X
</span></tt> unchanged.
</p>
32 <div class=
"section" id=
"id1149">
33 <h3><a class=
"subsection-title" href=
"#header" name=
"header">Header
</a></h3>
34 <pre class=
"literal-block">
35 #include
<<a href=
"../../../../boost/mpl/identity.hpp" class=
"header">boost/mpl/identity.hpp
</a>>
38 <div class=
"section" id=
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39 <h3><a class=
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"model-of">Model of
</a></h3>
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42 <div class=
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44 <table border=
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50 <thead valign=
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51 <tr><th class=
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</th>
52 <th class=
"head">Requirement
</th>
53 <th class=
"head">Description
</th>
57 <tr><td><tt class=
"literal"><span class=
"pre">X
</span></tt></td>
59 <td>An argument to be returned.
</td>
64 <div class=
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65 <h3><a class=
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"#expression-semantics" name=
"expression-semantics">Expression semantics
</a></h3>
66 <p>For an arbitrary type
<tt class=
"literal"><span class=
"pre">x
</span></tt>:
</p>
67 <pre class=
"literal-block">
68 typedef
<a href=
"./identity.html" class=
"identifier">identity
</a><x
>::type r;
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74 <tr class=
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</p>
77 <tr class=
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</th><td class=
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"first">Equivalent to
</p>
78 <pre class=
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83 <tr class=
"field"><th class=
"field-name">Postcondition:
</th><td class=
"field-body"><p class=
"first last"><tt class=
"literal"><span class=
"pre">is_same
<r,x
>::value
</span> <span class=
"pre">==
</span> <span class=
"pre">true
</span></tt>.
</p>
89 <div class=
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90 <h3><a class=
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</a></h3>
91 <pre class=
"literal-block">
92 typedef
<a href=
"./apply.html" class=
"identifier">apply
</a>< <a href=
"./identity.html" class=
"identifier">identity
</a><<a href=
"./placeholders.html" class=
"identifier">_1
</a>>, char
>::type t1;
93 typedef
<a href=
"./apply.html" class=
"identifier">apply
</a>< <a href=
"./identity.html" class=
"identifier">identity
</a><<a href=
"./placeholders.html" class=
"identifier">_2
</a>>, char,int
>::type t2;
95 <a href=
"./assert.html" class=
"identifier">BOOST_MPL_ASSERT
</a>(( is_same
< t1, char
> ));
96 <a href=
"./assert.html" class=
"identifier">BOOST_MPL_ASSERT
</a>(( is_same
< t2, int
> ));
99 <div class=
"section" id=
"id1154">
100 <h3><a class=
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"see-also">See also
</a></h3>
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</a>,
<a class=
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</a>,
<a class=
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"./trivial.html#label-metafunctions-trivial">Trivial Metafunctions
</a>,
<a class=
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</a>,
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</a></p>
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108 <td><div class=
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2001-
2009 Aleksey Gurtovoy and David Abrahams
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109 Distributed under the Boost Software License, Version
1.0. (See accompanying
110 file LICENSE_1_0.txt or copy at
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