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| <div class="titlepage"><div><div><h3 class="title"> |
| <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> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 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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