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<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./sequences.html" class="navigation-link">Sequences</a> / <a href="./intrinsic-metafunctions.html" class="navigation-link">Intrinsic Metafunctions</a> / <a href="./at.html" class="navigation-link">at</a></td>
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<div class="section" id="at">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1425">at</a></h1>
<div class="section" id="id177">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    , typename N
    &gt;
struct <a href="./at.html" class="identifier">at</a>
{
    typedef <em>unspecified</em> type;
};

template&lt;
      typename AssocSeq
    , typename Key
    , typename Default = <em>unspecified</em>
    &gt;
struct <a href="./at.html" class="identifier">at</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
</div>
<div class="section" id="id178">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a></span></tt> is an <a class="reference internal" href="./terminology.html#overloaded-name">overloaded name</a>:</p>
<ul class="simple">
<li><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;Sequence,N&gt;</span></tt> returns the <tt class="literal"><span class="pre">N</span></tt>-th element from the beginning of the
<a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">Sequence</span></tt>.</li>
<li><tt class="literal"><span class="pre"><a href="./at.html" class="identifier">at</a>&lt;AssocSeq,Key,Default&gt;</span></tt> returns the first element associated with <tt class="literal"><span class="pre">Key</span></tt>
in the <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">AssocSeq</span></tt>, or <tt class="literal"><span class="pre">Default</span></tt> if no such element
exists.</li>
</ul>
</div>
<div class="section" id="id179">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/at.hpp" class="header">boost/mpl/at.hpp</a>&gt;
</pre>
</div>
<div class="section" id="id180">
<h3><a class="subsection-title" href="#model-of" name="model-of">Model of</a></h3>
<p><a class="reference internal" href="./tag-dispatched-metafunction.html">Tag Dispatched Metafunction</a></p>
</div>
<div class="section" id="id181">
<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
<table border="1" class="docutils table">
<colgroup>
<col width="17%" />
<col width="30%" />
<col width="53%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="literal"><span class="pre">Sequence</span></tt></td>
<td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td>
<td>A sequence to be examined.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">AssocSeq</span></tt></td>
<td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td>
<td>A sequence to be examined.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">N</span></tt></td>
<td><a class="reference internal" href="./integral-constant.html">Integral Constant</a></td>
<td>An offset from the beginning of the sequence
specifying the element to be retrieved.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">Key</span></tt></td>
<td>Any type</td>
<td>A key for the element to be retrieved.</td>
</tr>
<tr><td><tt class="literal"><span class="pre">Default</span></tt></td>
<td>Any type</td>
<td>A default value to return if the element is
not found.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id182">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<div class="expression-semantics compound">
<p class="compound-first">For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">n</span></tt>:</p>
<pre class="compound-middle literal-block">
typedef <a href="./at.html" class="identifier">at</a>&lt;s,n&gt;::type t;
</pre>
<table class="compound-last docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Precondition:</th><td class="field-body"><p class="first"><tt class="literal"><span class="pre">0</span> <span class="pre">&lt;=</span> <span class="pre">n::value</span> <span class="pre">&lt;</span> <span class="pre"><a href="./size.html" class="identifier">size</a>&lt;s&gt;::value</span></tt>.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
<pre class="last literal-block">
typedef <a href="./deref.html" class="identifier">deref</a>&lt; <a href="./advance.html" class="identifier">advance</a>&lt; <a href="./begin.html" class="identifier">begin</a>&lt;s&gt;::type,n &gt;::type &gt;::type t;
</pre>
</td>
</tr>
</tbody>
</table>
</div>
<div class="expression-semantics compound">
<p class="compound-first">For any <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a> <tt class="literal"><span class="pre">s</span></tt>, and arbitrary types <tt class="literal"><span class="pre">key</span></tt> and <tt class="literal"><span class="pre">x</span></tt>:</p>
<pre class="compound-middle literal-block">
typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,x&gt;::type t;
</pre>
<table class="compound-middle docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body">A type.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre"><a href="./has-key.html" class="identifier">has_key</a>&lt;s,key&gt;::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is the value type associated with <tt class="literal"><span class="pre">key</span></tt>;
otherwise <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">x</span></tt>.</td>
</tr>
</tbody>
</table>
<!-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -->
<pre class="compound-middle literal-block">
typedef <a href="./at.html" class="identifier">at</a>&lt;s,key&gt;::type t;
</pre>
<table class="compound-last docutils field-list" frame="void" rules="none">
<col class="field-name" />
<col class="field-body" />
<tbody valign="top">
<tr class="field"><th class="field-name">Return type:</th><td class="field-body"><p class="first">A type.</p>
</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body"><p class="first">Equivalent to</p>
<pre class="last literal-block">
typedef <a href="./at.html" class="identifier">at</a>&lt;s,key,<a href="./void.html" class="identifier">void_</a>&gt;::type t;
</pre>
</td>
</tr>
</tbody>
</table>
</div>
</div>
<div class="section" id="complexity">
<h3>Complexity</h3>
<table border="1" class="docutils table">
<colgroup>
<col width="47%" />
<col width="53%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Sequence archetype</th>
<th class="head">Complexity</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a></td>
<td>Linear.</td>
</tr>
<tr><td><a class="reference internal" href="./random-access-sequence.html">Random Access Sequence</a></td>
<td>Amortized constant time.</td>
</tr>
<tr><td><a class="reference internal" href="./associative-sequence.html">Associative Sequence</a></td>
<td>Amortized constant time.</td>
</tr>
</tbody>
</table>
</div>
<div class="section" id="id183">
<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
<pre class="literal-block">
typedef <a href="./range-c.html" class="identifier">range_c</a>&lt;long,10,50&gt; range;
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;0&gt; &gt;::value), ==, 10 );
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;10&gt; &gt;::value), ==, 20 );
<a href="./assert-relation.html" class="identifier">BOOST_MPL_ASSERT_RELATION</a>( (<a href="./at.html" class="identifier">at</a>&lt; range, <a href="./int.html" class="identifier">int_</a>&lt;40&gt; &gt;::value), ==, 50 );
</pre>
<pre class="literal-block">
typedef <a href="./set.html" class="identifier">set</a>&lt; int const,long*,double &gt; s;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./at.html" class="identifier">at</a>&lt;s,char&gt;::type, <a href="./void.html" class="identifier">void_</a> &gt; ));
<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt; <a href="./at.html" class="identifier">at</a>&lt;s,int&gt;::type, int > ));
</pre>
</div>
<div class="section" id="id184">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./forward-sequence.html">Forward Sequence</a>, <a class="reference internal" href="./random-access-sequence.html">Random Access Sequence</a>, <a class="reference internal" href="./associative-sequence.html">Associative Sequence</a>, <a class="reference internal" href="./at-c.html">at_c</a>, <a class="reference internal" href="./front.html">front</a>, <a class="reference internal" href="./back.html">back</a></p>
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