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26 <div class=
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27 <a name=
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"A note about optional<bool>">A
28 note about optional
<bool
></a>
29 </h3></div></div></div>
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.
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>.
40 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>
41 instead of a
<code class=
"computeroutput"><span class=
"identifier">tribool
</span></code> is really
45 Second, although
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><></span></code> provides a contextual conversion
46 to
<code class=
"computeroutput"><span class=
"keyword">bool
</span></code> in C++
11, this falls
47 back to an implicit conversion on older compilers. This conversion refers
48 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
49 lead to subtle errors due to the implicit
<code class=
"computeroutput"><span class=
"keyword">bool
</span></code>
52 <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>
53 <span class=
"keyword">void
</span> <span class=
"identifier">bar
</span><span class=
"special">()
</span>
54 <span class=
"special">{
</span>
55 <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 <span class=
"comment">// The following intended to pass the value of 'v' to foo():
</span>
58 <span class=
"identifier">foo
</span><span class=
"special">(
</span><span class=
"identifier">v
</span><span class=
"special">);
</span>
59 <span class=
"comment">// But instead, the initialization state is passed
</span>
60 <span class=
"comment">// due to a typo: it should have been foo(*v).
</span>
61 <span class=
"special">}
</span>
64 The only implicit conversion is to
<code class=
"computeroutput"><span class=
"keyword">bool
</span></code>,
65 and it is safe in the sense that typical integral promotions don't apply
66 (i.e. if
<code class=
"computeroutput"><span class=
"identifier">foo
</span><span class=
"special">()
</span></code>
67 takes an
<code class=
"computeroutput"><span class=
"keyword">int
</span></code> instead, it won't
71 Third, mixed comparisons with
<code class=
"computeroutput"><span class=
"keyword">bool
</span></code>
72 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 <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 <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>
77 <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>
78 <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 <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>
81 <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>
82 <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 <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>
85 <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>
86 <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>
89 In other words, for
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><></span></code>, the following assertion does not
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>
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"left"></td>
97 <td align=
"right"><div class=
"copyright-footer">Copyright
© 2003-
2007 Fernando Luis Cacciola Carballal
<br>Copyright
© 2014-
2016 Andrzej Krzemie
ński
<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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