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53<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
54
55<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>
56
57<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>
58
59<span class="special">}</span> <span class="comment">// namespace boost</span>
60</pre>
61<p>
62 </p>
63<p>
64 Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
65 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,
66 trivially copyable class with disabled default constructor.
67 </p>
68<p>
69 Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
70 indicate an optional object that does not contain a value in initialization,
71 assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
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