ceph/src/boost/libs/optional/doc/html/boost_optional/a_note_about_optional_bool_.html

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  27 <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
  28     optional&lt;bool&gt;</a>
  29 </h2></div></div></div>
  30 <p>
  31       <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
  32       be used with special caution and consideration.
  33     </p>
  34 <p>
  35       First, it is functionally similar to a tristate boolean (false, maybe, true)
  36       &#8212;such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>&#8212;
  37       except that in a tristate boolean, the maybe state <span class="underline">represents
  38       a valid value</span>, unlike the corresponding state of an uninitialized
  39       <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
  40       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>
  41       instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
  42       needed.
  43     </p>
  44 <p>
  45       Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;&gt;</span></code>
  46       provides a contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
  47       in C++11, this falls back to an implicit conversion on older compilers. This
  48       conversion refers to the initialization state and not to the contained value.
  49       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
  50       lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
  51       conversion:
  52     </p>
  53 <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>
  54 <span class="keyword">void</span> <span class="identifier">bar</span><span class="special">()</span>
  55 <span class="special">{</span>
  56     <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>
  57
  58     <span class="comment">// The following intended to pass the value of 'v' to foo():</span>
  59     <span class="identifier">foo</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
  60     <span class="comment">// But instead, the initialization state is passed</span>
  61     <span class="comment">// due to a typo: it should have been foo(*v).</span>
  62 <span class="special">}</span>
  63 </pre>
  64 <p>
  65       The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
  66       and it is safe in the sense that typical integral promotions don't apply (i.e.
  67       if <code class="computeroutput"><span class="identifier">foo</span><span class="special">()</span></code>
  68       takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
  69       compile).
  70     </p>
  71 <p>
  72       Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
  73       work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
  74     </p>
  75 <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>
  76
  77 <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>
  78 <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>
  79 <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>
  80
  81 <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>
  82 <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>
  83 <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>
  84
  85 <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>
  86 <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>
  87 <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>
  88 </pre>
  89 <p>
  90       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:
  91     </p>
  92 <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>
  93 </pre>
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  97 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<p>
  98         Distributed under the Boost Software License, Version 1.0. (See accompanying
  99         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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