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docs: remove rendered HTML, reduce TOCSX
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Andrzej Krzemienski
@@ -2,7 +2,7 @@
|
||||
[quickbook 1.4]
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||||
[authors [Cacciola Carballal, Fernando Luis]]
|
||||
[copyright 2003-2007 Fernando Luis Cacciola Carballal]
|
||||
[copyright 2014-2023 Andrzej Krzemieński]
|
||||
[copyright 2014-2024 Andrzej Krzemieński]
|
||||
[category miscellaneous]
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||||
[id optional]
|
||||
[dirname optional]
|
||||
@@ -47,7 +47,7 @@ Distributed under the Boost Software License, Version 1.0.
|
||||
[/ Icons ]
|
||||
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[def __SPACE__ [$images/space.png]]
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[def __GO_TO__ [$images/callouts/R.png]]
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[def __GO_TO__ [$images/R.png]]
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[section Introduction]
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||||
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||||
@@ -26,6 +26,8 @@ install images
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images/opt_align2.png
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images/opt_align3.png
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images/opt_align4.png
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images/R.png
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images/space.png
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:
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<location>html/images
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;
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@@ -38,7 +40,7 @@ boostbook standalone
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||||
<format>html:<xsl:param>"boost.libraries=../../../../libs/libraries.htm"
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<xsl:param>"chapter.autolabel=0"
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||||
<xsl:param>"chunk.section.depth=8"
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<xsl:param>"toc.section.depth=2"
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<xsl:param>"toc.section.depth=1"
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<xsl:param>"toc.max.depth=2"
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<xsl:param>"generate.section.toc.level=1"
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<format>pdf:<xsl:param>"img.src.path=$(images)/"
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@@ -1,108 +0,0 @@
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<html>
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<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>A note about optional<bool></title>
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<link rel="next" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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||||
<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
|
||||
optional<bool></a>
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||||
</h2></div></div></div>
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<p>
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> should
|
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be used with special caution and consideration.
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</p>
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<p>
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First, it is functionally similar to a tristate boolean (false, maybe, true)
|
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—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
|
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<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>
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instead of a <code class="computeroutput"><span class="identifier">tribool</span></code> is really
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needed.
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</p>
|
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<p>
|
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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>
|
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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>
|
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<span class="special">{</span>
|
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<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>
|
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|
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<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>
|
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<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>
|
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<span class="special">}</span>
|
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</pre>
|
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<p>
|
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The only implicit conversion is to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
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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
|
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compile).
|
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</p>
|
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<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>
|
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|
||||
<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>
|
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<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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</pre>
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</div>
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<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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<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>
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</body>
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</html>
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@@ -1,130 +0,0 @@
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<html>
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<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Acknowledgements</title>
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
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<a name="boost_optional.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
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</h2></div></div></div>
|
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<h4>
|
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<a name="boost_optional.acknowledgements.h0"></a>
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<span class="phrase"><a name="boost_optional.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
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<li class="listitem">
|
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Peter Dimov suggested the name 'optional', and was the first to point out
|
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the need for aligned storage.
|
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</li>
|
||||
<li class="listitem">
|
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Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
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coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
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</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
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</li>
|
||||
<li class="listitem">
|
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Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
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optional, variant and any; and made other relevant comments.
|
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</li>
|
||||
<li class="listitem">
|
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Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
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</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgements.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
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</div></td>
|
||||
</tr></table>
|
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<hr>
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<div class="spirit-nav">
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</html>
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@@ -1,125 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Acknowledgments</title>
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.acknowledgments"></a><a class="link" href="acknowledgments.html" title="Acknowledgments">Acknowledgments</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgments.pre_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point out
|
||||
the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage techniques
|
||||
and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped the
|
||||
design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov, Kevlin
|
||||
Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.acknowledgments.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.acknowledgments.post_formal_review"></a></span><a class="link" href="acknowledgments.html#boost_optional.acknowledgments.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface and
|
||||
suggested the new interface which is currently used. He also started and
|
||||
fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make it
|
||||
a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on top
|
||||
of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to optional<>:
|
||||
the Maybe type constructor (analogy originally pointed out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped with
|
||||
the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
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<dt><span class="section"><a href="dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
|
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operations in older compilers</a></span></dt>
|
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<dt><span class="section"><a href="dependencies_and_portability/optional_reference_binding.html">Optional
|
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Reference Binding</a></span></dt>
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</dl></div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.dependencies_and_portability.dependencies"></a><a class="link" href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies" title="Dependencies">Dependencies</a>
|
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</h3></div></div></div>
|
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<p>
|
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The implementation uses the following other Boost modules:
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detail
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move
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<li class="listitem">
|
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mpl
|
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<li class="listitem">
|
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static_assert
|
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<li class="listitem">
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throw_exception
|
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type_traits
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<li class="listitem">
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utility
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</li>
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</ol></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
|
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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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</p>
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<a name="boost_optional.dependencies_and_portability.emplace_operations_in_older_compilers"></a><a class="link" href="emplace_operations_in_older_compilers.html" title="Emplace operations in older compilers">Emplace
|
||||
operations in older compilers</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<a name="optional_emplace_workaround"></a>Certain constructors and functions
|
||||
in the interface of <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
perform a 'perfect forwarding' of arguments:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
On compilers that do not support variadic templates, each of these functions
|
||||
is substituted with two overloads, one forwarding a single argument, the
|
||||
other forwarding zero arguments. This forms the following set:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">&&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
On compilers that do not support rvalue references, each of these functions
|
||||
is substituted with three overloads: taking <code class="computeroutput"><span class="keyword">const</span></code>
|
||||
and non-<code class="computeroutput"><span class="keyword">const</span></code> lvalue reference,
|
||||
and third forwarding zero arguments. This forms the following set:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_t</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special">(</span><span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Arg</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">(</span><span class="identifier">Arg</span><span class="special">&</span> <span class="identifier">arg</span><span class="special">);</span>
|
||||
<span class="keyword">void</span> <span class="identifier">emplace</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
This workaround addresses about 40% of all use cases. If this is insufficient,
|
||||
you need to resort to using <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<div class="section">
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||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="optional_reference_binding.html" title="Optional Reference Binding">Optional
|
||||
Reference Binding</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
<a name="optional_reference_binding"></a>A number of compilers incorrectly
|
||||
treat const lvalues of integral type as rvalues, and create an illegal temporary
|
||||
when binding to an lvalue reference to const in some expressions. This could
|
||||
result in creating an optional lvalue reference that is in fact bound to
|
||||
an unexpected temporary rather than to the intended object. In order to prevent
|
||||
hard to find run-time bugs, this library performs compile-time checks to
|
||||
prevent expressions that would otherwise bind an optional reference to an
|
||||
unexpected temporary. As a consequence, on certain compilers certain pieces
|
||||
of functionality in optional references are missing. In order to maintain
|
||||
a portability of your code across different compilers, it is recommended that
|
||||
you only stick to the minimum portable interface of optional references:
|
||||
prefer direct-initialization and copy assignment of optional references to
|
||||
copy-initialization and assignment from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or1</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// caution: not portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// caution: not portable</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
</pre>
|
||||
<p>
|
||||
Compilers known to have these deficiencies include GCC versions 4.2, 4.3,
|
||||
4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0.
|
||||
In order to check if your compiler correctly implements reference binding
|
||||
use this test program.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
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</div></td>
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<a name="boost_optional.development"></a><a class="link" href="development.html" title="Development">Development</a>
|
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</h2></div></div></div>
|
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<div class="toc"><dl class="toc">
|
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<dt><span class="section"><a href="development.html#boost_optional.development.the_models">The models</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_semantics">The semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="development.html#boost_optional.development.the_interface">The Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_models"></a><a class="link" href="development.html#boost_optional.development.the_models" title="The models">The models</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this variable
|
||||
an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and neither
|
||||
default nor value initialization applies, it is said that the object is
|
||||
<span class="bold"><strong>uninitialized</strong></span>. Its actual value exist but
|
||||
has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11). <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> intends
|
||||
to formalize the notion of initialization (or lack of it) allowing a program
|
||||
to test whether an object has been initialized and stating that access to
|
||||
the value of an uninitialized object is undefined behavior. That is, when
|
||||
a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and no initial value is given, the variable is <span class="emphasis"><em>formally</em></span>
|
||||
uninitialized. A formally uninitialized optional object has conceptually
|
||||
no value at all and this situation can be tested at runtime. It is formally
|
||||
<span class="emphasis"><em>undefined behavior</em></span> to try to access the value of an
|
||||
uninitialized optional. An uninitialized optional can be assigned a value,
|
||||
in which case its initialization state changes to initialized. Furthermore,
|
||||
given the formal treatment of initialization states in optional objects,
|
||||
it is even possible to reset an optional to <span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means that
|
||||
objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One of
|
||||
the typical ways in which this has been historically dealt with is via a
|
||||
special value: <code class="computeroutput"><span class="identifier">EOF</span></code>, <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>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can 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> library, this model
|
||||
can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>.
|
||||
There is precedent for a discriminated union as a model for an optional value:
|
||||
the <a href="http://www.haskell.org/" target="_top">Haskell</a> <span class="bold"><strong>Maybe</strong></span>
|
||||
built-in type constructor. Thus, a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that meaning,
|
||||
it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature of
|
||||
the additional <code class="computeroutput"><span class="identifier">nil_t</span></code> with
|
||||
respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> suggests
|
||||
an alternative model: a <span class="emphasis"><em>container</em></span> that either has a
|
||||
value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</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—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the <span class="emphasis"><em>exact</em></span>
|
||||
semantics required for a wrapper of optional values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the variant
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the container
|
||||
implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to access
|
||||
the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_semantics"></a><a class="link" href="development.html#boost_optional.development.the_semantics" title="The semantics">The semantics</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
are intended to be used in places where objects of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
would but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
purpose is to formalize the additional possibly uninitialized state. From
|
||||
the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> could
|
||||
be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
Of course, we can't do that in C++, so we need to compose the desired semantics
|
||||
using a different mechanism. Doing it the other way around, that is, making
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
|
||||
<span class="emphasis"><em>subtype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is not only conceptually wrong but also impractical: it is not allowed to
|
||||
derive from a non-class type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce a
|
||||
formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain a
|
||||
new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of some
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To assign
|
||||
to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by the
|
||||
type T):</strong></span> To compare wrapped object values taking into account
|
||||
the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects. (with
|
||||
whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the wrapped
|
||||
object (if any) and leave the wrapper in the uninitialized state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.development.the_interface"></a><a class="link" href="development.html#boost_optional.development.the_interface" title="The Interface">The Interface</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface, the
|
||||
following must be noted: Even if all the operations supported by an instance
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code> are defined for
|
||||
the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise valid)
|
||||
operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
itself is merely a <code class="computeroutput"><span class="identifier">T</span></code> wrapper
|
||||
(modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will be
|
||||
totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized, a
|
||||
different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects: The
|
||||
operators * and -></a>
|
||||
</h5>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value which
|
||||
is used to indicate that the pointer is not referring to any object at all.
|
||||
In other words, null pointer values convey the notion of nonexistent objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
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—from
|
||||
the days of C APIs to modern C++ libraries—to <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>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that access
|
||||
the pointee's value possibly undefined, therefore, expressions which use
|
||||
dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span> <span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code>
|
||||
and <code class="computeroutput"><span class="special">(</span> <span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly
|
||||
convey the notion of optionality, 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">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The problem
|
||||
resides in the shallow-copy of pointer semantics: if you need to effectively
|
||||
move or copy the object, pointers alone are not enough. The problem is that
|
||||
copies of pointers do not imply copies of pointees. For example, as was discussed
|
||||
in the motivation, pointers alone cannot be used to return optional objects
|
||||
from a function because the object must move outside from the function and
|
||||
into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort to
|
||||
dynamic allocation and use a smart pointer to automatically handle the details
|
||||
of this. For example, if a function is to optionally return an object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to the
|
||||
possible undefined value because of the idiomatic aid present in the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a> concept
|
||||
incarnated by pointers.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.development.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="development.html#boost_optional.development.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h5>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
|
||||
does not have shallow-copy so does not alias: two different optionals never
|
||||
refer to the <span class="emphasis"><em>same</em></span> value unless <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself is a reference (but may have <span class="emphasis"><em>equivalent</em></span> values).
|
||||
The difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and a pointer must be kept in mind, particularly because the semantics of
|
||||
relational operators are different: since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not compare
|
||||
pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="tutorial.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="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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</body>
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</html>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<link rel="prev" href="quick_start.html" title="Quick Start">
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<a accesskey="p" href="quick_start.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="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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<div class="section">
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||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.discussion"></a><a class="link" href="discussion.html" title="Discussion">Discussion</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="quick_start.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="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<div class="section">
|
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.examples"></a><a class="link" href="examples.html" title="Examples">Examples</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_local_variables">Optional
|
||||
local variables</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.optional_data_members">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction">Bypassing
|
||||
expensive unnecessary default construction</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_return_values"></a><a class="link" href="examples.html#boost_optional.examples.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>(</span><span class="identifier">queue</span><span class="special">.</span><span class="identifier">top</span><span class="special">());</span>
|
||||
<span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">>();</span> <span class="comment">// uninitialized</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">receive_async_message</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="identifier">rcv</span> <span class="special">;</span>
|
||||
<span class="comment">// The safe boolean conversion from 'rcv' is used here.</span>
|
||||
<span class="keyword">while</span> <span class="special">(</span> <span class="special">(</span><span class="identifier">rcv</span> <span class="special">=</span> <span class="identifier">get_async_input</span><span class="special">())</span> <span class="special">&&</span> <span class="special">!</span><span class="identifier">timeout</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="identifier">output</span><span class="special">(*</span><span class="identifier">rcv</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_local_variables"></a><a class="link" href="examples.html#boost_optional.examples.optional_local_variables" title="Optional local variables">Optional
|
||||
local variables</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">open</span><span class="special">()</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">database</span><span class="special">.</span><span class="identifier">lookup</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">can_ask_user</span> <span class="special">)</span>
|
||||
<span class="identifier">name</span> <span class="special">=</span> <span class="identifier">user</span><span class="special">.</span><span class="identifier">ask</span><span class="special">(</span><span class="identifier">employer_name</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">name</span> <span class="special">)</span>
|
||||
<span class="identifier">print</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">print</span><span class="special">(</span><span class="string">"employer's name not found!"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.optional_data_members"></a><a class="link" href="examples.html#boost_optional.examples.optional_data_members" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">figure</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">figure</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// data member 'm_clipping_rect' is uninitialized at this point.</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">clip_in_rect</span> <span class="special">(</span> <span class="identifier">rect</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rect</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="special">....</span>
|
||||
<span class="identifier">m_clipping_rect</span> <span class="special">=</span> <span class="identifier">rect</span> <span class="special">;</span> <span class="comment">// initialized here.</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">draw</span> <span class="special">(</span> <span class="identifier">canvas</span><span class="special">&</span> <span class="identifier">cvs</span> <span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">m_clipping_rect</span> <span class="special">)</span>
|
||||
<span class="identifier">do_clipping</span><span class="special">(*</span><span class="identifier">m_clipping_rect</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">cvs</span><span class="special">.</span><span class="identifier">drawXXX</span><span class="special">(..);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="comment">// this can return NULL.</span>
|
||||
<span class="identifier">rect</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_clipping_rect</span><span class="special">()</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">m_clipping_rect</span><span class="special">);</span> <span class="special">}</span>
|
||||
|
||||
<span class="keyword">private</span> <span class="special">:</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">rect</span><span class="special">></span> <span class="identifier">m_clipping_rect</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.examples.bypassing_expensive_unnecessary_default_construction"></a><a class="link" href="examples.html#boost_optional.examples.bypassing_expensive_unnecessary_default_construction" title="Bypassing expensive unnecessary default construction">Bypassing
|
||||
expensive unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">ExpensiveCtor</span> <span class="special">{</span> <span class="special">...</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">class</span> <span class="identifier">Fred</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">Fred</span><span class="special">()</span> <span class="special">:</span> <span class="identifier">mLargeVector</span><span class="special">(</span><span class="number">10000</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">ExpensiveCtor</span><span class="special">></span> <span class="special">></span> <span class="identifier">mLargeVector</span> <span class="special">;</span>
|
||||
<span class="special">}</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>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
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|
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<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
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<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is initialized
|
||||
or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s constructor
|
||||
and the assignment provide strong exception safety guarantee, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s assignment
|
||||
also provides strong exception safety guarantee; otherwise we only get the
|
||||
basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment never throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or assignment
|
||||
throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained value
|
||||
(if any) is destroyed. It doesn't call any assignment or move/copy constructor
|
||||
on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="boost_optional.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.exception_safety_guarantees.swap">Swap</a>
|
||||
</h4>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor (depending
|
||||
on the initialization state of the optional objects). Thus, if both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor offer
|
||||
strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code> on
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to achieve stronger exception safety guarantees.) In this case
|
||||
the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="a_note_about_optional_bool_.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="type_requirements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.implementation_notes"></a><a class="link" href="implementation_notes.html" title="Implementation Notes">Implementation Notes</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
currently implemented using a custom aligned storage facility built from <code class="computeroutput"><span class="identifier">alignment_of</span></code> and <code class="computeroutput"><span class="identifier">type_with_alignment</span></code>
|
||||
(both from Type Traits). It uses a separate boolean flag to indicate the initialization
|
||||
state. Placement new with <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy/move constructor and <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor are explicitly used to initialize, copy, move and destroy optional
|
||||
values. As a result, <code class="computeroutput"><span class="identifier">T</span></code>'s default
|
||||
constructor is effectively by-passed, but the exception guarantees are basic.
|
||||
It is planned to replace the current implementation with another with stronger
|
||||
exception safety, such as a future <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span></code>.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
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|
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<div class="spirit-nav">
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<a accesskey="p" href="type_requirements.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="dependencies_and_portability.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>In-Place Factories</title>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place Factories</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
One of the typical problems with wrappers and containers is that their interfaces
|
||||
usually provide an operation to initialize or assign the contained object as
|
||||
a copy of some other object. This not only requires the underlying type to
|
||||
be <a href="../../../../utility/CopyConstructible.html" target="_top">Copy Constructible</a>,
|
||||
but also requires the existence of a fully constructed object, often temporary,
|
||||
just to follow the copy from:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A solution to this problem is to support direct construction of the contained
|
||||
object right in the container's storage. In this scheme, the user only needs
|
||||
to supply the arguments to the constructor to use in the wrapped object construction.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A limitation of this method is that it doesn't scale well to wrapped objects
|
||||
with multiple constructors nor to generic code were the constructor overloads
|
||||
are unknown.
|
||||
</p>
|
||||
<p>
|
||||
The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
|
||||
and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
|
||||
are a family of classes which encapsulate an increasing number of arbitrary
|
||||
constructor parameters and supply a method to construct an object of a given
|
||||
type using those parameters at an address specified by the user via placement
|
||||
new.
|
||||
</p>
|
||||
<p>
|
||||
For example, one member of this family looks like:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
A wrapper class aware of this can use it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a template
|
||||
<code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code> member
|
||||
function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
allowed.
|
||||
</p>
|
||||
<p>
|
||||
This library provides an overloaded set of helper template functions to construct
|
||||
these factories without requiring unnecessary template parameters:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
||||
and <a href="../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<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="a_note_about_optional_bool_.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
</body>
|
||||
</html>
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||||
@@ -1,128 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Motivation</title>
|
||||
<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
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||||
<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
|
||||
<link rel="prev" href="../index.html" title="Chapter 1. Boost.Optional">
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<link rel="next" href="development.html" title="Development">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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||||
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td>
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<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>
|
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<hr>
|
||||
<div class="spirit-nav">
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||||
<a accesskey="p" href="../index.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="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not have
|
||||
a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or uses
|
||||
a runtime check and throws an exception if the postcondition is violated. This
|
||||
is a reasonable choice for example, for function (A), because the lack of a
|
||||
proper return value is directly related to an invalid parameter (out of domain
|
||||
argument), so it is appropriate to require the callee to supply only parameters
|
||||
in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail just
|
||||
because it can't find a value to return; so it is incorrect to consider such
|
||||
a situation an error and assert or throw an exception. This function must return,
|
||||
and somehow, must tell the callee that it is not returning a meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error to
|
||||
ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside itself,
|
||||
but in many applications, it is just impractical for performance reasons to
|
||||
treat this as an error (because detecting that the polygon has no area might
|
||||
be too expensive to be required to be tested previously), and either an arbitrary
|
||||
point (typically at infinity) is returned, or some efficient way to tell the
|
||||
callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved to
|
||||
communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>, points
|
||||
at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when such
|
||||
values do not exist. In these cases, the usual alternative is either to use
|
||||
a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code> in place
|
||||
of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound type, such
|
||||
as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the result
|
||||
which indicates if the result is meaningful, has the advantage that can be
|
||||
turned into a consistent idiom since the first element of the pair can be whatever
|
||||
the function would conceptually return. For example, the last two functions
|
||||
could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error prone
|
||||
since the user can easily use the function result (first element of the pair)
|
||||
without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="../index.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="development.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional references</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span>
|
||||
<span class="keyword">const</span><span class="special">&</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
However, since references are not real objects some restrictions apply and
|
||||
some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
treats it wrapped pseudo-object much as a real value, a true real reference
|
||||
is stored so aliasing will occur:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
will copy the references but all these references will nonetheless refer
|
||||
to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object rather
|
||||
than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. For more details see <a class="link" href="dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h4>
|
||||
<a name="boost_optional.optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.optional_references.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++ to
|
||||
extend the life time of a temporary they bind to. Optional references do not
|
||||
have this capability, therefore to avoid surprising effects it is not possible
|
||||
to initialize an optional references from a temporary. Optional rvalue references
|
||||
are disabled altogether. Also, the initialization and assignment of an optional
|
||||
reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
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</div></td>
|
||||
</tr></table>
|
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|
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<div class="spirit-nav">
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@@ -1,169 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.quick_start"></a><a class="link" href="quick_start.html" title="Quick Start">Quick Start</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/optional_data_members.html">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="quick_start.html#boost_optional.quick_start.optional_return_values" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exists no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a nonexistent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Finally, you can provide a callback to be called when trying to access the
|
||||
contained value fails:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">fallback_to_default</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">cerr</span> <span class="special"><<</span> <span class="string">"could not convert; using -1 instead"</span> <span class="special"><<</span> <span class="identifier">endl</span><span class="special">;</span>
|
||||
<span class="keyword">return</span> <span class="special">-</span><span class="number">1</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">l</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">fallback_to_default</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This will call the provided callback and return whatever the callback returns.
|
||||
The callback can have side effects: they will only be observed when the optional
|
||||
object does not contain a value.
|
||||
</p>
|
||||
<p>
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../index.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="quick_start/optional_automatic_variables.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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<a name="boost_optional.quick_start.bypassing_unnecessary_default_construction"></a><a class="link" href="bypassing_unnecessary_default_construction.html" title="Bypassing unnecessary default construction">Bypassing
|
||||
unnecessary default construction</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we have class <code class="computeroutput"><span class="identifier">Date</span></code>,
|
||||
which does not have a default constructor: there is no good candidate for
|
||||
a default date. We have a function that returns two dates in form of a <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">Date</span><span class="special">,</span> <span class="identifier">Date</span><span class="special">></span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In other place we want to use the result of <code class="computeroutput"><span class="identifier">getPeriod</span></code>,
|
||||
but want the two dates to be named: <code class="computeroutput"><span class="identifier">begin</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">end</span></code>. We want to implement
|
||||
something like 'multiple return values':
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">Date</span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span> <span class="comment">// Error: no default ctor!</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
The second line works already, this is the capability of <a href="../../../../../tuple/index.html" target="_top">Boost.Tuple</a>
|
||||
library, but the first line won't work. We could set some invented initial
|
||||
dates, but it is confusing and may be an unacceptable cost, given that these
|
||||
values will be overwritten in the next line anyway. This is where <code class="computeroutput"><span class="identifier">optional</span></code> can help:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Date</span><span class="special">></span> <span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">)</span> <span class="special">=</span> <span class="identifier">getPeriod</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
It works because inside <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">tie</span></code> a
|
||||
move-assignment from <code class="computeroutput"><span class="identifier">T</span></code> is
|
||||
invoked on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
which internally calls a move-constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="optional_data_members.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="storage_in_containers.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional
|
||||
automatic variables</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
We could write function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
in a slightly different manner, so that it has a single <code class="computeroutput"><span class="keyword">return</span></code>-statement:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="identifier">ans</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The default constructor of <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
creates an uninitialized optional object. Unlike with <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
you cannot have an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>
|
||||
in an indeterminate state. Its state is always well defined. Instruction
|
||||
<code class="computeroutput"><span class="identifier">ans</span> <span class="special">=</span>
|
||||
<span class="identifier">i</span></code> initializes the optional object.
|
||||
It uses the 'mixed' assignment from <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
In general, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>,
|
||||
when an assignment from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is invoked, it can do two things. If the optional object is not initialized
|
||||
(our case here), it initializes the contained value using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy constructor. If the optional object is already initialized, it assigns
|
||||
the new value to it using <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
copy assignment.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
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</div></td>
|
||||
</tr></table>
|
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<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="optional_data_members.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,94 +0,0 @@
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_data_members"></a><a class="link" href="optional_data_members.html" title="Optional data members">Optional
|
||||
data members</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization.
|
||||
This is because we do not want to perform an expensive initialization of
|
||||
our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at
|
||||
all) it is really used. We can do it this way:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">></span> <span class="identifier">resource_</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">// not thread-safe</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span>
|
||||
<span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor
|
||||
creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s
|
||||
default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>. In function
|
||||
<code class="computeroutput"><span class="identifier">getResource</span></code> we first check
|
||||
if <code class="computeroutput"><span class="identifier">resource_</span></code> is initialized.
|
||||
This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>,
|
||||
but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>.
|
||||
These two ways are equivalent. Function <code class="computeroutput"><span class="identifier">emplace</span></code>
|
||||
initializes the optional in-place by perfect-forwarding the arguments to
|
||||
the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>.
|
||||
No copy- or move-construction is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code>
|
||||
doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</p>
|
||||
<div class="note"><table border="0" summary="Note">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td>
|
||||
<th align="left">Note</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only
|
||||
available on compilers that support rvalue references and variadic templates.
|
||||
If your compiler does not support these features and you still need to
|
||||
avoid any move-constructions, use <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<a accesskey="p" href="optional_automatic_variables.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="bypassing_unnecessary_default_construction.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.optional_return_values"></a><a class="link" href="optional_return_values.html" title="Optional return values">Optional
|
||||
return values</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Let's write and use a converter function that converts an a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. It is possible that
|
||||
for a given string (e.g. <code class="computeroutput"><span class="string">"cat"</span></code>)
|
||||
there exist no value of type <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
capable of representing the conversion result. We do not consider such situation
|
||||
an error. We expect that the converter can be used only to check if the conversion
|
||||
is possible. A natural signature for this function can be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
All necessary functionality can be included with one header <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
The above function signature means that the function can either return a
|
||||
value of type <code class="computeroutput"><span class="keyword">int</span></code> or a flag
|
||||
indicating that no value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
is available. This does not indicate an error. It is like one additional
|
||||
value of <code class="computeroutput"><span class="keyword">int</span></code>. This is how we
|
||||
can use our function:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span> <span class="special">=</span> <span class="comment">/*... */</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span> <span class="comment">// move-construct</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oi</span><span class="special">)</span> <span class="comment">// contextual conversion to bool</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span> <span class="comment">// operator*</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to test if <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
contains a value, we use the contextual conversion to type <code class="computeroutput"><span class="keyword">bool</span></code>. Because of this we can combine the initialization
|
||||
of the optional object and the test into one instruction:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">))</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">oi</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
We extract the contained value with <code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code> (and with <code class="computeroutput"><span class="keyword">operator</span><span class="special">-></span></code> where it makes sense). An attempt to
|
||||
extract the contained value of an uninitialized optional object is an <span class="emphasis"><em>undefined
|
||||
behaviour</em></span> (UB). This implementation guards the call with <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span></code>. Therefore you should be sure
|
||||
that the contained value is there before extracting. For instance, the following
|
||||
code is reasonably UB-safe:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">convert</span><span class="special">(</span><span class="string">"100"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is because we know that string value <code class="computeroutput"><span class="string">"100"</span></code>
|
||||
converts to a valid value of <code class="computeroutput"><span class="keyword">int</span></code>.
|
||||
If you do not like this potential UB, you can use an alternative way of extracting
|
||||
the contained value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">try</span> <span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span> <span class="special">(</span><span class="keyword">const</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
|
||||
<span class="comment">// deal with it</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This version throws an exception upon an attempt to access a nonexistent
|
||||
contained value. If your way of dealing with the missing value is to use
|
||||
some default, like <code class="computeroutput"><span class="number">0</span></code>, there exists
|
||||
a yet another alternative:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">k</span> <span class="special">=</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">text</span><span class="special">).</span><span class="identifier">value_or</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This uses the <code class="computeroutput"><span class="identifier">atoi</span></code>-like approach
|
||||
to conversions: if <code class="computeroutput"><span class="identifier">text</span></code> does
|
||||
not represent an integral number just return <code class="computeroutput"><span class="number">0</span></code>.
|
||||
Now, let's consider how function <code class="computeroutput"><span class="identifier">convert</span></code>
|
||||
can be implemented.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&</span> <span class="identifier">text</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">(</span><span class="identifier">text</span><span class="special">);</span>
|
||||
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">if</span> <span class="special">((</span><span class="identifier">s</span> <span class="special">>></span> <span class="identifier">i</span><span class="special">)</span> <span class="special">&&</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">get</span><span class="special">()</span> <span class="special">==</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">char_traits</span><span class="special"><</span><span class="keyword">char</span><span class="special">>::</span><span class="identifier">eof</span><span class="special">())</span>
|
||||
<span class="keyword">return</span> <span class="identifier">i</span><span class="special">;</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="keyword">return</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Observe the two return statements. <code class="computeroutput"><span class="keyword">return</span>
|
||||
<span class="identifier">i</span></code> uses the converting constructor
|
||||
that can create <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
from <code class="computeroutput"><span class="identifier">T</span></code>. Thus constructed
|
||||
optional object is initialized and its value is a copy of <code class="computeroutput"><span class="identifier">i</span></code>.
|
||||
The other return statement uses another converting constructor from a special
|
||||
tag <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. It is used to indicate that we want
|
||||
to create an uninitialized optional object.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<a accesskey="p" href="../quick_start.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="optional_automatic_variables.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
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<hr>
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<a accesskey="p" href="bypassing_unnecessary_default_construction.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../quick_start.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="../../optional/tutorial.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.quick_start.storage_in_containers"></a><a class="link" href="storage_in_containers.html" title="Storage in containers">Storage
|
||||
in containers</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Suppose you want to ask users to choose some number (an <code class="computeroutput"><span class="keyword">int</span></code>).
|
||||
One of the valid responses is to choose nothing, which is represented by
|
||||
an uninitialized <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>.
|
||||
You want to make a histogram showing how many times each choice was made.
|
||||
You can use an <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">map</span><span class="special"><</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">choices</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">for</span> <span class="special">(</span><span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span> <span class="identifier">i</span> <span class="special"><</span> <span class="identifier">LIMIT</span><span class="special">;</span> <span class="special">++</span><span class="identifier">i</span><span class="special">)</span> <span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">readChoice</span><span class="special">();</span>
|
||||
<span class="special">++</span><span class="identifier">choices</span><span class="special">[</span><span class="identifier">choice</span><span class="special">];</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This works because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. In this case
|
||||
the state of being uninitialized is treated as a yet another value of <code class="computeroutput"><span class="identifier">T</span></code>, which is compared less than any value
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
the effect is to bind (for the first time) to the object. Clearly, there is
|
||||
no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the referenced
|
||||
object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h4>
|
||||
<a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h4>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has been
|
||||
chosen to provide <span class="bold"><strong>consistency among initialization states</strong></span>
|
||||
even at the expense of lack of consistency with the semantics of bare C++ references.
|
||||
It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives
|
||||
to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already <span class="emphasis"><em>initialized</em></span>?
|
||||
it would change the value of the referenced object (whatever that is); which
|
||||
is inconsistent with the other possible case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>
|
||||
does, you would never be able to use Optional's assignment without explicitly
|
||||
handling the previous initialization state unless your code is capable of functioning
|
||||
whether after the assignment, <code class="computeroutput"><span class="identifier">a</span></code>
|
||||
aliases the same object as <code class="computeroutput"><span class="identifier">b</span></code>
|
||||
or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is very
|
||||
likely that binding for the first time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a precondition
|
||||
that the lvalue must be already initialized. If it isn't, then binding for
|
||||
the first time is OK while rebinding is not which is IMO very unlikely. In
|
||||
such a scenario, you can assign the value itself directly, as in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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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</p>
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<div class="section">
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||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.acknowledgements"></a><a class="link" href="acknowledgements.html" title="Acknowledgements">Acknowledgements</a>
|
||||
</h3></div></div></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.acknowledgements.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.acknowledgements.pre_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.reference.acknowledgements.pre_formal_review">Pre-formal
|
||||
review</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Peter Dimov suggested the name 'optional', and was the first to point
|
||||
out the need for aligned storage.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor developed 'type_with_alignment', and later Eric Friedman
|
||||
coded 'aligned_storage', which are the core of the optional class implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Andrei Alexandrescu and Brian Parker also worked with aligned storage
|
||||
techniques and their work influenced the current implementation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Gennadiy Rozental made extensive and important comments which shaped
|
||||
the design.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vesa Karvonen and Douglas Gregor made quite useful comparisons between
|
||||
optional, variant and any; and made other relevant comments.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Douglas Gregor and Peter Dimov commented on comparisons and evaluation
|
||||
in boolean contexts.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Eric Friedman helped understand the issues involved with aligned storage,
|
||||
move/copy operations and exception safety.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Many others have participated with useful comments: Aleksey Gurotov,
|
||||
Kevlin Henney, David Abrahams, and others I can't recall.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.acknowledgements.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.acknowledgements.post_formal_review"></a></span><a class="link" href="acknowledgements.html#boost_optional.reference.acknowledgements.post_formal_review">Post-formal
|
||||
review</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
William Kempf carefully considered the originally proposed interface
|
||||
and suggested the new interface which is currently used. He also started
|
||||
and fueled the discussion about the analogy optional<>/smart pointer
|
||||
and about relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Peter Dimov, Joel de Guzman, David Abrahams, Tanton Gibbs and Ian Hanson
|
||||
focused on the relational semantics of optional (originally undefined);
|
||||
concluding with the fact that the pointer-like interface doesn't make
|
||||
it a pointer so it shall have deep relational operators.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Augustus Saunders also explored the different relational semantics between
|
||||
optional<> and a pointer and developed the OptionalPointee concept
|
||||
as an aid against potential conflicts on generic code.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman noticed that optional<> can be seen as an API on
|
||||
top of variant<T,nil_t>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Dave Gomboc explained the meaning and usage of the Haskell analog to
|
||||
optional<>: the Maybe type constructor (analogy originally pointed
|
||||
out by David Sankel).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Other comments were posted by Vincent Finn, Anthony Williams, Ed Brey,
|
||||
Rob Stewart, and others.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Joel de Guzman made the case for the support of references and helped
|
||||
with the proper semantics.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Mat Marcus shown the virtues of a value-oriented interface, influencing
|
||||
the current design, and contributed the idea of "none".
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Vladimir Batov's design of Boost.Convert library motivated the development
|
||||
of value accessors for <code class="computeroutput"><span class="identifier">optional</span></code>:
|
||||
functions <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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Reference Binding</a>
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<p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
operations on optional references might give adverse results: rather than
|
||||
binding a reference to a designated object they may create an unexpected
|
||||
temporary and bind to it. Compilers known to have these deficiencies include
|
||||
GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0,
|
||||
11.0, 12.0. On these compilers prefer using direct-initialization and copy
|
||||
assignment of optional references to copy-initialization and assignment
|
||||
from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or1</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&>(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
|
||||
</pre>
|
||||
<p>
|
||||
In order to check if your compiler correctly implements reference binding
|
||||
use this test program.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
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</div>
|
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<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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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||||
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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|
||||
</h4></div></div></div>
|
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<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">bad_optional_access</span> <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">logic_error</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
<span class="identifier">bad_optional_access</span><span class="special">();</span> <a class="link" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html#reference_bad_optional_access_constructor"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
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</div>
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<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 © 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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</p>
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||||
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">optional</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">swap</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="special">);</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">struct</span> <span class="identifier">optional_swap_should_use_default_constructor</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
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</p>
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|
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This header only contains declarations.
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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
|
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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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<a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
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<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
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|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
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|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="number">1</span><span class="special">+</span><span class="number">1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Creates an optional<int></span>
|
||||
</pre>
|
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</li>
|
||||
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|
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<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
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</p>
|
||||
<a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">condition</span><span class="special">,</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">calculate_foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
|
||||
<span class="keyword">return</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="identifier">is_not_nan_and_finite</span><span class="special">(</span><span class="identifier">val</span><span class="special">),</span><span class="identifier">val</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
|
||||
<span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
shall meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
|
||||
<code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
|
||||
<span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
|
||||
If only <code class="computeroutput"><span class="identifier">x</span></code> or <code class="computeroutput"><span class="identifier">y</span></code> is initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
|
||||
If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is compared unequal to any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equal
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> directly in generic code which expect
|
||||
to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
instead
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span> <span class="identifier">o1_</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o2</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span> <span class="special">);</span> <span class="comment">// Identity implies equality</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN_</span> <span class="special">);</span> <span class="comment">// Both uninitialized compare equal</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">// Initialized unequal to initialized.</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1_</span> <span class="special">);</span> <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">o2</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Expression <code class="computeroutput"><span class="special">*</span><span class="identifier">x</span> <span class="special"><</span> <span class="special">*</span><span class="identifier">y</span></code> shall be well-formed and its result
|
||||
shall be convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">(!</span><span class="identifier">y</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">false</span> <span class="special">:</span> <span class="special">(!</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">true</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="special">*</span><span class="identifier">y</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is ordered as less than any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equivalent
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> directly in generic code which
|
||||
expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead. <code class="computeroutput"><span class="identifier">T</span></code> need not
|
||||
be <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. Only
|
||||
single <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
is required. Other relational operations are defined in terms of this
|
||||
one. If <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
satisfies the axioms of <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> (transitivity,
|
||||
antisymmetry and irreflexivity), <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o0</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Identity implies equivalence</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN_</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Two uninitialized are equivalent</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN_</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">o0</span> <span class="special">);</span> <span class="comment">// Uninitialized is less than initialized</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o0</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o2</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Two initialized compare as (*lhs < *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o2</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">></span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">>=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
need not meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Lvalues of type <code class="computeroutput"><span class="identifier">T</span></code> shall be swappable and <code class="computeroutput"><span class="identifier">T</span></code> shall be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<p class="simpara">
|
||||
<span class="bold"><strong>Effects:</strong></span>
|
||||
</p>
|
||||
<div class="informaltable"><table class="table">
|
||||
<colgroup>
|
||||
<col>
|
||||
<col>
|
||||
<col>
|
||||
</colgroup>
|
||||
<thead><tr></tr></thead>
|
||||
<tbody>
|
||||
<tr>
|
||||
<td>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">),</span> <span class="special">*</span><span class="identifier">rhs</span><span class="special">)</span></code>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> as if direct-initializing
|
||||
an object of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
with the expression <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>, followed by <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>,
|
||||
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
contains a value and <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the expression
|
||||
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">))</span></code>,
|
||||
followed by <code class="computeroutput"><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value
|
||||
and <code class="computeroutput"><span class="identifier">rhs</span></code> contains
|
||||
a value
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
no effect
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
</tbody>
|
||||
</table></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> The states of <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
interchanged.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
|
||||
<code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code>
|
||||
throws. If only one is initialized, whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="special">)</span></code>
|
||||
throws.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def0</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def1</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">def1</span><span class="special">);</span> <span class="comment">// no-op</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span> <span class="comment">// Get back to original values</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span><span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optY</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_reference"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
refers to what <code class="computeroutput"><span class="identifier">y</span></code> referred
|
||||
to before the swap (if anything). <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
refers to whatever <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
referred to before the swap.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optX</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optY</span> <span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span> <span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optX</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optY</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">opt0</span><span class="special">,</span> <span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">opt0</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
</pre>
|
||||
[endsect]
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 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>
|
||||
</div></td>
|
||||
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|
||||
<hr>
|
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<div class="spirit-nav">
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<div class="section">
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||||
<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics____optional_references"></a><a class="link" href="detailed_semantics____optional_references.html" title="Detailed Semantics -- Optional References">Detailed
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Semantics -- Optional References</a>
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</h4></div></div></div>
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<p>
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||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_default_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><&>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">none_t</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> refers to nothing.
|
||||
</li></ul></div>
|
||||
</div>
|
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<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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 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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</p>
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</div></td>
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<div class="titlepage"><div><div><h4 class="title">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___free_functions"></a><a class="link" href="detailed_semantics___free_functions.html" title="Detailed Semantics - Free Functions">Detailed
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Semantics - Free Functions</a>
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</h4></div></div></div>
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<p>
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<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
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</p>
|
||||
<a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="number">1</span><span class="special">+</span><span class="number">1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Creates an optional<int></span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_rvalue"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span>
|
||||
<span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="identifier">T</span> <span class="special">&&</span>
|
||||
<span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">v</span><span class="special">))</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">condition</span><span class="special">,</span> <span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">calculate_foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
|
||||
<span class="keyword">return</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="identifier">is_not_nan_and_finite</span><span class="special">(</span><span class="identifier">val</span><span class="special">),</span><span class="identifier">val</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
|
||||
<span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_make_optional_bool_rvalue"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span>
|
||||
<span class="identifier">make_optional</span><span class="special">(</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span>
|
||||
<span class="special">)</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>>(</span><span class="identifier">condition</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">v</span><span class="special">))</span></code> for the <span class="emphasis"><em>deduced</em></span>
|
||||
type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
shall meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
|
||||
<code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
|
||||
<span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
|
||||
If only <code class="computeroutput"><span class="identifier">x</span></code> or <code class="computeroutput"><span class="identifier">y</span></code> is initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
|
||||
If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is compared unequal to any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equal
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> directly in generic code which expect
|
||||
to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
instead
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span> <span class="identifier">o1_</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o2</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span> <span class="special">);</span> <span class="comment">// Identity implies equality</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN_</span> <span class="special">);</span> <span class="comment">// Both uninitialized compare equal</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o1</span> <span class="special">);</span> <span class="comment">// Initialized unequal to initialized.</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1_</span> <span class="special">);</span> <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">o2</span> <span class="special">);</span> <span class="comment">//</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Expression <code class="computeroutput"><span class="special">*</span><span class="identifier">x</span> <span class="special"><</span> <span class="special">*</span><span class="identifier">y</span></code> shall be well-formed and its result
|
||||
shall be convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">(!</span><span class="identifier">y</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">false</span> <span class="special">:</span> <span class="special">(!</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">true</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="special">*</span><span class="identifier">y</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This definition guarantees
|
||||
that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
not containing a value is ordered as less than any <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> containing any value, and equivalent
|
||||
to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> not containing a value. Pointers
|
||||
have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code> directly in generic code which
|
||||
expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead. <code class="computeroutput"><span class="identifier">T</span></code> need not
|
||||
be <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. Only
|
||||
single <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
is required. Other relational operations are defined in terms of this
|
||||
one. If <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special"><</span></code>
|
||||
satisfies the axioms of <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> (transitivity,
|
||||
antisymmetry and irreflexivity), <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o0</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Identity implies equivalence</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">oN_</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Two uninitialized are equivalent</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN_</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special"><</span> <span class="identifier">o0</span> <span class="special">);</span> <span class="comment">// Uninitialized is less than initialized</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o0</span> <span class="special"><</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special"><</span> <span class="identifier">o2</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Two initialized compare as (*lhs < *rhs)</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special"><</span> <span class="identifier">o2</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special">==</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">></span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special"><=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">y</span> <span class="special"><</span>
|
||||
<span class="identifier">x</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">>=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
|
||||
<span class="identifier">x</span> <span class="special"><</span>
|
||||
<span class="identifier">y</span> <span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">==</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
need not meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
|
||||
<span class="special">!=</span> <span class="special">(</span>
|
||||
<span class="identifier">none_t</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span></code>
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_free_get_pointer"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">o</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">pointer_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">o</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">pointer_const_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_ptr</span><span class="special">()</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Nothing.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_free_get_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="identifier">def</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span> <span class="identifier">def</span>
|
||||
<span class="special">)</span> <span class="special">-></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_value_or</span><span class="special">(</span><span class="identifier">def</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> Nothing.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> Lvalues of type <code class="computeroutput"><span class="identifier">T</span></code> shall be swappable and <code class="computeroutput"><span class="identifier">T</span></code> shall be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<p class="simpara">
|
||||
<span class="bold"><strong>Effects:</strong></span>
|
||||
</p>
|
||||
<div class="informaltable"><table class="table">
|
||||
<colgroup>
|
||||
<col>
|
||||
<col>
|
||||
<col>
|
||||
</colgroup>
|
||||
<thead><tr></tr></thead>
|
||||
<tbody>
|
||||
<tr>
|
||||
<td>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
contains a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">),</span> <span class="special">*</span><span class="identifier">rhs</span><span class="special">)</span></code>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> as if direct-initializing
|
||||
an object of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
with the expression <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>, followed by <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>,
|
||||
<code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
|
||||
contains a value and <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>
|
||||
<p>
|
||||
<span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
does not contain a value</strong></span>
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
initializes the contained value of <code class="computeroutput"><span class="identifier">rhs</span></code>
|
||||
as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the expression
|
||||
<code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">))</span></code>,
|
||||
followed by <code class="computeroutput"><span class="identifier">val</span><span class="special">-></span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value
|
||||
and <code class="computeroutput"><span class="identifier">rhs</span></code> contains
|
||||
a value
|
||||
</p>
|
||||
</td>
|
||||
<td>
|
||||
<p>
|
||||
no effect
|
||||
</p>
|
||||
</td>
|
||||
</tr>
|
||||
</tbody>
|
||||
</table></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> The states of <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
interchanged.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
|
||||
<code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&,</span><span class="identifier">T</span><span class="special">&)</span></code>
|
||||
throws. If only one is initialized, whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="special">)</span></code>
|
||||
throws.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def0</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def1</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">def1</span><span class="special">);</span> <span class="comment">// no-op</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span> <span class="comment">// Get back to original values</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span><span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optY</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_swap_optional_reference"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
|
||||
<span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
refers to what <code class="computeroutput"><span class="identifier">y</span></code> referred
|
||||
to before the swap (if anything). <code class="computeroutput"><span class="identifier">y</span></code>
|
||||
refers to whatever <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
referred to before the swap.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt0</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optX</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">optY</span> <span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span> <span class="identifier">optY</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optX</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optY</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">opt0</span><span class="special">,</span> <span class="identifier">optX</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">opt0</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="detailed_semantics___optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.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="../header__boost_optional_hpp_.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</body>
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</html>
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@@ -1,552 +0,0 @@
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||||
<html>
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||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<title>Detailed Semantics - Optional References</title>
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<link rel="up" href="../../../optional/reference/header__boost_optional_optional_hpp_.html" title="Header <boost/optional/optional.hpp>">
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<link rel="prev" href="detailed_semantics___optional_values.html" title="Detailed Semantics - Optional Values">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../../index.html">Home</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="detailed_semantics___optional_values.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.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="detailed_semantics___free_functions.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___optional_references"></a><a class="link" href="detailed_semantics___optional_references.html" title="Detailed Semantics - Optional References">Detailed
|
||||
Semantics - Optional References</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_default_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">none_t</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>; <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> refers to nothing.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>; <code class="computeroutput"><span class="identifier">addressof</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This constructor
|
||||
does not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This constructor is declared
|
||||
<code class="computeroutput"><span class="keyword">explicit</span></code> on compilers
|
||||
that do not correctly support binding to const lvalues of integral types.
|
||||
For more details <a class="link" href="../../dependencies_and_portability/optional_reference_binding.html#optional_reference_binding">see here</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">vref</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span> <span class="comment">// mutate referee</span>
|
||||
<span class="identifier">assert</span> <span class="special">(*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span><span class="special">);</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_cond_value_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&&</span>
|
||||
<span class="identifier">r</span><span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">cond</span> <span class="special">?</span>
|
||||
<span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">nullptr</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">cond</span></code>; If <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>, <code class="computeroutput"><span class="identifier">addressof</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This constructor
|
||||
does not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_copy_ctor"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">uninit</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">uinit2</span> <span class="special">(</span> <span class="identifier">uninit</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">uninit2</span> <span class="special">==</span> <span class="identifier">uninit</span> <span class="special">);</span>
|
||||
|
||||
<span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">ref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">init</span><span class="special">(</span><span class="identifier">ref</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">init2</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init2</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">v</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init</span> <span class="special">==</span> <span class="number">3</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">init2</span> <span class="special">==</span> <span class="number">3</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_ctor_from_opt_U"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">optional</span> <span class="special">(</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="identifier">rhs</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">U</span><span class="special">&,</span> <span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span></code>
|
||||
is <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Initializes <code class="computeroutput"><span class="identifier">ref</span></code> with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_assign_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="keyword">nullptr</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">false</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<a name="reference_optional_ref_copy_assign"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="identifier">rhs</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Notes:</strong></span> This behaviour is called <span class="emphasis"><em>rebinding
|
||||
semantics</em></span>. See <a class="link" href="../../tutorial/optional_references/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">here</a>
|
||||
for details.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">def</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">def</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// binds 'def' to 'b' through 'rb' wrapped within 'orb'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def</span> <span class="special">==</span> <span class="identifier">b</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">*</span><span class="identifier">def</span> <span class="special">=</span> <span class="identifier">ora</span> <span class="special">;</span> <span class="comment">// changes the value of 'b' to a copy of the value of 'a'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">b</span> <span class="special">==</span> <span class="identifier">a</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orc</span><span class="special">(</span><span class="identifier">rc</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orc</span> <span class="special">;</span> <span class="comment">// REBINDS ora to 'c' through 'rc'</span>
|
||||
<span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">ora</span> <span class="special">==</span> <span class="number">4</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<a name="reference_optional_ref_assign_optional_U"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_convertible</span><span class="special"><</span><span class="identifier">U</span><span class="special">&,</span> <span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span></code>
|
||||
is <code class="computeroutput"><span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_assign_R"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">r</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This function does
|
||||
not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">def</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">def</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// binds 'def' to 'b' through 'rb'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def</span> <span class="special">==</span> <span class="identifier">b</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">*</span><span class="identifier">def</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> <span class="comment">// changes the value of 'b' to a copy of the value of 'a'</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">b</span> <span class="special">==</span> <span class="identifier">a</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
|
||||
<span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">rc</span> <span class="special">;</span> <span class="comment">// REBINDS to 'c' through 'rc'</span>
|
||||
<span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="number">4</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_emplace_R"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">emplace</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Assigns <code class="computeroutput"><span class="identifier">ref</span></code>
|
||||
with expression <code class="computeroutput"><span class="identifier">r</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed. This function does
|
||||
not participate in overload resolution if <code class="computeroutput"><span class="identifier">decay</span><span class="special"><</span><span class="identifier">R</span><span class="special">></span></code> is an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">*()</span>
|
||||
<span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Returns <code class="computeroutput"><span class="special">*</span><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws: </strong></span> Nothing.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Example:</strong></span>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">vref</span> <span class="special">);</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">vref2</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">opt</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">vref2</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">-></span>
|
||||
<span class="special">()</span> <span class="keyword">const</span>
|
||||
<span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">true</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects: </strong></span> Returns <code class="computeroutput"><span class="identifier">ref</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Throws: </strong></span> Nothing.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">*</span><span class="identifier">val</span> <span class="special">:</span> <span class="keyword">throw</span> <span class="identifier">bad_optional_access</span><span class="special">();</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span>
|
||||
<span class="identifier">r</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="identifier">R</span></code>
|
||||
is an lvalue reference, the program is ill-formed.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_value_or_eval"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span>
|
||||
<span class="identifier">f</span><span class="special">();</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> Unless <code class="computeroutput"><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">())</span></code> is an lvalue reference, the program
|
||||
is ill-formed.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">none</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> The return type of this function
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">))></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_flat_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">flat_map</span><span class="special">(</span>
|
||||
<span class="identifier">F</span> <span class="identifier">f</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="special">-></span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> The return type of expression
|
||||
<code class="computeroutput"><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>
|
||||
for some object or reference type <code class="computeroutput"><span class="identifier">U</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">none</span><span class="special">;</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> The return type of this function
|
||||
is <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get_ptr</span> <span class="special">()</span>
|
||||
<span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">ref</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="keyword">bool</span>
|
||||
<span class="special">()</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">ref</span><span class="special">)</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_operator_not"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="keyword">operator</span> <span class="special">!</span> <span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="keyword">bool</span><span class="special">(</span><span class="identifier">ref</span><span class="special">)</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_reset"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Same as <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">none</span></code>.
|
||||
</li></ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_reset_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span>
|
||||
<span class="identifier">r</span><span class="special">)</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">r</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_is_initialized"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
|
||||
</p>
|
||||
<a name="reference_optional_ref_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>::</span><span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span>
|
||||
<span class="special">)</span> <span class="keyword">const</span>
|
||||
<span class="keyword">noexcept</span><span class="special">;</span></code>
|
||||
</p></blockquote></div>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="identifier">value_or</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">R</span><span class="special">>(</span><span class="identifier">r</span><span class="special">);</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
|
||||
</li>
|
||||
</ul></div>
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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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This is an alias for header <a class="link" href="../../../optional/reference/header__boost_optional_optional_hpp_.html#boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional" title="Synopsis"><code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code></a>.
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|
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tags</a>
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</h4></div></div></div>
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<a name="reference_in_place_init"></a><a name="reference_in_place_init_if"></a><pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_t</span> <span class="special">{</span> <span class="comment">/* see below */</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_t</span> <span class="identifier">in_place_init</span> <span class="special">(</span> <span class="comment">/* see below */</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_if_t</span> <span class="special">{</span> <span class="comment">/*see below*/</span> <span class="special">}</span> <span class="special">;</span>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_if_t</span> <span class="identifier">in_place_init_if</span> <span class="special">(</span> <span class="comment">/*see below*/</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Classes <code class="computeroutput"><span class="identifier">in_place_init_t</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">in_place_init_if_t</span></code>
|
||||
are empty classes. Their purpose is to control overload resolution in the
|
||||
initialization of optional objects. They are empty, trivially copyable
|
||||
classes with disabled default constructor.
|
||||
</p>
|
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</div>
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<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 © 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
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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>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional_refs"></a><a class="link" href="header_optional_optional_refs.html" title="Optional References">Optional
|
||||
References</a>
|
||||
</h4></div></div></div>
|
||||
<a name="reference_operator_template_spec"></a><pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="comment">// specialization for lvalue references</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">reference_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">reference_const_type</span><span class="special">;</span> <span class="comment">// no const propagation</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">rval_reference_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">pointer_type</span><span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">pointer_const_type</span><span class="special">;</span> <span class="comment">// no const propagation</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_default_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_default_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">(</span><span class="keyword">bool</span> <span class="identifier">cond</span><span class="special">,</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_cond_value_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_copy_ctor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_ctor_from_opt_U"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_copy_assign"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">&></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_optional_U"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span><span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span><span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_assign_R"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">R</span><span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_emplace_R"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_or"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_value_or_eval"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_operator_not"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_reset"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_reset_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_is_initialized"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">R</span><span class="special">></span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">R</span> <span class="special">&&</span> <span class="identifier">r</span> <span class="special">)</span> <span class="identifier">constnoexcept</span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_references.html#reference_optional_ref_get_value_or_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">private</span><span class="special">:</span>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">ref</span><span class="special">;</span> <span class="comment">// exposition only</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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</html>
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@@ -1,153 +0,0 @@
|
||||
<html>
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||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.header__boost_optional_optional_hpp_.header_optional_optional_values"></a><a class="link" href="header_optional_optional_values.html" title="Optional Values">Optional
|
||||
Values</a>
|
||||
</h4></div></div></div>
|
||||
<a name="reference_operator_template"></a><pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="identifier">value_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&</span> <span class="identifier">reference_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">reference_const_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">rval_reference_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="special">*</span> <span class="identifier">pointer_type</span> <span class="special">;</span>
|
||||
<span class="keyword">typedef</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">pointer_const_type</span> <span class="special">;</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_in_place_init"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_in_place_init_if"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">&&...</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_asterisk_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_call"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or_eval</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_value_or_call_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_map_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">F</span><span class="special">></span> <span class="keyword">auto</span> <span class="identifier">flat_map</span><span class="special">(</span> <span class="identifier">F</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_flat_map_move"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics___optional_values.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="header_optional_in_place_init.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.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="header_optional_optional_refs.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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||||
</body>
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||||
</html>
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||||
@@ -1,72 +0,0 @@
|
||||
<html>
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||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Header <boost/optional/optional_io.hpp></title>
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/reference.html" title="Reference">
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<link rel="prev" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html" title="Detailed semantics">
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<link rel="next" href="io_header/io_semantics.html" title="Detailed semantics">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="header__boost_optional_bad_optional_access_hpp_/detailed_semantics.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/reference.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="io_header/io_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
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||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.io_header"></a><a class="link" href="io_header.html" title="Header <boost/optional/optional_io.hpp>">Header <boost/optional/optional_io.hpp></a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.reference.io_header.io_synop"></a><a class="link" href="io_header.html#boost_optional.reference.io_header.io_synop" title="Synopsis">Synopsis</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">istream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">ostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="keyword">const</span><span class="special">&);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_ostream_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span> <a class="link" href="io_header/io_semantics.html#reference_operator_istream"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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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>
|
||||
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>
|
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<div class="section">
|
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<div class="titlepage"><div><div><h4 class="title">
|
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<a name="boost_optional.reference.io_header.io_semantics"></a><a class="link" href="io_semantics.html" title="Detailed semantics">Detailed
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
<a name="reference_operator_ostream"></a><code class="computeroutput"><span class="keyword">template</span>
|
||||
<span class="special"><</span><span class="keyword">class</span>
|
||||
<span class="identifier">CharType</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code>
|
||||
<br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code> <br> <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span>
|
||||
<span class="identifier">out</span><span class="special">,</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string. The output contains the information about whether the optional
|
||||
object contains a value or not. If <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value, the output contains result of calling <code class="computeroutput"><span class="identifier">out</span> <span class="special"><<</span>
|
||||
<span class="special">*</span><span class="identifier">v</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_ostream_none"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br> <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">out</span><span class="special">,</span> <span class="identifier">none_t</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Outputs an implementation-defined
|
||||
string.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span> <a name="reference_operator_istream"></a>
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">CharType</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">CharTrait</span><span class="special">,</span>
|
||||
<span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span></code> <br> <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_ostream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span></code>
|
||||
<br> <code class="computeroutput"><span class="keyword">operator</span><span class="special">>>(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">basic_istream</span><span class="special"><</span><span class="identifier">CharType</span><span class="special">,</span> <span class="identifier">CharTrait</span><span class="special">>&</span> <span class="identifier">in</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">);</span></code>
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> and
|
||||
<code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Effect:</strong></span> Reads the value of optional
|
||||
object from <code class="computeroutput"><span class="identifier">in</span></code>. If
|
||||
the string representation indicates that the optional object should
|
||||
contain a value, <code class="computeroutput"><span class="identifier">v</span></code>
|
||||
contains a value and its contained value is obtained as if by default-constructing
|
||||
an object <code class="computeroutput"><span class="identifier">o</span></code> of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> and then calling
|
||||
<code class="computeroutput"><span class="identifier">in</span> <span class="special">>></span>
|
||||
<span class="identifier">o</span></code>; otherwise <code class="computeroutput"><span class="identifier">v</span></code> does not contain a value, and the
|
||||
previously contained value (if any) has been destroyed.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">out</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
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<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="../io_header.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../io_header.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="../header__boost_optional_optional_fwd_hpp_.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,183 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.reference.relnotes"></a><a class="link" href="relnotes.html" title="Release Notes">Release Notes</a>
|
||||
</h3></div></div></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_x_xx"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_x_xx">Boost Release
|
||||
X.XX</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Now <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> is specialized for reference
|
||||
parameters. This way the <code class="computeroutput"><span class="keyword">sizeof</span></code>
|
||||
of optional reference is that of a pointer, and a number of bugs is avoided.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_60">Boost Release
|
||||
1.60</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
again. Now it is a const object with internal linkage (as any other tag).
|
||||
This fixes <a href="https://svn.boost.org/trac/boost/ticket/11203" target="_top">Trac
|
||||
#11203</a>.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h2"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_59">Boost Release
|
||||
1.59</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
For C++03 compilers, added 0-argument overload for member function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>,
|
||||
and therewith removed the dependency on <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">in_place_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/11241" target="_top">Trac
|
||||
#11241</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h3"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_58">Boost Release
|
||||
1.58</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> is no longer convertible from
|
||||
literal <code class="computeroutput"><span class="number">0</span></code>. This avoids a
|
||||
bug where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span>
|
||||
<span class="identifier">oi</span> <span class="special">=</span>
|
||||
<span class="number">0</span><span class="special">;</span></code>
|
||||
would initialize an optional object with no contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved the trick that prevents streaming out <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
without header <code class="computeroutput"><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
by using safe-bool idiom. This addresses <a href="https://svn.boost.org/trac/boost/ticket/10825" target="_top">Trac
|
||||
#10825</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
IOStream operators are now mentioned in documentation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added a way to manually disable move semantics: just define macro <code class="computeroutput"><span class="identifier">BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES</span></code>.
|
||||
This can be used to work around <a href="https://svn.boost.org/trac/boost/ticket/10399" target="_top">Trac
|
||||
#10399</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
It is no longer possible to assign <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is not assignable or convertible to <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
(<a href="https://svn.boost.org/trac/boost/ticket/11087" target="_top">Trac #11087</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value accessors now work correctly on rvalues of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> (<a href="https://svn.boost.org/trac/boost/ticket/10839" target="_top">Trac
|
||||
#10839</a>).
|
||||
</li>
|
||||
</ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h4"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_57">Boost Release
|
||||
1.57</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<a href="https://github.com/boostorg/optional/pull/9" target="_top">Git pull #9</a>:
|
||||
<span class="emphasis"><em>"Supply <code class="computeroutput"><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
|
||||
</li></ul></div>
|
||||
<h5>
|
||||
<a name="boost_optional.reference.relnotes.h5"></a>
|
||||
<span class="phrase"><a name="boost_optional.reference.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.reference.relnotes.boost_release_1_56">Boost Release
|
||||
1.56</a>
|
||||
</h5>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added support for rvalue references. Now <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> works with moveable but non-copyable
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved <code class="computeroutput"><span class="identifier">swap</span></code> (now uses
|
||||
move operations),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>. This is the last of the requests from
|
||||
<a href="https://svn.boost.org/trac/boost/ticket/1841" target="_top">Trac #1841</a>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> is moveable,
|
||||
including conditional <code class="computeroutput"><span class="keyword">noexcept</span></code>
|
||||
specifications, which make it <code class="computeroutput"><span class="identifier">move_if_noexcept</span></code>-friendly,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Using explicit operator bool() on platforms that support it (<a href="https://svn.boost.org/trac/boost/ticket/4227" target="_top">Trac
|
||||
#4227</a>) (breaking change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Forward declaration of <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">ostream</span><span class="special">&,</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&)</span></code>
|
||||
to prevent inadvertent incorrect serialization of optional objects,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Removed deprecated function <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> from examples (<a href="https://svn.boost.org/trac/boost/ticket/9005" target="_top">Trac
|
||||
#9005</a>),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Equality comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
does not require that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
be EqualityComparable,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Optional rvalue references are explicitly disallowed,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Binding temporaries to optional references is explicitly disallowed (breaking
|
||||
change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
More ways to access the contained value, functions <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span><span class="special">()</span></code>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Updated and reorganized documentation, added tutorial and quick guide
|
||||
sections.
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../optional/reference.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="acknowledgements.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,384 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../index.html" title="Boost.Optional">
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<link rel="prev" href="dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">
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|
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|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td>
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<a accesskey="p" href="dependencies_and_portability/optional_reference_binding.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="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.relnotes"></a><a class="link" href="relnotes.html" title="Release Notes">Release Notes</a>
|
||||
</h2></div></div></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_79"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_79">Boost
|
||||
Release 1.79</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/98" target="_top">issue
|
||||
#98</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/92" target="_top">issue
|
||||
#92</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added support for <code class="computeroutput"><span class="identifier">BOOST_NO_IOSTREAM</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now aligned storage uses <code class="computeroutput"><span class="keyword">unsigned</span>
|
||||
<span class="keyword">char</span></code> rather than <code class="computeroutput"><span class="keyword">char</span></code>
|
||||
to avoid UB.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now using cv-unqualified <code class="computeroutput"><span class="identifier">value_type</span></code>
|
||||
with placement <code class="computeroutput"><span class="keyword">new</span></code> to avoid
|
||||
UB.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_76"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_76">Boost
|
||||
Release 1.76</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Fixed MSVC warning C4702.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h2"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_75"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_75">Boost
|
||||
Release 1.75</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> is <code class="computeroutput"><span class="keyword">constexpr</span></code>-declared.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/78" target="_top">issue
|
||||
#78</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h3"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_73"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_73">Boost
|
||||
Release 1.73</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/78" target="_top">issue
|
||||
#78</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> is now declared as an inline variable
|
||||
(on compilers that support it): there is only one instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> across all translation units.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed a number of compilation errors in GCC 4.4.7 in <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> for trivial <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
Thanks to Robert Leahy for the fix. For details see <a href="https://github.com/boostorg/optional/pull/80" target="_top">pr
|
||||
#78</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Now suppressing warning <code class="computeroutput"><span class="special">-</span><span class="identifier">Wweak</span><span class="special">-</span><span class="identifier">vtables</span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h4"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_69"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_69">Boost
|
||||
Release 1.69</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Remove deprecation mark from <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> method (without arguments).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/59" target="_top">issue
|
||||
#59</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed bug with initialization of certain wrapper types in clang with -std=c++03.
|
||||
See <a href="https://github.com/boostorg/optional/pull/64" target="_top">pr #64</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h5"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_68"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_68">Boost
|
||||
Release 1.68</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">has_value</span><span class="special">()</span></code> for compatibility with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">optional</span></code> (<a href="https://github.com/boostorg/optional/issues/52" target="_top">issue
|
||||
#52</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">map</span><span class="special">()</span></code> for transforming <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> into <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> using a function of type <code class="computeroutput"><span class="identifier">T</span> <span class="special">-></span> <span class="identifier">U</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added member function <code class="computeroutput"><span class="identifier">flat_map</span><span class="special">()</span></code> for transforming <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> into <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> using a function of type <code class="computeroutput"><span class="identifier">T</span> <span class="special">-></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h6"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_67"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_67">Boost
|
||||
Release 1.67</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://github.com/boostorg/optional/issues/46" target="_top">issue
|
||||
#46</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <code class="computeroutput"><span class="special">-</span><span class="identifier">Wzero</span><span class="special">-</span><span class="identifier">as</span><span class="special">-</span><span class="identifier">null</span><span class="special">-</span><span class="identifier">pointer</span><span class="special">-</span><span class="identifier">constant</span></code> warnings.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h7"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_66"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_66">Boost
|
||||
Release 1.66</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
On newer compilers <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
is now trivially-copyable for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
This uses a different storage (just <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
rather than <code class="computeroutput"><span class="identifier">aligned_storage</span></code>).
|
||||
We require the compiler to support defaulted functions.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> to get rid of the <code class="computeroutput"><span class="special">-</span><span class="identifier">Wmaybe</span><span class="special">-</span><span class="identifier">uninitialized</span></code> false-positive warning
|
||||
from GCC.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h8"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_63"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_63">Boost
|
||||
Release 1.63</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added two new in-place constructors. They work similarly to <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>
|
||||
functions: they initialize the contained value by perfect-forwarding the
|
||||
obtained arguments. One constructor always initializes the contained value,
|
||||
the other based on a boolean condition.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Syntax <code class="computeroutput"><span class="identifier">o</span> <span class="special">=</span>
|
||||
<span class="special">{}</span></code> now correctly un-initializes
|
||||
optional, just like in <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">optional</span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12203" target="_top">Trac #12203</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12563" target="_top">Trac #12563</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h9"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_62"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_62">Boost
|
||||
Release 1.62</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/12179" target="_top">Trac #12179</a>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h10"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_61"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_61">Boost
|
||||
Release 1.61</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Now <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> is specialized for reference
|
||||
parameters. This addresses a couple of issues:
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; ">
|
||||
<li class="listitem">
|
||||
the <code class="computeroutput"><span class="keyword">sizeof</span></code> of optional
|
||||
reference is that of a pointer,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
some bugs connected to copying optional references are gone,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
all run-time bugs caused by incorrect reference binding on some compilers
|
||||
are now turned into compile-time errors,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
you can swap optional references: it is like swapping pointers: shallow,
|
||||
underlying objects are not affected,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
optional references to abstract types work.
|
||||
</li>
|
||||
</ul></div>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Documented nested typedefs (<a href="https://svn.boost.org/trac/boost/ticket/5193" target="_top">Trac
|
||||
#5193</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Made the perfect-forwarding constructor SFINAE-friendly, which fixes <a href="https://svn.boost.org/trac/boost/ticket/12002" target="_top">Trac #12002</a>.
|
||||
However, this only works in the newer platforms that correctly implement
|
||||
C++11 <code class="computeroutput"><span class="special"><</span><span class="identifier">type_traits</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/10445" target="_top">Trac #10445</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h11"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_60"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_60">Boost
|
||||
Release 1.60</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
Changed the implementation of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
again. Now it is a const object with internal linkage (as any other tag).
|
||||
This fixes <a href="https://svn.boost.org/trac/boost/ticket/11203" target="_top">Trac
|
||||
#11203</a>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h12"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_59"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_59">Boost
|
||||
Release 1.59</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
For C++03 compilers, added 0-argument overload for member function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>,
|
||||
and therewith removed the dependency on <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">in_place_factory</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Fixed <a href="https://svn.boost.org/trac/boost/ticket/11241" target="_top">Trac #11241</a>.
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h13"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_58"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_58">Boost
|
||||
Release 1.58</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> is no longer convertible from
|
||||
literal <code class="computeroutput"><span class="number">0</span></code>. This avoids a bug
|
||||
where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">rational</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span></code> would
|
||||
initialize an optional object with no contained value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved the trick that prevents streaming out <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
without header <code class="computeroutput"><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span></code>
|
||||
by using safe-bool idiom. This addresses <a href="https://svn.boost.org/trac/boost/ticket/10825" target="_top">Trac
|
||||
#10825</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
IOStream operators are now mentioned in documentation.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added a way to manually disable move semantics: just define macro <code class="computeroutput"><span class="identifier">BOOST_OPTIONAL_CONFIG_NO_RVALUE_REFERENCES</span></code>.
|
||||
This can be used to work around <a href="https://svn.boost.org/trac/boost/ticket/10399" target="_top">Trac
|
||||
#10399</a>.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
It is no longer possible to assign <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is not assignable or convertible to <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
(<a href="https://svn.boost.org/trac/boost/ticket/11087" target="_top">Trac #11087</a>).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value accessors now work correctly on rvalues of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> (<a href="https://svn.boost.org/trac/boost/ticket/10839" target="_top">Trac
|
||||
#10839</a>).
|
||||
</li>
|
||||
</ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h14"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_57"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_57">Boost
|
||||
Release 1.57</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
|
||||
<a href="https://github.com/boostorg/optional/pull/9" target="_top">Git pull #9</a>:
|
||||
<span class="emphasis"><em>"Supply <code class="computeroutput"><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to fix C++03 compile error on <code class="computeroutput"><span class="identifier">logic_error</span><span class="special">(</span><span class="string">"..."</span><span class="special">)</span></code>"</em></span>.
|
||||
</li></ul></div>
|
||||
<h4>
|
||||
<a name="boost_optional.relnotes.h15"></a>
|
||||
<span class="phrase"><a name="boost_optional.relnotes.boost_release_1_56"></a></span><a class="link" href="relnotes.html#boost_optional.relnotes.boost_release_1_56">Boost
|
||||
Release 1.56</a>
|
||||
</h4>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Added support for rvalue references. Now <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> works with moveable but non-copyable
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>'s,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Improved <code class="computeroutput"><span class="identifier">swap</span></code> (now uses
|
||||
move operations),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Added function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>. This is the last of the requests from
|
||||
<a href="https://svn.boost.org/trac/boost/ticket/1841" target="_top">Trac #1841</a>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> is moveable, including
|
||||
conditional <code class="computeroutput"><span class="keyword">noexcept</span></code> specifications,
|
||||
which make it <code class="computeroutput"><span class="identifier">move_if_noexcept</span></code>-friendly,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Using explicit operator bool() on platforms that support it (<a href="https://svn.boost.org/trac/boost/ticket/4227" target="_top">Trac
|
||||
#4227</a>) (breaking change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Forward declaration of <code class="computeroutput"><span class="keyword">operator</span><span class="special"><<(</span><span class="identifier">ostream</span><span class="special">&,</span> <span class="identifier">optional</span>
|
||||
<span class="keyword">const</span><span class="special">&)</span></code>
|
||||
to prevent inadvertent incorrect serialization of optional objects,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Removed deprecated function <code class="computeroutput"><span class="identifier">reset</span><span class="special">()</span></code> from examples (<a href="https://svn.boost.org/trac/boost/ticket/9005" target="_top">Trac
|
||||
#9005</a>),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Equality comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
|
||||
does not require that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
be EqualityComparable,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Optional rvalue references are explicitly disallowed,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Binding temporaries to optional references is explicitly disallowed (breaking
|
||||
change),
|
||||
</li>
|
||||
<li class="listitem">
|
||||
More ways to access the contained value, functions <code class="computeroutput"><span class="identifier">value</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code>, <code class="computeroutput"><span class="identifier">value_or_eval</span><span class="special">()</span></code>,
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Updated and reorganized documentation, added tutorial and quick guide sections.
|
||||
</li>
|
||||
</ul></div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
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<html>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="boost_optional.synopsis"></a><a class="link" href="synopsis.html" title="Synopsis">Synopsis</a>
|
||||
</h2></div></div></div>
|
||||
<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">public</span> <span class="special">:</span>
|
||||
|
||||
<span class="comment">// (If T is of reference type, the parameters and results by reference are by value)</span>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_move_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// [new in 1.34]</span>
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_move_constructor_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="keyword">explicit</span> <span class="identifier">optional</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_constructor_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_none_t"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">(</span><span class="emphasis"><em>see below</em></span><span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">>&&</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_move_equal_other_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <span class="identifier">emplace</span> <span class="special">(</span> <span class="identifier">Args</span><span class="special">...&&</span> <span class="identifier">args</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_emplace"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">f</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_equal_factory"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="keyword">operator</span> <span class="special">->()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_arrow"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="keyword">operator</span> <span class="special">*()</span> <span class="special">&&;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_asterisk"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">value</span><span class="special">()</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">T</span> <span class="identifier">value_or</span><span class="special">(</span> <span class="identifier">U</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">&&</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_value_or"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">explicit</span> <span class="keyword">operator</span> <span class="keyword">bool</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_bool"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_operator_not"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// deprecated methods</span>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">void</span> <span class="identifier">reset</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_reset_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">is_initialized</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_is_initialized"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="comment">// (deprecated)</span>
|
||||
<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_not_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_less_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">></span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_greater_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_less_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_value_or_value"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_optional_get_ptr"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="detailed_semantics.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview">Design Overview</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_models">The
|
||||
models</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics">The
|
||||
semantics</a></span></dt>
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.design_overview.the_interface">The
|
||||
Interface</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<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>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can 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>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</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—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_semantics" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
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—from the days of C APIs to modern C++ libraries—to <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>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
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">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept. This concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>, <code class="computeroutput"><span class="special">-></span></code>
|
||||
and contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
to convey the notion of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../utility/OptionalPointee.html" target="_top">OptionalPointee</a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="tutorial.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="warning"><table border="0" summary="Warning">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
|
||||
<th align="left">Warning</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</div>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="quick_start.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="synopsis.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,108 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>A note about optional<bool></title>
|
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<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="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.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 optional<bool></a>
|
||||
</h3></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, 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"><</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>
|
||||
<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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 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>
|
||||
</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="exception_safety_guarantees.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
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</body>
|
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</html>
|
||||
@@ -1,184 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<title>Design Overview</title>
|
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
|
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
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<link rel="prev" href="../../optional/tutorial.html" title="Tutorial">
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<link rel="next" href="design_overview/the_semantics.html" title="The semantics">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
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</tr></table>
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<hr>
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<div class="spirit-nav">
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<a accesskey="p" href="../../optional/tutorial.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="design_overview/the_semantics.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview"></a><a class="link" href="design_overview.html" title="Design Overview">Design Overview</a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_models"></a><a class="link" href="design_overview.html#boost_optional.tutorial.design_overview.the_models" title="The models">The
|
||||
models</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
In C++, we can <span class="emphasis"><em>declare</em></span> an object (a variable) of type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>, and we can give this
|
||||
variable an <span class="emphasis"><em>initial value</em></span> (through an <span class="emphasis"><em>initializer</em></span>.
|
||||
(cf. 8.5)). When a declaration includes a non-empty initializer (an initial
|
||||
value is given), it is said that the object has been initialized. If the
|
||||
declaration uses an empty initializer (no initial value is given), and
|
||||
neither default nor value initialization applies, it is said that the object
|
||||
is <span class="bold"><strong>uninitialized</strong></span>. Its actual value exist
|
||||
but has an <span class="emphasis"><em>indeterminate initial value</em></span> (cf. 8.5/11).
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
intends to formalize the notion of initialization (or lack of it) allowing
|
||||
a program to test whether an object has been initialized and stating that
|
||||
access to the value of an uninitialized object is undefined behavior. That
|
||||
is, when a variable is declared as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and no initial value is given, the
|
||||
variable is <span class="emphasis"><em>formally</em></span> uninitialized. A formally uninitialized
|
||||
optional object has conceptually no value at all and this situation can
|
||||
be tested at runtime. It is formally <span class="emphasis"><em>undefined behavior</em></span>
|
||||
to try to access the value of an uninitialized optional. An uninitialized
|
||||
optional can be assigned a value, in which case its initialization state
|
||||
changes to initialized. Furthermore, given the formal treatment of initialization
|
||||
states in optional objects, it is even possible to reset an optional to
|
||||
<span class="emphasis"><em>uninitialized</em></span>.
|
||||
</p>
|
||||
<p>
|
||||
In C++ there is no formal notion of uninitialized objects, which means
|
||||
that objects always have an initial value even if indeterminate. As discussed
|
||||
on the previous section, this has a drawback because you need additional
|
||||
information to tell if an object has been effectively initialized. One
|
||||
of the typical ways in which this has been historically dealt with is via
|
||||
a special value: <code class="computeroutput"><span class="identifier">EOF</span></code>,
|
||||
<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>—where <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
is some stateless POD—can 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>
|
||||
library, this model can be implemented in terms of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code>. There is precedent for a discriminated
|
||||
union as a model for an optional value: the <a href="http://www.haskell.org/" target="_top">Haskell</a>
|
||||
<span class="bold"><strong>Maybe</strong></span> built-in type constructor. Thus,
|
||||
a discriminated union <code class="computeroutput"><span class="identifier">T</span><span class="special">+</span><span class="identifier">nil_t</span></code>
|
||||
serves as a conceptual foundation.
|
||||
</p>
|
||||
<p>
|
||||
A <code class="computeroutput"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">nil_t</span><span class="special">></span></code> follows naturally from the traditional
|
||||
idiom of extending the range of possible values adding an additional sentinel
|
||||
value with the special meaning of <span class="emphasis"><em>Nothing</em></span>. However,
|
||||
this additional <span class="emphasis"><em>Nothing</em></span> value is largely irrelevant
|
||||
for our purpose since our goal is to formalize the notion of uninitialized
|
||||
objects and, while a special extended value can be used to convey that
|
||||
meaning, it is not strictly necessary in order to do so.
|
||||
</p>
|
||||
<p>
|
||||
The observation made in the last paragraph about the irrelevant nature
|
||||
of the additional <code class="computeroutput"><span class="identifier">nil_t</span></code>
|
||||
with respect to <span class="underline">purpose</span> of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
suggests an alternative model: a <span class="emphasis"><em>container</em></span> that either
|
||||
has a value of <code class="computeroutput"><span class="identifier">T</span></code> or nothing.
|
||||
</p>
|
||||
<p>
|
||||
As of this writing I don't know of any precedent for a variable-size fixed-capacity
|
||||
(of 1) stack-based container model for optional values, yet I believe this
|
||||
is the consequence of the lack of practical implementations of such a container
|
||||
rather than an inherent shortcoming of the container model.
|
||||
</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—i.e.
|
||||
possibly uninitialized—objects. For instance, these models show the
|
||||
<span class="emphasis"><em>exact</em></span> semantics required for a wrapper of optional
|
||||
values:
|
||||
</p>
|
||||
<p>
|
||||
Discriminated-union:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
variant implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between variants matches both current types and values
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>initialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the variant's current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
it is modeling an <span class="emphasis"><em>uninitialized</em></span> optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the variant's current type is <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
models testing if the optional is initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from a variant when its current type is not <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
models the undefined behavior of trying to access the value of an uninitialized
|
||||
optional
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Single-element container:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-copy</strong></span> semantics: copies of the
|
||||
container implies copies of the value.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>deep-relational</strong></span> semantics: comparisons
|
||||
between containers compare container size and if match, contained value
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is not empty (contains an object of type <code class="computeroutput"><span class="identifier">T</span></code>), it is modeling an <span class="emphasis"><em>initialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
If the container is empty, it is modeling an <span class="emphasis"><em>uninitialized</em></span>
|
||||
optional.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Testing if the container is empty models testing if the optional is
|
||||
initialized
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Trying to extract a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
from an empty container models the undefined behavior of trying to
|
||||
access the value of an uninitialized optional
|
||||
</li>
|
||||
</ul></div>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
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|
||||
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|
||||
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<head>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface"></a><a class="link" href="the_interface.html" title="The Interface">The
|
||||
Interface</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Since the purpose of optional is to allow us to use objects with a formal
|
||||
uninitialized additional state, the interface could try to follow the interface
|
||||
of the underlying <code class="computeroutput"><span class="identifier">T</span></code> type
|
||||
as much as possible. In order to choose the proper degree of adoption of
|
||||
the native <code class="computeroutput"><span class="identifier">T</span></code> interface,
|
||||
the following must be noted: Even if all the operations supported by an
|
||||
instance of type <code class="computeroutput"><span class="identifier">T</span></code> are
|
||||
defined for the entire range of values for such a type, an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
extends such a set of values with a new value for which most (otherwise
|
||||
valid) operations are not defined in terms of <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Furthermore, since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> itself is merely a <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
wrapper (modeling a <code class="computeroutput"><span class="identifier">T</span></code> supertype),
|
||||
any attempt to define such operations upon uninitialized optionals will
|
||||
be totally artificial w.r.t. <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
This library chooses an interface which follows from <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
interface only for those operations which are well defined (w.r.t the type
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>) even if any of the operands
|
||||
are uninitialized. These operations include: construction, copy-construction,
|
||||
assignment, swap and relational operations.
|
||||
</p>
|
||||
<p>
|
||||
For the value access operations, which are undefined (w.r.t the type <code class="computeroutput"><span class="identifier">T</span></code>) when the operand is uninitialized,
|
||||
a different interface is chosen (which will be explained next).
|
||||
</p>
|
||||
<p>
|
||||
Also, the presence of the possibly uninitialized state requires additional
|
||||
operations not provided by <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
itself which are supported by a special interface.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.lexically_hinted_value_access_in_the_presence_of_possibly_uninitialized_optional_objects__the_operators___and___gt_">Lexically-hinted
|
||||
Value Access in the presence of possibly uninitialized optional objects:
|
||||
The operators * and -></a>
|
||||
</h6>
|
||||
<p>
|
||||
A relevant feature of a pointer is that it can have a <span class="bold"><strong>null
|
||||
pointer value</strong></span>. This is a <span class="emphasis"><em>special</em></span> value
|
||||
which is used to indicate that the pointer is not referring to any object
|
||||
at all. In other words, null pointer values convey the notion of nonexistent
|
||||
objects.
|
||||
</p>
|
||||
<p>
|
||||
This meaning of the null pointer value allowed pointers to became a <span class="emphasis"><em>de
|
||||
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—from the days of C APIs to modern C++ libraries—to <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>
|
||||
<p>
|
||||
The possible presence of a null pointer value makes the operations that
|
||||
access the pointee's value possibly undefined, therefore, expressions which
|
||||
use dereference and access operators, such as: <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="special">*</span><span class="identifier">p</span>
|
||||
<span class="special">=</span> <span class="number">2</span> <span class="special">)</span></code> and <code class="computeroutput"><span class="special">(</span>
|
||||
<span class="identifier">p</span><span class="special">-></span><span class="identifier">foo</span><span class="special">()</span> <span class="special">)</span></code>, implicitly convey the notion of optionality,
|
||||
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">-></span></code> tell by themselves
|
||||
—without any additional context— that the expression will be undefined
|
||||
unless the implied pointee actually exist.
|
||||
</p>
|
||||
<p>
|
||||
Such a <span class="emphasis"><em>de facto</em></span> idiom for referring to optional objects
|
||||
can be formalized in the form of a concept: the <a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a> concept. This
|
||||
concept captures the syntactic usage of operators <code class="computeroutput"><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="special">-></span></code> and contextual conversion
|
||||
to <code class="computeroutput"><span class="keyword">bool</span></code> to convey the notion
|
||||
of optionality.
|
||||
</p>
|
||||
<p>
|
||||
However, pointers are good to <span class="underline">refer</span>
|
||||
to optional objects, but not particularly good to handle the optional objects
|
||||
in all other respects, such as initializing or moving/copying them. The
|
||||
problem resides in the shallow-copy of pointer semantics: if you need to
|
||||
effectively move or copy the object, pointers alone are not enough. The
|
||||
problem is that copies of pointers do not imply copies of pointees. For
|
||||
example, as was discussed in the motivation, pointers alone cannot be used
|
||||
to return optional objects from a function because the object must move
|
||||
outside from the function and into the caller's context.
|
||||
</p>
|
||||
<p>
|
||||
A solution to the shallow-copy problem that is often used is to resort
|
||||
to dynamic allocation and use a smart pointer to automatically handle the
|
||||
details of this. For example, if a function is to optionally return an
|
||||
object <code class="computeroutput"><span class="identifier">X</span></code>, it can use <code class="computeroutput"><span class="identifier">shared_ptr</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code>
|
||||
as the return value. However, this requires dynamic allocation of <code class="computeroutput"><span class="identifier">X</span></code>. If <code class="computeroutput"><span class="identifier">X</span></code>
|
||||
is a built-in or small POD, this technique is very poor in terms of required
|
||||
resources. Optional objects are essentially values so it is very convenient
|
||||
to be able to use automatic storage and deep-copy semantics to manipulate
|
||||
optional values just as we do with ordinary values. Pointers do not have
|
||||
this semantics, so are inappropriate for the initialization and transport
|
||||
of optional values, yet are quite convenient for handling the access to
|
||||
the possible undefined value because of the idiomatic aid present in the
|
||||
<a href="../../../../../../utility/OptionalPointee.html" target="_top"><code class="computeroutput"><span class="identifier">OptionalPointee</span></code></a>
|
||||
concept incarnated by pointers.
|
||||
</p>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.design_overview.the_interface.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee"></a></span><a class="link" href="the_interface.html#boost_optional.tutorial.design_overview.the_interface.optional_lt_t_gt__as_a_model_of_optionalpointee">Optional<T>
|
||||
as a model of OptionalPointee</a>
|
||||
</h6>
|
||||
<p>
|
||||
For value access operations <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> uses operators <code class="computeroutput"><span class="special">*</span></code>
|
||||
and <code class="computeroutput"><span class="special">-></span></code> to lexically warn
|
||||
about the possibly uninitialized state appealing to the familiar pointer
|
||||
semantics w.r.t. to null pointers.
|
||||
</p>
|
||||
<div class="caution"><table border="0" summary="Caution">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../../doc/src/images/caution.png"></td>
|
||||
<th align="left">Caution</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
However, it is particularly important to note that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> objects are not pointers. <span class="underline"><code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> is not, and does not model, a
|
||||
pointer</span>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<p>
|
||||
For instance, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> does not have shallow-copy so does
|
||||
not alias: two different optionals never refer to the <span class="emphasis"><em>same</em></span>
|
||||
value unless <code class="computeroutput"><span class="identifier">T</span></code> itself is
|
||||
a reference (but may have <span class="emphasis"><em>equivalent</em></span> values). The
|
||||
difference between an <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> and a pointer must be kept in mind,
|
||||
particularly because the semantics of relational operators are different:
|
||||
since <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is a value-wrapper, relational operators are deep: they compare optional
|
||||
values; but relational operators for pointers are shallow: they do not
|
||||
compare pointee values. As a result, you might be able to replace <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
by <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span></code>
|
||||
on some situations but not always. Specifically, on generic code written
|
||||
for both, you cannot use relational operators directly, and must use the
|
||||
template functions <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
|
||||
and <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
|
||||
instead.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="the_semantics.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.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="../when_to_use_optional.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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|
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<html>
|
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
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<a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.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="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
|
||||
semantics</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
|
||||
objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
|
||||
but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
|
||||
possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
plus the additional semantics corresponding to this special state. As such,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
|
||||
so we need to compose the desired semantics using a different mechanism.
|
||||
Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
|
||||
wrong but also impractical: it is not allowed to derive from a non-class
|
||||
type, such as a built-in type.
|
||||
</p>
|
||||
<p>
|
||||
We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Default Construction:</strong></span> To introduce
|
||||
a formally uninitialized wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Construction via copy:</strong></span>
|
||||
To introduce a formally initialized wrapped object whose value is obtained
|
||||
as a copy of some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
|
||||
a new yet equivalent wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
|
||||
To assign a value to the wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
|
||||
To initialize the wrapped object with a value obtained as a copy of
|
||||
some object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon initialized):</strong></span> To
|
||||
assign to the wrapped object the value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
|
||||
initialize the wrapped object with value of another wrapped object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Deep Relational Operations (when supported by
|
||||
the type T):</strong></span> To compare wrapped object values taking into
|
||||
account the presence of uninitialized states.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
|
||||
object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Initialization state query:</strong></span> To determine
|
||||
if the object is formally initialized or not.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
|
||||
(with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="bold"><strong>De-initialization:</strong></span> To release the
|
||||
wrapped object (if any) and leave the wrapper in the uninitialized
|
||||
state.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Additional operations are useful, such as converting constructors and converting
|
||||
assignments, in-place construction and assignment, and safe value access
|
||||
via a pointer to the wrapped object or null.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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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</p>
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</div></td>
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<a name="boost_optional.tutorial.exception_safety_guarantees"></a><a class="link" href="exception_safety_guarantees.html" title="Exception Safety Guarantees">Exception
|
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Safety Guarantees</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
This library assumes that <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
destructor does not throw exceptions. If it does, the behaviour of many operations
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
undefined.
|
||||
</p>
|
||||
<p>
|
||||
The following mutating operations never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">none_t</span>
|
||||
<span class="special">)</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
In addition, the following constructors and the destructor never throw exceptions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">()</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">optional</span><span class="special">(</span>
|
||||
<span class="identifier">none_t</span> <span class="special">)</span>
|
||||
<span class="keyword">noexcept</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Regarding the following assignment functions:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">InPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory</span><span class="special">></span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
|
||||
<span class="keyword">const</span><span class="special">&</span>
|
||||
<span class="special">)</span> </code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
They forward calls to the corresponding <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructors or assignments (depending on whether the optional object is
|
||||
initialized or not); so if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
constructor and the assignment provide strong exception safety guarantee,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also provides strong exception safety guarantee; otherwise we
|
||||
only get the basic guarantee. Additionally, if both involved <code class="computeroutput"><span class="identifier">T</span></code>'s constructor and the assignment never
|
||||
throw, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s
|
||||
assignment also never throws.
|
||||
</p>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">T</span></code>'s constructor or
|
||||
assignment throws, assignments to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
do not throw anything else on its own. A throw during assignment never changes
|
||||
the initialization state of any optional object involved:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt1</span><span class="special">(</span><span class="identifier">val1</span><span class="special">);</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">opt2</span><span class="special">(</span><span class="identifier">val2</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
|
||||
<span class="keyword">try</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt2</span><span class="special">;</span> <span class="comment">// throws</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">catch</span><span class="special">(...)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">opt2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This also applies to move assignments/constructors. However, move operations
|
||||
are made no-throw more often.
|
||||
</p>
|
||||
<p>
|
||||
Operation <code class="computeroutput"><span class="identifier">emplace</span></code> provides
|
||||
basic exception safety guarantee. If it throws, the optional object becomes
|
||||
uninitialized regardless of its initial state, and its previous contained
|
||||
value (if any) is destroyed. It doesn't call any assignment or move/copy
|
||||
constructor on <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.exception_safety_guarantees.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.exception_safety_guarantees.swap"></a></span><a class="link" href="exception_safety_guarantees.html#boost_optional.tutorial.exception_safety_guarantees.swap">Swap</a>
|
||||
</h5>
|
||||
<p>
|
||||
Unless <code class="computeroutput"><span class="identifier">swap</span></code> on optional is
|
||||
customized, its primary implementation forwards calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> or move constructor
|
||||
(depending on the initialization state of the optional objects). Thus, if
|
||||
both <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
and move constructor never throw, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> never
|
||||
throws. similarly, if both <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and move constructor
|
||||
offer strong guarantee, <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
on <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> also
|
||||
offers a strong guarantee.
|
||||
</p>
|
||||
<p>
|
||||
In case <code class="computeroutput"><span class="identifier">swap</span></code> on optional
|
||||
is customized, the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
move constructor are replaced with the calls to <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
default constructor followed by <code class="computeroutput"><span class="identifier">swap</span></code>.
|
||||
(This is more useful on older compilers that do not support move semantics,
|
||||
when one wants to achieve stronger exception safety guarantees.) In this
|
||||
case the exception safety guarantees for <code class="computeroutput"><span class="identifier">swap</span></code>
|
||||
are reliant on the guarantees of <code class="computeroutput"><span class="identifier">T</span></code>'s
|
||||
<code class="computeroutput"><span class="identifier">swap</span></code> and default constructor
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
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</div></td>
|
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</tr></table>
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<div class="titlepage"><div><div><h3 class="title">
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<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,
|
||||
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"><</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>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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<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>
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||||
<html>
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||||
<head>
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||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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|
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.false_positive_with__wmaybe_uninitialized"></a><a class="link" href="false_positive_with__wmaybe_uninitialized.html" title="False positive with -Wmaybe-uninitialized">False
|
||||
positive with -Wmaybe-uninitialized</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Sometimes on GCC compilers below version 5.1 you may get an -Wmaybe-uninitialized
|
||||
warning when compiling with option -02 on a perfectly valid <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> usage. For instance in this
|
||||
program:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getitem</span><span class="special">();</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*[])</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">getitem</span><span class="special">();</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">argc</span> <span class="special">></span> <span class="number">0</span><span class="special">)</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="identifier">argc</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">!=</span> <span class="identifier">b</span><span class="special">)</span>
|
||||
<span class="keyword">return</span> <span class="number">1</span><span class="special">;</span>
|
||||
|
||||
<span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is a bug in the compiler. As a workaround (provided in <a href="http://stackoverflow.com/questions/21755206/how-to-get-around-gcc-void-b-4-may-be-used-uninitialized-in-this-funct" target="_top">this
|
||||
Stack Overflow question</a>) use the following way of initializing
|
||||
an optional containing no value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">make_optional</span><span class="special">(</span><span class="keyword">false</span><span class="special">,</span> <span class="keyword">int</span><span class="special">());</span>
|
||||
</pre>
|
||||
<p>
|
||||
This is obviously redundant, but makes the warning disappear.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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<a accesskey="p" href="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.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>
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<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.mixed_relational_comparisons"></a><a class="link" href="mixed_relational_comparisons.html" title="Mixed relational comparisons">Mixed
|
||||
relational comparisons</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
Because <code class="computeroutput"><span class="identifier">T</span></code> is convertible
|
||||
to <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
and because <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> when <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>, you can sometimes
|
||||
get an unexpected runtime result where you would rather expect a compiler
|
||||
error:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="identifier">Flight_plan</span><span class="special">::</span><span class="identifier">weight</span><span class="special">();</span> <span class="comment">// sometimes no weight can be returned</span>
|
||||
|
||||
<span class="keyword">bool</span> <span class="identifier">is_aircraft_too_heavy</span><span class="special">(</span><span class="identifier">Flight_plan</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">p</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">return</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">weight</span><span class="special">()</span> <span class="special">></span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">aircraft</span><span class="special">().</span><span class="identifier">max_weight</span><span class="special">();</span> <span class="comment">// compiles!</span>
|
||||
<span class="special">}</span> <span class="comment">// returns false when the optional contains no value </span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
||||
<div class="spirit-nav">
|
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@@ -1,63 +0,0 @@
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<html>
|
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<head>
|
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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|
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<div class="section">
|
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<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.gotchas.moved_from__optional_"></a><a class="link" href="moved_from__optional_.html" title="Moved-from optional">Moved-from
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code></a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
When an optional object that contains a value is moved from (is a source
|
||||
of move constructor or assignment) it still contains a value and its contained
|
||||
value is left in a moved-from state. This can be illustrated with the following
|
||||
example.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opi</span> <span class="special">{</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">make_unique</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="number">1</span><span class="special">)};</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">opj</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">opi</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">opi</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(*</span><span class="identifier">opi</span> <span class="special">==</span> <span class="keyword">nullptr</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
Quite a lot of people expect that when an object that contains a value
|
||||
is moved from, its contained value should be destroyed. This is not so,
|
||||
for performance reasons. Current semantics allow the implementation of
|
||||
<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to be trivially copyable when <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is trivial.
|
||||
</p>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
||||
<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
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<div class="spirit-nav">
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<a accesskey="p" href="../gotchas.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../gotchas.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="mixed_relational_comparisons.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
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<a name="boost_optional.tutorial.in_place_factories"></a><a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
One of the typical problems with wrappers and containers is that their interfaces
|
||||
usually provide an operation to initialize or assign the contained object
|
||||
as a copy of some other object. This not only requires the underlying type
|
||||
to be <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, but also requires
|
||||
the existence of a fully constructed object, often temporary, just to follow
|
||||
the copy from:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="special">(</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Temporary object created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A solution to this problem is to support direct construction of the contained
|
||||
object right in the container's storage. In this scheme, the user only needs
|
||||
to supply the arguments to the constructor to use in the wrapped object construction.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="keyword">int</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">a1</span><span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">a0</span><span class="special">,</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
A limitation of this method is that it doesn't scale well to wrapped objects
|
||||
with multiple constructors nor to generic code were the constructor overloads
|
||||
are unknown.
|
||||
</p>
|
||||
<p>
|
||||
The solution presented in this library is the family of <span class="bold"><strong>InPlaceFactories</strong></span>
|
||||
and <span class="bold"><strong>TypedInPlaceFactories</strong></span>. These factories
|
||||
are a family of classes which encapsulate an increasing number of arbitrary
|
||||
constructor parameters and supply a method to construct an object of a given
|
||||
type using those parameters at an address specified by the user via placement
|
||||
new.
|
||||
</p>
|
||||
<p>
|
||||
For example, one member of this family looks like:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">A1</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">TypedInPlaceFactory2</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">A0</span> <span class="identifier">m_a0</span> <span class="special">;</span> <span class="identifier">A1</span> <span class="identifier">m_a1</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">TypedInPlaceFactory2</span><span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A1</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a1</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">m_a0</span><span class="special">(</span><span class="identifier">a0</span><span class="special">),</span> <span class="identifier">m_a1</span><span class="special">(</span><span class="identifier">a1</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">construct</span> <span class="special">(</span> <span class="keyword">void</span><span class="special">*</span> <span class="identifier">p</span> <span class="special">)</span> <span class="special">{</span> <span class="keyword">new</span> <span class="special">(</span><span class="identifier">p</span><span class="special">)</span> <span class="identifier">T</span><span class="special">(</span><span class="identifier">m_a0</span><span class="special">,</span><span class="identifier">m_a1</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
A wrapper class aware of this can use it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a TypedInPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">TypedInPlaceFactory2</span><span class="special"><</span><span class="identifier">X</span><span class="special">,</span><span class="keyword">int</span><span class="special">,</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">))</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are divided in two groups:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
<span class="underline">TypedInPlaceFactories</span>: those which
|
||||
take the target type as a primary template parameter.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
<span class="underline">InPlaceFactories</span>: those with a
|
||||
template <code class="computeroutput"><span class="identifier">construct</span><span class="special">(</span><span class="keyword">void</span><span class="special">*)</span></code>
|
||||
member function taking the target type.
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Within each group, all the family members differ only in the number of parameters
|
||||
allowed.
|
||||
</p>
|
||||
<p>
|
||||
This library provides an overloaded set of helper template functions to construct
|
||||
these factories without requiring unnecessary template parameters:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">InPlaceFactoryN</span> <span class="special"><</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">,</span><span class="keyword">class</span> <span class="identifier">A0</span><span class="special">,...,</span><span class="keyword">class</span> <span class="identifier">AN</span><span class="special">></span>
|
||||
<span class="identifier">TypedInPlaceFactoryN</span> <span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="identifier">A0</span><span class="special">,...,</span><span class="identifier">AN</span><span class="special">></span> <span class="identifier">in_place</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="identifier">A0</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">a0</span><span class="special">,</span> <span class="special">...,</span> <span class="identifier">AN</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">aN</span><span class="special">)</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
In-place factories can be used generically by the wrapper and user as follows:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">W</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span> <span class="identifier">wrapped_</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">public</span><span class="special">:</span>
|
||||
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">X</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span> <span class="special">)</span> <span class="special">:</span> <span class="identifier">wrapped_</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">{}</span>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span> <span class="keyword">class</span> <span class="identifier">InPlaceFactory</span> <span class="special">></span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">fac</span> <span class="special">)</span> <span class="special">{</span> <span class="identifier">fac</span><span class="special">.</span><span class="keyword">template</span> <span class="special"><</span><span class="identifier">X</span><span class="special">></span><span class="identifier">construct</span><span class="special">(&</span><span class="identifier">wrapped_</span><span class="special">)</span> <span class="special">;</span> <span class="special">}</span>
|
||||
|
||||
<span class="special">}</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">void</span> <span class="identifier">foo</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="comment">// Wrapped object constructed in-place via a InPlaceFactory.</span>
|
||||
<span class="comment">// No temporary created.</span>
|
||||
<span class="identifier">W</span> <span class="special">(</span> <span class="identifier">in_place</span><span class="special">(</span><span class="number">123</span><span class="special">,</span><span class="string">"hello"</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
The factories are implemented in the headers: <a href="../../../../../../boost/utility/in_place_factory.hpp" target="_top">in_place_factory.hpp</a>
|
||||
and <a href="../../../../../../boost/utility/typed_in_place_factory.hpp" target="_top">typed_in_place_factory.hpp</a>
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="optional_references/rebinding_semantics_for_assignment_of_optional_references.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.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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</div>
|
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</body>
|
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</html>
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@@ -1,87 +0,0 @@
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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|
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
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<link rel="prev" href="relational_operators.html" title="Relational operators">
|
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<link rel="next" href="optional_references.html" title="Optional references">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<a accesskey="p" href="relational_operators.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="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.io_operators"></a><a class="link" href="io_operators.html" title="IO operators">IO operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is possible to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
with IO streams, provided that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
can be used with streams. IOStream operators are defined in a separate header.
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">iostream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">o1</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cin</span> <span class="special">>></span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
<p>
|
||||
The current implementation does not guarantee any particular output. What
|
||||
it guarantees is that if streaming out and then back in <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
gives the same value, then streaming out and then back in <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will also give back the same result:
|
||||
</p>
|
||||
<p>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">sstream</span><span class="special">></span>
|
||||
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">/</span><span class="identifier">optional_io</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">,</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">x1</span><span class="special">,</span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">stringstream</span> <span class="identifier">s</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special"><<</span> <span class="identifier">o1</span> <span class="special"><<</span> <span class="identifier">oN</span><span class="special">;</span>
|
||||
<span class="identifier">s</span> <span class="special">>></span> <span class="identifier">x1</span> <span class="special">>></span> <span class="identifier">x2</span><span class="special">;</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">x1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span> <span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">x2</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="relational_operators.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="optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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|
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</body>
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||||
</html>
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||||
@@ -1,129 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
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<title>Motivation</title>
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<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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<link rel="prev" href="../../optional/tutorial.html" title="Tutorial">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
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<hr>
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||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.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="design_overview.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.motivation"></a><a class="link" href="motivation.html" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../../optional/tutorial.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="design_overview.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,117 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||||
<title>Optional references</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="io_operators.html" title="IO operators">
|
||||
<link rel="next" href="optional_references/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">
|
||||
</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>
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||||
<td align="center"><a href="../../../../../../index.html">Home</a></td>
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<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>
|
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<td align="center"><a href="../../../../../../more/index.htm">More</a></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
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|
||||
<a accesskey="p" href="io_operators.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="optional_references/rebinding_semantics_for_assignment_of_optional_references.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.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional
|
||||
references</a>
|
||||
</h3></div></div></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references.overview"></a><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview" title="Overview">Overview</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
However, since references are not real objects some restrictions apply
|
||||
and some operations are not available in this case:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Converting constructors
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Converting assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace construction
|
||||
</li>
|
||||
<li class="listitem">
|
||||
InPlace assignment
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access via pointer
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> treats it wrapped pseudo-object
|
||||
much as a real value, a true real reference is stored so aliasing will
|
||||
occur:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> will copy the references but
|
||||
all these references will nonetheless refer to the same object.
|
||||
</li>
|
||||
<li class="listitem">
|
||||
Value-access will actually provide access to the referenced object
|
||||
rather than the reference itself.
|
||||
</li>
|
||||
</ul></div>
|
||||
<div class="caution"><table border="0" summary="Caution">
|
||||
<tr>
|
||||
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
|
||||
<th align="left">Caution</th>
|
||||
</tr>
|
||||
<tr><td align="left" valign="top"><p>
|
||||
On compilers that do not conform to Standard C++ rules of reference binding,
|
||||
some operations on optional references are disabled in order to prevent
|
||||
subtle bugs. For more details see <a class="link" href="../dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
|
||||
and Portability section</a>.
|
||||
</p></td></tr>
|
||||
</table></div>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.optional_references.overview.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.optional_references.overview.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview.rvalue_references">Rvalue
|
||||
references</a>
|
||||
</h6>
|
||||
<p>
|
||||
Rvalue references and lvalue references to const have the ability in C++
|
||||
to extend the life time of a temporary they bind to. Optional references
|
||||
do not have this capability, therefore to avoid surprising effects it is
|
||||
not possible to initialize an optional references from a temporary. Optional
|
||||
rvalue references are disabled altogether. Also, the initialization and
|
||||
assignment of an optional reference to const from rvalue reference is disabled.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
|
||||
</pre>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="io_operators.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="optional_references/rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<html>
|
||||
<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
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<title>Rebinding semantics for assignment of optional references</title>
|
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<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../../index.html" title="Boost.Optional">
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<link rel="up" href="../optional_references.html" title="Optional references">
|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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||||
<a accesskey="p" href="../optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../optional_references.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="../in_place_factories.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h4 class="title">
|
||||
<a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h4></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> the effect is to bind (for the
|
||||
first time) to the object. Clearly, there is no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h6>
|
||||
<a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h6>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has
|
||||
been chosen to provide <span class="bold"><strong>consistency among initialization
|
||||
states</strong></span> even at the expense of lack of consistency with the semantics
|
||||
of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives to behave as much as possible
|
||||
as <code class="computeroutput"><span class="identifier">U</span></code> does whenever it is
|
||||
initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
|
||||
<span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
|
||||
object (whatever that is); which is inconsistent with the other possible
|
||||
case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use
|
||||
Optional's assignment without explicitly handling the previous initialization
|
||||
state unless your code is capable of functioning whether after the assignment,
|
||||
<code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
|
||||
as <code class="computeroutput"><span class="identifier">b</span></code> or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is
|
||||
very likely that binding for the first time isn't either. In such case,
|
||||
assignment to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> shall be prohibited. It is quite
|
||||
possible that in such a scenario it is a precondition that the lvalue must
|
||||
be already initialized. If it isn't, then binding for the first time is
|
||||
OK while rebinding is not which is IMO very unlikely. In such a scenario,
|
||||
you can assign the value itself directly, as in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
|
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@@ -1,265 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<a accesskey="p" href="type_requirements.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="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.performance_considerations"></a><a class="link" href="performance_considerations.html" title="Performance considerations">Performance
|
||||
considerations</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Technical details aside, the memory layout of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
for a generic <code class="computeroutput"><span class="identifier">T</span></code> is more-less
|
||||
this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">aligned_storage_t</span><span class="special"><</span><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">t</span><span class="special">),</span> <span class="keyword">alignof</span><span class="special">(</span><span class="identifier">T</span><span class="special">)></span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Lifetime of the <code class="computeroutput"><span class="identifier">T</span></code> inside
|
||||
<code class="computeroutput"><span class="identifier">_storage</span></code> is manually controlled
|
||||
with placement-<code class="computeroutput"><span class="keyword">new</span></code>s and pseudo-destructor
|
||||
calls. However, for scalar <code class="computeroutput"><span class="identifier">T</span></code>s
|
||||
we use a different way of storage, by simply holding a <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
We call it a <span class="emphasis"><em>direct</em></span> storage. This makes <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a
|
||||
trivially-copyable type for scalar <code class="computeroutput"><span class="identifier">T</span></code>s.
|
||||
This only works for compilers that support defaulted functions (including
|
||||
defaulted move assignment and constructor). On compilers without defaulted
|
||||
functions we still use the direct storage, but <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is no longer recognized as trivially-copyable. Apart from scalar types, we
|
||||
leave the programmer a way of customizing her type, so that it is recognized
|
||||
by <code class="computeroutput"><span class="identifier">optional</span></code> as candidate
|
||||
for optimized storage, by specializing type trait <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional_config</span><span class="special">::</span><span class="identifier">optional_uses_direct_storage_for</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="comment">// not trivial</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">X</span><span class="special">()</span> <span class="special">{}</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">optional_config</span> <span class="special">{</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><></span> <span class="keyword">struct</span> <span class="identifier">optional_uses_direct_storage_for</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span> <span class="special">:</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">true_type</span> <span class="special">{};</span>
|
||||
|
||||
<span class="special">}}</span>
|
||||
</pre>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.performance_considerations.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.controlling_the_size"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.controlling_the_size">Controlling
|
||||
the size</a>
|
||||
</h5>
|
||||
<p>
|
||||
For the purpose of the following analysis, considering memory layouts, we
|
||||
can think of it as:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_initialized</span><span class="special">;</span>
|
||||
<span class="identifier">T</span> <span class="identifier">_storage</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Given type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>, and
|
||||
assuming that <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="keyword">int</span><span class="special">)</span> <span class="special">==</span>
|
||||
<span class="number">4</span></code>, we will get <code class="computeroutput"><span class="keyword">sizeof</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>)</span>
|
||||
<span class="special">==</span> <span class="number">8</span></code>.
|
||||
This is so because of the alignment rules, for our two members we get the
|
||||
following alignment:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align1.png" alt="opt_align1"></span>
|
||||
</p>
|
||||
<p>
|
||||
This means you can fit twice as many <code class="computeroutput"><span class="keyword">int</span></code>s
|
||||
as <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>s into
|
||||
the same space of memory. Therefore, if the size of the objects is critical
|
||||
for your application (e.g., because you want to utilize your CPU cache in
|
||||
order to gain performance) and you have determined you are willing to trade
|
||||
the code clarity, it is recommended that you simply go with type <code class="computeroutput"><span class="keyword">int</span></code> and use some 'magic value' to represent
|
||||
<span class="emphasis"><em>not-an-int</em></span>, or use something like <a href="https://github.com/akrzemi1/markable" target="_top"><code class="computeroutput"><span class="identifier">markable</span></code></a> library.
|
||||
</p>
|
||||
<p>
|
||||
Even if you cannot spare any value of <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
to represent <span class="emphasis"><em>not-an-int</em></span> (e.g., because every value is
|
||||
useful, or you do want to signal <span class="emphasis"><em>not-an-int</em></span> explicitly),
|
||||
at least for <code class="computeroutput"><span class="identifier">Trivial</span></code> types
|
||||
you should consider storing the value and the <code class="computeroutput"><span class="keyword">bool</span></code>
|
||||
flag representing the <span class="emphasis"><em>null-state</em></span> separately. Consider
|
||||
the following class:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Its memory layout can be depicted as follows:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align2.png" alt="opt_align2"></span>
|
||||
</p>
|
||||
<p>
|
||||
This is exactly the same as if we had the following members:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
But when they are stored separately, we at least have an option to reorder
|
||||
them like this:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_min</span><span class="special">;</span>
|
||||
<span class="keyword">bool</span> <span class="identifier">_has_max</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
Which gives us the following layout (and smaller total size):
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align3.png" alt="opt_align3"></span>
|
||||
</p>
|
||||
<p>
|
||||
Sometimes it requires detailed consideration what data we make optional.
|
||||
In our case above, if we determine that both minimum and maximum value can
|
||||
be provided or not provided together, but one is never provided without the
|
||||
other, we can make only one optional member:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">Limits</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_min</span><span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">_max</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
|
||||
<span class="keyword">struct</span> <span class="identifier">Record</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Limits</span><span class="special">></span> <span class="identifier">_limits</span><span class="special">;</span>
|
||||
<span class="special">};</span>
|
||||
</pre>
|
||||
<p>
|
||||
This would give us the following layout:
|
||||
</p>
|
||||
<p>
|
||||
<span class="inlinemediaobject"><img src="../../images/opt_align4.png" alt="opt_align4"></span>
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.performance_considerations.h1"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.performance_considerations.optional_function_parameters"></a></span><a class="link" href="performance_considerations.html#boost_optional.tutorial.performance_considerations.optional_function_parameters">Optional
|
||||
function parameters</a>
|
||||
</h5>
|
||||
<p>
|
||||
Having function parameters of type <code class="computeroutput"><span class="keyword">const</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span></code>
|
||||
may incur certain unexpected run-time cost connected to copy construction
|
||||
of <code class="computeroutput"><span class="identifier">T</span></code>. Consider the following
|
||||
code.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">>&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// copy constructor of Big</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
No copy elision or move semantics can save us from copying type <code class="computeroutput"><span class="identifier">Big</span></code> here. Not that we need any copy, but
|
||||
this is how <code class="computeroutput"><span class="identifier">optional</span></code> works.
|
||||
In order to avoid copying in this case, one could provide second overload
|
||||
of <code class="computeroutput"><span class="identifier">fun</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&</span> <span class="identifier">v</span><span class="special">)</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">doSomethingWith</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy: second overload selected</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
Alternatively, you could consider using an optional reference instead:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">void</span> <span class="identifier">fun</span><span class="special">(</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Big</span><span class="special">&></span> <span class="identifier">v</span><span class="special">)</span> <span class="comment">// note where the reference is</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">v</span><span class="special">)</span> <span class="identifier">doSomethingWith</span><span class="special">(*</span><span class="identifier">v</span><span class="special">);</span>
|
||||
<span class="keyword">else</span> <span class="identifier">doSomethingElse</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">Big</span><span class="special">></span> <span class="identifier">ov</span><span class="special">;</span>
|
||||
<span class="identifier">Big</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">ov</span><span class="special">);</span> <span class="comment">// doesn't compile</span>
|
||||
<span class="identifier">fun</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span> <span class="comment">// no copy</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="type_requirements.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="../../optional/reference.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
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||||
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||||
</html>
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||||
@@ -1,149 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
|
||||
<title>Rebinding semantics for assignment of optional references</title>
|
||||
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<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
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||||
<link rel="prev" href="optional_references.html" title="Optional references">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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||||
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<a accesskey="p" href="optional_references.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="in_place_factories.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding
|
||||
semantics for assignment of optional references</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
the effect is to bind (for the first time) to the object. Clearly, there
|
||||
is no other choice.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If you assign to a bare C++ reference, the assignment is forwarded to the
|
||||
referenced object; its value changes but the reference is never rebound.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span>
|
||||
<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span>
|
||||
</pre>
|
||||
<p>
|
||||
Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>,
|
||||
the effect is to <span class="bold"><strong>rebind</strong></span> to the new object
|
||||
instead of assigning the referee. This is unlike bare C++ references.
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
|
||||
<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span>
|
||||
<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span>
|
||||
</pre>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a>
|
||||
</h5>
|
||||
<p>
|
||||
Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> references has been
|
||||
chosen to provide <span class="bold"><strong>consistency among initialization
|
||||
states</strong></span> even at the expense of lack of consistency with the semantics
|
||||
of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code>
|
||||
strives to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>,
|
||||
doing so would result in inconsistent behavior w.r.t to the lvalue initialization
|
||||
state.
|
||||
</p>
|
||||
<p>
|
||||
Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
forwarding assignment to the referenced object (thus changing the referenced
|
||||
object value but not rebinding), and consider the following code:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span>
|
||||
<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span>
|
||||
<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span>
|
||||
<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
What does the assignment do?
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>,
|
||||
the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code>
|
||||
(we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>).
|
||||
But what if <code class="computeroutput"><span class="identifier">a</span></code> is already
|
||||
<span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced
|
||||
object (whatever that is); which is inconsistent with the other possible
|
||||
case.
|
||||
</p>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use
|
||||
Optional's assignment without explicitly handling the previous initialization
|
||||
state unless your code is capable of functioning whether after the assignment,
|
||||
<code class="computeroutput"><span class="identifier">a</span></code> aliases the same object
|
||||
as <code class="computeroutput"><span class="identifier">b</span></code> or not.
|
||||
</p>
|
||||
<p>
|
||||
That is, you would have to discriminate in order to be consistent.
|
||||
</p>
|
||||
<p>
|
||||
If in your code rebinding to another object is not an option, then it is
|
||||
very likely that binding for the first time isn't either. In such case, assignment
|
||||
to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
|
||||
shall be prohibited. It is quite possible that in such a scenario it is a
|
||||
precondition that the lvalue must be already initialized. If it isn't, then
|
||||
binding for the first time is OK while rebinding is not which is IMO very
|
||||
unlikely. In such a scenario, you can assign the value itself directly, as
|
||||
in:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span>
|
||||
<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span>
|
||||
</pre>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
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|
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||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
|
||||
operators</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
<a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
|
||||
objects containing a value compare in the same way as their contained values.
|
||||
The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is treated as a distinct value, equal to itself, and unequal to any value
|
||||
of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
|
||||
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="identifier">o1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="identifier">o0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
as well as from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> implies the existence and semantics
|
||||
of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> as
|
||||
well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="number">1</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
This mixed comparison has a practical interpretation, which is occasionally
|
||||
useful:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">ask_user</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">choice</span> <span class="special">==</span> <span class="number">2</span><span class="special">)</span>
|
||||
<span class="identifier">start_procedure_2</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the above example, the meaning of the comparison is 'user chose number
|
||||
2'. If user chose nothing, he didn't choose number 2.
|
||||
</p>
|
||||
<p>
|
||||
In case where <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
|
||||
required that <code class="computeroutput"><span class="identifier">T</span></code> be <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
|
||||
</p>
|
||||
<p>
|
||||
In a similar manner, type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. The optional
|
||||
object containing no value is compared less than any value of <code class="computeroutput"><span class="identifier">T</span></code>. To illustrate this, if the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">size_t</span></code> is {<code class="computeroutput"><span class="number">0</span></code>, <code class="computeroutput"><span class="number">1</span></code>,
|
||||
<code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
|
||||
of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span></code>
|
||||
is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, <code class="computeroutput"><span class="number">0</span></code>,
|
||||
<code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
|
||||
...}. This order does not have a practical interpretation. The goal is to
|
||||
have any semantically correct default ordering in order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> to
|
||||
be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is usable).
|
||||
</p>
|
||||
<p>
|
||||
Mixed relational operators are the only case where the contained value of
|
||||
an optional object can be inspected without the usage of value accessing
|
||||
function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="when_to_use_optional.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="io_operators.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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||||
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|
||||
</body>
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||||
</html>
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||||
@@ -1,109 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||||
<title>Type requirements</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../../index.html" title="Boost.Optional">
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<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
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|
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../boost.png"></td>
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<td align="center"><a href="../../../../../../index.html">Home</a></td>
|
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<td align="center"><a href="../../../../../../libs/libraries.htm">Libraries</a></td>
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<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="exception_safety_guarantees.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="performance_considerations.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.type_requirements"></a><a class="link" href="type_requirements.html" title="Type requirements">Type requirements</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
The very minimum requirement of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is that <code class="computeroutput"><span class="identifier">T</span></code> is a complete type
|
||||
and that it has a publicly accessible destructor. <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
doesn't even need to be constructible. You can use a very minimum interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span> <span class="comment">// uninitialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">o</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// check if initialized</span>
|
||||
<span class="identifier">assert</span><span class="special">(!</span><span class="identifier">o</span><span class="special">);</span> <span class="comment">//</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// always throws</span>
|
||||
</pre>
|
||||
<p>
|
||||
But this is practically useless. In order for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to be able to do anything useful and offer all the spectrum of ways of accessing
|
||||
the contained value, <code class="computeroutput"><span class="identifier">T</span></code> needs
|
||||
to have at least one accessible constructor. In that case you need to initialize
|
||||
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>, or if your compiler does not support it,
|
||||
resort to <a class="link" href="in_place_factories.html" title="In-Place Factories">In-Place
|
||||
Factories</a>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span><span class="special">;</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"T"</span><span class="special">,</span> <span class="string">"ctor"</span><span class="special">,</span> <span class="string">"params"</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>,
|
||||
<code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
can be easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and be passed by value:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>();</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a>
|
||||
and can be easily initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">v</span><span class="special">;</span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">o</span><span class="special">;</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is not <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>, it is still possible to
|
||||
reset the value of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
using function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">></span> <span class="identifier">o</span> <span class="special">=</span> <span class="identifier">make_T</span><span class="special">();</span>
|
||||
<span class="identifier">o</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">make_another_T</span><span class="special">());</span>
|
||||
</pre>
|
||||
<p>
|
||||
If <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>
|
||||
(both <code class="computeroutput"><span class="identifier">MoveConstructible</span></code> and
|
||||
<code class="computeroutput"><span class="identifier">MoveAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Moveable</span></code> and additionally
|
||||
can be constructed and assigned from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Similarly, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Copyable</span></code> (both <a href="../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>) then <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is also <code class="computeroutput"><span class="identifier">Copyable</span></code> and additionally
|
||||
can be constructed and assigned from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="emphasis"><em>is not</em></span>
|
||||
required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>.
|
||||
</p>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="exception_safety_guarantees.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="performance_considerations.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
|
||||
</div>
|
||||
</body>
|
||||
</html>
|
||||
@@ -1,142 +0,0 @@
|
||||
<html>
|
||||
<head>
|
||||
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
|
||||
<title>When to use Optional</title>
|
||||
<link rel="stylesheet" href="../../../../../../doc/src/boostbook.css" type="text/css">
|
||||
<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
|
||||
<link rel="home" href="../../index.html" title="Boost.Optional">
|
||||
<link rel="up" href="../../optional/tutorial.html" title="Tutorial">
|
||||
<link rel="prev" href="design_overview/the_interface.html" title="The Interface">
|
||||
<link rel="next" href="relational_operators.html" title="Relational operators">
|
||||
</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="design_overview/the_interface.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="relational_operators.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.when_to_use_optional"></a><a class="link" href="when_to_use_optional.html" title="When to use Optional">When to
|
||||
use Optional</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
It is recommended to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
in situations where there is exactly one, clear (to all parties) reason for
|
||||
having no value of type <code class="computeroutput"><span class="identifier">T</span></code>,
|
||||
and where the lack of value is as natural as having any regular value of
|
||||
<code class="computeroutput"><span class="identifier">T</span></code>. One example of such situation
|
||||
is asking the user in some GUI form to optionally specify some limit on an
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> value, but the user is allowed
|
||||
to say 'I want the number not to be constrained by the maximum'. For another
|
||||
example, consider a config parameter specifying how many threads the application
|
||||
should launch. Leaving this parameter unspecified means that the application
|
||||
should decide itself. For yet another example, consider a function returning
|
||||
the index of the smallest element in a <code class="computeroutput"><span class="identifier">vector</span></code>.
|
||||
We need to be prepared for the situation, where the <code class="computeroutput"><span class="identifier">vector</span></code>
|
||||
is empty. Therefore a natural signature for such function would be:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="identifier">optional</span><span class="special"><</span><span class="identifier">size_t</span><span class="special">></span> <span class="identifier">find_smallest_elem</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">vec</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
Here, having received an empty <code class="computeroutput"><span class="identifier">vec</span></code>
|
||||
and having no <code class="computeroutput"><span class="identifier">size_t</span></code> to return
|
||||
is not a <span class="emphasis"><em>failure</em></span> but a <span class="emphasis"><em>normal</em></span>,
|
||||
albeit irregular, situation.
|
||||
</p>
|
||||
<p>
|
||||
Another typical situation is to indicate that we do not have a value yet,
|
||||
but we expect to have it later. This notion can be used in implementing solutions
|
||||
like lazy initialization or a two-phase initialization.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code> can be used to take
|
||||
a non-<a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a> type <code class="computeroutput"><span class="identifier">T</span></code> and create a sibling type with a default
|
||||
constructor. This is a way to add a <span class="emphasis"><em>null-state</em></span> to any
|
||||
type that doesn't have it already.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes type <code class="computeroutput"><span class="identifier">T</span></code> already
|
||||
provides a built-in null-state, but it may still be useful to wrap it into
|
||||
<code class="computeroutput"><span class="identifier">optional</span></code>. Consider <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span></code>.
|
||||
When you read a piece of text from a GUI form or a DB table, it is hardly
|
||||
ever that the empty string indicates anything else but a missing text. And
|
||||
some data bases do not even distinguish between a null string entry and a
|
||||
non-null string of length 0. Still, it may be practical to use <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">string</span><span class="special">></span></code>
|
||||
to indicate in the returned type that we want to treat the empty string in
|
||||
a special dedicated program path:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">if</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></span> <span class="identifier">name</span> <span class="special">=</span> <span class="identifier">ask_user_name</span><span class="special">())</span> <span class="special">{</span>
|
||||
<span class="identifier">assert</span><span class="special">(*</span><span class="identifier">name</span> <span class="special">!=</span> <span class="string">""</span><span class="special">);</span>
|
||||
<span class="identifier">logon_as</span><span class="special">(*</span><span class="identifier">name</span><span class="special">);</span>
|
||||
<span class="special">}</span>
|
||||
<span class="keyword">else</span> <span class="special">{</span>
|
||||
<span class="identifier">skip_logon</span><span class="special">();</span>
|
||||
<span class="special">}</span>
|
||||
</pre>
|
||||
<p>
|
||||
In the example above, the assertion indicates that if we choose to use this
|
||||
technique, we must translate the empty string state to an optional object
|
||||
with no contained value (inside function <code class="computeroutput"><span class="identifier">ask_user_name</span></code>).
|
||||
</p>
|
||||
<h5>
|
||||
<a name="boost_optional.tutorial.when_to_use_optional.h0"></a>
|
||||
<span class="phrase"><a name="boost_optional.tutorial.when_to_use_optional.not_recommended_usages"></a></span><a class="link" href="when_to_use_optional.html#boost_optional.tutorial.when_to_use_optional.not_recommended_usages">Not
|
||||
recommended usages</a>
|
||||
</h5>
|
||||
<p>
|
||||
It is not recommended to use <code class="computeroutput"><span class="identifier">optional</span></code>
|
||||
to indicate that we were not able to compute a value because of a <span class="emphasis"><em>failure</em></span>.
|
||||
It is difficult to define what a failure is, but it usually has one common
|
||||
characteristic: an associated information on the cause of the failure. This
|
||||
can be the type and member data of an exception object, or an error code.
|
||||
It is a bad design to signal a failure and not inform about the cause. If
|
||||
you do not want to use exceptions, and do not like the fact that by returning
|
||||
error codes you cannot return the computed value, you can use <a href="https://github.com/ptal/Boost.Expected" target="_top">Expected</a>
|
||||
library. It is sort of <a href="../../../../../variant/index.html" target="_top">Boost.Variant</a>
|
||||
that contains either a computed value or a reason why the computation failed.
|
||||
</p>
|
||||
<p>
|
||||
Sometimes the distinction into what is a failure and what is a valid but
|
||||
irregular result is blurry and depends on a particular usage and personal
|
||||
preference. Consider a function that converts a <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
to an <code class="computeroutput"><span class="keyword">int</span></code>. Is it a failure that
|
||||
you cannot convert? It might in some cases, but in other you may call it
|
||||
exactly for the purpose of figuring out if a given <code class="computeroutput"><span class="identifier">string</span></code>
|
||||
is convertible, and you are not even interested in the resulting value. Sometimes
|
||||
when a conversion fails you may not consider it a failure, but you need to
|
||||
know why it cannot be converted; for instance at which character it is determined
|
||||
that the conversion is impossible. In this case returning <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
will not suffice. Finally, there is a use case where an input string that
|
||||
does not represent an <code class="computeroutput"><span class="keyword">int</span></code> is
|
||||
not a failure condition, but during the conversion we use resources whose
|
||||
acquisition may fail. In that case the natural representation is to both
|
||||
return <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code> and
|
||||
signal failure:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">convert1</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// throws</span>
|
||||
<span class="identifier">expected</span><span class="special"><</span><span class="identifier">ErrorT</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>></span> <span class="identifier">convert2</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&</span> <span class="identifier">str</span><span class="special">);</span> <span class="comment">// return either optional or error</span>
|
||||
</pre>
|
||||
</div>
|
||||
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
|
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<td align="left"></td>
|
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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>
|
||||
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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</p>
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|
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<p>
|
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At the very minimum for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
to work with a minimum interface it is required that <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
has a publicly accessible no-throw destructor. In that case you need to initialize
|
||||
the optional object with function <code class="computeroutput"><span class="identifier">emplace</span><span class="special">()</span></code> or use <span class="bold"><strong>InPlaceFactories</strong></span>.
|
||||
Additionally, if <code class="computeroutput"><span class="identifier">T</span></code> is <code class="computeroutput"><span class="identifier">Moveable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
|
||||
is also <code class="computeroutput"><span class="identifier">Moveable</span></code> and can be
|
||||
easily initialized from an rvalue of type <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
and be passed by value. Additionally, if <code class="computeroutput"><span class="identifier">T</span></code>
|
||||
is <code class="computeroutput"><span class="identifier">Copyable</span></code>, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> is
|
||||
also <code class="computeroutput"><span class="identifier">Copyable</span></code> and can be easily
|
||||
initialized from an lvalue of type <code class="computeroutput"><span class="identifier">T</span></code>.
|
||||
</p>
|
||||
<p>
|
||||
<code class="computeroutput"><span class="identifier">T</span></code> <span class="underline">is
|
||||
not</span> required to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top">Default
|
||||
Constructible</a>.
|
||||
</p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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||||
<div><h2 class="title">
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<a name="optional"></a>Boost.Optional</h2></div>
|
||||
<div><div class="author"><h3 class="author">
|
||||
<span class="firstname">Fernando Luis</span> <span class="surname">Cacciola Carballal</span>
|
||||
</h3></div></div>
|
||||
<div><p class="copyright">Copyright © 2003-2007 Fernando Luis Cacciola Carballal</p></div>
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<div><p class="copyright">Copyright © 2014-2021 Andrzej Krzemieński</p></div>
|
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<div><div class="legalnotice">
|
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<a name="optional.legal"></a><p>
|
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Distributed under the Boost Software License, Version 1.0. (See accompanying
|
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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></div>
|
||||
</div></div>
|
||||
<div class="toc">
|
||||
<p><b>Table of Contents</b></p>
|
||||
<dl class="toc">
|
||||
<dt><span class="section"><a href="index.html#optional.introduction">Introduction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html">Quick Start</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start.html#boost_optional.quick_start.optional_return_values">Optional
|
||||
return values</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_automatic_variables.html">Optional
|
||||
automatic variables</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/optional_data_members.html">Optional
|
||||
data members</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/bypassing_unnecessary_default_construction.html">Bypassing
|
||||
unnecessary default construction</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/quick_start/storage_in_containers.html">Storage
|
||||
in containers</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/tutorial.html">Tutorial</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="optional/reference.html">Reference</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="optional/reference.html#boost_optional.reference.header__boost_none_hpp_">Header
|
||||
<boost/none.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_bad_optional_access_hpp_.html">Header
|
||||
<boost/optional/bad_optional_access.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html">Header
|
||||
<boost/optional/optional_fwd.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="optional/reference/header__boost_optional_optional_hpp_.html">Header
|
||||
<boost/optional/optional.hpp></a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/reference/header__boost_optional_hpp_.html">Header
|
||||
<boost/optional.hpp></a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html">Dependencies
|
||||
and Portability</a></span></dt>
|
||||
<dd><dl>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability.html#boost_optional.dependencies_and_portability.dependencies">Dependencies</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/emplace_operations_in_older_compilers.html">Emplace
|
||||
operations in older compilers</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/dependencies_and_portability/optional_reference_binding.html">Optional
|
||||
Reference Binding</a></span></dt>
|
||||
</dl></dd>
|
||||
<dt><span class="section"><a href="boost_optional/relnotes.html">Release Notes</a></span></dt>
|
||||
<dt><span class="section"><a href="boost_optional/acknowledgements.html">Acknowledgements</a></span></dt>
|
||||
</dl>
|
||||
</div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.introduction"></a><a class="link" href="index.html#optional.introduction" title="Introduction">Introduction</a>
|
||||
</h2></div></div></div>
|
||||
<p>
|
||||
Class template <code class="computeroutput"><span class="identifier">optional</span></code> is
|
||||
a wrapper for representing 'optional' (or 'nullable') objects who may not (yet)
|
||||
contain a valid value. Optional objects offer full value semantics; they are
|
||||
good for passing by value and usage inside STL containers. This is a header-only
|
||||
library.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h0"></a>
|
||||
<span class="phrase"><a name="optional.introduction.problem"></a></span><a class="link" href="index.html#optional.introduction.problem">Problem</a>
|
||||
</h4>
|
||||
<p>
|
||||
Suppose we want to read a parameter from a config file which represents some
|
||||
integral value, let's call it <code class="computeroutput"><span class="string">"MaxValue"</span></code>.
|
||||
It is possible that this parameter is not specified; such situation is no error.
|
||||
It is valid to not specify the parameter and in that case the program is supposed
|
||||
to behave slightly differently. Also, suppose that any possible value of type
|
||||
<code class="computeroutput"><span class="keyword">int</span></code> is a valid value for <code class="computeroutput"><span class="string">"MaxValue"</span></code>, so we cannot just use
|
||||
<code class="computeroutput"><span class="special">-</span><span class="number">1</span></code>
|
||||
to represent the absence of the parameter in the config file.
|
||||
</p>
|
||||
<h4>
|
||||
<a name="optional.introduction.h1"></a>
|
||||
<span class="phrase"><a name="optional.introduction.solution"></a></span><a class="link" href="index.html#optional.introduction.solution">Solution</a>
|
||||
</h4>
|
||||
<p>
|
||||
This is how you solve it with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code>:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
|
||||
|
||||
<span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">name</span><span class="special">);</span> <span class="comment">// return either an int or a `not-an-int`</span>
|
||||
|
||||
<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
|
||||
<span class="special">{</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="identifier">getConfigParam</span><span class="special">(</span><span class="string">"MaxValue"</span><span class="special">))</span> <span class="comment">// did I get a real int?</span>
|
||||
<span class="identifier">runWithMax</span><span class="special">(*</span><span class="identifier">oi</span><span class="special">);</span> <span class="comment">// use my int</span>
|
||||
<span class="keyword">else</span>
|
||||
<span class="identifier">runWithNoMax</span><span class="special">();</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"><p><small>Last revised: March 26, 2022 at 22:47:53 GMT</small></p></td>
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<dt><span class="section"><a href="reference.html#boost_optional.reference.header__boost_none_hpp_">Header
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<dt><span class="section"><a href="../boost_optional/reference/io_header.html">Header <boost/optional/optional_io.hpp></a></span></dt>
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<boost/optional/optional_fwd.hpp></a></span></dt>
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<dt><span class="section"><a href="reference/header__boost_optional_optional_hpp_.html">Header
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<boost/optional/optional.hpp></a></span></dt>
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<dt><span class="section"><a href="../boost_optional/reference/header__boost_optional_hpp_.html">Header
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<boost/optional.hpp></a></span></dt>
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<p>
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</p>
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
||||
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<span class="keyword">class</span> <span class="identifier">none_t</span> <span class="special">{/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/};</span>
|
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||||
<span class="keyword">inline</span> <span class="keyword">constexpr</span> <span class="identifier">none_t</span> <span class="identifier">none</span> <span class="special">(/*</span> <span class="identifier">see</span> <span class="identifier">below</span> <span class="special">*/);</span>
|
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|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
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</pre>
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<p>
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</p>
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||||
<p>
|
||||
Class <code class="computeroutput"><span class="identifier">none_t</span></code> is meant to
|
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serve as a tag for selecting appropriate overloads of from <code class="computeroutput"><span class="identifier">optional</span></code>'s interface. It is an empty,
|
||||
trivially copyable class with disabled default constructor.
|
||||
</p>
|
||||
<p>
|
||||
Constant <code class="computeroutput"><span class="identifier">none</span></code> is used to
|
||||
indicate an optional object that does not contain a value in initialization,
|
||||
assignment and relational operations of <code class="computeroutput"><span class="identifier">optional</span></code>.
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</p>
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<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 © 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
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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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<pre class="programlisting"><code class="computeroutput"><span class="comment">// In Header: <</span></code><a href="../../../../../../boost/optional/optional.hpp" target="_top">boost/optional/optional.hpp</a><span class="comment">></span>
|
||||
|
||||
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
|
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|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_t</span> <span class="special">{</span> <span class="comment">/* see below */</span> <span class="special">}</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html#reference_in_place_init"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_t</span> <span class="identifier">in_place_init</span> <span class="special">(</span> <span class="comment">/* see below */</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">class</span> <span class="identifier">in_place_init_if_t</span> <span class="special">{</span> <span class="comment">/*see below*/</span> <span class="special">}</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html#reference_in_place_init_if"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
<span class="keyword">const</span> <span class="identifier">in_place_init_if_t</span> <span class="identifier">in_place_init_if</span> <span class="special">(</span> <span class="comment">/*see below*/</span> <span class="special">)</span> <span class="special">;</span>
|
||||
|
||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_optional_values.html#reference_operator_template"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
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||||
<span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">class</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_optional_refs.html#reference_operator_template_spec"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
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||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_not_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_less_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">></span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_greater_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special"><=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_less_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">>=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_greater_or_equal_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">bool</span> <span class="keyword">operator</span> <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span> <span class="keyword">noexcept</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_operator_compare_not_equal_optional_none"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_rvalue"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_bool_value"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">decay_t</span><span class="special"><</span><span class="identifier">T</span><span class="special">>></span> <span class="identifier">make_optional</span> <span class="special">(</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="special">&&</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_make_optional_bool_rvalue"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_const_type</span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_value_or"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">-></span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">reference_type</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_value_or"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>*</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html#reference_optional_get"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_pointer"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">auto</span> <span class="identifier">get_pointer</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="special">&</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">-></span> <span class="emphasis"><em>see below</em></span><span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_free_get_pointer"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_swap_optional_optional"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span> <span class="keyword">inline</span> <span class="keyword">void</span> <span class="identifier">swap</span><span class="special">(</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&>&</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span> <a class="link" href="../../boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___free_functions.html#reference_swap_optional_reference"><span class="inlinemediaobject"><img src="../../images/callouts/R.png" alt="R"></span></a>
|
||||
|
||||
<span class="special">}</span> <span class="comment">// namespace boost</span>
|
||||
</pre>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
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<div class="spirit-nav">
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<a accesskey="p" href="../../boost_optional/reference/header__boost_optional_optional_fwd_hpp_.html"><img src="../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../reference.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="../../boost_optional/reference/header__boost_optional_optional_hpp_/header_optional_in_place_init.html"><img src="../../../../../../doc/src/images/next.png" alt="Next"></a>
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@@ -1,153 +0,0 @@
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
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<title>Tutorial</title>
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<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets Vsnapshot">
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<link rel="home" href="../index.html" title="Boost.Optional">
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<link rel="prev" href="../boost_optional/quick_start/storage_in_containers.html" title="Storage in containers">
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<div class="section">
|
||||
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="optional.tutorial"></a><a class="link" href="tutorial.html" title="Tutorial">Tutorial</a>
|
||||
</h2></div></div></div>
|
||||
<div class="toc"><dl class="toc">
|
||||
<dt><span class="section"><a href="tutorial.html#boost_optional.tutorial.motivation">Motivation</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/design_overview.html">Design Overview</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/when_to_use_optional.html">When to
|
||||
use Optional</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/relational_operators.html">Relational
|
||||
operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/io_operators.html">IO operators</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/optional_references.html">Optional
|
||||
references</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/in_place_factories.html">In-Place
|
||||
Factories</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/gotchas.html">Gotchas</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/exception_safety_guarantees.html">Exception
|
||||
Safety Guarantees</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/type_requirements.html">Type requirements</a></span></dt>
|
||||
<dt><span class="section"><a href="../boost_optional/tutorial/performance_considerations.html">Performance
|
||||
considerations</a></span></dt>
|
||||
</dl></div>
|
||||
<div class="section">
|
||||
<div class="titlepage"><div><div><h3 class="title">
|
||||
<a name="boost_optional.tutorial.motivation"></a><a class="link" href="tutorial.html#boost_optional.tutorial.motivation" title="Motivation">Motivation</a>
|
||||
</h3></div></div></div>
|
||||
<p>
|
||||
Consider these functions which should return a value but which might not
|
||||
have a value to return:
|
||||
</p>
|
||||
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
|
||||
<li class="listitem">
|
||||
(A) <code class="computeroutput"><span class="keyword">double</span> <span class="identifier">sqrt</span><span class="special">(</span><span class="keyword">double</span> <span class="identifier">n</span> <span class="special">);</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(B) <code class="computeroutput"><span class="keyword">char</span> <span class="identifier">get_async_input</span><span class="special">();</span></code>
|
||||
</li>
|
||||
<li class="listitem">
|
||||
(C) <code class="computeroutput"><span class="identifier">point</span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span></code>
|
||||
</li>
|
||||
</ul></div>
|
||||
<p>
|
||||
There are different approaches to the issue of not having a value to return.
|
||||
</p>
|
||||
<p>
|
||||
A typical approach is to consider the existence of a valid return value as
|
||||
a postcondition, so that if the function cannot compute the value to return,
|
||||
it has either undefined behavior (and can use assert in a debug build) or
|
||||
uses a runtime check and throws an exception if the postcondition is violated.
|
||||
This is a reasonable choice for example, for function (A), because the lack
|
||||
of a proper return value is directly related to an invalid parameter (out
|
||||
of domain argument), so it is appropriate to require the callee to supply
|
||||
only parameters in a valid domain for execution to continue normally.
|
||||
</p>
|
||||
<p>
|
||||
However, function (B), because of its asynchronous nature, does not fail
|
||||
just because it can't find a value to return; so it is incorrect to consider
|
||||
such a situation an error and assert or throw an exception. This function
|
||||
must return, and somehow, must tell the callee that it is not returning a
|
||||
meaningful value.
|
||||
</p>
|
||||
<p>
|
||||
A similar situation occurs with function (C): it is conceptually an error
|
||||
to ask a <span class="emphasis"><em>null-area</em></span> polygon to return a point inside
|
||||
itself, but in many applications, it is just impractical for performance
|
||||
reasons to treat this as an error (because detecting that the polygon has
|
||||
no area might be too expensive to be required to be tested previously), and
|
||||
either an arbitrary point (typically at infinity) is returned, or some efficient
|
||||
way to tell the callee that there is no such point is used.
|
||||
</p>
|
||||
<p>
|
||||
There are various mechanisms to let functions communicate that the returned
|
||||
value is not valid. One such mechanism, which is quite common since it has
|
||||
zero or negligible overhead, is to use a special value which is reserved
|
||||
to communicate this. Classical examples of such special values are <code class="computeroutput"><span class="identifier">EOF</span></code>, <code class="computeroutput"><span class="identifier">string</span><span class="special">::</span><span class="identifier">npos</span></code>,
|
||||
points at infinity, etc...
|
||||
</p>
|
||||
<p>
|
||||
When those values exist, i.e. the return type can hold all meaningful values
|
||||
<span class="emphasis"><em>plus</em></span> the <span class="emphasis"><em>signal</em></span> value, this mechanism
|
||||
is quite appropriate and well known. Unfortunately, there are cases when
|
||||
such values do not exist. In these cases, the usual alternative is either
|
||||
to use a wider type, such as <code class="computeroutput"><span class="keyword">int</span></code>
|
||||
in place of <code class="computeroutput"><span class="keyword">char</span></code>; or a compound
|
||||
type, such as <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>.
|
||||
</p>
|
||||
<p>
|
||||
Returning a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span></code>, thus attaching a boolean flag to the
|
||||
result which indicates if the result is meaningful, has the advantage that
|
||||
can be turned into a consistent idiom since the first element of the pair
|
||||
can be whatever the function would conceptually return. For example, the
|
||||
last two functions could have the following interface:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="keyword">char</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">get_async_input</span><span class="special">();</span>
|
||||
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">polygon</span><span class="special">::</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
</pre>
|
||||
<p>
|
||||
These functions use a consistent interface for dealing with possibly nonexistent
|
||||
results:
|
||||
</p>
|
||||
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">point</span><span class="special">,</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">poly</span><span class="special">.</span><span class="identifier">get_any_point_effectively_inside</span><span class="special">();</span>
|
||||
<span class="keyword">if</span> <span class="special">(</span> <span class="identifier">p</span><span class="special">.</span><span class="identifier">second</span> <span class="special">)</span>
|
||||
<span class="identifier">flood_fill</span><span class="special">(</span><span class="identifier">p</span><span class="special">.</span><span class="identifier">first</span><span class="special">);</span>
|
||||
</pre>
|
||||
<p>
|
||||
However, not only is this quite a burden syntactically, it is also error
|
||||
prone since the user can easily use the function result (first element of
|
||||
the pair) without ever checking if it has a valid value.
|
||||
</p>
|
||||
<p>
|
||||
Clearly, we need a better idiom.
|
||||
</p>
|
||||
</div>
|
||||
</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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2021 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>
|
||||
</div></td>
|
||||
</tr></table>
|
||||
<hr>
|
||||
<div class="spirit-nav">
|
||||
<a accesskey="p" href="../boost_optional/quick_start/storage_in_containers.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="../boost_optional/tutorial/design_overview.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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