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4 | <title>A note about optional<bool></title> | |
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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
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<bool>">A note about | |
28 | optional<bool></a> | |
29 | </h2></div></div></div> | |
30 | <p> | |
31 | <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></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 | —such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>— | |
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"><</span><span class="keyword">bool</span><span class="special">></span></code>. It | |
40 | 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> | |
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"><></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"><</span><span class="keyword">bool</span><span class="special">></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"><</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> | |
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"><</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> | |
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"><></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> | |
94 | </div> | |
95 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
96 | <td align="left"></td> | |
97 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<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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