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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_empty"></a><a class="link" href="is_empty.html" title="is_empty"> is_empty</a>
</h3></div></div></div>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_empty</span> <span class="special">:</span> <span class="keyword">public</span> <em class="replaceable"><code><a class="link" href="integral_constant.html" title="integral_constant">true_type</a>-or-<a class="link" href="integral_constant.html" title="integral_constant">false_type</a></code></em> <span class="special">{};</span>
</pre>
<p>
<span class="bold"><strong>Inherits:</strong></span> If T is an empty class type then
inherits from <a class="link" href="integral_constant.html" title="integral_constant">true_type</a>,
otherwise inherits from <a class="link" href="integral_constant.html" title="integral_constant">false_type</a>.
</p>
<p>
<span class="bold"><strong>C++ Standard Reference:</strong></span> 10p5.
</p>
<p>
<span class="bold"><strong>Header:</strong></span> <code class="computeroutput"> <span class="preprocessor">#include</span>
<span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">is_empty</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
or <code class="computeroutput"> <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">type_traits</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
</p>
<p>
<span class="bold"><strong>Compiler Compatibility:</strong></span> In order to correctly
detect empty classes this trait relies on either:
</p>
<div class="itemizedlist"><ul type="disc">
<li>
the compiler implementing zero sized empty base classes, or
</li>
<li>
the compiler providing <a class="link" href="../intrinsics.html" title="Support for Compiler Intrinsics">intrinsics</a>
to detect empty classes.
</li>
</ul></div>
<p>
Can not be used with incomplete types.
</p>
<p>
Can not be used with union types, until is_union can be made to work.
</p>
<p>
If the compiler does not support partial-specialization of class templates,
then this template can not be used with abstract types.
</p>
<p>
<span class="bold"><strong>Examples:</strong></span>
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
Given: <code class="computeroutput"><span class="keyword">struct</span> <span class="identifier">empty_class</span>
<span class="special">{};</span></code>
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_empty</span><span class="special"><</span><span class="identifier">empty_class</span><span class="special">></span></code>
inherits from <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_empty</span><span class="special"><</span><span class="identifier">empty_class</span> <span class="keyword">const</span><span class="special">>::</span><span class="identifier">type</span></code>
is the type <code class="computeroutput"><a class="link" href="integral_constant.html" title="integral_constant">true_type</a></code>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_empty</span><span class="special"><</span><span class="identifier">empty_class</span><span class="special">>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>true</em></span>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_empty</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value_type</span></code> is the type <code class="computeroutput"><span class="keyword">bool</span></code>.
</p></blockquote></div>
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