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Andrzej Krzemienski e47a017009 added o.map()
2018-06-23 18:27:14 +02:00

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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_optional.tutorial.gotchas"></a><a class="link" href="gotchas.html" title="Gotchas">Gotchas</a>
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<div class="titlepage"><div><div><h4 class="title">
<a name="boost_optional.tutorial.gotchas.a_note_about_optional_bool_"></a><a class="link" href="gotchas.html#boost_optional.tutorial.gotchas.a_note_about_optional_bool_" title="A note about optional&lt;bool&gt;">A
note about optional&lt;bool&gt;</a>
</h4></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; 2014-2018 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>)
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