2016-09-17 01:25:12 +02:00
< html >
< head >
2020-10-24 17:17:10 +02:00
< meta http-equiv = "Content-Type" content = "text/html; charset=UTF-8" >
2016-09-17 01:25:12 +02:00
< title > Gotchas< / title >
< link rel = "stylesheet" href = "../../../../../../doc/src/boostbook.css" type = "text/css" >
2018-03-24 00:04:15 +01:00
< meta name = "generator" content = "DocBook XSL Stylesheets Vsnapshot" >
2016-09-17 01:25:12 +02:00
< link rel = "home" href = "../../index.html" title = "Boost.Optional" >
< link rel = "up" href = "../../optional/tutorial.html" title = "Tutorial" >
< link rel = "prev" href = "in_place_factories.html" title = "In-Place Factories" >
< link rel = "next" href = "gotchas/moved_from__optional_.html" title = "Moved-from optional" >
< / head >
< body bgcolor = "white" text = "black" link = "#0000FF" vlink = "#840084" alink = "#0000FF" >
< table cellpadding = "2" width = "100%" > < tr >
< td valign = "top" > < img alt = "Boost C++ Libraries" width = "277" height = "86" src = "../../../../../../boost.png" > < / td >
< td align = "center" > < a href = "../../../../../../index.html" > Home< / a > < / td >
< td align = "center" > < a href = "../../../../../../libs/libraries.htm" > Libraries< / a > < / td >
< td align = "center" > < a href = "http://www.boost.org/users/people.html" > People< / a > < / td >
< td align = "center" > < a href = "http://www.boost.org/users/faq.html" > FAQ< / a > < / td >
< td align = "center" > < a href = "../../../../../../more/index.htm" > More< / a > < / td >
< / tr > < / table >
< hr >
< div class = "spirit-nav" >
< a accesskey = "p" href = "in_place_factories.html" > < img src = "../../../../../../doc/src/images/prev.png" alt = "Prev" > < / a > < a accesskey = "u" href = "../../optional/tutorial.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 = "gotchas/moved_from__optional_.html" > < img src = "../../../../../../doc/src/images/next.png" alt = "Next" > < / a >
< / div >
< div class = "section" >
< 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 >
< / h3 > < / div > < / div > < / div >
< div class = "section" >
< 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<bool>" > A
note about optional< bool> < / a >
< / h4 > < / div > < / div > < / div >
< 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,
2020-10-24 17:17:10 +02:00
true) —such as < a href = "../../../../../../doc/html/tribool.html" target = "_top" > boost::tribool< / a > —
2016-09-17 01:25:12 +02:00
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 >
< / pre >
< / div >
< / div >
< table xmlns:rev = "http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width = "100%" > < tr >
< td align = "left" > < / td >
2020-12-18 11:13:47 -06:00
< td align = "right" > < div class = "copyright-footer" > Copyright © 2003-2007 Fernando Luis Cacciola Carballal< br > Copyright © 2014-2020 Andrzej Krzemieński< p >
2016-09-17 01:25:12 +02:00
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 > )
< / p >
< / div > < / td >
< / tr > < / table >
< hr >
< div class = "spirit-nav" >
< a accesskey = "p" href = "in_place_factories.html" > < img src = "../../../../../../doc/src/images/prev.png" alt = "Prev" > < / a > < a accesskey = "u" href = "../../optional/tutorial.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 = "gotchas/moved_from__optional_.html" > < img src = "../../../../../../doc/src/images/next.png" alt = "Next" > < / a >
< / div >
< / body >
< / html >
