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<div class="section" id="empty">
<h1><a class="toc-backref" href="./intrinsic-metafunctions.html#id1430">empty</a></h1>
<div class="section" id="id220">
<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
<pre class="literal-block">
template&lt;
      typename Sequence
    &gt;
struct <a href="./empty.html" class="identifier">empty</a>
{
    typedef <em>unspecified</em> type;
};
</pre>
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<div class="section" id="id221">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>Returns an <a class="reference internal" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> such that <tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt> if
and only if the sequence is empty.</p>
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<div class="section" id="id222">
<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
<pre class="literal-block">
#include &lt;<a href="../../../../boost/mpl/empty.hpp" class="header">boost/mpl/empty.hpp</a>&gt;
</pre>
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<div class="section" id="id223">
<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>
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<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
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<tr><th class="head">Parameter</th>
<th class="head">Requirement</th>
<th class="head">Description</th>
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<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 test.</td>
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</tbody>
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<div class="section" id="id225">
<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
<p>For any <a class="reference internal" href="./forward-sequence.html">Forward Sequence</a> <tt class="literal"><span class="pre">s</span></tt>:</p>
<pre class="literal-block">
typedef <a href="./empty.html" class="identifier">empty</a>&lt;s&gt;::type c;
</pre>
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<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Boolean <a class="reference internal" href="./integral-constant.html">Integral Constant</a>.</td>
</tr>
<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">Equivalent to <tt class="literal"><span class="pre">typedef</span> <span class="pre">is_same&lt;</span> <span class="pre"><a href="./begin.html" class="identifier">begin</a>&lt;s&gt;::type,<a href="./end.html" class="identifier">end</a>&lt;s&gt;::type</span> <span class="pre">&gt;::type</span> <span class="pre">c;</span></tt>.</td>
</tr>
<tr class="field"><th class="field-name">Postcondition:</th><td class="field-body"><tt class="literal"><span class="pre"><a href="./empty.html" class="identifier">empty</a>&lt;s&gt;::value</span> <span class="pre">==</span> <span class="pre">(</span> <span class="pre"><a href="./size.html" class="identifier">size</a>&lt;s&gt;::value</span> <span class="pre">==</span> <span class="pre">0</span> <span class="pre">)</span></tt>.</td>
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<div class="section" id="id226">
<h3><a class="subsection-title" href="#complexity" name="complexity">Complexity</a></h3>
<p>Amortized constant time.</p>
</div>
<div class="section" id="id227">
<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;int,0,0&gt; empty_range;
typedef <a href="./vector.html" class="identifier">vector</a>&lt;long,float,double&gt; types;

<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>( <a href="./empty.html" class="identifier">empty</a>&lt;empty_range&gt; );
<a href="./assert-not.html" class="identifier">BOOST_MPL_ASSERT_NOT</a>( <a href="./empty.html" class="identifier">empty</a>&lt;types&gt; );
</pre>
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<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="./integral-constant.html">Integral Constant</a>, <a class="reference internal" href="./size.html">size</a>, <a class="reference internal" href="./begin.html">begin</a> / <a class="reference internal" href="./end.html">end</a></p>
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