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<title>A note about optional&lt;bool&gt;</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_optional.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional&lt;bool&gt;">A note about
    optional&lt;bool&gt;</a>
</h2></div></div></div>
<p>
      <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code> should
      be used with special caution and consideration.
    </p>
<p>
      First, it is functionally similar to a tristate boolean (false, maybe, true)
      &#8212;such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>&#8212;
      except that in a tristate boolean, the maybe state <span class="underline">represents
      a valid value</span>, unlike the corresponding state of an uninitialized
      <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>. It
      should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>
      instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
      needed.
    </p>
<p>
      Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>
      provides a contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
      in C++11, this falls back to an implicit conversion on older compilers. This
      conversion refers to the initialization state and not to the contained value.
      Using <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code> can
      lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
      conversion:
    </p>
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
<span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
<span class="special">{</span>
    <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="keyword">try</span><span class="special">();</span>

    <span class="comment">// The following intended to pass the value of 'v' to foo():</span>
    <span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
    <span class="comment">// But instead, the initialization state is passed</span>
    <span class="comment">// due to a typo: it should have been foo(*v).</span>
<span class="special">}</span>
</pre>
<p>
      The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
      and it is safe in the sense that typical integral promotions don't apply (i.e.
      if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
      takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
      compile).
    </p>
<p>
      Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
      work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
    </p>
<pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>

<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>  <span class="comment">// renders true</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>  <span class="comment">// renders false!</span>

<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>  <span class="comment">// renders false</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>  <span class="comment">// renders false</span>

<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span>   <span class="comment">// renders false</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span>  <span class="comment">// renders false</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span>   <span class="comment">// renders true</span>
</pre>
<p>
      In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>, the following assertion does not hold:
    </p>
<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
</pre>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 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>)
      </p>
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