ceph/src/boost/libs/optional/doc/html/boost_optional/reference/header__boost_optional_optional_hpp_/detailed_semantics___optional_values.html

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  27 <a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___optional_values"></a><a class="link" href="detailed_semantics___optional_values.html" title="Detailed Semantics - Optional Values">Detailed
  28         Semantics - Optional Values</a>
  29 </h4></div></div></div>
  30 <div class="note"><table border="0" summary="Note">
  31 <tr>
  32 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
  33 <th align="left">Note</th>
  34 </tr>
  35 <tr><td align="left" valign="top"><p>
  36             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
  37             as sample code. It is not implied that the type <code class="computeroutput"><span class="identifier">T</span></code>
  38             must support each particular expression but that if the expression is
  39             supported, the implied condition holds.
  40           </p></td></tr>
  41 </table></div>
  42 <p>
  43           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  44         </p>
  45 <a name="reference_optional_constructor"></a><div class="blockquote"><blockquote class="blockquote"><p>
  46             <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>
  47             <span class="keyword">noexcept</span><span class="special">;</span></code>
  48           </p></blockquote></div>
  49 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  50 <li class="listitem">
  51               <span class="bold"><strong>Effect:</strong></span> Default-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
  52             </li>
  53 <li class="listitem">
  54               <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>.
  55             </li>
  56 <li class="listitem">
  57               <span class="bold"><strong>Notes:</strong></span> T's default constructor <span class="underline">is not</span> called.
  58             </li>
  59 <li class="listitem">
  60               <span class="bold"><strong>Example:</strong></span>
  61 <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>
  62 <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>
  63 </pre>
  64             </li>
  65 </ul></div>
  66 <p>
  67           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  68         </p>
  69 <a name="reference_optional_constructor_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
  70             <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>
  71             <span class="identifier">none_t</span> <span class="special">)</span>
  72             <span class="keyword">noexcept</span><span class="special">;</span></code>
  73           </p></blockquote></div>
  74 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  75 <li class="listitem">
  76               <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code> uninitialized.
  77             </li>
  78 <li class="listitem">
  79               <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>.
  80             </li>
  81 <li class="listitem">
  82               <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>'s
  83               default constructor <span class="underline">is not</span> called.
  84               The expression <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
  85               denotes an instance of <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none_t</span></code>
  86               that can be used as the parameter.
  87             </li>
  88 <li class="listitem">
  89               <span class="bold"><strong>Example:</strong></span>
  90 <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>
  91 <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>
  92 <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>
  93 </pre>
  94             </li>
  95 </ul></div>
  96 <p>
  97           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  98         </p>
  99 <a name="reference_optional_constructor_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 100             <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>
 101             <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>
 102           </p></blockquote></div>
 103 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 104 <li class="listitem">
 105               <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>
 106               is <code class="computeroutput"><span class="keyword">true</span></code>.
 107             </li>
 108 <li class="listitem">
 109               <span class="bold"><strong>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 110             </li>
 111 <li class="listitem">
 112               <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>
 113               and its value is a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">v</span></code>.
 114             </li>
 115 <li class="listitem">
 116               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span>
 117               <span class="special">)</span></code> throws.
 118             </li>
 119 <li class="listitem">
 120               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span>
 121               <span class="special">)</span></code> is called.
 122             </li>
 123 <li class="listitem">
 124               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 125               be thrown during <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span>
 126               <span class="special">);</span></code> in that case, this constructor
 127               has no effect.
 128             </li>
 129 <li class="listitem">
 130               <span class="bold"><strong>Example:</strong></span>
 131 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 132 <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>
 133 <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>
 134 </pre>
 135             </li>
 136 </ul></div>
 137 <p>
 138           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 139         </p>
 140 <a name="reference_optional_constructor_move_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 141             <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>
 142             <span class="identifier">T</span><span class="special">&amp;&amp;</span>
 143             <span class="identifier">v</span> <span class="special">)</span></code>
 144           </p></blockquote></div>
 145 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 146 <li class="listitem">
 147               <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>
 148               is <code class="computeroutput"><span class="keyword">true</span></code>.
 149             </li>
 150 <li class="listitem">
 151               <span class="bold"><strong>Effect:</strong></span> Directly-Move-Constructs an
 152               <code class="computeroutput"><span class="identifier">optional</span></code>.
 153             </li>
 154 <li class="listitem">
 155               <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>
 156               and its value is move-constructed from <code class="computeroutput"><span class="identifier">v</span></code>.
 157             </li>
 158 <li class="listitem">
 159               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 160               throws.
 161             </li>
 162 <li class="listitem">
 163               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 164               is called.
 165             </li>
 166 <li class="listitem">
 167               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 168               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>
 169               in that case, the state of <code class="computeroutput"><span class="identifier">v</span></code>
 170               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>.
 171             </li>
 172 <li class="listitem">
 173               <span class="bold"><strong>Example:</strong></span>
 174 <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>
 175 <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>
 176 <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>
 177 </pre>
 178             </li>
 179 </ul></div>
 180 <p>
 181           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 182         </p>
 183 <a name="reference_optional_constructor_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 184             <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>
 185             <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>
 186           </p></blockquote></div>
 187 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 188               If condition is true, same as:
 189             </li></ul></div>
 190 <div class="blockquote"><blockquote class="blockquote"><p>
 191             <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>
 192             <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>
 193           </p></blockquote></div>
 194 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 195               otherwise, same as:
 196             </li></ul></div>
 197 <div class="blockquote"><blockquote class="blockquote"><p>
 198             <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></code>
 199           </p></blockquote></div>
 200 <p>
 201           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 202         </p>
 203 <a name="reference_optional_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 204             <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>
 205             <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span>
 206             <span class="special">);</span></code>
 207           </p></blockquote></div>
 208 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 209 <li class="listitem">
 210               <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>
 211               is <code class="computeroutput"><span class="keyword">true</span></code>.
 212             </li>
 213 <li class="listitem">
 214               <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 215             </li>
 216 <li class="listitem">
 217               <span class="bold"><strong>Postconditions:</strong></span> If rhs is initialized,
 218               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 219               is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the
 220               value of <code class="computeroutput"><span class="identifier">rhs</span></code>; else
 221               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 222               is uninitialized.
 223             </li>
 224 <li class="listitem">
 225               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span>
 226               <span class="special">)</span></code> throws.
 227             </li>
 228 <li class="listitem">
 229               <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>
 230               is called.
 231             </li>
 232 <li class="listitem">
 233               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 234               be thrown during <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span>
 235               <span class="special">);</span></code> in that case, this constructor
 236               has no effect.
 237             </li>
 238 <li class="listitem">
 239               <span class="bold"><strong>Example:</strong></span>
 240 <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>
 241 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 242
 243 <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>
 244 <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>
 245
 246 <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>
 247 <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>
 248
 249 <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>
 250 <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>
 251 </pre>
 252             </li>
 253 </ul></div>
 254 <p>
 255           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 256         </p>
 257 <a name="reference_optional_move_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 258             <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>
 259             <span class="identifier">optional</span><span class="special">&amp;&amp;</span>
 260             <span class="identifier">rhs</span> <span class="special">)</span>
 261             <span class="keyword">noexcept</span><span class="special">(</span></code><span class="emphasis"><em>see
 262             below</em></span><code class="computeroutput"><span class="special">);</span></code>
 263           </p></blockquote></div>
 264 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 265 <li class="listitem">
 266               <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>
 267               is <code class="computeroutput"><span class="keyword">true</span></code>.
 268             </li>
 269 <li class="listitem">
 270               <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 271             </li>
 272 <li class="listitem">
 273               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 274               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 275               is initialized and its value is move constructed from <code class="computeroutput"><span class="identifier">rhs</span></code>; else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
 276             </li>
 277 <li class="listitem">
 278               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 279               throws.
 280             </li>
 281 <li class="listitem">
 282               <span class="bold"><strong>Remarks:</strong></span> The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code> is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span></code>.
 283             </li>
 284 <li class="listitem">
 285               <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 286               is initialized, <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">T</span> <span class="special">&amp;&amp;</span>
 287               <span class="special">)</span></code> is called.
 288             </li>
 289 <li class="listitem">
 290               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 291               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>
 292               in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
 293               initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
 294               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>.
 295             </li>
 296 <li class="listitem">
 297               <span class="bold"><strong>Example:</strong></span>
 298 <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>
 299 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
 300
 301 <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>
 302 <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>
 303
 304 <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>
 305 <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>
 306
 307 <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>
 308 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">);</span>
 309 <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>
 310 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init2</span> <span class="special">);</span>
 311 <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>
 312 </pre>
 313             </li>
 314 </ul></div>
 315 <p>
 316           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 317         </p>
 318 <a name="reference_optional_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 319             <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><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>
 320             <span class="special">);</span></code>
 321           </p></blockquote></div>
 322 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 323 <li class="listitem">
 324               <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 325             </li>
 326 <li class="listitem">
 327               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 328               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 329               is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the
 330               value of rhs converted to type <code class="computeroutput"><span class="identifier">T</span></code>;
 331               else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 332               is uninitialized.
 333             </li>
 334 <li class="listitem">
 335               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span>
 336               <span class="special">)</span></code> throws.
 337             </li>
 338 <li class="listitem">
 339               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span>
 340               <span class="special">)</span></code> is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized, which requires
 341               a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
 342               to <code class="computeroutput"><span class="identifier">T</span></code>.
 343             </li>
 344 <li class="listitem">
 345               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 346               be thrown during <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span>
 347               <span class="special">);</span></code> in that case, this constructor
 348               has no effect.
 349             </li>
 350 <li class="listitem">
 351               <span class="bold"><strong>Example:</strong></span>
 352 <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>
 353 <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>
 354
 355 <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>
 356 <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>
 357 </pre>
 358             </li>
 359 </ul></div>
 360 <p>
 361           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 362         </p>
 363 <a name="reference_optional_move_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 364             <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><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> <span class="identifier">rhs</span>
 365             <span class="special">);</span></code>
 366           </p></blockquote></div>
 367 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 368 <li class="listitem">
 369               <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
 370             </li>
 371 <li class="listitem">
 372               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
 373               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 374               is initialized and its value is move-constructed from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
 375               else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 376               is uninitialized.
 377             </li>
 378 <li class="listitem">
 379               <span class="bold"><strong>Throws:</strong></span> Whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 380               throws.
 381             </li>
 382 <li class="listitem">
 383               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span> <span class="identifier">U</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 384               is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is
 385               initialized, which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
 386               to <code class="computeroutput"><span class="identifier">T</span></code>.
 387             </li>
 388 <li class="listitem">
 389               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 390               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>
 391               in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
 392               initialized and the value of <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by exception safety
 393               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>
 394               <span class="special">)</span></code>.
 395             </li>
 396 <li class="listitem">
 397               <span class="bold"><strong>Example:</strong></span>
 398 <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>
 399 <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>
 400
 401 <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>
 402 <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>
 403 </pre>
 404             </li>
 405 </ul></div>
 406 <p>
 407           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 408         </p>
 409 <a name="reference_optional_in_place_init"></a><div class="blockquote"><blockquote class="blockquote"><p>
 410             <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>
 411             <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="identifier">optional</span><span class="special">(</span> <span class="identifier">in_place_init_t</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&amp;&amp;...</span> <span class="identifier">ars</span>
 412             <span class="special">);</span></code>
 413           </p></blockquote></div>
 414 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 415 <li class="listitem">
 416               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_constructible_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&amp;&amp;...&gt;</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
 417             </li>
 418 <li class="listitem">
 419               <span class="bold"><strong>Effect:</strong></span> Initializes the contained
 420               value as if direct-non-list-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the arguments <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>.
 421             </li>
 422 <li class="listitem">
 423               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
 424             </li>
 425 <li class="listitem">
 426               <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
 427               selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
 428             </li>
 429 <li class="listitem">
 430               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
 431               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
 432               On compilers that do not suppor variadic templates or rvalue references,
 433               this constuctor is available in limited functionality. For details
 434               <a class="link" href="../../dependencies_and_portability/emplace_operations_in_older_compilers.html#optional_emplace_workaround">see here</a>.
 435             </li>
 436 <li class="listitem">
 437               <span class="bold"><strong>Example:</strong></span>
 438 <pre class="programlisting"><span class="comment">// creates an std::mutex using its default constructor</span>
 439 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">mutex</span><span class="special">&gt;</span> <span class="identifier">om</span> <span class="special">{</span><span class="identifier">in_place_init</span><span class="special">};</span>
 440 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">om</span><span class="special">);</span>
 441
 442 <span class="comment">// creates a unique_lock by calling unique_lock(*om, std::defer_lock)</span>
 443 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">unique_lock</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">mutex</span><span class="special">&gt;&gt;</span> <span class="identifier">ol</span> <span class="special">{</span><span class="identifier">in_place_init</span><span class="special">,</span> <span class="special">*</span><span class="identifier">om</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">defer_lock</span><span class="special">};</span>
 444 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">ol</span><span class="special">);</span>
 445 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">ol</span><span class="special">-&gt;</span><span class="identifier">owns_lock</span><span class="special">());</span>
 446 </pre>
 447             </li>
 448 </ul></div>
 449 <p>
 450           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 451         </p>
 452 <a name="reference_optional_in_place_init_if"></a><div class="blockquote"><blockquote class="blockquote"><p>
 453             <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>
 454             <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="identifier">optional</span><span class="special">(</span> <span class="identifier">in_place_init_if_t</span><span class="special">,</span> <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span>
 455             <span class="identifier">Args</span><span class="special">&amp;&amp;...</span>
 456             <span class="identifier">ars</span> <span class="special">);</span></code>
 457           </p></blockquote></div>
 458 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 459 <li class="listitem">
 460               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">is_constructible_v</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Args</span><span class="special">&amp;&amp;...&gt;</span></code> is <code class="computeroutput"><span class="keyword">true</span></code>.
 461             </li>
 462 <li class="listitem">
 463               <span class="bold"><strong>Effect:</strong></span> If <code class="computeroutput"><span class="identifier">condition</span></code>
 464               is <code class="computeroutput"><span class="keyword">true</span></code>, initializes the
 465               contained value as if direct-non-list-initializing an object of type
 466               <code class="computeroutput"><span class="identifier">T</span></code> with the arguments
 467               <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>.
 468             </li>
 469 <li class="listitem">
 470               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">condition</span></code>.
 471             </li>
 472 <li class="listitem">
 473               <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
 474               selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
 475             </li>
 476 <li class="listitem">
 477               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
 478               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
 479               On compilers that do not suppor variadic templates or rvalue references,
 480               this constuctor is available in limited functionality. For details
 481               <a class="link" href="../../dependencies_and_portability/emplace_operations_in_older_compilers.html#optional_emplace_workaround">see here</a>.
 482             </li>
 483 <li class="listitem">
 484               <span class="bold"><strong>Example:</strong></span>
 485 <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">vector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;&gt;</span> <span class="identifier">ov1</span> <span class="special">{</span><span class="identifier">in_place_init_if</span><span class="special">,</span> <span class="keyword">false</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="string">"A"</span><span class="special">};</span>
 486 <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">ov1</span><span class="special">);</span>
 487
 488 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">vector</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;&gt;</span> <span class="identifier">ov2</span> <span class="special">{</span><span class="identifier">in_place_init_if</span><span class="special">,</span> <span class="keyword">true</span><span class="special">,</span> <span class="number">3</span><span class="special">,</span> <span class="string">"A"</span><span class="special">};</span>
 489 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">ov2</span><span class="special">);</span>
 490 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">ov2</span><span class="special">-&gt;</span><span class="identifier">size</span><span class="special">()</span> <span class="special">==</span> <span class="number">3</span><span class="special">);</span>
 491 </pre>
 492             </li>
 493 </ul></div>
 494 <p>
 495           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 496         </p>
 497 <a name="reference_optional_constructor_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
 498             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">InPlaceFactory</span><span class="special">&gt;</span>
 499             <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="identifier">optional</span><span class="special">(</span> <span class="identifier">InPlaceFactory</span>
 500             <span class="keyword">const</span><span class="special">&amp;</span>
 501             <span class="identifier">f</span> <span class="special">);</span></code>
 502           </p></blockquote></div>
 503 <div class="blockquote"><blockquote class="blockquote"><p>
 504             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span>
 505             <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="identifier">optional</span><span class="special">(</span> <span class="identifier">TypedInPlaceFactory</span>
 506             <span class="keyword">const</span><span class="special">&amp;</span>
 507             <span class="identifier">f</span> <span class="special">);</span></code>
 508           </p></blockquote></div>
 509 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 510 <li class="listitem">
 511               <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code> with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained from the factory.
 512             </li>
 513 <li class="listitem">
 514               <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>
 515               and its value is <span class="emphasis"><em>directly given</em></span> from the factory
 516               <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value
 517               <span class="underline">is not copied</span>).
 518             </li>
 519 <li class="listitem">
 520               <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code> constructor called by the factory
 521               throws.
 522             </li>
 523 <li class="listitem">
 524               <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="../../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
 525               Factories</a>
 526             </li>
 527 <li class="listitem">
 528               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
 529               be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
 530               constructor used by the factory; in that case, this constructor has
 531               no effect.
 532             </li>
 533 <li class="listitem">
 534               <span class="bold"><strong>Example:</strong></span>
 535 <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>
 536
 537 <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>
 538
 539 <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>
 540 <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>
 541
 542 <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>
 543 <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>
 544 </pre>
 545             </li>
 546 </ul></div>
 547 <p>
 548           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 549         </p>
 550 <a name="reference_optional_operator_equal_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
 551             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 552             <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>
 553             <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
 554           </p></blockquote></div>
 555 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 556 <li class="listitem">
 557               <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
 558               value.
 559             </li>
 560 <li class="listitem">
 561               <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
 562             </li>
 563 </ul></div>
 564 <p>
 565           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 566         </p>
 567 <a name="reference_optional_operator_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 568             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 569             <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="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span> <span class="special">)</span> <span class="special">;</span></code>
 570           </p></blockquote></div>
 571 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 572 <li class="listitem">
 573               <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>.
 574             </li>
 575 <li class="listitem">
 576               <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
 577               a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">rhs</span></code>.
 578             </li>
 579 <li class="listitem">
 580               <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>
 581               <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>
 582               throws.
 583             </li>
 584 <li class="listitem">
 585               <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s assignment operator is used,
 586               otherwise, its copy-constructor is used.
 587             </li>
 588 <li class="listitem">
 589               <span class="bold"><strong>Exception Safety:</strong></span> In the event of
 590               an exception, the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 591               as far as <code class="computeroutput"><span class="identifier">optional</span></code>
 592               is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
 593               <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
 594               If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 595               is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 596               <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.
 597             </li>
 598 <li class="listitem">
 599               <span class="bold"><strong>Example:</strong></span>
 600 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
 601 <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>
 602 <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>
 603
 604 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">;</span>
 605 <span class="identifier">def</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
 606 <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>
 607 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
 608 <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>
 609 </pre>
 610             </li>
 611 </ul></div>
 612 <p>
 613           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 614         </p>
 615 <a name="reference_optional_operator_move_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
 616             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 617             <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> <span class="identifier">rhs</span>
 618             <span class="special">)</span> <span class="special">;</span></code>
 619           </p></blockquote></div>
 620 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 621 <li class="listitem">
 622               <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>.
 623             </li>
 624 <li class="listitem">
 625               <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
 626               moved from <code class="computeroutput"><span class="identifier">rhs</span></code>.
 627             </li>
 628 <li class="listitem">
 629               <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>
 630               or <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span><span class="special">(</span><span class="identifier">T</span> <span class="special">&amp;&amp;)</span></code>
 631               throws.
 632             </li>
 633 <li class="listitem">
 634               <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> was initialized, <code class="computeroutput"><span class="identifier">T</span></code>'s move-assignment operator is used,
 635               otherwise, its move-constructor is used.
 636             </li>
 637 <li class="listitem">
 638               <span class="bold"><strong>Exception Safety:</strong></span> In the event of
 639               an exception, the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is unchanged and its value unspecified
 640               as far as <code class="computeroutput"><span class="identifier">optional</span></code>
 641               is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
 642               <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
 643               If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 644               is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
 645               <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.
 646             </li>
 647 <li class="listitem">
 648               <span class="bold"><strong>Example:</strong></span>
 649 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
 650 <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>
 651 <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>
 652
 653 <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>
 654 <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>
 655 <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>
 656 <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>
 657 <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>
 658 </pre>
 659             </li>
 660 </ul></div>
 661 <p>
 662           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 663         </p>
 664 <a name="reference_optional_operator_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 665             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 666             <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">optional</span>
 667             <span class="keyword">const</span><span class="special">&amp;</span>
 668             <span class="identifier">rhs</span> <span class="special">)</span>
 669             <span class="special">;</span></code>
 670           </p></blockquote></div>
 671 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 672 <li class="listitem">
 673               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
 674               is <a href="../../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">CopyAssignable</span></code>.
 675             </li>
 676 <li class="listitem">
 677 <p class="simpara">
 678               <span class="bold"><strong>Effects:</strong></span>
 679             </p>
 680 <div class="informaltable"><table class="table">
 681 <colgroup>
 682 <col>
 683 <col>
 684 <col>
 685 </colgroup>
 686 <thead><tr></tr></thead>
 687 <tbody>
 688 <tr>
 689 <td>
 690                     </td>
 691 <td>
 692                       <p>
 693                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
 694                       </p>
 695                     </td>
 696 <td>
 697                       <p>
 698                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
 699                       </p>
 700                     </td>
 701 </tr>
 702 <tr>
 703 <td>
 704                       <p>
 705                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 706                         contains a value</strong></span>
 707                       </p>
 708                     </td>
 709 <td>
 710                       <p>
 711                         assigns <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 712                         to the contained value
 713                       </p>
 714                     </td>
 715 <td>
 716                       <p>
 717                         initializes the contained value as if direct-initializing
 718                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
 719                         with <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 720                       </p>
 721                     </td>
 722 </tr>
 723 <tr>
 724 <td>
 725                       <p>
 726                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 727                         does not contain a value</strong></span>
 728                       </p>
 729                     </td>
 730 <td>
 731                       <p>
 732                         destroys the contained value by calling <code class="computeroutput"><span class="identifier">val</span><span class="special">-&gt;</span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>
 733                       </p>
 734                     </td>
 735 <td>
 736                       <p>
 737                         no effect
 738                       </p>
 739                     </td>
 740 </tr>
 741 </tbody>
 742 </table></div>
 743 </li>
 744 <li class="listitem">
 745               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>;
 746             </li>
 747 <li class="listitem">
 748               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
 749             </li>
 750 <li class="listitem">
 751               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
 752               is thrown, the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code>
 753               remains unchanged. If an exception is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s copy constructor, no effect.
 754               If an exception is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
 755               copy assignment, the state of its contained value is as defined by
 756               the exception safety guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s
 757               copy assignment.
 758             </li>
 759 <li class="listitem">
 760               <span class="bold"><strong>Example:</strong></span>
 761 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 762 <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>
 763 <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>
 764
 765 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
 766 <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>
 767 <span class="comment">// previous value (copy of 'v') destroyed from within 'opt'.</span>
 768 </pre>
 769             </li>
 770 </ul></div>
 771 <p>
 772           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 773         </p>
 774 <a name="reference_optional_operator_move_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 775             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
 776             <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">optional</span><span class="special">&amp;&amp;</span> <span class="identifier">rhs</span>
 777             <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>
 778           </p></blockquote></div>
 779 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 780 <li class="listitem">
 781               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
 782               is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
 783               and <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
 784             </li>
 785 <li class="listitem">
 786 <p class="simpara">
 787               <span class="bold"><strong>Effects:</strong></span>
 788             </p>
 789 <div class="informaltable"><table class="table">
 790 <colgroup>
 791 <col>
 792 <col>
 793 <col>
 794 </colgroup>
 795 <thead><tr></tr></thead>
 796 <tbody>
 797 <tr>
 798 <td>
 799                     </td>
 800 <td>
 801                       <p>
 802                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
 803                       </p>
 804                     </td>
 805 <td>
 806                       <p>
 807                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
 808                       </p>
 809                     </td>
 810 </tr>
 811 <tr>
 812 <td>
 813                       <p>
 814                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 815                         contains a value</strong></span>
 816                       </p>
 817                     </td>
 818 <td>
 819                       <p>
 820                         assigns <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code> to the contained value
 821                       </p>
 822                     </td>
 823 <td>
 824                       <p>
 825                         initializes the contained value as if direct-initializing
 826                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
 827                         with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>
 828                       </p>
 829                     </td>
 830 </tr>
 831 <tr>
 832 <td>
 833                       <p>
 834                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 835                         does not contain a value</strong></span>
 836                       </p>
 837                     </td>
 838 <td>
 839                       <p>
 840                         destroys the contained value by calling <code class="computeroutput"><span class="identifier">val</span><span class="special">-&gt;</span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>
 841                       </p>
 842                     </td>
 843 <td>
 844                       <p>
 845                         no effect
 846                       </p>
 847                     </td>
 848 </tr>
 849 </tbody>
 850 </table></div>
 851 </li>
 852 <li class="listitem">
 853               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>;
 854             </li>
 855 <li class="listitem">
 856               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
 857             </li>
 858 <li class="listitem">
 859               <span class="bold"><strong>Remarks:</strong></span> The expression inside <code class="computeroutput"><span class="keyword">noexcept</span></code> is equivalent to <code class="computeroutput"><span class="identifier">is_nothrow_move_constructible</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">value</span> <span class="special">&amp;&amp;</span>
 860               <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>.
 861             </li>
 862 <li class="listitem">
 863               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
 864               is thrown, the initialization state of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="identifier">rhs</span></code>
 865               remains unchanged. If an exception is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s move constructor, the state of
 866               <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 867               is determined by the exception safety guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s
 868               move constructor. If an exception is thrown during the call to T's
 869               move-assignment, the state of <code class="computeroutput"><span class="special">**</span><span class="keyword">this</span></code> and <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code> is determined by the exception
 870               safety guarantee of T's move assignment.
 871             </li>
 872 <li class="listitem">
 873               <span class="bold"><strong>Example:</strong></span>
 874 <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>
 875 <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>
 876
 877 <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
 878 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">;</span>
 879 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
 880 <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>
 881 </pre>
 882             </li>
 883 </ul></div>
 884 <p>
 885           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 886         </p>
 887 <a name="reference_optional_operator_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 888             <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>
 889             <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">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>
 890             <span class="special">)</span> <span class="special">;</span></code>
 891           </p></blockquote></div>
 892 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 893 <li class="listitem">
 894 <p class="simpara">
 895               <span class="bold"><strong>Effect:</strong></span>
 896             </p>
 897 <div class="informaltable"><table class="table">
 898 <colgroup>
 899 <col>
 900 <col>
 901 <col>
 902 </colgroup>
 903 <thead><tr></tr></thead>
 904 <tbody>
 905 <tr>
 906 <td>
 907                     </td>
 908 <td>
 909                       <p>
 910                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
 911                       </p>
 912                     </td>
 913 <td>
 914                       <p>
 915                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
 916                       </p>
 917                     </td>
 918 </tr>
 919 <tr>
 920 <td>
 921                       <p>
 922                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 923                         contains a value</strong></span>
 924                       </p>
 925                     </td>
 926 <td>
 927                       <p>
 928                         assigns <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 929                         to the contained value
 930                       </p>
 931                     </td>
 932 <td>
 933                       <p>
 934                         initializes the contained value as if direct-initializing
 935                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
 936                         with <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
 937                       </p>
 938                     </td>
 939 </tr>
 940 <tr>
 941 <td>
 942                       <p>
 943                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 944                         does not contain a value</strong></span>
 945                       </p>
 946                     </td>
 947 <td>
 948                       <p>
 949                         destroys the contained value by calling <code class="computeroutput"><span class="identifier">val</span><span class="special">-&gt;</span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>
 950                       </p>
 951                     </td>
 952 <td>
 953                       <p>
 954                         no effect
 955                       </p>
 956                     </td>
 957 </tr>
 958 </tbody>
 959 </table></div>
 960 </li>
 961 <li class="listitem">
 962               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
 963             </li>
 964 <li class="listitem">
 965               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
 966             </li>
 967 <li class="listitem">
 968               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
 969               is thrown, the result of the expression <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code> remains unchanged. If an exception
 970               is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
 971               constructor, no effect. If an exception is thrown during the call to
 972               <code class="computeroutput"><span class="identifier">T</span></code>'s assignment, the
 973               state of its contained value is as defined by the exception safety
 974               guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s copy
 975               assignment.
 976             </li>
 977 <li class="listitem">
 978               <span class="bold"><strong>Example:</strong></span>
 979 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
 980 <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>
 981 <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>
 982
 983 <span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt0</span> <span class="special">;</span>
 984 <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>
 985 </pre>
 986             </li>
 987 </ul></div>
 988 <p>
 989           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 990         </p>
 991 <a name="reference_optional_operator_move_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 992             <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>
 993             <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">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;&amp;&amp;</span> <span class="identifier">rhs</span>
 994             <span class="special">)</span> <span class="special">;</span></code>
 995           </p></blockquote></div>
 996 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 997 <li class="listitem">
 998 <p class="simpara">
 999               <span class="bold"><strong>Effect:</strong></span>
1000             </p>
1001 <div class="informaltable"><table class="table">
1002 <colgroup>
1003 <col>
1004 <col>
1005 <col>
1006 </colgroup>
1007 <thead><tr></tr></thead>
1008 <tbody>
1009 <tr>
1010 <td>
1011                     </td>
1012 <td>
1013                       <p>
1014                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
1015                       </p>
1016                     </td>
1017 <td>
1018                       <p>
1019                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
1020                       </p>
1021                     </td>
1022 </tr>
1023 <tr>
1024 <td>
1025                       <p>
1026                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
1027                         contains a value</strong></span>
1028                       </p>
1029                     </td>
1030 <td>
1031                       <p>
1032                         assigns <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code> to the contained value
1033                       </p>
1034                     </td>
1035 <td>
1036                       <p>
1037                         initializes the contained value as if direct-initializing
1038                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
1039                         with <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>
1040                       </p>
1041                     </td>
1042 </tr>
1043 <tr>
1044 <td>
1045                       <p>
1046                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
1047                         does not contain a value</strong></span>
1048                       </p>
1049                     </td>
1050 <td>
1051                       <p>
1052                         destroys the contained value by calling <code class="computeroutput"><span class="identifier">val</span><span class="special">-&gt;</span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>
1053                       </p>
1054                     </td>
1055 <td>
1056                       <p>
1057                         no effect
1058                       </p>
1059                     </td>
1060 </tr>
1061 </tbody>
1062 </table></div>
1063 </li>
1064 <li class="listitem">
1065               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
1066             </li>
1067 <li class="listitem">
1068               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">rhs</span><span class="special">)</span> <span class="special">==</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code>.
1069             </li>
1070 <li class="listitem">
1071               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
1072               is thrown, the result of the expression <code class="computeroutput"><span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span></code> remains unchanged. If an exception
1073               is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
1074               constructor, no effect. If an exception is thrown during the call to
1075               <code class="computeroutput"><span class="identifier">T</span></code>'s assignment, the
1076               state of its contained value is as defined by the exception safety
1077               guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s copy
1078               assignment.
1079             </li>
1080 <li class="listitem">
1081               <span class="bold"><strong>Example:</strong></span>
1082 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1083 <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>
1084 <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>
1085
1086 <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>
1087 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
1088 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt1</span> <span class="special">)</span>
1089 <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>
1090 </pre>
1091             </li>
1092 </ul></div>
1093 <p>
1094           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1095         </p>
1096 <a name="reference_optional_emplace"></a><div class="blockquote"><blockquote class="blockquote"><p>
1097             <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>
1098             <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">emplace</span><span class="special">(</span> <span class="identifier">Args</span><span class="special">&amp;&amp;...</span> <span class="identifier">args</span>
1099             <span class="special">);</span></code>
1100           </p></blockquote></div>
1101 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1102 <li class="listitem">
1103               <span class="bold"><strong>Requires:</strong></span> The compiler supports rvalue
1104               references and variadic templates.
1105             </li>
1106 <li class="listitem">
1107               <span class="bold"><strong>Effect:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized calls <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span> <span class="special">=</span> <span class="identifier">none</span></code>.
1108               Then initializes in-place the contained value as if direct-initializing
1109               an object of type <code class="computeroutput"><span class="identifier">T</span></code>
1110               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>.
1111             </li>
1112 <li class="listitem">
1113               <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>.
1114             </li>
1115 <li class="listitem">
1116               <span class="bold"><strong>Throws:</strong></span> Whatever the selected <code class="computeroutput"><span class="identifier">T</span></code>'s constructor throws.
1117             </li>
1118 <li class="listitem">
1119               <span class="bold"><strong>Exception Safety:</strong></span> If an exception
1120               is thrown during the initialization of <code class="computeroutput"><span class="identifier">T</span></code>,
1121               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
1122               is <span class="emphasis"><em>uninitialized</em></span>.
1123             </li>
1124 <li class="listitem">
1125               <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1126               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
1127               or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
1128               On compilers that do not suppor variadic templates or rvalue references,
1129               this function is available in limited functionality. For details <a class="link" href="../../dependencies_and_portability/emplace_operations_in_older_compilers.html#optional_emplace_workaround">see here</a>.
1130             </li>
1131 <li class="listitem">
1132               <span class="bold"><strong>Example:</strong></span>
1133 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1134 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">opt</span><span class="special">;</span>
1135 <span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="number">0</span><span class="special">);</span>  <span class="comment">// create in-place using ctor T(int)</span>
1136 <span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">();</span>   <span class="comment">// destroy previous and default-construct another T</span>
1137 <span class="identifier">opt</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="identifier">v</span><span class="special">);</span>  <span class="comment">// destroy and copy-construct in-place (no assignment called)</span>
1138 </pre>
1139             </li>
1140 </ul></div>
1141 <p>
1142           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1143         </p>
1144 <a name="reference_optional_operator_equal_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
1145             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">InPlaceFactory</span><span class="special">&gt;</span>
1146             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span>
1147             <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="identifier">InPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">);</span></code>
1148           </p></blockquote></div>
1149 <div class="blockquote"><blockquote class="blockquote"><p>
1150             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="identifier">TypedInPlaceFactory</span><span class="special">&gt;</span>
1151             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span>
1152             <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="identifier">TypedInPlaceFactory</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">f</span> <span class="special">);</span></code>
1153           </p></blockquote></div>
1154 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1155 <li class="listitem">
1156               <span class="bold"><strong>Effect:</strong></span> Assigns an <code class="computeroutput"><span class="identifier">optional</span></code>
1157               with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
1158               from the factory.
1159             </li>
1160 <li class="listitem">
1161               <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>
1162               and its value is <span class="emphasis"><em>directly given</em></span> from the factory
1163               <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value
1164               <span class="underline">is not copied</span>).
1165             </li>
1166 <li class="listitem">
1167               <span class="bold"><strong>Throws:</strong></span> Whatever the <code class="computeroutput"><span class="identifier">T</span></code> constructor called by the factory
1168               throws.
1169             </li>
1170 <li class="listitem">
1171               <span class="bold"><strong>Notes:</strong></span> See <a class="link" href="../../tutorial/in_place_factories.html" title="In-Place Factories">In-Place
1172               Factories</a>
1173             </li>
1174 <li class="listitem">
1175               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
1176               be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
1177               constructor used by the factory; in that case, the <code class="computeroutput"><span class="identifier">optional</span></code>
1178               object will be reset to be <span class="emphasis"><em>uninitialized</em></span>.
1179             </li>
1180 </ul></div>
1181 <p>
1182           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1183         </p>
1184 <a name="reference_optional_reset_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1185             <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="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>
1186           </p></blockquote></div>
1187 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1188               <span class="bold"><strong>Deprecated:</strong></span> same as <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span>
1189               <span class="special">(</span> <span class="identifier">T</span>
1190               <span class="keyword">const</span><span class="special">&amp;</span>
1191               <span class="identifier">v</span><span class="special">)</span>
1192               <span class="special">;</span></code>
1193             </li></ul></div>
1194 <p>
1195           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1196         </p>
1197 <a name="reference_optional_reset"></a><div class="blockquote"><blockquote class="blockquote"><p>
1198             <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>
1199             <span class="special">;</span></code>
1200           </p></blockquote></div>
1201 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1202               <span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">operator</span><span class="special">=(</span>
1203               <span class="identifier">none_t</span> <span class="special">);</span></code>
1204             </li></ul></div>
1205 <p>
1206           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1207         </p>
1208 <a name="reference_optional_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
1209             <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">get</span><span class="special">()</span> <span class="keyword">const</span> <span class="special">;</span></code>
1210           </p></blockquote></div>
1211 <div class="blockquote"><blockquote class="blockquote"><p>
1212             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1213             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">get</span><span class="special">()</span> <span class="special">;</span></code>
1214           </p></blockquote></div>
1215 <div class="blockquote"><blockquote class="blockquote"><p>
1216             <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
1217             <span class="keyword">const</span><span class="special">&amp;</span>
1218             <span class="identifier">get</span> <span class="special">(</span>
1219             <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="special">)</span> <span class="special">;</span></code>
1220           </p></blockquote></div>
1221 <div class="blockquote"><blockquote class="blockquote"><p>
1222             <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span><span class="special">&amp;</span> <span class="identifier">get</span>
1223             <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="special">&amp;)</span>
1224             <span class="special">;</span></code>
1225           </p></blockquote></div>
1226 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1227 <li class="listitem">
1228               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1229             </li>
1230 <li class="listitem">
1231               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1232               value
1233             </li>
1234 <li class="listitem">
1235               <span class="bold"><strong>Throws:</strong></span> Nothing.
1236             </li>
1237 <li class="listitem">
1238               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
1239               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1240             </li>
1241 </ul></div>
1242 <p>
1243           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1244         </p>
1245 <a name="reference_optional_operator_asterisk"></a><div class="blockquote"><blockquote class="blockquote"><p>
1246             <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="keyword">operator</span><span class="special">*()</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1247           </p></blockquote></div>
1248 <div class="blockquote"><blockquote class="blockquote"><p>
1249             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1250             <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>
1251             <span class="special">&amp;;</span></code>
1252           </p></blockquote></div>
1253 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1254 <li class="listitem">
1255               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
1256             </li>
1257 <li class="listitem">
1258               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1259               value
1260             </li>
1261 <li class="listitem">
1262               <span class="bold"><strong>Throws:</strong></span> Nothing.
1263             </li>
1264 <li class="listitem">
1265               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
1266               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1267               On compilers that do not support ref-qualifiers on member functions
1268               these two overloads are replaced with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code> and non-<code class="computeroutput"><span class="keyword">const</span></code>
1269               member functions.
1270             </li>
1271 <li class="listitem">
1272               <span class="bold"><strong>Example:</strong></span>
1273 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
1274 <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>
1275 <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>
1276 <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>
1277 <span class="identifier">T</span> <span class="identifier">w</span> <span class="special">;</span>
1278 <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">w</span> <span class="special">;</span>
1279 <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>
1280 </pre>
1281             </li>
1282 </ul></div>
1283 <p>
1284           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1285         </p>
1286 <a name="reference_optional_operator_asterisk_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
1287             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
1288             <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>
1289             <span class="special">&amp;&amp;;</span></code>
1290           </p></blockquote></div>
1291 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1292 <li class="listitem">
1293               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value.
1294             </li>
1295 <li class="listitem">
1296               <span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">val</span><span class="special">);</span></code>.
1297             </li>
1298 <li class="listitem">
1299               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
1300               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1301               On compilers that do not support ref-qualifiers on member functions
1302               this overload is not present.
1303             </li>
1304 </ul></div>
1305 <p>
1306           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1307         </p>
1308 <a name="reference_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1309             <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>
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">&gt;::</span><span class="identifier">value</span><span class="special">()</span> <span class="special">&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>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="special">*</span><span class="identifier">val</span> <span class="special">:</span> <span class="keyword">throw</span> <span class="identifier">bad_optional_access</span><span class="special">();</span></code>.
1318             </li>
1319 <li class="listitem">
1320               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1321               ref-qualifiers on member functions these two overloads are replaced
1322               with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
1323               and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
1324             </li>
1325 <li class="listitem">
1326               <span class="bold"><strong>Example:</strong></span>
1327 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
1328 <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>
1329 <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>
1330
1331 <span class="keyword">try</span> <span class="special">{</span>
1332   <span class="identifier">o0</span><span class="special">.</span><span class="identifier">value</span><span class="special">();</span> <span class="comment">// throws</span>
1333   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">false</span> <span class="special">);</span>
1334 <span class="special">}</span>
1335 <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>
1336   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
1337 <span class="special">}</span>
1338 </pre>
1339             </li>
1340 </ul></div>
1341 <p>
1342           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1343         </p>
1344 <a name="reference_optional_value_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
1345             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
1346             <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>
1347           </p></blockquote></div>
1348 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1349 <li class="listitem">
1350               <span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">return</span> <span class="keyword">bool</span><span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="special">?</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">val</span><span class="special">)</span> <span class="special">:</span> <span class="keyword">throw</span> <span class="identifier">bad_optional_access</span><span class="special">();</span></code>.
1351             </li>
1352 <li class="listitem">
1353               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1354               ref-qualifiers on member functions this overload is not present.
1355             </li>
1356 </ul></div>
1357 <p>
1358           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1359         </p>
1360 <a name="reference_optional_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
1361             <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>
1362             <span class="identifier">v</span><span class="special">)</span>
1363             <span class="keyword">const</span><span class="special">&amp;</span>
1364             <span class="special">;</span></code>
1365           </p></blockquote></div>
1366 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1367 <li class="listitem">
1368               <span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span>
1369               <span class="identifier">std</span><span class="special">::</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>.
1370             </li>
1371 <li class="listitem">
1372               <span class="bold"><strong>Remarks:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code>
1373               is not <a href="../../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> or <code class="computeroutput"><span class="identifier">U</span> <span class="special">&amp;&amp;</span></code>
1374               is not convertible to <code class="computeroutput"><span class="identifier">T</span></code>,
1375               the program is ill-formed.
1376             </li>
1377 <li class="listitem">
1378               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1379               ref-qualifiers on member functions this overload is replaced with the
1380               <code class="computeroutput"><span class="keyword">const</span></code>-qualified member
1381               function. On compilers without rvalue reference support the type of
1382               <code class="computeroutput"><span class="identifier">v</span></code> becomes <code class="computeroutput"><span class="identifier">U</span> <span class="keyword">const</span><span class="special">&amp;</span></code>.
1383             </li>
1384 </ul></div>
1385 <p>
1386           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1387         </p>
1388 <a name="reference_optional_value_or_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
1389             <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>
1390             <span class="identifier">v</span><span class="special">)</span>
1391             <span class="special">&amp;&amp;</span> <span class="special">;</span></code>
1392           </p></blockquote></div>
1393 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1394 <li class="listitem">
1395               <span class="bold"><strong>Effects:</strong></span> Equivalent to <code class="computeroutput"><span class="keyword">if</span> <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">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="keyword">else</span> <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</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>.
1396             </li>
1397 <li class="listitem">
1398               <span class="bold"><strong>Remarks:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code>
1399               is not <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
1400               or <code class="computeroutput"><span class="identifier">U</span> <span class="special">&amp;&amp;</span></code>
1401               is not convertible to <code class="computeroutput"><span class="identifier">T</span></code>,
1402               the program is ill-formed.
1403             </li>
1404 <li class="listitem">
1405               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1406               ref-qualifiers on member functions this overload is not present.
1407             </li>
1408 </ul></div>
1409 <p>
1410           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1411         </p>
1412 <a name="reference_optional_value_or_call"></a><div class="blockquote"><blockquote class="blockquote"><p>
1413             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">F</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_eval</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">;</span></code>
1414           </p></blockquote></div>
1415 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1416 <li class="listitem">
1417               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1418               is <a href="../../../../../../utility/CopyConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">CopyConstructible</span></code></a> and <code class="computeroutput"><span class="identifier">F</span></code> models a <a href="http://www.sgi.com/tech/stl/Generator.html" target="_top"><code class="computeroutput"><span class="identifier">Generator</span></code></a> whose result type
1419               is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
1420             </li>
1421 <li class="listitem">
1422               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
1423               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="special">**</span><span class="keyword">this</span><span class="special">;</span> <span class="keyword">else</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">();</span></code>.
1424             </li>
1425 <li class="listitem">
1426               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1427               ref-qualifiers on member functions this overload is replaced with the
1428               <code class="computeroutput"><span class="keyword">const</span></code>-qualified member
1429               function.
1430             </li>
1431 <li class="listitem">
1432               <span class="bold"><strong>Example:</strong></span>
1433 <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">complain_and_0</span><span class="special">()</span>
1434 <span class="special">{</span>
1435   <span class="identifier">clog</span> <span class="special">&lt;&lt;</span> <span class="string">"no value returned, using default"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
1436   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
1437 <span class="special">}</span>
1438
1439 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">o1</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span>
1440 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">oN</span> <span class="special">=</span> <span class="identifier">none</span><span class="special">;</span>
1441
1442 <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="identifier">o1</span><span class="special">.</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">complain_and_0</span><span class="special">);</span> <span class="comment">// fun not called</span>
1443 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">1</span><span class="special">);</span>
1444
1445 <span class="keyword">int</span> <span class="identifier">j</span> <span class="special">=</span> <span class="identifier">oN</span><span class="special">.</span><span class="identifier">value_or_eval</span><span class="special">(</span><span class="identifier">complain_and_0</span><span class="special">);</span> <span class="comment">// fun called</span>
1446 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">i</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
1447 </pre>
1448             </li>
1449 </ul></div>
1450 <p>
1451           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1452         </p>
1453 <a name="reference_optional_value_or_call_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
1454             <code class="computeroutput"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">F</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_eval</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&amp;&amp;</span>
1455             <span class="special">;</span></code>
1456           </p></blockquote></div>
1457 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1458 <li class="listitem">
1459               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
1460               is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
1461               and <code class="computeroutput"><span class="identifier">F</span></code> models a <a href="http://www.sgi.com/tech/stl/Generator.html" target="_top"><code class="computeroutput"><span class="identifier">Generator</span></code></a>
1462               whose result type is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
1463             </li>
1464 <li class="listitem">
1465               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
1466               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span>
1467               <span class="identifier">f</span><span class="special">();</span></code>.
1468             </li>
1469 <li class="listitem">
1470               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1471               ref-qualifiers on member functions this overload is not present.
1472             </li>
1473 </ul></div>
1474 <p>
1475           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1476         </p>
1477 <a name="reference_optional_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
1478             <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">get_value_or</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="keyword">const</span> <span class="special">;</span></code>
1479           </p></blockquote></div>
1480 <div class="blockquote"><blockquote class="blockquote"><p>
1481             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
1482             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">get_value_or</span><span class="special">(</span>
1483             <span class="identifier">T</span><span class="special">&amp;</span>
1484             <span class="keyword">default</span> <span class="special">)</span>
1485             <span class="special">;</span></code>
1486           </p></blockquote></div>
1487 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1488 <li class="listitem">
1489               <span class="bold"><strong>Deprecated:</strong></span> Use <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code> instead.
1490             </li>
1491 <li class="listitem">
1492               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
1493               value, if any, or <code class="computeroutput"><span class="keyword">default</span></code>.
1494             </li>
1495 <li class="listitem">
1496               <span class="bold"><strong>Throws:</strong></span> Nothing.
1497             </li>
1498 <li class="listitem">
1499               <span class="bold"><strong>Example:</strong></span>
1500 <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>
1501 <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>
1502 <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>
1503 <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>
1504
1505 <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>
1506 <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">opt</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>
1507 <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>
1508 <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>
1509 </pre>
1510             </li>
1511 </ul></div>
1512 <p>
1513           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1514         </p>
1515 <a name="reference_optional_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
1516             <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><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>
1517           </p></blockquote></div>
1518 <div class="blockquote"><blockquote class="blockquote"><p>
1519             <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
1520             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">get_ptr</span><span class="special">()</span>
1521             <span class="special">;</span></code>
1522           </p></blockquote></div>
1523 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1524 <li class="listitem">
1525               <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
1526               contained value; else <code class="computeroutput"><span class="number">0</span></code>
1527               (<span class="emphasis"><em>null</em></span>).
1528             </li>
1529 <li class="listitem">
1530               <span class="bold"><strong>Throws:</strong></span> Nothing.
1531             </li>
1532 <li class="listitem">
1533               <span class="bold"><strong>Notes:</strong></span> The contained value is permanently
1534               stored within <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>,
1535               so you should not hold nor delete this pointer
1536             </li>
1537 <li class="listitem">
1538               <span class="bold"><strong>Example:</strong></span>
1539 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
1540 <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>
1541 <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>
1542 <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>
1543 <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>
1544 <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>
1545 <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>
1546 </pre>
1547             </li>
1548 </ul></div>
1549 <p>
1550           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1551         </p>
1552 <a name="reference_optional_operator_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
1553             <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><span class="special">&gt;::</span><span class="keyword">operator</span>
1554             <span class="special">-&gt;()</span> <span class="keyword">const</span>
1555             <span class="special">;</span></code>
1556           </p></blockquote></div>
1557 <div class="blockquote"><blockquote class="blockquote"><p>
1558             <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
1559             <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">-&gt;()</span>
1560             <span class="special">;</span></code>
1561           </p></blockquote></div>
1562 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1563 <li class="listitem">
1564               <span class="bold"><strong>Requires: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
1565             </li>
1566 <li class="listitem">
1567               <span class="bold"><strong>Returns:</strong></span> A pointer to the contained
1568               value.
1569             </li>
1570 <li class="listitem">
1571               <span class="bold"><strong>Throws:</strong></span> Nothing.
1572             </li>
1573 <li class="listitem">
1574               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
1575               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
1576             </li>
1577 <li class="listitem">
1578               <span class="bold"><strong>Example:</strong></span>
1579 <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>
1580 <span class="identifier">X</span> <span class="identifier">x</span> <span class="special">;</span>
1581 <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>
1582 <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>
1583 </pre>
1584             </li>
1585 </ul></div>
1586 <p>
1587           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1588         </p>
1589 <a name="reference_optional_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
1590             <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>
1591             <span class="keyword">bool</span><span class="special">()</span>
1592             <span class="keyword">const</span> <span class="keyword">noexcept</span>
1593             <span class="special">;</span></code>
1594           </p></blockquote></div>
1595 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1596 <li class="listitem">
1597               <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>.
1598             </li>
1599 <li class="listitem">
1600               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
1601               explicit conversion operators this falls back to safe-bool idiom.
1602             </li>
1603 <li class="listitem">
1604               <span class="bold"><strong>Example:</strong></span>
1605 <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>
1606 <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>
1607 <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>
1608 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">);</span>
1609 <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>
1610 </pre>
1611             </li>
1612 </ul></div>
1613 <p>
1614           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1615         </p>
1616 <a name="reference_optional_operator_not"></a><div class="blockquote"><blockquote class="blockquote"><p>
1617             <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>
1618             <span class="special">;</span></code>
1619           </p></blockquote></div>
1620 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
1621 <li class="listitem">
1622               <span class="bold"><strong>Returns:</strong></span> If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>; else <code class="computeroutput"><span class="keyword">false</span></code>.
1623             </li>
1624 <li class="listitem">
1625               <span class="bold"><strong>Notes:</strong></span> This operator is provided for
1626               those compilers which can't use the <span class="emphasis"><em>unspecified-bool-type
1627               operator</em></span> in certain boolean contexts.
1628             </li>
1629 <li class="listitem">
1630               <span class="bold"><strong>Example:</strong></span>
1631 <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>
1632 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">opt</span> <span class="special">);</span>
1633 <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">some_T</span> <span class="special">;</span>
1634
1635 <span class="comment">// Notice the "double-bang" idiom here.</span>
1636 <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>
1637 </pre>
1638             </li>
1639 </ul></div>
1640 <p>
1641           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
1642         </p>
1643 <a name="reference_optional_is_initialized"></a><div class="blockquote"><blockquote class="blockquote"><p>
1644             <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> <span class="keyword">const</span> <span class="special">;</span></code>
1645           </p></blockquote></div>
1646 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
1647               <span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">explicit</span> <span class="keyword">operator</span>
1648               <span class="keyword">bool</span> <span class="special">()</span>
1649               <span class="special">;</span></code>
1650             </li></ul></div>
1651 </div>
1652 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
1653 <td align="left"></td>
1654 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
1655         Distributed under the Boost Software License, Version 1.0. (See accompanying
1656         file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
1657       </p>
1658 </div></td>
1659 </tr></table>
1660 <hr>
1661 <div class="spirit-nav">
1662 <a accesskey="p" href="header_optional_optional_refs.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../../../optional/reference/header__boost_optional_optional_hpp_.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="detailed_semantics___optional_references.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
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