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60 lines
2.4 KiB
HTML
60 lines
2.4 KiB
HTML
<!doctype html public "-//ietf//dtd html//en">
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<html><head><title>boost::mpl::Reference/front</title>
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<link rel="stylesheet" href="../mpl_wiki.css">
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</head><body bgcolor="white">
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<h1><a href="../Table_of_Content.html"><img src="../mpl_logo.jpg" alt="[Home]" border=0 align="right"></a>front</h1><h3>Synopsis</h3>
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<p>
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<pre>
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template<
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typename Sequence
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>
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struct front
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{
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typedef <em>unspecified</em> type;
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};
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</pre>
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<p>
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<h3>Description</h3>
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<p>
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Returns a type identical to the first element in the sequence.
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<p>
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<h3>Definition</h3>
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<p>
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<pre>
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#include "<a href="../../../../../\boost/mpl/front.hpp">boost/mpl/front.hpp</a>"
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</pre>
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<p>
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<h3>Parameters</h3>
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<table border="1">
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<tr><th> Parameter </th><th> Requirement </th><th> Description </th></tr>
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<tr><td><code>Sequence</code></td><td>A model of <a href="../Forward_Sequence.html">Forward Sequence</a></td><td>A sequence to be examined. </td></tr>
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</table>
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<p>
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<h3>Expression semantics</h3>
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<p>
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<table border="1">
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<tr><th> Expression </th><th> Expression type </th><th> Precondition </th><th> Semantics </th><th> Postcondition </th></tr>
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<tr><td><code>typedef front<Sequence>::type t;</code></td><td>A type</td><td><code>empty<Sequence>::type::value == false</code></td><td>Equivalent to <code>typedef begin<Sequence>::type::type t;</code></td><td></td></tr>
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</table>
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<p>
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<h3>Complexity</h3>
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<p>
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Amortized constant time.
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<p>
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<h3>Example</h3>
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<p>
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<pre>
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typedef <a href="../Reference/list.html">list</a><long>::type types1;
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typedef list<int,long>::type types2;
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typedef list<char,int,long>::type types3;
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<p>
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BOOST_MPL_ASSERT_IS_SAME(front<types1>::type, long);
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BOOST_MPL_ASSERT_IS_SAME(front<types2>::type, int);
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BOOST_MPL_ASSERT_IS_SAME(front<types3>::type, char);
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</pre>
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<p>
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<h3>See also</h3>
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<p>
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<a href="../Forward_Sequence.html">Forward Sequence</a>, <code><a href="../Reference/back.html">back</a></code>, <code><a href="../Reference/at.html">at</a></code>, <code><a href="../Reference/push_front.html">push_front</a></code>, <code><a href="../Reference/begin.html">begin</a></code>, <code><a href="../Reference/empty.html">empty</a></code>
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<p><hr>
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<a href="../Table_of_Content.html">Table of Content</a> | <a href="../Reference.html">Reference</a><br>Last edited July 17, 2002 3:48 am</body></html> |