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< 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<bool>" > A
note about optional< bool> </ a >
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< 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,
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true) —such as < a href = "../../../../../../doc/html/tribool.html" target = "_top" > boost::tribool</ a > —
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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 >
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< td align = "right" >< div class = "copyright-footer" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal< br > Copyright © 2014-2021 Andrzej Krzemieński< p >
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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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