boost::none is constexpr-declared

This commit is contained in:
Andrzej Krzemienski
2020-10-24 17:17:10 +02:00
parent 2b08711ec6
commit 7e0b8145b5
45 changed files with 139 additions and 120 deletions

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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Design Overview</title>
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<code class="computeroutput"><span class="identifier">npos</span></code>, -1, etc... This is
equivalent to adding the special value to the set of possible values of
a given type. This super set of <code class="computeroutput"><span class="identifier">T</span></code>
plus some <span class="emphasis"><em>nil_t</em></span>&#8212;where <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless POD&#8212;can be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
plus some <span class="emphasis"><em>nil_t</em></span>where <code class="computeroutput"><span class="identifier">nil_t</span></code>
is some stateless PODcan be modeled in modern languages as a <span class="bold"><strong>discriminated union</strong></span> of T and nil_t. Discriminated
unions are often called <span class="emphasis"><em>variants</em></span>. A variant has a
<span class="emphasis"><em>current type</em></span>, which in our case is either <code class="computeroutput"><span class="identifier">T</span></code> or <code class="computeroutput"><span class="identifier">nil_t</span></code>.
Using the <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
@ -100,8 +100,8 @@
</p>
<p>
In any event, both the discriminated-union or the single-element container
models serve as a conceptual ground for a class representing optional&#8212;i.e.
possibly uninitialized&#8212;objects. For instance, these models show the
models serve as a conceptual ground for a class representing optionali.e.
possibly uninitializedobjects. For instance, these models show the
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
values:
</p>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>The Interface</title>
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facto</em></span> standard for handling optional objects because all you
have to do to refer to a value which you don't really have is to use a
null pointer value of the appropriate type. Pointers have been used for
decades&#8212;from the days of C APIs to modern C++ libraries&#8212;to <span class="emphasis"><em>refer</em></span>
decadesfrom the days of C APIs to modern C++ librariesto <span class="emphasis"><em>refer</em></span>
to optional (that is, possibly nonexistent) objects; particularly as optional
arguments to a function, but also quite often as optional data members.
</p>
@ -93,7 +93,7 @@
and this information is tied to the <span class="emphasis"><em>syntax</em></span> of the
expressions. That is, the presence of operators <code class="computeroutput"><span class="special">*</span></code>
and <code class="computeroutput"><span class="special">-&gt;</span></code> tell by themselves
&#8212;without any additional context&#8212; that the expression will be undefined
without any additional context that the expression will be undefined
unless the implied pointee actually exist.
</p>
<p>
@ -173,7 +173,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>The semantics</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>Exception Safety Guarantees</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>Gotchas</title>
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</p>
<p>
First, it is functionally similar to a tristate boolean (false, maybe,
true) &#8212;such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>&#8212;
true) such as <a href="../../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>
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">&lt;</span><span class="keyword">bool</span><span class="special">&gt;</span></code>.
@ -98,7 +98,7 @@
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>False positive with -Wmaybe-uninitialized</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Mixed relational comparisons</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>Moved-from optional</title>
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</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>In-Place Factories</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>IO operators</title>
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</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<html>
<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Optional references</title>
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</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Rebinding semantics for assignment of optional references</title>
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@ -133,7 +133,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>Performance considerations</title>
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@ -251,7 +251,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<head>
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<title>Relational operators</title>
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@ -99,7 +99,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>Type requirements</title>
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@ -95,7 +95,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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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<title>When to use Optional</title>
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@ -128,7 +128,7 @@
</div>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p>
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>