ceph/src/boost/libs/optional/doc/html/boost_optional/detailed_semantics.html

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  24 </div>
  25 <div class="section">
  26 <div class="titlepage"><div><div><h2 class="title" style="clear: both">
  27 <a name="boost_optional.detailed_semantics"></a><a class="link" href="detailed_semantics.html" title="Detailed Semantics">Detailed Semantics</a>
  28 </h2></div></div></div>
  29 <p>
  30       Because <code class="computeroutput"><span class="identifier">T</span></code> might be of reference
  31       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
  32       distinguished using the following convention:
  33     </p>
  34 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  35 <li class="listitem">
  36           If the entry reads: <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code><span class="emphasis"><em>(not
  37           a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span></code>, the
  38           description corresponds only to the case where <code class="computeroutput"><span class="identifier">T</span></code>
  39           is not of reference type.
  40         </li>
  41 <li class="listitem">
  42           If the entry reads: <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>, the description corresponds only
  43           to the case where <code class="computeroutput"><span class="identifier">T</span></code> is
  44           of reference type.
  45         </li>
  46 <li class="listitem">
  47           If the entry reads: <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>, the description is the same for both
  48           cases.
  49         </li>
  50 </ul></div>
  51 <div class="note"><table border="0" summary="Note">
  52 <tr>
  53 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../doc/src/images/note.png"></td>
  54 <th align="left">Note</th>
  55 </tr>
  56 <tr><td align="left" valign="top"><p>
  57         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
  58         as sample code. It is not implied that the type <code class="computeroutput"><span class="identifier">T</span></code>
  59         must support each particular expression but that if the expression is supported,
  60         the implied condition holds.
  61       </p></td></tr>
  62 </table></div>
  63 <p>
  64       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
  65     </p>
  66 <h4>
  67 <a name="boost_optional.detailed_semantics.h0"></a>
  68       <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
  69       class member functions</a>
  70     </h4>
  71 <p>
  72       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
  73     </p>
  74 <a name="reference_optional_constructor"></a><div class="blockquote"><blockquote class="blockquote"><p>
  75         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">()</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
  76       </p></blockquote></div>
  77 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  78 <li class="listitem">
  79           <span class="bold"><strong>Effect:</strong></span> Default-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
  80         </li>
  81 <li class="listitem">
  82           <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>.
  83         </li>
  84 <li class="listitem">
  85           <span class="bold"><strong>Notes:</strong></span> T's default constructor <span class="underline">is not</span> called.
  86         </li>
  87 <li class="listitem">
  88           <span class="bold"><strong>Example:</strong></span>
  89 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
  90 <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>
  91 </pre>
  92         </li>
  93 </ul></div>
  94 <p>
  95       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
  96     </p>
  97 <a name="reference_optional_constructor_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
  98         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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>
  99       </p></blockquote></div>
 100 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 101 <li class="listitem">
 102           <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>
 103           uninitialized.
 104         </li>
 105 <li class="listitem">
 106           <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>.
 107         </li>
 108 <li class="listitem">
 109           <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>'s
 110           default constructor <span class="underline">is not</span> called.
 111           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.
 112         </li>
 113 <li class="listitem">
 114           <span class="bold"><strong>Example:</strong></span>
 115 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">&gt;</span>
 116 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">n</span><span class="special">(</span><span class="identifier">none</span><span class="special">)</span> <span class="special">;</span>
 117 <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>
 118 </pre>
 119         </li>
 120 </ul></div>
 121 <p>
 122       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 123     </p>
 124 <a name="reference_optional_constructor_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 125         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">)</span></code>
 126       </p></blockquote></div>
 127 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 128 <li class="listitem">
 129           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_copy_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 130           is <code class="computeroutput"><span class="keyword">true</span></code>.
 131         </li>
 132 <li class="listitem">
 133           <span class="bold"><strong>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 134         </li>
 135 <li class="listitem">
 136           <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>
 137           and its value is a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">v</span></code>.
 138         </li>
 139 <li class="listitem">
 140           <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>
 141           <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 142           throws.
 143         </li>
 144 <li class="listitem">
 145           <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>
 146           <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 147           is called.
 148         </li>
 149 <li class="listitem">
 150           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 151           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>
 152           <span class="keyword">const</span><span class="special">&amp;</span>
 153           <span class="special">);</span></code> in that case, this constructor
 154           has no effect.
 155         </li>
 156 <li class="listitem">
 157           <span class="bold"><strong>Example:</strong></span>
 158 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 159 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
 160 <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>
 161 </pre>
 162         </li>
 163 </ul></div>
 164 <p>
 165       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 166     </p>
 167 <div class="blockquote"><blockquote class="blockquote"><p>
 168         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">ref</span> <span class="special">)</span></code>
 169       </p></blockquote></div>
 170 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 171 <li class="listitem">
 172           <span class="bold"><strong>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 173         </li>
 174 <li class="listitem">
 175           <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>
 176           and its value is an instance of an internal type wrapping the reference
 177           <code class="computeroutput"><span class="identifier">ref</span></code>.
 178         </li>
 179 <li class="listitem">
 180           <span class="bold"><strong>Throws:</strong></span> Nothing.
 181         </li>
 182 <li class="listitem">
 183           <span class="bold"><strong>Example:</strong></span>
 184 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 185 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
 186 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">vref</span><span class="special">);</span>
 187 <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>
 188 <span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span> <span class="comment">// mutate referee</span>
 189 <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>
 190 </pre>
 191         </li>
 192 </ul></div>
 193 <p>
 194       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 195     </p>
 196 <a name="reference_optional_constructor_move_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 197         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 198         <span class="identifier">v</span> <span class="special">)</span></code>
 199       </p></blockquote></div>
 200 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 201 <li class="listitem">
 202           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 203           is <code class="computeroutput"><span class="keyword">true</span></code>.
 204         </li>
 205 <li class="listitem">
 206           <span class="bold"><strong>Effect:</strong></span> Directly-Move-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 207         </li>
 208 <li class="listitem">
 209           <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>
 210           and its value is move-constructed from <code class="computeroutput"><span class="identifier">v</span></code>.
 211         </li>
 212 <li class="listitem">
 213           <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>
 214           <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 215           <span class="special">)</span></code> throws.
 216         </li>
 217 <li class="listitem">
 218           <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>
 219           <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 220           <span class="special">)</span></code> is called.
 221         </li>
 222 <li class="listitem">
 223           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 224           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">&amp;&amp;</span> <span class="special">);</span></code>
 225           in that case, the state of <code class="computeroutput"><span class="identifier">v</span></code>
 226           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">&amp;&amp;)</span></code>.
 227         </li>
 228 <li class="listitem">
 229           <span class="bold"><strong>Example:</strong></span>
 230 <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>
 231 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
 232 <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>
 233 </pre>
 234         </li>
 235 </ul></div>
 236 <p>
 237       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 238     </p>
 239 <div class="blockquote"><blockquote class="blockquote"><p>
 240         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="identifier">ref</span> <span class="special">)</span> <span class="special">=</span> <span class="keyword">delete</span></code>
 241       </p></blockquote></div>
 242 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 243           <span class="bold"><strong>Notes:</strong></span> This constructor is deleted
 244         </li></ul></div>
 245 <p>
 246       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 247     </p>
 248 <a name="reference_optional_constructor_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 249         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</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">&amp;</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span></code>
 250       </p></blockquote></div>
 251 <div class="blockquote"><blockquote class="blockquote"><p>
 252         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</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">&amp;</span>
 253         <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span></code>
 254       </p></blockquote></div>
 255 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 256           If condition is true, same as:
 257         </li></ul></div>
 258 <div class="blockquote"><blockquote class="blockquote"><p>
 259         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">)</span></code>
 260       </p></blockquote></div>
 261 <div class="blockquote"><blockquote class="blockquote"><p>
 262         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="special">::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;</span>
 263         <span class="identifier">v</span> <span class="special">)</span></code>
 264       </p></blockquote></div>
 265 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 266           otherwise, same as:
 267         </li></ul></div>
 268 <div class="blockquote"><blockquote class="blockquote"><p>
 269         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">()</span></code>
 270       </p></blockquote></div>
 271 <div class="blockquote"><blockquote class="blockquote"><p>
 272         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="special">::</span><span class="identifier">optional</span><span class="special">()</span></code>
 273       </p></blockquote></div>
 274 <p>
 275       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 276     </p>
 277 <a name="reference_optional_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 278         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span> <span class="special">);</span></code>
 279       </p></blockquote></div>
 280 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 281 <li class="listitem">
 282           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_copy_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 283           is <code class="computeroutput"><span class="keyword">true</span></code>.
 284         </li>
 285 <li class="listitem">
 286           <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 287         </li>
 288 <li class="listitem">
 289           <span class="bold"><strong>Postconditions:</strong></span> If rhs is initialized,
 290           <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 291           is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
 292           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
 293           uninitialized.
 294         </li>
 295 <li class="listitem">
 296           <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>
 297           <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 298           throws.
 299         </li>
 300 <li class="listitem">
 301           <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">&amp;</span> <span class="special">)</span></code>
 302           is called.
 303         </li>
 304 <li class="listitem">
 305           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 306           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>
 307           <span class="keyword">const</span><span class="special">&amp;</span>
 308           <span class="special">);</span></code> in that case, this constructor
 309           has no effect.
 310         </li>
 311 <li class="listitem">
 312           <span class="bold"><strong>Example:</strong></span>
 313 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">uninit</span> <span class="special">;</span>
 314 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 315
 316 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">uinit2</span> <span class="special">(</span> <span class="identifier">uninit</span> <span class="special">)</span> <span class="special">;</span>
 317 <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>
 318
 319 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
 320 <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>
 321
 322 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">init2</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">)</span> <span class="special">;</span>
 323 <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>
 324 </pre>
 325         </li>
 326 </ul></div>
 327 <p>
 328       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 329     </p>
 330 <div class="blockquote"><blockquote class="blockquote"><p>
 331         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span> <span class="special">);</span></code>
 332       </p></blockquote></div>
 333 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 334 <li class="listitem">
 335           <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 336         </li>
 337 <li class="listitem">
 338           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 339           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 340           is initialized and its value is another reference to the same object referenced
 341           by <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
 342           else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 343           is uninitialized.
 344         </li>
 345 <li class="listitem">
 346           <span class="bold"><strong>Throws:</strong></span> Nothing.
 347         </li>
 348 <li class="listitem">
 349           <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 350           is initialized, both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 351           and <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 352           will reefer to the same object (they alias).
 353         </li>
 354 <li class="listitem">
 355           <span class="bold"><strong>Example:</strong></span>
 356 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">uninit</span> <span class="special">;</span>
 357 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 358
 359 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">uinit2</span> <span class="special">(</span> <span class="identifier">uninit</span> <span class="special">)</span> <span class="special">;</span>
 360 <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>
 361
 362 <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">&amp;</span> <span class="identifier">ref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
 363 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">init</span><span class="special">(</span><span class="identifier">ref</span><span class="special">);</span>
 364 <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>
 365
 366 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">init2</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">)</span> <span class="special">;</span>
 367 <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>
 368
 369 <span class="identifier">v</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
 370
 371 <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>
 372 <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>
 373 </pre>
 374         </li>
 375 </ul></div>
 376 <p>
 377       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 378     </p>
 379 <a name="reference_optional_move_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 380         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span> </code><span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span><span class="special">&amp;&amp;</span> <span class="identifier">rhs</span>
 381         <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>
 382       </p></blockquote></div>
 383 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 384 <li class="listitem">
 385           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>
 386           is <code class="computeroutput"><span class="keyword">true</span></code>.
 387         </li>
 388 <li class="listitem">
 389           <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 390         </li>
 391 <li class="listitem">
 392           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 393           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 394           is initialized and its value is move constructed from <code class="computeroutput"><span class="identifier">rhs</span></code>;
 395           else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 396           is uninitialized.
 397         </li>
 398 <li class="listitem">
 399           <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>
 400           <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 401           <span class="special">)</span></code> throws.
 402         </li>
 403 <li class="listitem">
 404           <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 405           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>
 406           <span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 407           is called. The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code>
 408           is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>.
 409         </li>
 410 <li class="listitem">
 411           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 412           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">&amp;&amp;</span> <span class="special">);</span></code>
 413           in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
 414           initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
 415           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">&amp;&amp;)</span></code>.
 416         </li>
 417 <li class="listitem">
 418           <span class="bold"><strong>Example:</strong></span>
 419 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</span> <span class="identifier">uninit</span> <span class="special">;</span>
 420 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 421
 422 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</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>
 423 <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>
 424
 425 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</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>
 426 <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>
 427
 428 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&gt;</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>
 429 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">);</span>
 430 <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>
 431 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init2</span> <span class="special">);</span>
 432 <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>
 433 </pre>
 434         </li>
 435 </ul></div>
 436 <p>
 437       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 438     </p>
 439 <div class="blockquote"><blockquote class="blockquote"><p>
 440         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span> <span class="special">&amp;&amp;</span>
 441         <span class="identifier">rhs</span> <span class="special">);</span></code>
 442       </p></blockquote></div>
 443 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 444 <li class="listitem">
 445           <span class="bold"><strong>Effect:</strong></span> Move-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 446         </li>
 447 <li class="listitem">
 448           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 449           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 450           is initialized and its value is another reference to the same object referenced
 451           by <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
 452           else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 453           is uninitialized.
 454         </li>
 455 <li class="listitem">
 456           <span class="bold"><strong>Throws:</strong></span> Nothing.
 457         </li>
 458 <li class="listitem">
 459           <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 460           is initialized, both <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 461           and <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 462           will reefer to the same object (they alias).
 463         </li>
 464 <li class="listitem">
 465           <span class="bold"><strong>Example:</strong></span>
 466 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;&gt;</span> <span class="identifier">uninit</span> <span class="special">;</span>
 467 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 468
 469 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;&gt;</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>
 470 <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>
 471
 472 <span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
 473 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;&gt;</span> <span class="identifier">init</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
 474 <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>
 475
 476 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_ptr</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;&gt;</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>
 477 <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>
 478
 479 <span class="special">*</span><span class="identifier">v</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span>
 480
 481 <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>
 482 <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>
 483 </pre>
 484         </li>
 485 </ul></div>
 486 <p>
 487       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 488     </p>
 489 <a name="reference_optional_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 490         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="keyword">explicit</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
 491         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span> <span class="special">);</span></code>
 492       </p></blockquote></div>
 493 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 494 <li class="listitem">
 495           <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 496         </li>
 497 <li class="listitem">
 498           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 499           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 500           is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
 501           of rhs converted to type <code class="computeroutput"><span class="identifier">T</span></code>;
 502           else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 503           is uninitialized.
 504         </li>
 505 <li class="listitem">
 506           <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>
 507           <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 508           throws.
 509         </li>
 510 <li class="listitem">
 511           <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>
 512           <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 513           is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized,
 514           which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
 515           to <code class="computeroutput"><span class="identifier">T</span></code>.
 516         </li>
 517 <li class="listitem">
 518           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 519           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>
 520           <span class="keyword">const</span><span class="special">&amp;</span>
 521           <span class="special">);</span></code> in that case, this constructor
 522           has no effect.
 523         </li>
 524 <li class="listitem">
 525           <span class="bold"><strong>Example:</strong></span>
 526 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">123.4</span><span class="special">);</span>
 527 <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>
 528
 529 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
 530 <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>
 531 </pre>
 532         </li>
 533 </ul></div>
 534 <p>
 535       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 536     </p>
 537 <a name="reference_optional_move_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 538         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="keyword">explicit</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
 539         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;&amp;&amp;</span>
 540         <span class="identifier">rhs</span> <span class="special">);</span></code>
 541       </p></blockquote></div>
 542 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 543 <li class="listitem">
 544           <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 545         </li>
 546 <li class="listitem">
 547           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 548           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 549           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.
 550         </li>
 551 <li class="listitem">
 552           <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>
 553           <span class="identifier">U</span><span class="special">&amp;&amp;</span>
 554           <span class="special">)</span></code> throws.
 555         </li>
 556 <li class="listitem">
 557           <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>
 558           <span class="identifier">U</span><span class="special">&amp;&amp;</span>
 559           <span class="special">)</span></code> is called if <code class="computeroutput"><span class="identifier">rhs</span></code>
 560           is initialized, which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
 561           to <code class="computeroutput"><span class="identifier">T</span></code>.
 562         </li>
 563 <li class="listitem">
 564           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 565           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">&amp;&amp;</span> <span class="special">);</span></code>
 566           in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
 567           initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
 568           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">&amp;&amp;</span> <span class="special">)</span></code>.
 569         </li>
 570 <li class="listitem">
 571           <span class="bold"><strong>Example:</strong></span>
 572 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">x</span><span class="special">(</span><span class="number">123.4</span><span class="special">);</span>
 573 <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>
 574
 575 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</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>
 576 <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>
 577 </pre>
 578         </li>
 579 </ul></div>
 580 <p>
 581       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 582     </p>
 583 <a name="reference_optional_constructor_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
 584         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">InPlaceFactory</span><span class="special">&gt;</span>
 585         <span class="keyword">explicit</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
 586         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">);</span></code>
 587       </p></blockquote></div>
 588 <div class="blockquote"><blockquote class="blockquote"><p>
 589         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span>
 590         <span class="keyword">explicit</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
 591         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">optional</span><span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">);</span></code>
 592       </p></blockquote></div>
 593 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 594 <li class="listitem">
 595           <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>
 596           with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
 597           from the factory.
 598         </li>
 599 <li class="listitem">
 600           <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>
 601           and its value is <span class="emphasis"><em>directly given</em></span> from the factory
 602           <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value <span class="underline">is not copied</span>).
 603         </li>
 604 <li class="listitem">
 605           <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code>
 606           constructor called by the factory throws.
 607         </li>
 608 <li class="listitem">
 609           <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="tutorial/in_place_factories.html" title="In-Place Factories">In-Place
 610           Factories</a>
 611         </li>
 612 <li class="listitem">
 613           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 614           be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
 615           constructor used by the factory; in that case, this constructor has no
 616           effect.
 617         </li>
 618 <li class="listitem">
 619           <span class="bold"><strong>Example:</strong></span>
 620 <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>
 621
 622 <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>
 623
 624 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</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>
 625 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;</span> <span class="identifier">y</span><span class="special">(</span> <span class="identifier">in_place</span><span class="special">&lt;</span><span class="identifier">C</span><span class="special">&gt;(</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>
 626
 627 <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>
 628 <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>
 629 </pre>
 630         </li>
 631 </ul></div>
 632 <p>
 633       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 634     </p>
 635 <a name="reference_optional_operator_equal_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
 636         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 637         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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>
 638       </p></blockquote></div>
 639 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 640 <li class="listitem">
 641           <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
 642           value.
 643         </li>
 644 <li class="listitem">
 645           <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
 646         </li>
 647 </ul></div>
 648 <p>
 649       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 650     </p>
 651 <a name="reference_optional_operator_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 652         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 653         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span>
 654         <span class="keyword">const</span><span class="special">&amp;</span>
 655         <span class="identifier">rhs</span> <span class="special">)</span>
 656         <span class="special">;</span></code>
 657       </p></blockquote></div>
 658 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 659 <li class="listitem">
 660           <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>.
 661         </li>
 662 <li class="listitem">
 663           <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
 664           <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">rhs</span></code>.
 665         </li>
 666 <li class="listitem">
 667           <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">&amp;</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">&amp;)</span></code> throws.
 668         </li>
 669 <li class="listitem">
 670           <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
 671           assignment operator is used, otherwise, its copy-constructor is used.
 672         </li>
 673 <li class="listitem">
 674           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
 675           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 676           as far as <code class="computeroutput"><span class="identifier">optional</span></code> is concerned
 677           (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>).
 678           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 679           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 680           <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.
 681         </li>
 682 <li class="listitem">
 683           <span class="bold"><strong>Example:</strong></span>
 684 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
 685 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 686 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
 687
 688 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">;</span>
 689 <span class="identifier">def</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
 690 <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>
 691 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
 692 <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>
 693 </pre>
 694         </li>
 695 </ul></div>
 696 <p>
 697       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 698     </p>
 699 <div class="blockquote"><blockquote class="blockquote"><p>
 700         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;&amp;</span>
 701         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;</span>
 702         <span class="identifier">rhs</span> <span class="special">)</span>
 703         <span class="special">;</span></code>
 704       </p></blockquote></div>
 705 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 706 <li class="listitem">
 707           <span class="bold"><strong>Effect:</strong></span> (Re)binds the wrapped reference.
 708         </li>
 709 <li class="listitem">
 710           <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
 711           same object referenced by <code class="computeroutput"><span class="identifier">rhs</span></code>.
 712         </li>
 713 <li class="listitem">
 714           <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>
 715           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>
 716           for details on this behavior.
 717         </li>
 718 <li class="listitem">
 719           <span class="bold"><strong>Example:</strong></span>
 720 <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>
 721 <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
 722 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
 723 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
 724 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 725 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
 726
 727 <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>
 728 <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>
 729 <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>
 730 <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>
 731 <span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
 732 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
 733 <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>
 734 <span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
 735 <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>
 736 </pre>
 737         </li>
 738 </ul></div>
 739 <p>
 740       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 741     </p>
 742 <a name="reference_optional_operator_move_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 743         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 744         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="identifier">rhs</span>
 745         <span class="special">)</span> <span class="special">;</span></code>
 746       </p></blockquote></div>
 747 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 748 <li class="listitem">
 749           <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>.
 750         </li>
 751 <li class="listitem">
 752           <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
 753           from <code class="computeroutput"><span class="identifier">rhs</span></code>.
 754         </li>
 755 <li class="listitem">
 756           <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">&amp;&amp;</span> <span class="special">)</span></code>
 757           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>
 758           <span class="special">&amp;&amp;)</span></code> throws.
 759         </li>
 760 <li class="listitem">
 761           <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
 762           move-assignment operator is used, otherwise, its move-constructor is used.
 763         </li>
 764 <li class="listitem">
 765           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
 766           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 767           as far as <code class="computeroutput"><span class="identifier">optional</span></code> is concerned
 768           (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>).
 769           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 770           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 771           <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.
 772         </li>
 773 <li class="listitem">
 774           <span class="bold"><strong>Example:</strong></span>
 775 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
 776 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 777 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span>
 778
 779 <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>
 780 <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>
 781 <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>
 782 <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>
 783 <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>
 784 </pre>
 785         </li>
 786 </ul></div>
 787 <p>
 788       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 789     </p>
 790 <div class="blockquote"><blockquote class="blockquote"><p>
 791         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;&amp;</span>
 792         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 793         <span class="identifier">rhs</span> <span class="special">)</span>
 794         <span class="special">=</span> <span class="keyword">delete</span><span class="special">;</span></code>
 795       </p></blockquote></div>
 796 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 797           <span class="bold"><strong>Notes:</strong></span> This assignment operator is deleted.
 798         </li></ul></div>
 799 <p>
 800       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 801     </p>
 802 <a name="reference_optional_operator_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 803         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 804         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span>
 805         <span class="keyword">const</span><span class="special">&amp;</span>
 806         <span class="identifier">rhs</span> <span class="special">)</span>
 807         <span class="special">;</span></code>
 808       </p></blockquote></div>
 809 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 810 <li class="listitem">
 811           <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>.
 812         </li>
 813 <li class="listitem">
 814           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 815           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 816           is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
 817           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
 818           uninitialized.
 819         </li>
 820 <li class="listitem">
 821           <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">&amp;)</span></code>
 822           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>
 823           <span class="keyword">const</span><span class="special">&amp;</span>
 824           <span class="special">)</span></code> throws.
 825         </li>
 826 <li class="listitem">
 827           <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>
 828           are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
 829           <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
 830           [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
 831           <span class="emphasis"><em>copy constructor</em></span> is called.
 832         </li>
 833 <li class="listitem">
 834           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
 835           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 836           as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
 837           <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
 838           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 839           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 840           <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.
 841         </li>
 842 <li class="listitem">
 843           <span class="bold"><strong>Example:</strong></span>
 844 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 845 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
 846 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 847
 848 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
 849 <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>
 850 <span class="comment">// previous value (copy of 'v') destroyed from within 'opt'.</span>
 851 </pre>
 852         </li>
 853 </ul></div>
 854 <p>
 855       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 856     </p>
 857 <div class="blockquote"><blockquote class="blockquote"><p>
 858         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span>
 859         <span class="keyword">const</span><span class="special">&amp;</span>
 860         <span class="identifier">rhs</span> <span class="special">)</span>
 861         <span class="special">;</span></code>
 862       </p></blockquote></div>
 863 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 864 <li class="listitem">
 865           <span class="bold"><strong>Effect:</strong></span> (Re)binds thee wrapped reference.
 866         </li>
 867 <li class="listitem">
 868           <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
 869           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
 870           object).
 871         </li>
 872 <li class="listitem">
 873           <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>,
 874           <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 875           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>
 876           for details on this behavior.
 877         </li>
 878 <li class="listitem">
 879           <span class="bold"><strong>Example:</strong></span>
 880 <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>
 881 <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
 882 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span>
 883 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span>
 884 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 885 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span>
 886 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span>
 887
 888 <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>
 889 <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>
 890 <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>
 891 <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>
 892 <span class="keyword">int</span> <span class="identifier">c</span> <span class="special">=</span> <span class="number">3</span><span class="special">;</span>
 893 <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">rc</span> <span class="special">=</span> <span class="identifier">c</span> <span class="special">;</span>
 894 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">orc</span><span class="special">(</span><span class="identifier">rc</span><span class="special">)</span> <span class="special">;</span>
 895 <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>
 896 <span class="identifier">c</span> <span class="special">=</span> <span class="number">4</span> <span class="special">;</span>
 897 <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>
 898 </pre>
 899         </li>
 900 </ul></div>
 901 <p>
 902       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 903     </p>
 904 <a name="reference_optional_operator_move_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 905         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 906         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&amp;&amp;</span> <span class="identifier">rhs</span>
 907         <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>
 908       </p></blockquote></div>
 909 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 910 <li class="listitem">
 911           <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>.
 912         </li>
 913 <li class="listitem">
 914           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 915           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 916           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>
 917           remains initialized; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
 918         </li>
 919 <li class="listitem">
 920           <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">&amp;&amp;</span>
 921           <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>
 922           <span class="identifier">T</span> <span class="special">&amp;&amp;</span>
 923           <span class="special">)</span></code> throws.
 924         </li>
 925 <li class="listitem">
 926           <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>
 927           are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
 928           <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
 929           initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
 930           is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s [destructor]
 931           is called. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 932           is initially uninitialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
 933           is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>move
 934           constructor</em></span> is called. The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code>
 935           is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span>
 936           <span class="special">&amp;&amp;</span> <span class="identifier">is_nothrow_move_assignable</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>.
 937         </li>
 938 <li class="listitem">
 939           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
 940           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 941           as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
 942           <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
 943           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 944           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 945           <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.
 946         </li>
 947 <li class="listitem">
 948           <span class="bold"><strong>Example:</strong></span>
 949 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
 950 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
 951
 952 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
 953 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">;</span>
 954 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
 955 <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>
 956 </pre>
 957         </li>
 958 </ul></div>
 959 <p>
 960       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 961     </p>
 962 <div class="blockquote"><blockquote class="blockquote"><p>
 963         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;&amp;&amp;</span>
 964         <span class="identifier">rhs</span> <span class="special">)</span>
 965         <span class="special">;</span></code>
 966       </p></blockquote></div>
 967 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 968           <span class="bold"><strong>Effect:</strong></span> Same as <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span> <span class="special">)</span></code>.
 969         </li></ul></div>
 970 <p>
 971       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
 972     </p>
 973 <a name="reference_optional_operator_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 974         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&amp;</span>
 975         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span>
 976         <span class="keyword">const</span><span class="special">&amp;</span>
 977         <span class="identifier">rhs</span> <span class="special">)</span>
 978         <span class="special">;</span></code>
 979       </p></blockquote></div>
 980 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 981 <li class="listitem">
 982           <span class="bold"><strong>Effect:</strong></span> Assigns another convertible optional
 983           to an optional.
 984         </li>
 985 <li class="listitem">
 986           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 987           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 988           is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the value
 989           of <code class="computeroutput"><span class="identifier">rhs</span></code> <span class="emphasis"><em>converted</em></span>
 990           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
 991           uninitialized.
 992         </li>
 993 <li class="listitem">
 994           <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">&amp;</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>
 995           <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span></code>
 996           throws.
 997         </li>
 998 <li class="listitem">
 999           <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,
1000           <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>assignment
1001           operator</em></span> (from <code class="computeroutput"><span class="identifier">U</span></code>)
1002           is used. If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1003           is initially initialized but <code class="computeroutput"><span class="identifier">rhs</span></code>
1004           is uninitialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
1005           <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
1006           is initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s <span class="emphasis"><em>converting
1007           constructor</em></span> (from <code class="computeroutput"><span class="identifier">U</span></code>)
1008           is called.
1009         </li>
1010 <li class="listitem">
1011           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
1012           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
1013           as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
1014           <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
1015           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1016           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
1017           converting constructor fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
1018         </li>
1019 <li class="listitem">
1020           <span class="bold"><strong>Example:</strong></span>
1021 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1022 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt0</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
1023 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">opt1</span><span class="special">;</span>
1024
1025 <span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt0</span> <span class="special">;</span>
1026 <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">&lt;</span><span class="identifier">U</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
1027 </pre>
1028         </li>
1029 </ul></div>
1030 <p>
1031       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1032     </p>
1033 <a name="reference_optional_operator_move_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1034         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">optional</span><span class="special">&amp;</span>
1035         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;&amp;&amp;</span>
1036         <span class="identifier">rhs</span> <span class="special">)</span>
1037         <span class="special">;</span></code>
1038       </p></blockquote></div>
1039 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1040 <li class="listitem">
1041           <span class="bold"><strong>Effect:</strong></span> Move-assigns another convertible
1042           optional to an optional.
1043         </li>
1044 <li class="listitem">
1045           <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
1046           is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1047           is initialized and its value is moved from the value of <code class="computeroutput"><span class="identifier">rhs</span></code>;
1048           else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1049           is uninitialized.
1050         </li>
1051 <li class="listitem">
1052           <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">&amp;&amp;</span> <span class="special">)</span></code>
1053           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">&amp;&amp;</span> <span class="special">)</span></code>
1054           throws.
1055         </li>
1056 <li class="listitem">
1057           <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>
1058           are initially initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s
1059           <span class="emphasis"><em>assignment operator</em></span> (from <code class="computeroutput"><span class="identifier">U</span><span class="special">&amp;&amp;</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
1060           <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
1061           <span class="emphasis"><em>converting constructor</em></span> (from <code class="computeroutput"><span class="identifier">U</span><span class="special">&amp;&amp;</span></code>) is called.
1062         </li>
1063 <li class="listitem">
1064           <span class="bold"><strong>Exception Safety:</strong></span> In the event of an exception,
1065           the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
1066           as far as optional is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
1067           <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
1068           If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1069           is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
1070           converting constructor fails, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is left properly uninitialized.
1071         </li>
1072 <li class="listitem">
1073           <span class="bold"><strong>Example:</strong></span>
1074 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1075 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt0</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
1076 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">opt1</span><span class="special">;</span>
1077
1078 <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>
1079 <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">&lt;</span><span class="identifier">U</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
1080 </pre>
1081         </li>
1082 </ul></div>
1083 <p>
1084       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1085     </p>
1086 <a name="reference_optional_emplace"></a><div class="blockquote"><blockquote class="blockquote"><p>
1087         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span><span class="special">...</span> <span class="identifier">Args</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1088         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">emplace</span><span class="special">(</span> <span class="identifier">Args</span><span class="special">...&amp;&amp;</span>
1089         <span class="identifier">args</span> <span class="special">);</span></code>
1090       </p></blockquote></div>
1091 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1092 <li class="listitem">
1093           <span class="bold"><strong>Requires:</strong></span> The compiler supports rvalue
1094           references and variadic templates.
1095         </li>
1096 <li class="listitem">
1097           <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>
1098           <span class="identifier">none</span></code>. Then initializes in-place
1099           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">&lt;</span><span class="identifier">Args</span><span class="special">&gt;(</span><span class="identifier">args</span><span class="special">)...</span></code>.
1100         </li>
1101 <li class="listitem">
1102           <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>.
1103         </li>
1104 <li class="listitem">
1105           <span class="bold"><strong>Throws:</strong></span> Whatever the selected <code class="computeroutput"><span class="identifier">T</span></code>'s constructor throws.
1106         </li>
1107 <li class="listitem">
1108           <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1109           need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
1110           or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
1111         </li>
1112 <li class="listitem">
1113           <span class="bold"><strong>Exception Safety:</strong></span> If an exception is thrown
1114           during the initialization of <code class="computeroutput"><span class="identifier">T</span></code>,
1115           <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1116           is <span class="emphasis"><em>uninitialized</em></span>.
1117         </li>
1118 </ul></div>
1119 <p>
1120       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1121     </p>
1122 <a name="reference_optional_operator_equal_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
1123         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">InPlaceFactory</span><span class="special">&gt;</span>
1124         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">InPlaceFactory</span>
1125         <span class="keyword">const</span><span class="special">&amp;</span>
1126         <span class="identifier">f</span> <span class="special">);</span></code>
1127       </p></blockquote></div>
1128 <div class="blockquote"><blockquote class="blockquote"><p>
1129         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span>
1130         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">TypedInPlaceFactory</span>
1131         <span class="keyword">const</span><span class="special">&amp;</span>
1132         <span class="identifier">f</span> <span class="special">);</span></code>
1133       </p></blockquote></div>
1134 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1135 <li class="listitem">
1136           <span class="bold"><strong>Effect:</strong></span> Assigns an <code class="computeroutput"><span class="identifier">optional</span></code>
1137           with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
1138           from the factory.
1139         </li>
1140 <li class="listitem">
1141           <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>
1142           and its value is <span class="emphasis"><em>directly given</em></span> from the factory
1143           <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value <span class="underline">is not copied</span>).
1144         </li>
1145 <li class="listitem">
1146           <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code>
1147           constructor called by the factory throws.
1148         </li>
1149 <li class="listitem">
1150           <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="tutorial/in_place_factories.html" title="In-Place Factories">In-Place
1151           Factories</a>
1152         </li>
1153 <li class="listitem">
1154           <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
1155           be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
1156           constructor used by the factory; in that case, the <code class="computeroutput"><span class="identifier">optional</span></code>
1157           object will be reset to be <span class="emphasis"><em>uninitialized</em></span>.
1158         </li>
1159 </ul></div>
1160 <p>
1161       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1162     </p>
1163 <a name="reference_optional_reset_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1164         <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1165         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">reset</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span></code>
1166       </p></blockquote></div>
1167 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1168           <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>
1169           <span class="keyword">const</span><span class="special">&amp;</span>
1170           <span class="identifier">v</span><span class="special">)</span>
1171           <span class="special">;</span></code>
1172         </li></ul></div>
1173 <p>
1174       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1175     </p>
1176 <a name="reference_optional_reset"></a><div class="blockquote"><blockquote class="blockquote"><p>
1177         <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reset</span><span class="special">()</span> <span class="keyword">noexcept</span> <span class="special">;</span></code>
1178       </p></blockquote></div>
1179 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1180           <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>
1181         </li></ul></div>
1182 <p>
1183       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1184     </p>
1185 <a name="reference_optional_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
1186         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1187         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
1188       </p></blockquote></div>
1189 <div class="blockquote"><blockquote class="blockquote"><p>
1190         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1191         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get</span><span class="special">()</span> <span class="special">;</span></code>
1192       </p></blockquote></div>
1193 <div class="blockquote"><blockquote class="blockquote"><p>
1194         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
1195         <span class="keyword">const</span><span class="special">&amp;</span>
1196         <span class="identifier">get</span> <span class="special">(</span>
1197         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span> <span class="special">;</span></code>
1198       </p></blockquote></div>
1199 <div class="blockquote"><blockquote class="blockquote"><p>
1200         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1201         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span>
1202         <span class="special">&amp;)</span> <span class="special">;</span></code>
1203       </p></blockquote></div>
1204 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1205 <li class="listitem">
1206           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1207         </li>
1208 <li class="listitem">
1209           <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1210           value
1211         </li>
1212 <li class="listitem">
1213           <span class="bold"><strong>Throws:</strong></span> Nothing.
1214         </li>
1215 <li class="listitem">
1216           <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
1217           <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1218         </li>
1219 </ul></div>
1220 <p>
1221       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1222     </p>
1223 <div class="blockquote"><blockquote class="blockquote"><p>
1224         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
1225       </p></blockquote></div>
1226 <div class="blockquote"><blockquote class="blockquote"><p>
1227         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1228         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="identifier">get</span><span class="special">()</span> <span class="special">;</span></code>
1229       </p></blockquote></div>
1230 <div class="blockquote"><blockquote class="blockquote"><p>
1231         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
1232         <span class="keyword">const</span><span class="special">&amp;</span>
1233         <span class="identifier">get</span> <span class="special">(</span>
1234         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span> <span class="special">;</span></code>
1235       </p></blockquote></div>
1236 <div class="blockquote"><blockquote class="blockquote"><p>
1237         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">get</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span>
1238         <span class="special">&amp;)</span> <span class="special">;</span></code>
1239       </p></blockquote></div>
1240 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1241 <li class="listitem">
1242           <span class="bold"><strong>Requires: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1243         </li>
1244 <li class="listitem">
1245           <span class="bold"><strong>Returns:</strong></span> <span class="underline">The</span>
1246           reference contained.
1247         </li>
1248 <li class="listitem">
1249           <span class="bold"><strong>Throws:</strong></span> Nothing.
1250         </li>
1251 <li class="listitem">
1252           <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
1253           <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1254         </li>
1255 </ul></div>
1256 <p>
1257       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1258     </p>
1259 <a name="reference_optional_operator_asterisk"></a><div class="blockquote"><blockquote class="blockquote"><p>
1260         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1261         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1262       </p></blockquote></div>
1263 <div class="blockquote"><blockquote class="blockquote"><p>
1264         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1265         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&amp;;</span></code>
1266       </p></blockquote></div>
1267 <div class="blockquote"><blockquote class="blockquote"><p>
1268         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
1269         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&amp;&amp;;</span></code>
1270       </p></blockquote></div>
1271 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1272 <li class="listitem">
1273           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1274         </li>
1275 <li class="listitem">
1276           <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1277           value
1278         </li>
1279 <li class="listitem">
1280           <span class="bold"><strong>Throws:</strong></span> Nothing.
1281         </li>
1282 <li class="listitem">
1283           <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
1284           <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1285           On compilers that do not support ref-qualifiers on member functions these
1286           three overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
1287           and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
1288         </li>
1289 <li class="listitem">
1290           <span class="bold"><strong>Example:</strong></span>
1291 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
1292 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">v</span> <span class="special">);</span>
1293 <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">u</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">opt</span><span class="special">;</span>
1294 <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>
1295 <span class="identifier">T</span> <span class="identifier">w</span> <span class="special">;</span>
1296 <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">w</span> <span class="special">;</span>
1297 <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>
1298 </pre>
1299         </li>
1300 </ul></div>
1301 <p>
1302       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1303     </p>
1304 <div class="blockquote"><blockquote class="blockquote"><p>
1305         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1306       </p></blockquote></div>
1307 <div class="blockquote"><blockquote class="blockquote"><p>
1308         <code class="computeroutput"><span class="identifier">T</span> <span class="special">&amp;</span>
1309         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&amp;</span> <span class="special">;</span></code>
1310       </p></blockquote></div>
1311 <div class="blockquote"><blockquote class="blockquote"><p>
1312         <code class="computeroutput"><span class="identifier">T</span> <span class="special">&amp;</span>
1313         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;::</span><span class="keyword">operator</span><span class="special">*()</span> <span class="special">&amp;&amp;</span> <span class="special">;</span></code>
1314       </p></blockquote></div>
1315 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1316 <li class="listitem">
1317           <span class="bold"><strong>Requires: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1318         </li>
1319 <li class="listitem">
1320           <span class="bold"><strong>Returns:</strong></span> <span class="underline">The</span>
1321           reference contained.
1322         </li>
1323 <li class="listitem">
1324           <span class="bold"><strong>Throws:</strong></span> Nothing.
1325         </li>
1326 <li class="listitem">
1327           <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
1328           <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1329           On compilers that do not support ref-qualifiers on member functions these
1330           three overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
1331           and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
1332         </li>
1333 <li class="listitem">
1334           <span class="bold"><strong>Example:</strong></span>
1335 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
1336 <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">vref</span> <span class="special">=</span> <span class="identifier">v</span> <span class="special">;</span>
1337 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">vref</span> <span class="special">);</span>
1338 <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">vref2</span> <span class="special">=</span> <span class="special">*</span><span class="identifier">opt</span><span class="special">;</span>
1339 <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>
1340 <span class="special">++</span> <span class="identifier">v</span> <span class="special">;</span>
1341 <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>
1342 </pre>
1343         </li>
1344 </ul></div>
1345 <p>
1346       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1347     </p>
1348 <a name="reference_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1349         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">()</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1350       </p></blockquote></div>
1351 <div class="blockquote"><blockquote class="blockquote"><p>
1352         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1353         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&amp;</span> <span class="special">;</span></code>
1354       </p></blockquote></div>
1355 <div class="blockquote"><blockquote class="blockquote"><p>
1356         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
1357         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&amp;&amp;</span> <span class="special">;</span></code>
1358       </p></blockquote></div>
1359 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1360 <li class="listitem">
1361           <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1362           value, if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1363           is initialized.
1364         </li>
1365 <li class="listitem">
1366           <span class="bold"><strong>Throws:</strong></span> An instance of <code class="computeroutput"><span class="identifier">bad_optional_access</span></code>,
1367           if <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1368           is not initialized.
1369         </li>
1370 <li class="listitem">
1371           <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1372           ref-qualifiers on member functions these three overloads are replaced with
1373           the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
1374           and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
1375         </li>
1376 <li class="listitem">
1377           <span class="bold"><strong>Example:</strong></span>
1378 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
1379 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
1380 <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>
1381
1382 <span class="keyword">try</span> <span class="special">{</span>
1383   <span class="identifier">o0</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// throws</span>
1384   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">false</span> <span class="special">);</span>
1385 <span class="special">}</span>
1386 <span class="keyword">catch</span><span class="special">(</span><span class="identifier">bad_optional_access</span><span class="special">&amp;)</span> <span class="special">{</span>
1387   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
1388 <span class="special">}</span>
1389 </pre>
1390           <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1391         </li>
1392 </ul></div>
1393 <a name="reference_optional_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
1394         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">T</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_or</span><span class="special">(</span><span class="identifier">U</span> <span class="special">&amp;&amp;</span>
1395         <span class="identifier">v</span><span class="special">)</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1396       </p></blockquote></div>
1397 <div class="blockquote"><blockquote class="blockquote"><p>
1398         <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">U</span><span class="special">&gt;</span> <span class="identifier">T</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value_or</span><span class="special">(</span><span class="identifier">U</span> <span class="special">&amp;&amp;</span>
1399         <span class="identifier">v</span><span class="special">)</span> <span class="special">&amp;&amp;</span> <span class="special">;</span></code>
1400       </p></blockquote></div>
1401 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1402 <li class="listitem">
1403           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1404           is CopyConstructible.
1405         </li>
1406 <li class="listitem">
1407           <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">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">forward</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">))</span></code>.
1408           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>
1409           <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>
1410           <span class="keyword">static_cast</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">forward</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">))</span></code>.
1411         </li>
1412 <li class="listitem">
1413           <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the selected
1414           constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
1415         </li>
1416 <li class="listitem">
1417           <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1418           ref-qualifiers on member functions these three overloads are replaced with
1419           the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
1420           and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
1421           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>
1422           <span class="keyword">const</span><span class="special">&amp;</span></code>.
1423         </li>
1424 </ul></div>
1425 <p>
1426       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1427     </p>
1428 <a name="reference_optional_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1429         <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1430         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get_value_or</span><span class="special">(</span>
1431         <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="keyword">default</span><span class="special">)</span> <span class="keyword">const</span> <span class="special">;</span></code>
1432       </p></blockquote></div>
1433 <div class="blockquote"><blockquote class="blockquote"><p>
1434         <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1435         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get_value_or</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;</span>
1436         <span class="keyword">default</span> <span class="special">)</span>
1437         <span class="special">;</span></code>
1438       </p></blockquote></div>
1439 <div class="blockquote"><blockquote class="blockquote"><p>
1440         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
1441         <span class="keyword">const</span><span class="special">&amp;</span>
1442         <span class="identifier">get_optional_value_or</span> <span class="special">(</span>
1443         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">o</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span></code>
1444       </p></blockquote></div>
1445 <div class="blockquote"><blockquote class="blockquote"><p>
1446         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">get_optional_value_or</span>
1447         <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1448         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;&amp;</span>
1449         <span class="identifier">o</span><span class="special">,</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="keyword">default</span> <span class="special">)</span> <span class="special">;</span></code>
1450       </p></blockquote></div>
1451 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1452 <li class="listitem">
1453           <span class="bold"><strong>Deprecated:</strong></span> Use <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code> instead.
1454         </li>
1455 <li class="listitem">
1456           <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1457           value, if any, or <code class="computeroutput"><span class="keyword">default</span></code>.
1458         </li>
1459 <li class="listitem">
1460           <span class="bold"><strong>Throws:</strong></span> Nothing.
1461         </li>
1462 <li class="listitem">
1463           <span class="bold"><strong>Example:</strong></span>
1464 <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>
1465 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span><span class="special">;</span>
1466 <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</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>
1467 <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>
1468
1469 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">v</span> <span class="special">);</span>
1470 <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</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>
1471 <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>
1472 <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>
1473 </pre>
1474         </li>
1475 </ul></div>
1476 <p>
1477       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1478     </p>
1479 <a name="reference_optional_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
1480         <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">&lt;</span><span class="identifier">T</span></code>
1481         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get_ptr</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
1482       </p></blockquote></div>
1483 <div class="blockquote"><blockquote class="blockquote"><p>
1484         <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
1485         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="identifier">get_ptr</span><span class="special">()</span> <span class="special">;</span></code>
1486       </p></blockquote></div>
1487 <div class="blockquote"><blockquote class="blockquote"><p>
1488         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
1489         <span class="keyword">const</span><span class="special">*</span> <span class="identifier">get_pointer</span> <span class="special">(</span>
1490         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">)</span> <span class="special">;</span></code>
1491       </p></blockquote></div>
1492 <div class="blockquote"><blockquote class="blockquote"><p>
1493         <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span><span class="special">*</span> <span class="identifier">get_pointer</span>
1494         <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code>
1495         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span>
1496         <span class="special">&amp;)</span> <span class="special">;</span></code>
1497       </p></blockquote></div>
1498 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1499 <li class="listitem">
1500           <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
1501           value; else <code class="computeroutput"><span class="number">0</span></code> (<span class="emphasis"><em>null</em></span>).
1502         </li>
1503 <li class="listitem">
1504           <span class="bold"><strong>Throws:</strong></span> Nothing.
1505         </li>
1506 <li class="listitem">
1507           <span class="bold"><strong>Notes:</strong></span> The contained value is permanently
1508           stored within <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>,
1509           so you should not hold nor delete this pointer
1510         </li>
1511 <li class="listitem">
1512           <span class="bold"><strong>Example:</strong></span>
1513 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1514 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
1515 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span> <span class="identifier">copt</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>
1516 <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>
1517 <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>
1518 <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>
1519 <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>
1520 </pre>
1521         </li>
1522 </ul></div>
1523 <p>
1524       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1525     </p>
1526 <a name="reference_optional_operator_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
1527         <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">&lt;</span><span class="identifier">T</span></code>
1528         <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span> <span class="special">-&gt;()</span>
1529         <span class="keyword">const</span> <span class="special">;</span></code>
1530       </p></blockquote></div>
1531 <div class="blockquote"><blockquote class="blockquote"><p>
1532         <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
1533         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;::</span><span class="keyword">operator</span> <span class="special">-&gt;()</span> <span class="special">;</span></code>
1534       </p></blockquote></div>
1535 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1536 <li class="listitem">
1537           <span class="bold"><strong>Requires: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
1538         </li>
1539 <li class="listitem">
1540           <span class="bold"><strong>Returns:</strong></span> A pointer to the contained value.
1541         </li>
1542 <li class="listitem">
1543           <span class="bold"><strong>Throws:</strong></span> Nothing.
1544         </li>
1545 <li class="listitem">
1546           <span class="bold"><strong>Notes:</strong></span> The requirement is asserted via
1547           <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1548         </li>
1549 <li class="listitem">
1550           <span class="bold"><strong>Example:</strong></span>
1551 <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>
1552 <span class="identifier">X</span> <span class="identifier">x</span> <span class="special">;</span>
1553 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">X</span><span class="special">&gt;</span> <span class="identifier">opt</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
1554 <span class="identifier">opt</span><span class="special">-&gt;</span><span class="identifier">mdata</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span>
1555 </pre>
1556         </li>
1557 </ul></div>
1558 <p>
1559       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1560     </p>
1561 <a name="reference_optional_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
1562         <code class="computeroutput"><span class="keyword">explicit</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</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>
1563       </p></blockquote></div>
1564 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1565 <li class="listitem">
1566           <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>.
1567         </li>
1568 <li class="listitem">
1569           <span class="bold"><strong>Throws:</strong></span> Nothing.
1570         </li>
1571 <li class="listitem">
1572           <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1573           explicit conversion operators this falls back to safe-bool idiom.
1574         </li>
1575 <li class="listitem">
1576           <span class="bold"><strong>Example:</strong></span>
1577 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
1578 <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>
1579 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span> <span class="special">(</span> <span class="identifier">v</span> <span class="special">)</span> <span class="special">;</span>
1580 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">);</span>
1581 <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>
1582 </pre>
1583         </li>
1584 </ul></div>
1585 <p>
1586       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1587     </p>
1588 <a name="reference_optional_operator_not"></a><div class="blockquote"><blockquote class="blockquote"><p>
1589         <code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">!()</span> <span class="keyword">noexcept</span> <span class="special">;</span></code>
1590       </p></blockquote></div>
1591 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1592 <li class="listitem">
1593           <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>;
1594           else <code class="computeroutput"><span class="keyword">false</span></code>.
1595         </li>
1596 <li class="listitem">
1597           <span class="bold"><strong>Notes:</strong></span> This operator is provided for those
1598           compilers which can't use the <span class="emphasis"><em>unspecified-bool-type operator</em></span>
1599           in certain boolean contexts.
1600         </li>
1601 <li class="listitem">
1602           <span class="bold"><strong>Example:</strong></span>
1603 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span> <span class="special">;</span>
1604 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">opt</span> <span class="special">);</span>
1605 <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">some_T</span> <span class="special">;</span>
1606
1607 <span class="comment">// Notice the "double-bang" idiom here.</span>
1608 <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>
1609 </pre>
1610         </li>
1611 </ul></div>
1612 <p>
1613       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1614     </p>
1615 <a name="reference_optional_is_initialized"></a><div class="blockquote"><blockquote class="blockquote"><p>
1616         <code class="computeroutput"><span class="keyword">bool</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">is_initialized</span><span class="special">()</span>
1617         <span class="keyword">const</span> <span class="special">;</span></code>
1618       </p></blockquote></div>
1619 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1620           <span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">explicit</span>
1621           <span class="keyword">operator</span> <span class="keyword">bool</span>
1622           <span class="special">()</span> <span class="special">;</span></code>
1623         </li></ul></div>
1624 <p>
1625       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1626     </p>
1627 <h4>
1628 <a name="boost_optional.detailed_semantics.h1"></a>
1629       <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
1630       functions</a>
1631     </h4>
1632 <p>
1633       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1634     </p>
1635 <a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1636         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</span> <span class="identifier">make_optional</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">)</span></code>
1637       </p></blockquote></div>
1638 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1639 <li class="listitem">
1640           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span> type
1641           <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
1642         </li>
1643 <li class="listitem">
1644           <span class="bold"><strong>Example:</strong></span>
1645 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
1646
1647 <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&lt;int&gt;</span>
1648 </pre>
1649         </li>
1650 </ul></div>
1651 <p>
1652       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1653     </p>
1654 <a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1655         <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span></code> <span class="emphasis"><em>(not a ref)</em></span><code class="computeroutput"><span class="special">&gt;</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">&amp;</span> <span class="identifier">v</span> <span class="special">)</span></code>
1656       </p></blockquote></div>
1657 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1658 <li class="listitem">
1659           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">condition</span><span class="special">,</span><span class="identifier">v</span><span class="special">)</span></code>
1660           for the <span class="emphasis"><em>deduced</em></span> type <code class="computeroutput"><span class="identifier">T</span></code>
1661           of <code class="computeroutput"><span class="identifier">v</span></code>.
1662         </li>
1663 <li class="listitem">
1664           <span class="bold"><strong>Example:</strong></span>
1665 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">calculate_foo</span><span class="special">()</span>
1666 <span class="special">{</span>
1667   <span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
1668   <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>
1669 <span class="special">}</span>
1670
1671 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
1672 <span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
1673   <span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
1674 </pre>
1675         </li>
1676 </ul></div>
1677 <p>
1678       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1679     </p>
1680 <a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1681         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1682         <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1683         <span class="keyword">const</span><span class="special">&amp;</span>
1684         <span class="identifier">y</span> <span class="special">);</span></code>
1685       </p></blockquote></div>
1686 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1687 <li class="listitem">
1688           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1689           shall meet requirements of <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>.
1690         </li>
1691 <li class="listitem">
1692           <span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
1693           and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
1694           <code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
1695           <span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>. If
1696           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>.
1697           If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
1698         </li>
1699 <li class="listitem">
1700           <span class="bold"><strong>Throws:</strong></span> Nothing.
1701         </li>
1702 <li class="listitem">
1703           <span class="bold"><strong>Notes:</strong></span> Pointers have shallow relational
1704           operators while <code class="computeroutput"><span class="identifier">optional</span></code>
1705           has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span>
1706           <span class="special">==</span></code> directly in generic code which
1707           expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
1708           instead
1709         </li>
1710 <li class="listitem">
1711           <span class="bold"><strong>Example:</strong></span>
1712 <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>
1713 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
1714 <span class="identifier">T</span> <span class="identifier">z</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
1715 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def0</span> <span class="special">;</span>
1716 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def1</span> <span class="special">;</span>
1717 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
1718 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
1719 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optZ</span><span class="special">(</span><span class="identifier">z</span><span class="special">);</span>
1720
1721 <span class="comment">// Identity always hold</span>
1722 <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>
1723 <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>
1724
1725 <span class="comment">// Both uninitialized compare equal</span>
1726 <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>
1727
1728 <span class="comment">// Only one initialized compare unequal.</span>
1729 <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>
1730
1731 <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
1732 <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>
1733 <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>
1734 </pre>
1735         </li>
1736 </ul></div>
1737 <p>
1738       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1739     </p>
1740 <a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1741         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1742         <span class="special">&lt;</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1743         <span class="keyword">const</span><span class="special">&amp;</span>
1744         <span class="identifier">y</span> <span class="special">);</span></code>
1745       </p></blockquote></div>
1746 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1747 <li class="listitem">
1748           <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1749           shall meet requirements of <code class="computeroutput"><span class="identifier">LessThanComparable</span></code>.
1750         </li>
1751 <li class="listitem">
1752           <span class="bold"><strong>Returns:</strong></span> If <code class="computeroutput"><span class="identifier">y</span></code>
1753           is not initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
1754           If <code class="computeroutput"><span class="identifier">y</span></code> is initialized and
1755           <code class="computeroutput"><span class="identifier">x</span></code> is not initialized,
1756           <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>
1757           are initialized, <code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
1758           <span class="special">&lt;</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
1759         </li>
1760 <li class="listitem">
1761           <span class="bold"><strong>Throws:</strong></span> Nothing.
1762         </li>
1763 <li class="listitem">
1764           <span class="bold"><strong>Notes:</strong></span> Pointers have shallow relational
1765           operators while <code class="computeroutput"><span class="identifier">optional</span></code>
1766           has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span>
1767           <span class="special">&lt;</span></code> directly in generic code which
1768           expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</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>
1769           instead.
1770         </li>
1771 <li class="listitem">
1772           <span class="bold"><strong>Example:</strong></span>
1773 <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>
1774 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">34</span><span class="special">);</span>
1775 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def</span> <span class="special">;</span>
1776 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
1777 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
1778
1779 <span class="comment">// Identity always hold</span>
1780 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">def</span> <span class="special">&lt;</span> <span class="identifier">def</span><span class="special">)</span> <span class="special">);</span>
1781 <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>
1782
1783 <span class="comment">// Both uninitialized compare equal</span>
1784 <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>
1785
1786 <span class="comment">// Only one initialized compare unequal.</span>
1787 <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>
1788
1789 <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
1790 <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>
1791 <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>
1792 </pre>
1793         </li>
1794 </ul></div>
1795 <p>
1796       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1797     </p>
1798 <a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1799         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1800         <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1801         <span class="keyword">const</span><span class="special">&amp;</span>
1802         <span class="identifier">y</span> <span class="special">);</span></code>
1803       </p></blockquote></div>
1804 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1805 <li class="listitem">
1806           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
1807           <span class="identifier">x</span> <span class="special">==</span>
1808           <span class="identifier">y</span> <span class="special">);</span></code>
1809         </li>
1810 <li class="listitem">
1811           <span class="bold"><strong>Throws:</strong></span> Nothing.
1812         </li>
1813 </ul></div>
1814 <p>
1815       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1816     </p>
1817 <a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1818         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1819         <span class="special">&gt;</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1820         <span class="keyword">const</span><span class="special">&amp;</span>
1821         <span class="identifier">y</span> <span class="special">);</span></code>
1822       </p></blockquote></div>
1823 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1824 <li class="listitem">
1825           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
1826           <span class="identifier">y</span> <span class="special">&lt;</span>
1827           <span class="identifier">x</span> <span class="special">);</span></code>
1828         </li>
1829 <li class="listitem">
1830           <span class="bold"><strong>Throws:</strong></span> Nothing.
1831         </li>
1832 </ul></div>
1833 <p>
1834       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1835     </p>
1836 <a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1837         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1838         <span class="special">&lt;=</span> <span class="special">(</span>
1839         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1840         <span class="keyword">const</span><span class="special">&amp;</span>
1841         <span class="identifier">y</span> <span class="special">);</span></code>
1842       </p></blockquote></div>
1843 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1844 <li class="listitem">
1845           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
1846           <span class="identifier">y</span> <span class="special">&lt;</span>
1847           <span class="identifier">x</span> <span class="special">);</span></code>
1848         </li>
1849 <li class="listitem">
1850           <span class="bold"><strong>Throws:</strong></span> Nothing.
1851         </li>
1852 </ul></div>
1853 <p>
1854       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1855     </p>
1856 <a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1857         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1858         <span class="special">&gt;=</span> <span class="special">(</span>
1859         <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span>
1860         <span class="keyword">const</span><span class="special">&amp;</span>
1861         <span class="identifier">y</span> <span class="special">);</span></code>
1862       </p></blockquote></div>
1863 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1864 <li class="listitem">
1865           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
1866           <span class="identifier">x</span><span class="special">&lt;</span><span class="identifier">y</span> <span class="special">);</span></code>
1867         </li>
1868 <li class="listitem">
1869           <span class="bold"><strong>Throws:</strong></span> Nothing.
1870         </li>
1871 </ul></div>
1872 <a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
1873         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1874         <span class="special">==</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span>
1875         <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
1876       </p></blockquote></div>
1877 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1878 <li class="listitem">
1879           <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
1880         </li>
1881 <li class="listitem">
1882           <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1883           need not meet requirements of <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>.
1884         </li>
1885 </ul></div>
1886 <p>
1887       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1888     </p>
1889 <a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
1890         <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
1891         <span class="special">!=</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span>
1892         <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
1893       </p></blockquote></div>
1894 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1895           <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
1896           <span class="identifier">x</span> <span class="special">==</span>
1897           <span class="identifier">y</span> <span class="special">);</span></code>
1898         </li></ul></div>
1899 <p>
1900       <span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
1901     </p>
1902 <a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
1903         <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
1904         <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span>
1905         <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">y</span> <span class="special">)</span> <span class="special">;</span></code>
1906       </p></blockquote></div>
1907 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1908 <li class="listitem">
1909           <span class="bold"><strong>Effect:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
1910           and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
1911           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>.
1912           If only one is initialized, say <code class="computeroutput"><span class="identifier">x</span></code>,
1913           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.
1914         </li>
1915 <li class="listitem">
1916           <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>
1917           interchanged.
1918         </li>
1919 <li class="listitem">
1920           <span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
1921           <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;,</span><span class="identifier">T</span><span class="special">&amp;)</span></code>
1922           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">&amp;&amp;</span>
1923           <span class="special">)</span></code> throws.
1924         </li>
1925 <li class="listitem">
1926           <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">&amp;,</span><span class="identifier">T</span><span class="special">&amp;)</span></code>
1927           is used unqualified but with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">swap</span></code>
1928           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>
1929           <span class="identifier">T</span><span class="special">&amp;&amp;</span>
1930           <span class="special">)</span></code> is called.
1931         </li>
1932 <li class="listitem">
1933           <span class="bold"><strong>Exception Safety:</strong></span> If both are initialized,
1934           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">&amp;,</span><span class="identifier">T</span><span class="special">&amp;)</span></code>.
1935           If only one is initialized, it has the same basic guarantee as <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="keyword">operator</span><span class="special">=</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span>
1936           <span class="special">)</span></code>.
1937         </li>
1938 <li class="listitem">
1939           <span class="bold"><strong>Example:</strong></span>
1940 <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>
1941 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
1942 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def0</span> <span class="special">;</span>
1943 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def1</span> <span class="special">;</span>
1944 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
1945 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
1946
1947 <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>
1948
1949 <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>
1950 <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>
1951 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
1952
1953 <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>
1954
1955 <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>
1956 <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>
1957 <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>
1958 </pre>
1959         </li>
1960 </ul></div>
1961 </div>
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1965         Distributed under the Boost Software License, Version 1.0. (See accompanying
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