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<title>is_virtual_base_of</title>
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<div class="section boost_typetraits_reference_is_virtual_base_of">
<div class="titlepage"><div><div><h3 class="title">
<a name="boost_typetraits.reference.is_virtual_base_of"></a><a class="link" href="is_virtual_base_of.html" title="is_virtual_base_of">is_virtual_base_of</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">Base</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">Derived</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">is_virtual_base_of</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 Base is a virtual base class
of type Derived 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>
Types <code class="computeroutput"><span class="identifier">Base</span></code> and <code class="computeroutput"><span class="identifier">Derived</span></code> must not be incomplete types.
</p>
<p>
<span class="bold"><strong>C++ Standard Reference:</strong></span> 10.
</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_virtual_base_of</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> this trait also
requires a working <a class="link" href="is_base_of.html" title="is_base_of">is_base_of</a>
trait.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
There are a small number of cases where it's simply not possible for this
trait to work, and where attempting to instantiate the trait will cause
compiler errors (see bug report <a href="https://svn.boost.org/trac/boost/ticket/3730" target="_top">#3730</a>).
Further more the issues may well be compiler specific. In this situation
the user should supply a full specialization of the trait to work around
the problem.
</p></td></tr>
</table></div>
<p>
<span class="bold"><strong>Examples:</strong></span>
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
Given: <code class="computeroutput"> <span class="keyword">class</span> <span class="identifier">Base</span><span class="special">{};</span> <span class="keyword">class</span> <span class="identifier">Derived</span> <span class="special">:</span> <span class="keyword">public</span> <span class="keyword">virtual</span>
<span class="identifier">Base</span><span class="special">{};</span></code>
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_virtual_base_of</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">Derived</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_virtual_base_of</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">Derived</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_virtual_base_of</span><span class="special"><</span><span class="identifier">Base</span><span class="special">,</span> <span class="identifier">Derived</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_virtual_base_of</span><span class="special"><</span><span class="identifier">SomeClassType</span><span class="special">,</span>
<span class="identifier">SomeClassType</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_virtual_base_of</span><span class="special"><</span><span class="identifier">NotAClassType</span><span class="special">,</span>
<span class="identifier">NotAClassType</span><span class="special">>::</span><span class="identifier">value</span></code> is an integral constant expression
that evaluates to <span class="emphasis"><em>false</em></span>.
</p></blockquote></div>
<div class="blockquote"><blockquote class="blockquote"><p>
<code class="computeroutput"><span class="identifier">is_virtual_base_of</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">U</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>
</div>
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<td align="right"><div class="copyright-footer">Copyright © 2000, 2011 Adobe Systems Inc, David Abrahams,
Frederic Bron, Steve Cleary, Beman Dawes, Aleksey Gurtovoy, Howard Hinnant,
Jesse Jones, Mat Marcus, Itay Maman, John Maddock, Alexander Nasonov, Thorsten
Ottosen, Roman Perepelitsa, Robert Ramey, Jeremy Siek, Robert Stewart and Steven
Watanabe<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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