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- <title>A note about optional<bool></title>
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- <a name="boost_optional.tutorial.a_note_about_optional_bool_"></a><a class="link" href="a_note_about_optional_bool_.html" title="A note about optional<bool>">A
- note about optional<bool></a>
- </h3></div></div></div>
- <p>
- <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should
- be used with special caution and consideration.
- </p>
- <p>
- First, it is functionally similar to a tristate boolean (false, maybe, true)
- —such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
- 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"><</span><span class="keyword">bool</span><span class="special">></span></code>.
- It should be carefully considered if an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></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"><></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"><</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="keyword">bool</span><span class="special">></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"><</span><span class="keyword">bool</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="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"><></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>
- </div>
- <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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<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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