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+<p>
+</p>
+<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
+
+<span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/};</span>
+
+<span class="keyword">const</span> <span class="identifier">none_t</span> <span class="identifier">none</span> <span class="special">(/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/);</span>
+
+<span class="special">}</span> <span class="comment">// namespace boost</span>
+</pre>
+<p>
+        </p>
+<p>
+          Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
+          serve as a tag for selecting appropriate overloads of from <code class="computeroutput"><span class="identifier">optional</span></code>'s interface. It is an empty,
+          trivially copyable class with disabled default constructor.
+        </p>
+<p>
+          Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
+          indicate an optional object that does not contain a value in initialization,
+          assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
+        </p>
+</div>
+</div>
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+<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
+<td align="left"></td>
+<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<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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