3 <meta http-equiv=
"Content-Type" content=
"text/html; charset=US-ASCII">
4 <title>A note about optional
<bool
></title>
5 <link rel=
"stylesheet" href=
"../../../../../doc/src/boostbook.css" type=
"text/css">
6 <meta name=
"generator" content=
"DocBook XSL Stylesheets V1.78.1">
7 <link rel=
"home" href=
"../index.html" title=
"Chapter 1. Boost.Optional">
8 <link rel=
"up" href=
"../index.html" title=
"Chapter 1. Boost.Optional">
9 <link rel=
"prev" href=
"detailed_semantics.html" title=
"Detailed Semantics">
10 <link rel=
"next" href=
"exception_safety_guarantees.html" title=
"Exception Safety Guarantees">
12 <body bgcolor=
"white" text=
"black" link=
"#0000FF" vlink=
"#840084" alink=
"#0000FF">
13 <table cellpadding=
"2" width=
"100%"><tr>
14 <td valign=
"top"><img alt=
"Boost C++ Libraries" width=
"277" height=
"86" src=
"../../../../../boost.png"></td>
15 <td align=
"center"><a href=
"../../../../../index.html">Home
</a></td>
16 <td align=
"center"><a href=
"../../../../../libs/libraries.htm">Libraries
</a></td>
17 <td align=
"center"><a href=
"http://www.boost.org/users/people.html">People
</a></td>
18 <td align=
"center"><a href=
"http://www.boost.org/users/faq.html">FAQ
</a></td>
19 <td align=
"center"><a href=
"../../../../../more/index.htm">More
</a></td>
22 <div class=
"spirit-nav">
23 <a accesskey=
"p" href=
"detailed_semantics.html"><img src=
"../../../../../doc/src/images/prev.png" alt=
"Prev"></a><a accesskey=
"u" href=
"../index.html"><img src=
"../../../../../doc/src/images/up.png" alt=
"Up"></a><a accesskey=
"h" href=
"../index.html"><img src=
"../../../../../doc/src/images/home.png" alt=
"Home"></a><a accesskey=
"n" href=
"exception_safety_guarantees.html"><img src=
"../../../../../doc/src/images/next.png" alt=
"Next"></a>
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>
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>. 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
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>
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>
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>
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
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:
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>
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>
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>
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>
90 In other words, for
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><></span></code>, the following assertion does not hold:
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>
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 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>)
104 <div class=
"spirit-nav">
105 <a accesskey=
"p" href=
"detailed_semantics.html"><img src=
"../../../../../doc/src/images/prev.png" alt=
"Prev"></a><a accesskey=
"u" href=
"../index.html"><img src=
"../../../../../doc/src/images/up.png" alt=
"Up"></a><a accesskey=
"h" href=
"../index.html"><img src=
"../../../../../doc/src/images/home.png" alt=
"Home"></a><a accesskey=
"n" href=
"exception_safety_guarantees.html"><img src=
"../../../../../doc/src/images/next.png" alt=
"Next"></a>