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+<html>
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
+<title>Detailed Semantics</title>
+<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
+<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
+<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
+<link rel="up" href="../index.html" title="Chapter 1. Boost.Optional">
+<link rel="prev" href="synopsis.html" title="Synopsis">
+<link rel="next" href="dependencies_and_portability.html" title="Dependencies and Portability">
+</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="synopsis.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>
+</div>
+<div class="section">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="boost_optional.detailed_semantics"></a><a class="link" href="detailed_semantics.html" title="Detailed Semantics">Detailed Semantics</a>
+</h2></div></div></div>
+<p>
+ Because <code class="computeroutput"><span class="identifier">T</span></code> might be of reference
+ type, in the sequel, those entries whose semantic depends on <code class="computeroutput"><span class="identifier">T</span></code> being of reference type or not will be
+ distinguished using the following convention:
+ </p>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ If the entry reads: <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></code>, the
+ description corresponds only to the case where <code class="computeroutput"><span class="identifier">T</span></code>
+ is not of reference type.
+ </li>
+<li class="listitem">
+ If the entry reads: <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>, the description corresponds only
+ to the case where <code class="computeroutput"><span class="identifier">T</span></code> is
+ of reference type.
+ </li>
+<li class="listitem">
+ If the entry reads: <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>, the description is the same for both
+ cases.
+ </li>
+</ul></div>
+<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>
+ The following section contains various <code class="computeroutput"><span class="identifier">assert</span><span class="special">()</span></code> which are used only to show the postconditions
+ as sample code. It is not implied that the type <code class="computeroutput"><span class="identifier">T</span></code>
+ must support each particular expression but that if the expression is supported,
+ the implied condition holds.
+ </p></td></tr>
+</table></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<h4>
+<a name="boost_optional.detailed_semantics.h0"></a>
+ <span class="phrase"><a name="boost_optional.detailed_semantics.optional_class_member_functions"></a></span><a class="link" href="detailed_semantics.html#boost_optional.detailed_semantics.optional_class_member_functions">optional
+ class member functions</a>
+ </h4>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_constructor"></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="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Default-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">uninitialized</span>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> T's default constructor <span class="underline">is not</span> called.
+ </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">def</span> <span class="special">;</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="special">;</span>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_constructor_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="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>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>
+ uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">uninitialized</span>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>'s
+ default constructor <span class="underline">is not</span> called.
+ The expression <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> denotes an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code> that can be used as the parameter.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Example:</strong></span>
+<pre class="programlisting"><span class="preprocessor">#include</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">hpp</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">n</span><span class="special">(</span><span class="identifier">none</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">n</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_constructor_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">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>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_copy_constructible</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>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>
+ and its value is a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">v</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes: </strong></span> <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="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <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="keyword">const</span><span class="special">&</span>
+ <span class="special">);</span></code> in that case, this constructor
+ has no effect.
+ </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">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">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>
+<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">T</span><span class="special">&</span> <span class="identifier">ref</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>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>
+ and its value is an instance of an internal type wrapping the reference
+ <code class="computeroutput"><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">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_constructor_move_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">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>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_move_constructible</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>Effect:</strong></span> Directly-Move-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>
+ and its value is move-constructed from <code class="computeroutput"><span class="identifier">v</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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>Notes: </strong></span> <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> is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <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>
+ in that case, the state of <code class="computeroutput"><span class="identifier">v</span></code>
+ is determined by exception safety guarantees for <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></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Example:</strong></span>
+<pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v1</span><span class="special">,</span> <span class="identifier">v2</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">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">v1</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">v2</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>
+<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">T</span><span class="special">&&</span> <span class="identifier">ref</span> <span class="special">)</span> <span class="special">=</span> <span class="keyword">delete</span></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> This constructor is deleted
+ </li></ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_constructor_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">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></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="special">::</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="special">&</span>
+ <span class="identifier">v</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">
+ If condition is true, same as:
+ </li></ul></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><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></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="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></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
+ otherwise, same as:
+ </li></ul></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> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="identifier">optional</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="special">::</span><span class="identifier">optional</span><span class="special">()</span></code>
+ </p></blockquote></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_constructor_optional"></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">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></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_copy_constructible</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>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If rhs is initialized,
+ <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
+ of <code class="computeroutput"><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is
+ uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If rhs is initialized, <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="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <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="keyword">const</span><span class="special">&</span>
+ <span class="special">);</span></code> in that case, this constructor
+ has no effect.
+ </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">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">T</span><span class="special">(</span><span class="number">2</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">init</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="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="identifier">init2</span> <span class="special">==</span> <span class="identifier">init</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>
+<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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is another reference to the same object referenced
+ by <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
+ else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ and <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
+ will reefer to the same object (they alias).
+ </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_move_constructor_optional"></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">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></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> <code class="computeroutput"><span class="identifier">is_move_constructible</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>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is move constructed from <code class="computeroutput"><span class="identifier">rhs</span></code>;
+ else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <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>
+ is called. The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code>
+ is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <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>
+ in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
+ initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
+ of <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></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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">uninit</span><span class="special">)</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">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="identifier">T</span><span class="special">>></span> <span class="identifier">init</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">uniqye_ptr</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="number">2</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">init</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="special">)</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="identifier">T</span><span class="special">>></span> <span class="identifier">init2</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">init</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init</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="keyword">nullptr</span> <span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init2</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">T</span><span class="special">(</span><span class="number">2</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>
+<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="special">&&</span>
+ <span class="identifier">rhs</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>Effect:</strong></span> Move-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is another reference to the same object referenced
+ by <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
+ else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ and <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
+ will reefer to the same object (they alias).
+ </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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">uninit</span><span class="special">)</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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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="keyword">new</span> <span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">))</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="identifier">T</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="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">std</span><span class="special">::</span><span class="identifier">unique_ptr</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">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(</span><span class="identifier">init</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">init2</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="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_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
+ of rhs converted to type <code class="computeroutput"><span class="identifier">T</span></code>;
+ else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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">U</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span>
+ <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized,
+ which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
+ to <code class="computeroutput"><span class="identifier">T</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span>
+ <span class="keyword">const</span><span class="special">&</span>
+ <span class="special">);</span></code> in that case, this constructor
+ has no effect.
+ </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">x</span><span class="special">(</span><span class="number">123.4</span><span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">x</span> <span class="special">==</span> <span class="number">123.4</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">y</span><span class="special">(</span><span class="identifier">x</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">y</span> <span class="special">==</span> <span class="number">123</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_move_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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="identifier">rhs</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>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is move constructed from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> 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">U</span><span class="special">&&</span>
+ <span class="special">)</span></code> throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span>
+ <span class="identifier">U</span><span class="special">&&</span>
+ <span class="special">)</span></code> is called if <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
+ to <code class="computeroutput"><span class="identifier">T</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&&</span> <span class="special">);</span></code>
+ in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
+ initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
+ guarantee of <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&&</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="keyword">double</span><span class="special">></span> <span class="identifier">x</span><span class="special">(</span><span class="number">123.4</span><span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">x</span> <span class="special">==</span> <span class="number">123.4</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">y</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">x</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">y</span> <span class="special">==</span> <span class="number">123</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_constructor_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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">T</span></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</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></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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">T</span></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>
+ with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
+ from the factory.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>
+ and its value is <span class="emphasis"><em>directly given</em></span> from the factory
+ <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value <span class="underline">is not copied</span>).
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code>
+ constructor called by the factory throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="tutorial/in_place_factories.html" title="In-Place Factories">In-Place
+ Factories</a>
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
+ constructor used by the factory; in that case, this constructor has no
+ effect.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Example:</strong></span>
+<pre class="programlisting"><span class="keyword">class</span> <span class="identifier">C</span> <span class="special">{</span> <span class="identifier">C</span> <span class="special">(</span> <span class="keyword">char</span><span class="special">,</span> <span class="keyword">double</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="identifier">C</span> <span class="identifier">v</span><span class="special">(</span><span class="char">'A'</span><span class="special">,</span><span class="number">123.4</span><span class="special">,</span><span class="string">"hello"</span><span class="special">);</span>
+
+<span class="identifier">optional</span><span class="special"><</span><span class="identifier">C</span><span class="special">></span> <span class="identifier">x</span><span class="special">(</span> <span class="identifier">in_place</span> <span class="special">(</span><span class="char">'A'</span><span class="special">,</span> <span class="number">123.4</span><span class="special">,</span> <span class="string">"hello"</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// InPlaceFactory used</span>
+<span class="identifier">optional</span><span class="special"><</span><span class="identifier">C</span><span class="special">></span> <span class="identifier">y</span><span class="special">(</span> <span class="identifier">in_place</span><span class="special"><</span><span class="identifier">C</span><span class="special">>(</span><span class="char">'A'</span><span class="special">,</span> <span class="number">123.4</span><span class="special">,</span> <span class="string">"hello"</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// TypedInPlaceFactory used</span>
+
+<span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">x</span> <span class="special">==</span> <span class="identifier">v</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">y</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_operator_equal_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">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>Effect:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized destroys its contained
+ value.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_operator_equal_value"></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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">rhs</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>Effect:</strong></span> Assigns the value <code class="computeroutput"><span class="identifier">rhs</span></code> to an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized and its value is a
+ <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">rhs</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code> or <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="keyword">const</span><span class="special">&)</span></code> throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ assignment operator is used, otherwise, its copy-constructor is used.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as <code class="computeroutput"><span class="identifier">optional</span></code> is concerned
+ (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>copy constructor</em></span> fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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="identifier">optional</span><span class="special"><</span><span class="identifier">T</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="identifier">T</span><span class="special">></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
+
+<span class="identifier">T</span> <span class="identifier">y</span><span class="special">;</span>
+<span class="identifier">def</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">def</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span>
+<span class="identifier">opt</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">opt</span> <span class="special">==</span> <span class="identifier">y</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>
+<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">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="special">&</span>
+ <span class="identifier">rhs</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>Effect:</strong></span> (Re)binds the wrapped reference.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized and it references the
+ same object referenced by <code class="computeroutput"><span class="identifier">rhs</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized, it is <span class="emphasis"><em>rebound</em></span>
+ to the new object. See <a class="link" href="tutorial/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">here</a>
+ for details on this behavior.
+ </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_operator_move_equal_value"></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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">rhs</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>Effect:</strong></span> Moves the value <code class="computeroutput"><span class="identifier">rhs</span></code> to an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized and its value is moved
+ from <code class="computeroutput"><span class="identifier">rhs</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">T</span><span class="special">&&</span> <span class="special">)</span></code>
+ or <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></code> throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ move-assignment operator is used, otherwise, its move-constructor is used.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as <code class="computeroutput"><span class="identifier">optional</span></code> is concerned
+ (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>move constructor</em></span> fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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="identifier">optional</span><span class="special"><</span><span class="identifier">T</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="identifier">T</span><span class="special">></span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
+
+<span class="identifier">T</span> <span class="identifier">y1</span><span class="special">,</span> <span class="identifier">y2</span><span class="special">,</span> <span class="identifier">yR</span><span class="special">;</span>
+<span class="identifier">def</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">y1</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">def</span> <span class="special">==</span> <span class="identifier">yR</span> <span class="special">)</span> <span class="special">;</span>
+<span class="identifier">opt</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">y2</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">opt</span> <span class="special">==</span> <span class="identifier">yR</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>
+<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">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="special">&&</span>
+ <span class="identifier">rhs</span> <span class="special">)</span>
+ <span class="special">=</span> <span class="keyword">delete</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>Notes:</strong></span> This assignment operator is deleted.
+ </li></ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_operator_equal_optional"></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Assigns another <code class="computeroutput"><span class="identifier">optional</span></code> to an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
+ of <code class="computeroutput"><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is
+ uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&)</span></code>
+ or <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="keyword">const</span><span class="special">&</span>
+ <span class="special">)</span></code> throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code>
+ are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>assignment operator</em></span> is used. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code> is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ [destructor] is called. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initially uninitialized but <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>copy constructor</em></span> is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>copy constructor</em></span> fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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">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">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">def</span> <span class="special">;</span>
+
+<span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</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="special">;</span>
+<span class="comment">// previous value (copy of 'v') destroyed from within 'opt'.</span>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<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="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">T</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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> (Re)binds thee wrapped reference.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized and it references the
+ same object referenced by <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>; otherwise, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized (and references no
+ object).
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized and so is <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>,
+ <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is <span class="emphasis"><em>rebound</em></span> to the new object. See <a class="link" href="tutorial/rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">here</a>
+ for details on this behavior.
+ </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>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_operator_move_equal_optional"></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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></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>Effect:</strong></span> Move-assigns another <code class="computeroutput"><span class="identifier">optional</span></code> to an <code class="computeroutput"><span class="identifier">optional</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is moved from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>, <code class="computeroutput"><span class="identifier">rhs</span></code>
+ remains initialized; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&&</span>
+ <span class="special">)</span></code> or <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>Notes:</strong></span> If both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code>
+ are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>move assignment operator</em></span> is used. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is
+ initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s [destructor]
+ is called. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>move
+ constructor</em></span> is called. The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code>
+ is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</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="identifier">is_nothrow_move_assignable</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">value</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>move constructor</em></span> fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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">opt</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="special">;</span>
+<span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">def</span> <span class="special">;</span>
+
+<span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">;</span>
+<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">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">opt</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="special">)</span> <span class="special">;</span>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<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="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">T</span><span class="special">&>&&</span>
+ <span class="identifier">rhs</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>Effect:</strong></span> Same as <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="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_operator_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Assigns another convertible optional
+ to an optional.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
+ of <code class="computeroutput"><span class="identifier">rhs</span></code> <span class="emphasis"><em>converted</em></span>
+ to type <code class="computeroutput"><span class="identifier">T</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is
+ uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code> or <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span>
+ <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span></code>
+ throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and rhs are initially initialized,
+ <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>assignment
+ operator</em></span> (from <code class="computeroutput"><span class="identifier">U</span></code>)
+ is used. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>destructor</em></span> is called. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initially uninitialized but rhs
+ is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>converting
+ constructor</em></span> (from <code class="computeroutput"><span class="identifier">U</span></code>)
+ is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ converting constructor fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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">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">v</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">opt1</span><span class="special">;</span>
+
+<span class="identifier">opt1</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">opt1</span> <span class="special">==</span> <span class="keyword">static_cast</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>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_operator_move_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Move-assigns another convertible
+ optional to an optional.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
+ is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized and its value is moved from the value of <code class="computeroutput"><span class="identifier">rhs</span></code>;
+ else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is uninitialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">U</span><span class="special">&&</span> <span class="special">)</span></code>
+ or <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&&</span> <span class="special">)</span></code>
+ throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> If both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code>
+ are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>assignment operator</em></span> (from <code class="computeroutput"><span class="identifier">U</span><span class="special">&&</span></code>) is used. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code> is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>destructor</em></span> is called. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initially uninitialized but <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <span class="emphasis"><em>converting constructor</em></span> (from <code class="computeroutput"><span class="identifier">U</span><span class="special">&&</span></code>) is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
+ the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
+ as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
+ <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
+ If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
+ converting constructor fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
+ </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">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">v</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">opt1</span><span class="special">;</span>
+
+<span class="identifier">opt1</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">opt0</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">opt1</span> <span class="special">==</span> <span class="keyword">static_cast</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>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_emplace"></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="special">...</span> <span class="identifier">Args</span><span class="special">></span> <span class="keyword">void</span> <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">emplace</span><span class="special">(</span> <span class="identifier">Args</span><span class="special">...&&</span>
+ <span class="identifier">args</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> The compiler supports rvalue
+ references and variadic templates.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized calls <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span>
+ <span class="identifier">none</span></code>. Then initializes in-place
+ the contained value as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">Args</span><span class="special">>(</span><span class="identifier">args</span><span class="special">)...</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever the selected <code class="computeroutput"><span class="identifier">T</span></code>'s constructor throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
+ need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
+ or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> If an exception is thrown
+ during the initialization of <code class="computeroutput"><span class="identifier">T</span></code>,
+ <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is <span class="emphasis"><em>uninitialized</em></span>.
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_operator_equal_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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="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="keyword">operator</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></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">template</span><span class="special"><</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="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="keyword">operator</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></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ <span class="bold"><strong>Effect:</strong></span> Assigns an <code class="computeroutput"><span class="identifier">optional</span></code>
+ with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
+ from the factory.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is <span class="underline">initialized</span>
+ and its value is <span class="emphasis"><em>directly given</em></span> from the factory
+ <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value <span class="underline">is not copied</span>).
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code>
+ constructor called by the factory throws.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="tutorial/in_place_factories.html" title="In-Place Factories">In-Place
+ Factories</a>
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
+ be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
+ constructor used by the factory; in that case, the <code class="computeroutput"><span class="identifier">optional</span></code>
+ object will be reset to be <span class="emphasis"><em>uninitialized</em></span>.
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_reset_value"></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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><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="identifier">v</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>Deprecated:</strong></span> same as <code class="computeroutput"><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></code>
+ </li></ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_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>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">none_t</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_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <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">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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="identifier">get</span><span class="special">()</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></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> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> A reference to the contained
+ value
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
+ <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</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="identifier">get</span><span class="special">()</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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="special">)</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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="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="special">*</span><span class="keyword">this</span></code> is initialized
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> <span class="underline">The</span>
+ reference contained.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
+ <code class="computeroutput"><span class="identifier">BOOST_ASSERT</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_operator_asterisk"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <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="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">&</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</span><span class="special">*()</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</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> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> A reference to the contained
+ value
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
+ <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
+ On compilers that do not support ref-qualifiers on member functions these
+ three overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
+ and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
+ </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">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">v</span> <span class="special">);</span>
+<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">u</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">u</span> <span class="special">==</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
+<span class="identifier">T</span> <span class="identifier">w</span> <span class="special">;</span>
+<span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">w</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">w</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>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</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> <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="special">&</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="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> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> <span class="underline">The</span>
+ reference contained.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
+ <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
+ On compilers that do not support ref-qualifiers on member functions these
+ three overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
+ and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
+ </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_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</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> <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="identifier">value</span><span class="special">()</span> <span class="special">&</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="identifier">value</span><span class="special">()</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>Returns:</strong></span> A reference to the contained
+ value, if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is initialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> An instance of <code class="computeroutput"><span class="identifier">bad_optional_access</span></code>,
+ if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
+ is not initialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
+ ref-qualifiers on member functions these three overloads are replaced with
+ the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
+ and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
+ </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">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">o1</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="identifier">o1</span><span class="special">.</span><span class="identifier">value</span><span class="special">()</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="identifier">o0</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// throws</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="keyword">catch</span><span class="special">(</span><span class="identifier">bad_optional_access</span><span class="special">&)</span> <span class="special">{</span>
+ <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
+<span class="special">}</span>
+</pre>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </li>
+</ul></div>
+<a name="reference_optional_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">U</span><span class="special">></span> <span class="identifier">T</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">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></code>
+ </p></blockquote></div>
+<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">T</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">U</span> <span class="special">&&</span>
+ <span class="identifier">v</span><span class="special">)</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> <code class="computeroutput"><span class="identifier">T</span></code>
+ is CopyConstructible.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> First overload: <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="special">**</span><span class="keyword">this</span> <span class="special">:</span> <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
+ second overload: <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">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span> <span class="special">:</span>
+ <span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">T</span><span class="special">>(</span><span class="identifier">forward</span><span class="special"><</span><span class="identifier">U</span><span class="special">>(</span><span class="identifier">v</span><span class="special">))</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the selected
+ constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
+ ref-qualifiers on member functions these three overloads are replaced with
+ the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
+ and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
+ On compilers without rvalue reference support the type of <code class="computeroutput"><span class="identifier">v</span></code> becomes <code class="computeroutput"><span class="identifier">U</span>
+ <span class="keyword">const</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_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <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">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></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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&</span>
+ <span class="keyword">default</span> <span class="special">)</span>
+ <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">o</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></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>&</span>
+ <span class="identifier">o</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&</span> <span class="keyword">default</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>Deprecated:</strong></span> Use <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code> instead.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> A reference to the contained
+ value, if any, or <code class="computeroutput"><span class="keyword">default</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">z</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">def</span><span class="special">;</span>
+<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">y</span> <span class="special">=</span> <span class="identifier">def</span><span class="special">.</span><span class="identifier">get_value_or</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">y</span> <span class="special">==</span> <span class="identifier">z</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">opt</span> <span class="special">(</span> <span class="identifier">v</span> <span class="special">);</span>
+<span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">u</span> <span class="special">=</span> <span class="identifier">get_optional_value_or</span><span class="special">(</span><span class="identifier">opt</span><span class="special">,</span><span class="identifier">z</span><span class="special">);</span>
+<span class="identifier">assert</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="identifier">assert</span> <span class="special">(</span> <span class="identifier">u</span> <span class="special">!=</span> <span class="identifier">z</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_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <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">get_ptr</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="special">)</span> <span class="special">;</span></code>
+ </p></blockquote></div>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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></code>
+ <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">></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>Returns:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized, a pointer to the contained
+ value; else <code class="computeroutput"><span class="number">0</span></code> (<span class="emphasis"><em>null</em></span>).
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The contained value is permanently
+ stored within <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>,
+ so you should not hold nor delete this pointer
+ </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">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">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="keyword">const</span> <span class="identifier">copt</span><span class="special">(</span><span class="identifier">v</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">opt</span><span class="special">.</span><span class="identifier">get_ptr</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">cp</span> <span class="special">=</span> <span class="identifier">copt</span><span class="special">.</span><span class="identifier">get_ptr</span><span class="special">();</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">p</span> <span class="special">==</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">opt</span><span class="special">)</span> <span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">cp</span> <span class="special">==</span> <span class="identifier">get_pointer</span><span class="special">(</span><span class="identifier">copt</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_optional_operator_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">*</span> <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="keyword">operator</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></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">>::</span><span class="keyword">operator</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> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> A pointer to the contained value.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
+ <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Example:</strong></span>
+<pre class="programlisting"><span class="keyword">struct</span> <span class="identifier">X</span> <span class="special">{</span> <span class="keyword">int</span> <span class="identifier">mdata</span> <span class="special">;</span> <span class="special">}</span> <span class="special">;</span>
+<span class="identifier">X</span> <span class="identifier">x</span> <span class="special">;</span>
+<span class="identifier">optional</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span> <span class="identifier">opt</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
+<span class="identifier">opt</span><span class="special">-></span><span class="identifier">mdata</span> <span class="special">=</span> <span class="number">2</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_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
+ <code class="computeroutput"><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="keyword">operator</span> <span class="keyword">bool</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>Returns:</strong></span> <code class="computeroutput"><span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">!=</span> <span class="number">0</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
+ explicit conversion operators this falls back to safe-bool idiom.
+ </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">def</span> <span class="special">;</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def</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">opt</span> <span class="special">(</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
+<span class="identifier">assert</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">opt</span> <span class="special">!=</span> <span class="number">0</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_operator_not"></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="keyword">operator</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> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>;
+ else <code class="computeroutput"><span class="keyword">false</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> This operator is provided for those
+ compilers which can't use the <span class="emphasis"><em>unspecified-bool-type operator</em></span>
+ in certain boolean contexts.
+ </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">opt</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="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">some_T</span> <span class="special">;</span>
+
+<span class="comment">// Notice the "double-bang" idiom here.</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="special">;</span>
+</pre>
+ </li>
+</ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<a name="reference_optional_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="special">;</span></code>
+ </p></blockquote></div>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
+ <span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">explicit</span>
+ <span class="keyword">operator</span> <span class="keyword">bool</span>
+ <span class="special">()</span> <span class="special">;</span></code>
+ </li></ul></div>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </p>
+<h4>
+<a name="boost_optional.detailed_semantics.h1"></a>
+ <span class="phrase"><a name="boost_optional.detailed_semantics.free_functions"></a></span><a class="link" href="detailed_semantics.html#boost_optional.detailed_semantics.free_functions">Free
+ functions</a>
+ </h4>
+<p>
+ <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
+ </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></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="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_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 <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>.
+ </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>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> 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">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">12</span><span class="special">);</span>
+<span class="identifier">T</span> <span class="identifier">z</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">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span> <span class="identifier">optZ</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span>
+
+<span class="comment">// Identity always hold</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">def0</span> <span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">optX</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">def0</span> <span class="special">==</span> <span class="identifier">def1</span> <span class="special">);</span>
+
+<span class="comment">// Only one initialized compare unequal.</span>
+<span class="identifier">assert</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">// Both initialized compare as (*lhs == *rhs)</span>
+<span class="identifier">assert</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="special">;</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">optX</span> <span class="special">!=</span> <span class="identifier">optZ</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_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> <code class="computeroutput"><span class="identifier">T</span></code>
+ shall meet requirements of <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Returns:</strong></span> If <code class="computeroutput"><span class="identifier">y</span></code>
+ is not initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
+ If <code class="computeroutput"><span class="identifier">y</span></code> is initialized and
+ <code class="computeroutput"><span class="identifier">x</span></code> is not initialized,
+ <code class="computeroutput"><span class="keyword">true</span></code>. 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>.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Notes:</strong></span> 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.
+ </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">34</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">def</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="comment">// Identity always hold</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">def</span><span class="special">)</span> <span class="special">);</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">optX</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">def0</span> <span class="special">==</span> <span class="identifier">def1</span> <span class="special">);</span>
+
+<span class="comment">// Only one initialized compare unequal.</span>
+<span class="identifier">assert</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">// Both initialized compare as (*lhs == *rhs)</span>
+<span class="identifier">assert</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="special">;</span>
+<span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">optX</span> <span class="special">!=</span> <span class="identifier">optZ</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>
+<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_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>
+<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_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>
+<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_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>
+<li class="listitem">
+ <span class="bold"><strong>Throws:</strong></span> Nothing.
+ </li>
+</ul></div>
+<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="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 <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>.
+ </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="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>Effect:</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,
+ calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*</span><span class="identifier">x</span><span class="special">,*</span><span class="identifier">y</span><span class="special">)</span></code> using <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>.
+ If only one is initialized, say <code class="computeroutput"><span class="identifier">x</span></code>,
+ calls: <code class="computeroutput"><span class="identifier">y</span><span class="special">.</span><span class="identifier">reset</span><span class="special">(*</span><span class="identifier">x</span><span class="special">);</span> <span class="identifier">x</span><span class="special">.</span><span class="identifier">reset</span><span class="special">();</span></code> If none is initialized, does nothing.
+ </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>Notes:</strong></span> If both are initialized, <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>
+ is used unqualified but with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
+ introduced in scope. If only one is initialized, <code class="computeroutput"><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code> and <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> is called.
+ </li>
+<li class="listitem">
+ <span class="bold"><strong>Exception Safety:</strong></span> If both are initialized,
+ this operation has the exception safety guarantees of <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>.
+ If only one is initialized, it has the same basic guarantee as <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="special">&&</span>
+ <span class="special">)</span></code>.
+ </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>
+</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="synopsis.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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