Home Explore Help
Register Sign In
devn00b
/
EQ2EMu
9
3
Fork 0
Files Issues 59
Tree: 9d9a5ec294
Branches Tags
EQ2EMu
/
EQ2
/
source
/
depends
/
boost_1_72_0
/
libs
/
optional
/
doc
/
html
/
boost_optional
/
reference
/
header__boost_optional_optional_hpp_
/
detailed_semantics___optional_values.html

detailed_semantics___optional_values.html 173 KB
History Raw

123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139140141142143144145146147148149150151152153154155156157158159160161162163164165166167168169170171172173174175176177178179180181182183184185186187188189190191192193194195196197198199200201202203204205206207208209210211212213214215216217218219220221222223224225226227228229230231232233234235236237238239240241242243244245246247248249250251252253254255256257258259260261262263264265266267268269270271272273274275276277278279280281282283284285286287288289290291292293294295296297298299300301302303304305306307308309310311312313314315316317318319320321322323324325326327328329330331332333334335336337338339340341342343344345346347348349350351352353354355356357358359360361362363364365366367368369370371372373374375376377378379380381382383384385386387388389390391392393394395396397398399400401402403404405406407408409410411412413414415416417418419420421422423424425426427428429430431432433434435436437438439440441442443444445446447448449450451452453454455456457458459460461462463464465466467468469470471472473474475476477478479480481482483484485486487488489490491492493494495496497498499500501502503504505506507508509510511512513514515516517518519520521522523524525526527528529530531532533534535536537538539540541542543544545546547548549550551552553554555556557558559560561562563564565566567568569570571572573574575576577578579580581582583584585586587588589590591592593594595596597598599600601602603604605606607608609610611612613614615616617618619620621622623624625626627628629630631632633634635636637638639640641642643644645646647648649650651652653654655656657658659660661662663664665666667668669670671672673674675676677678679680681682683684685686687688689690691692693694695696697698699700701702703704705706707708709710711712713714715716717718719720721722723724725726727728729730731732733734735736737738739740741742743744745746747748749750751752753754755756757758759760761762763764765766767768769770771772773774775776777778779780781782783784785786787788789790791792793794795796797798799800801802803804805806807808809810811812813814815816817818819820821822823824825826827828829830831832833834835836837838839840841842843844845846847848849850851852853854855856857858859860861862863864865866867868869870871872873874875876877878879880881882883884885886887888889890891892893894895896897898899900901902903904905906907908909910911912913914915916917918919920921922923924925926927928929930931932933934935936937938939940941942943944945946947948949950951952953954955956957958959960961962963964965966967968969970971972973974975976977978979980981982983984985986987988989990991992993994995996997998999100010011002100310041005100610071008100910101011101210131014101510161017101810191020102110221023102410251026102710281029103010311032103310341035103610371038103910401041104210431044104510461047104810491050105110521053105410551056105710581059106010611062106310641065106610671068106910701071107210731074107510761077107810791080108110821083108410851086108710881089109010911092109310941095109610971098109911001101110211031104110511061107110811091110111111121113111411151116111711181119112011211122112311241125112611271128112911301131113211331134113511361137113811391140114111421143114411451146114711481149115011511152115311541155115611571158115911601161116211631164116511661167116811691170117111721173117411751176117711781179118011811182118311841185118611871188118911901191119211931194119511961197119811991200120112021203120412051206120712081209121012111212121312141215121612171218121912201221122212231224122512261227122812291230123112321233123412351236123712381239124012411242124312441245124612471248124912501251125212531254125512561257125812591260126112621263126412651266126712681269127012711272127312741275127612771278127912801281128212831284128512861287128812891290129112921293129412951296129712981299130013011302130313041305130613071308130913101311131213131314131513161317131813191320132113221323132413251326132713281329133013311332133313341335133613371338133913401341134213431344134513461347134813491350135113521353135413551356135713581359136013611362136313641365136613671368136913701371137213731374137513761377137813791380138113821383138413851386138713881389139013911392139313941395139613971398139914001401140214031404140514061407140814091410141114121413141414151416141714181419142014211422142314241425142614271428142914301431143214331434143514361437143814391440144114421443144414451446144714481449145014511452145314541455145614571458145914601461146214631464146514661467146814691470147114721473147414751476147714781479148014811482148314841485148614871488148914901491149214931494149514961497149814991500150115021503150415051506150715081509151015111512151315141515151615171518151915201521152215231524152515261527152815291530153115321533153415351536153715381539154015411542154315441545154615471548154915501551155215531554155515561557155815591560156115621563156415651566156715681569157015711572157315741575157615771578157915801581158215831584158515861587158815891590159115921593159415951596159715981599160016011602160316041605160616071608160916101611161216131614161516161617161816191620162116221623162416251626162716281629163016311632163316341635163616371638163916401641164216431644164516461647164816491650165116521653165416551656165716581659166016611662166316641665166616671668166916701671167216731674167516761677167816791680168116821683168416851686168716881689169016911692169316941695169616971698169917001701170217031704170517061707170817091710171117121713171417151716171717181719172017211722172317241725172617271728172917301731173217331734173517361737173817391740174117421743174417451746174717481749175017511752175317541755175617571758175917601761176217631764176517661767176817691770177117721773177417751776177717781779178017811782178317841785178617871788 
  1. <html>
    2. 

  2. <head>
    3. 

  3. <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
    4. 

  4. <title>Detailed Semantics - Optional Values</title>
    5. 

  5. <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css">
    6. 

  6. <meta name="generator" content="DocBook XSL Stylesheets V1.79.1">
    7. 

  7. <link rel="home" href="../../../index.html" title="Boost.Optional">
    8. 

  8. <link rel="up" href="../../../optional/reference/header__boost_optional_optional_hpp_.html" title="Header &lt;boost/optional/optional.hpp&gt;">
    9. 

  9. <link rel="prev" href="header_optional_optional_refs.html" title="Optional References">
    10. 

  10. <link rel="next" href="detailed_semantics___optional_references.html" title="Detailed Semantics - Optional References">
    11. 

  11. </head>
    12. 

  12. <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
    13. 

  13. <table cellpadding="2" width="100%"><tr>
    14. 

  14. <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td>
    15. 

  15. <td align="center"><a href="../../../../../../../index.html">Home</a></td>
    16. 

  16. <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td>
    17. 

  17. <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
    18. 

  18. <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
    19. 

  19. <td align="center"><a href="../../../../../../../more/index.htm">More</a></td>
    20. 

  20. </tr></table>
    21. 

  21. <hr>
    22. 

  22. <div class="spirit-nav">
    23. 

  23. <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>
    24. 

  24. </div>
    25. 

  25. <div class="section">
    26. 

  26. <div class="titlepage"><div><div><h4 class="title">
    27. 

  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. 

  28.         Semantics - Optional Values</a>
    29. 

  29. </h4></div></div></div>
    30. 

  30. <div class="note"><table border="0" summary="Note">
    31. 

  31. <tr>
    32. 

  32. <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../../doc/src/images/note.png"></td>
    33. 

  33. <th align="left">Note</th>
    34. 

  34. </tr>
    35. 

  35. <tr><td align="left" valign="top"><p>
    36. 

  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. 

  37.             as sample code. It is not implied that the type <code class="computeroutput"><span class="identifier">T</span></code>
    38. 

  38.             must support each particular expression but that if the expression is
    39. 

  39.             supported, the implied condition holds.
    40. 

  40.           </p></td></tr>
    41. 

  41. </table></div>
    42. 

  42. <p>
    43. 

  43.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    44. 

  44.         </p>
    45. 

  45. <a name="reference_optional_constructor"></a><div class="blockquote"><blockquote class="blockquote"><p>
    46. 

  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. 

  47.             <span class="keyword">noexcept</span><span class="special">;</span></code>
    48. 

  48.           </p></blockquote></div>
    49. 

  49. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    50. 

  50. <li class="listitem">
    51. 

  51.               <span class="bold"><strong>Effect:</strong></span> Default-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    52. 

  52.             </li>
    53. 

  53. <li class="listitem">
    54. 

  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. 

  55.             </li>
    56. 

  56. <li class="listitem">
    57. 

  57.               <span class="bold"><strong>Notes:</strong></span> T's default constructor <span class="underline">is not</span> called.
    58. 

  58.             </li>
    59. 

  59. <li class="listitem">
    60. 

  60.               <span class="bold"><strong>Example:</strong></span>
    61. 

  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. 

  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. 

  63. </pre>
    64. 

  64.             </li>
    65. 

  65. </ul></div>
    66. 

  66. <p>
    67. 

  67.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    68. 

  68.         </p>
    69. 

  69. <a name="reference_optional_constructor_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
    70. 

  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. 

  71.             <span class="identifier">none_t</span> <span class="special">)</span>
    72. 

  72.             <span class="keyword">noexcept</span><span class="special">;</span></code>
    73. 

  73.           </p></blockquote></div>
    74. 

  74. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    75. 

  75. <li class="listitem">
    76. 

  76.               <span class="bold"><strong>Effect:</strong></span> Constructs an <code class="computeroutput"><span class="identifier">optional</span></code> uninitialized.
    77. 

  77.             </li>
    78. 

  78. <li class="listitem">
    79. 

  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. 

  80.             </li>
    81. 

  81. <li class="listitem">
    82. 

  82.               <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>'s
    83. 

  83.               default constructor <span class="underline">is not</span> called.
    84. 

  84.               The expression <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>
    85. 

  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. 

  86.               that can be used as the parameter.
    87. 

  87.             </li>
    88. 

  88. <li class="listitem">
    89. 

  89.               <span class="bold"><strong>Example:</strong></span>
    90. 

  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. 

  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. 

  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. 

  93. </pre>
    94. 

  94.             </li>
    95. 

  95. </ul></div>
    96. 

  96. <p>
    97. 

  97.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    98. 

  98.         </p>
    99. 

  99. <a name="reference_optional_constructor_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    100. 

  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. 

  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. 

  102.           </p></blockquote></div>
    103. 

  103. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    104. 

  104. <li class="listitem">
    105. 

  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. 

  106.               is <code class="computeroutput"><span class="keyword">true</span></code>.
    107. 

  107.             </li>
    108. 

  108. <li class="listitem">
    109. 

  109.               <span class="bold"><strong>Effect:</strong></span> Directly-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    110. 

  110.             </li>
    111. 

  111. <li class="listitem">
    112. 

  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. 

  113.               and its value is a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">v</span></code>.
    114. 

  114.             </li>
    115. 

  115. <li class="listitem">
    116. 

  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. 

  117.               <span class="special">)</span></code> throws.
    118. 

  118.             </li>
    119. 

  119. <li class="listitem">
    120. 

  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. 

  121.               <span class="special">)</span></code> is called.
    122. 

  122.             </li>
    123. 

  123. <li class="listitem">
    124. 

  124.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    125. 

  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. 

  126.               <span class="special">);</span></code> in that case, this constructor
    127. 

  127.               has no effect.
    128. 

  128.             </li>
    129. 

  129. <li class="listitem">
    130. 

  130.               <span class="bold"><strong>Example:</strong></span>
    131. 

  131. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    132. 

  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. 

  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. 

  134. </pre>
    135. 

  135.             </li>
    136. 

  136. </ul></div>
    137. 

  137. <p>
    138. 

  138.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    139. 

  139.         </p>
    140. 

  140. <a name="reference_optional_constructor_move_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    141. 

  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. 

  142.             <span class="identifier">T</span><span class="special">&amp;&amp;</span>
    143. 

  143.             <span class="identifier">v</span> <span class="special">)</span></code>
    144. 

  144.           </p></blockquote></div>
    145. 

  145. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    146. 

  146. <li class="listitem">
    147. 

  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. 

  148.               is <code class="computeroutput"><span class="keyword">true</span></code>.
    149. 

  149.             </li>
    150. 

  150. <li class="listitem">
    151. 

  151.               <span class="bold"><strong>Effect:</strong></span> Directly-Move-Constructs an
    152. 

  152.               <code class="computeroutput"><span class="identifier">optional</span></code>.
    153. 

  153.             </li>
    154. 

  154. <li class="listitem">
    155. 

  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. 

  156.               and its value is move-constructed from <code class="computeroutput"><span class="identifier">v</span></code>.
    157. 

  157.             </li>
    158. 

  158. <li class="listitem">
    159. 

  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. 

  160.               throws.
    161. 

  161.             </li>
    162. 

  162. <li class="listitem">
    163. 

  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. 

  164.               is called.
    165. 

  165.             </li>
    166. 

  166. <li class="listitem">
    167. 

  167.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    168. 

  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. 

  169.               in that case, the state of <code class="computeroutput"><span class="identifier">v</span></code>
    170. 

  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. 

  171.             </li>
    172. 

  172. <li class="listitem">
    173. 

  173.               <span class="bold"><strong>Example:</strong></span>
    174. 

  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. 

  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. 

  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. 

  177. </pre>
    178. 

  178.             </li>
    179. 

  179. </ul></div>
    180. 

  180. <p>
    181. 

  181.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    182. 

  182.         </p>
    183. 

  183. <a name="reference_optional_constructor_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    184. 

  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. 

  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. 

  186.           </p></blockquote></div>
    187. 

  187. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
    188. 

  188.               If condition is true, same as:
    189. 

  189.             </li></ul></div>
    190. 

  190. <div class="blockquote"><blockquote class="blockquote"><p>
    191. 

  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. 

  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. 

  193.           </p></blockquote></div>
    194. 

  194. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
    195. 

  195.               otherwise, same as:
    196. 

  196.             </li></ul></div>
    197. 

  197. <div class="blockquote"><blockquote class="blockquote"><p>
    198. 

  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. 

  199.           </p></blockquote></div>
    200. 

  200. <p>
    201. 

  201.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    202. 

  202.         </p>
    203. 

  203. <a name="reference_optional_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    204. 

  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. 

  205.             <span class="identifier">optional</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">rhs</span>
    206. 

  206.             <span class="special">);</span></code>
    207. 

  207.           </p></blockquote></div>
    208. 

  208. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    209. 

  209. <li class="listitem">
    210. 

  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. 

  211.               is <code class="computeroutput"><span class="keyword">true</span></code>.
    212. 

  212.             </li>
    213. 

  213. <li class="listitem">
    214. 

  214.               <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    215. 

  215.             </li>
    216. 

  216. <li class="listitem">
    217. 

  217.               <span class="bold"><strong>Postconditions:</strong></span> If rhs is initialized,
    218. 

  218.               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    219. 

  219.               is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the
    220. 

  220.               value of <code class="computeroutput"><span class="identifier">rhs</span></code>; else
    221. 

  221.               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    222. 

  222.               is uninitialized.
    223. 

  223.             </li>
    224. 

  224. <li class="listitem">
    225. 

  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. 

  226.               <span class="special">)</span></code> throws.
    227. 

  227.             </li>
    228. 

  228. <li class="listitem">
    229. 

  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. 

  230.               is called.
    231. 

  231.             </li>
    232. 

  232. <li class="listitem">
    233. 

  233.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    234. 

  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. 

  235.               <span class="special">);</span></code> in that case, this constructor
    236. 

  236.               has no effect.
    237. 

  237.             </li>
    238. 

  238. <li class="listitem">
    239. 

  239.               <span class="bold"><strong>Example:</strong></span>
    240. 

  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. 

  241. <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
    242. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  251. </pre>
    252. 

  252.             </li>
    253. 

  253. </ul></div>
    254. 

  254. <p>
    255. 

  255.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    256. 

  256.         </p>
    257. 

  257. <a name="reference_optional_move_constructor_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    258. 

  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. 

  259.             <span class="identifier">optional</span><span class="special">&amp;&amp;</span>
    260. 

  260.             <span class="identifier">rhs</span> <span class="special">)</span>
    261. 

  261.             <span class="keyword">noexcept</span><span class="special">(</span></code><span class="emphasis"><em>see
    262. 

  262.             below</em></span><code class="computeroutput"><span class="special">);</span></code>
    263. 

  263.           </p></blockquote></div>
    264. 

  264. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    265. 

  265. <li class="listitem">
    266. 

  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. 

  267.               is <code class="computeroutput"><span class="keyword">true</span></code>.
    268. 

  268.             </li>
    269. 

  269. <li class="listitem">
    270. 

  270.               <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    271. 

  271.             </li>
    272. 

  272. <li class="listitem">
    273. 

  273.               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
    274. 

  274.               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    275. 

  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. 

  276.             </li>
    277. 

  277. <li class="listitem">
    278. 

  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. 

  279.               throws.
    280. 

  280.             </li>
    281. 

  281. <li class="listitem">
    282. 

  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. 

  283.             </li>
    284. 

  284. <li class="listitem">
    285. 

  285.               <span class="bold"><strong>Notes:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
    286. 

  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. 

  287.               <span class="special">)</span></code> is called.
    288. 

  288.             </li>
    289. 

  289. <li class="listitem">
    290. 

  290.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    291. 

  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. 

  292.               in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
    293. 

  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. 

  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. 

  295.             </li>
    296. 

  296. <li class="listitem">
    297. 

  297.               <span class="bold"><strong>Example:</strong></span>
    298. 

  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. 

  299. <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">uninit</span><span class="special">);</span>
    300. 

  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. 

  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. 

  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. 

  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. 

  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. 

  308. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init</span> <span class="special">);</span>
    309. 

  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. 

  310. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">init2</span> <span class="special">);</span>
    311. 

  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. 

  312. </pre>
    313. 

  313.             </li>
    314. 

  314. </ul></div>
    315. 

  315. <p>
    316. 

  316.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    317. 

  317.         </p>
    318. 

  318. <a name="reference_optional_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    319. 

  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. 

  320.             <span class="special">);</span></code>
    321. 

  321.           </p></blockquote></div>
    322. 

  322. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    323. 

  323. <li class="listitem">
    324. 

  324.               <span class="bold"><strong>Effect:</strong></span> Copy-Constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    325. 

  325.             </li>
    326. 

  326. <li class="listitem">
    327. 

  327.               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
    328. 

  328.               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    329. 

  329.               is initialized and its value is a <span class="emphasis"><em>copy</em></span> of the
    330. 

  330.               value of rhs converted to type <code class="computeroutput"><span class="identifier">T</span></code>;
    331. 

  331.               else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    332. 

  332.               is uninitialized.
    333. 

  333.             </li>
    334. 

  334. <li class="listitem">
    335. 

  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. 

  336.               <span class="special">)</span></code> throws.
    337. 

  337.             </li>
    338. 

  338. <li class="listitem">
    339. 

  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. 

  340.               <span class="special">)</span></code> is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is initialized, which requires
    341. 

  341.               a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
    342. 

  342.               to <code class="computeroutput"><span class="identifier">T</span></code>.
    343. 

  343.             </li>
    344. 

  344. <li class="listitem">
    345. 

  345.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    346. 

  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. 

  347.               <span class="special">);</span></code> in that case, this constructor
    348. 

  348.               has no effect.
    349. 

  349.             </li>
    350. 

  350. <li class="listitem">
    351. 

  351.               <span class="bold"><strong>Example:</strong></span>
    352. 

  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. 

  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. 

  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. 

  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. 

  357. </pre>
    358. 

  358.             </li>
    359. 

  359. </ul></div>
    360. 

  360. <p>
    361. 

  361.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    362. 

  362.         </p>
    363. 

  363. <a name="reference_optional_move_constructor_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    364. 

  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. 

  365.             <span class="special">);</span></code>
    366. 

  366.           </p></blockquote></div>
    367. 

  367. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    368. 

  368. <li class="listitem">
    369. 

  369.               <span class="bold"><strong>Effect:</strong></span> Move-constructs an <code class="computeroutput"><span class="identifier">optional</span></code>.
    370. 

  370.             </li>
    371. 

  371. <li class="listitem">
    372. 

  372.               <span class="bold"><strong>Postconditions:</strong></span> If <code class="computeroutput"><span class="identifier">rhs</span></code>
    373. 

  373.               is initialized, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    374. 

  374.               is initialized and its value is move-constructed from <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>;
    375. 

  375.               else <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    376. 

  376.               is uninitialized.
    377. 

  377.             </li>
    378. 

  378. <li class="listitem">
    379. 

  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. 

  380.               throws.
    381. 

  381.             </li>
    382. 

  382. <li class="listitem">
    383. 

  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. 

  384.               is called if <code class="computeroutput"><span class="identifier">rhs</span></code> is
    385. 

  385.               initialized, which requires a valid conversion from <code class="computeroutput"><span class="identifier">U</span></code>
    386. 

  386.               to <code class="computeroutput"><span class="identifier">T</span></code>.
    387. 

  387.             </li>
    388. 

  388. <li class="listitem">
    389. 

  389.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    390. 

  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. 

  391.               in that case, <code class="computeroutput"><span class="identifier">rhs</span></code> remains
    392. 

  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. 

  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. 

  394.               <span class="special">)</span></code>.
    395. 

  395.             </li>
    396. 

  396. <li class="listitem">
    397. 

  397.               <span class="bold"><strong>Example:</strong></span>
    398. 

  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. 

  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. 

  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. 

  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. 

  403. </pre>
    404. 

  404.             </li>
    405. 

  405. </ul></div>
    406. 

  406. <p>
    407. 

  407.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    408. 

  408.         </p>
    409. 

  409. <a name="reference_optional_in_place_init"></a><div class="blockquote"><blockquote class="blockquote"><p>
    410. 

  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. 

  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. 

  412.             <span class="special">);</span></code>
    413. 

  413.           </p></blockquote></div>
    414. 

  414. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    415. 

  415. <li class="listitem">
    416. 

  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. 

  417.             </li>
    418. 

  418. <li class="listitem">
    419. 

  419.               <span class="bold"><strong>Effect:</strong></span> Initializes the contained
    420. 

  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. 

  421.             </li>
    422. 

  422. <li class="listitem">
    423. 

  423.               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
    424. 

  424.             </li>
    425. 

  425. <li class="listitem">
    426. 

  426.               <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
    427. 

  427.               selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
    428. 

  428.             </li>
    429. 

  429. <li class="listitem">
    430. 

  430.               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    431. 

  431.               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
    432. 

  432.               On compilers that do not suppor variadic templates or rvalue references,
    433. 

  433.               this constuctor is available in limited functionality. For details
    434. 

  434.               <a class="link" href="../../dependencies_and_portability/emplace_operations_in_older_compilers.html#optional_emplace_workaround">see here</a>.
    435. 

  435.             </li>
    436. 

  436. <li class="listitem">
    437. 

  437.               <span class="bold"><strong>Example:</strong></span>
    438. 

  438. <pre class="programlisting"><span class="comment">// creates an std::mutex using its default constructor</span>
    439. 

  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. 

  440. <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">om</span><span class="special">);</span>
    441. 

  441. 

  442. <span class="comment">// creates a unique_lock by calling unique_lock(*om, std::defer_lock)</span>
    443. 

  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. 

  444. <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">ol</span><span class="special">);</span>
    445. 

  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. 

  446. </pre>
    447. 

  447.             </li>
    448. 

  448. </ul></div>
    449. 

  449. <p>
    450. 

  450.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    451. 

  451.         </p>
    452. 

  452. <a name="reference_optional_in_place_init_if"></a><div class="blockquote"><blockquote class="blockquote"><p>
    453. 

  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. 

  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. 

  455.             <span class="identifier">Args</span><span class="special">&amp;&amp;...</span>
    456. 

  456.             <span class="identifier">ars</span> <span class="special">);</span></code>
    457. 

  457.           </p></blockquote></div>
    458. 

  458. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    459. 

  459. <li class="listitem">
    460. 

  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. 

  461.             </li>
    462. 

  462. <li class="listitem">
    463. 

  463.               <span class="bold"><strong>Effect:</strong></span> If <code class="computeroutput"><span class="identifier">condition</span></code>
    464. 

  464.               is <code class="computeroutput"><span class="keyword">true</span></code>, initializes the
    465. 

  465.               contained value as if direct-non-list-initializing an object of type
    466. 

  466.               <code class="computeroutput"><span class="identifier">T</span></code> with the arguments
    467. 

  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. 

  468.             </li>
    469. 

  469. <li class="listitem">
    470. 

  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. 

  471.             </li>
    472. 

  472. <li class="listitem">
    473. 

  473.               <span class="bold"><strong>Throws:</strong></span> Any exception thrown by the
    474. 

  474.               selected constructor of <code class="computeroutput"><span class="identifier">T</span></code>.
    475. 

  475.             </li>
    476. 

  476. <li class="listitem">
    477. 

  477.               <span class="bold"><strong>Notes: </strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    478. 

  478.               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
    479. 

  479.               On compilers that do not suppor variadic templates or rvalue references,
    480. 

  480.               this constuctor is available in limited functionality. For details
    481. 

  481.               <a class="link" href="../../dependencies_and_portability/emplace_operations_in_older_compilers.html#optional_emplace_workaround">see here</a>.
    482. 

  482.             </li>
    483. 

  483. <li class="listitem">
    484. 

  484.               <span class="bold"><strong>Example:</strong></span>
    485. 

  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. 

  486. <span class="identifier">assert</span> <span class="special">(!</span><span class="identifier">ov1</span><span class="special">);</span>
    487. 

  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. 

  489. <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">ov2</span><span class="special">);</span>
    490. 

  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. 

  491. </pre>
    492. 

  492.             </li>
    493. 

  493. </ul></div>
    494. 

  494. <p>
    495. 

  495.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    496. 

  496.         </p>
    497. 

  497. <a name="reference_optional_constructor_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
    498. 

  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. 

  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. 

  500.             <span class="keyword">const</span><span class="special">&amp;</span>
    501. 

  501.             <span class="identifier">f</span> <span class="special">);</span></code>
    502. 

  502.           </p></blockquote></div>
    503. 

  503. <div class="blockquote"><blockquote class="blockquote"><p>
    504. 

  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. 

  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. 

  506.             <span class="keyword">const</span><span class="special">&amp;</span>
    507. 

  507.             <span class="identifier">f</span> <span class="special">);</span></code>
    508. 

  508.           </p></blockquote></div>
    509. 

  509. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    510. 

  510. <li class="listitem">
    511. 

  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. 

  512.             </li>
    513. 

  513. <li class="listitem">
    514. 

  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. 

  515.               and its value is <span class="emphasis"><em>directly given</em></span> from the factory
    516. 

  516.               <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value
    517. 

  517.               <span class="underline">is not copied</span>).
    518. 

  518.             </li>
    519. 

  519. <li class="listitem">
    520. 

  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. 

  521.               throws.
    522. 

  522.             </li>
    523. 

  523. <li class="listitem">
    524. 

  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. 

  525.               Factories</a>
    526. 

  526.             </li>
    527. 

  527. <li class="listitem">
    528. 

  528.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    529. 

  529.               be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
    530. 

  530.               constructor used by the factory; in that case, this constructor has
    531. 

  531.               no effect.
    532. 

  532.             </li>
    533. 

  533. <li class="listitem">
    534. 

  534.               <span class="bold"><strong>Example:</strong></span>
    535. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  544. </pre>
    545. 

  545.             </li>
    546. 

  546. </ul></div>
    547. 

  547. <p>
    548. 

  548.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    549. 

  549.         </p>
    550. 

  550. <a name="reference_optional_operator_equal_none_t"></a><div class="blockquote"><blockquote class="blockquote"><p>
    551. 

  551.             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
    552. 

  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. 

  553.             <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
    554. 

  554.           </p></blockquote></div>
    555. 

  555. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    556. 

  556. <li class="listitem">
    557. 

  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. 

  558.               value.
    559. 

  559.             </li>
    560. 

  560. <li class="listitem">
    561. 

  561.               <span class="bold"><strong>Postconditions: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is uninitialized.
    562. 

  562.             </li>
    563. 

  563. </ul></div>
    564. 

  564. <p>
    565. 

  565.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    566. 

  566.         </p>
    567. 

  567. <a name="reference_optional_operator_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    568. 

  568.             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
    569. 

  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. 

  570.           </p></blockquote></div>
    571. 

  571. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    572. 

  572. <li class="listitem">
    573. 

  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. 

  574.             </li>
    575. 

  575. <li class="listitem">
    576. 

  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. 

  577.               a <span class="emphasis"><em>copy</em></span> of <code class="computeroutput"><span class="identifier">rhs</span></code>.
    578. 

  578.             </li>
    579. 

  579. <li class="listitem">
    580. 

  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. 

  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. 

  582.               throws.
    583. 

  583.             </li>
    584. 

  584. <li class="listitem">
    585. 

  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. 

  586.               otherwise, its copy-constructor is used.
    587. 

  587.             </li>
    588. 

  588. <li class="listitem">
    589. 

  589.               <span class="bold"><strong>Exception Safety:</strong></span> In the event of
    590. 

  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. 

  591.               as far as <code class="computeroutput"><span class="identifier">optional</span></code>
    592. 

  592.               is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
    593. 

  593.               <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
    594. 

  594.               If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    595. 

  595.               is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
    596. 

  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. 

  597.             </li>
    598. 

  598. <li class="listitem">
    599. 

  599.               <span class="bold"><strong>Example:</strong></span>
    600. 

  600. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
    601. 

  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. 

  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. 

  603. 

  604. <span class="identifier">T</span> <span class="identifier">y</span><span class="special">;</span>
    605. 

  605. <span class="identifier">def</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
    606. 

  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. 

  607. <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">y</span> <span class="special">;</span>
    608. 

  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. 

  609. </pre>
    610. 

  610.             </li>
    611. 

  611. </ul></div>
    612. 

  612. <p>
    613. 

  613.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    614. 

  614.         </p>
    615. 

  615. <a name="reference_optional_operator_move_equal_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    616. 

  616.             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
    617. 

  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. 

  618.             <span class="special">)</span> <span class="special">;</span></code>
    619. 

  619.           </p></blockquote></div>
    620. 

  620. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    621. 

  621. <li class="listitem">
    622. 

  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. 

  623.             </li>
    624. 

  624. <li class="listitem">
    625. 

  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. 

  626.               moved from <code class="computeroutput"><span class="identifier">rhs</span></code>.
    627. 

  627.             </li>
    628. 

  628. <li class="listitem">
    629. 

  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. 

  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. 

  631.               throws.
    632. 

  632.             </li>
    633. 

  633. <li class="listitem">
    634. 

  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. 

  635.               otherwise, its move-constructor is used.
    636. 

  636.             </li>
    637. 

  637. <li class="listitem">
    638. 

  638.               <span class="bold"><strong>Exception Safety:</strong></span> In the event of
    639. 

  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. 

  640.               as far as <code class="computeroutput"><span class="identifier">optional</span></code>
    641. 

  641.               is concerned (it is up to <code class="computeroutput"><span class="identifier">T</span></code>'s
    642. 

  642.               <code class="computeroutput"><span class="keyword">operator</span><span class="special">=()</span></code>).
    643. 

  643.               If <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    644. 

  644.               is initially uninitialized and <code class="computeroutput"><span class="identifier">T</span></code>'s
    645. 

  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. 

  646.             </li>
    647. 

  647. <li class="listitem">
    648. 

  648.               <span class="bold"><strong>Example:</strong></span>
    649. 

  649. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">;</span>
    650. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  658. </pre>
    659. 

  659.             </li>
    660. 

  660. </ul></div>
    661. 

  661. <p>
    662. 

  662.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    663. 

  663.         </p>
    664. 

  664. <a name="reference_optional_operator_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    665. 

  665.             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
    666. 

  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. 

  667.             <span class="keyword">const</span><span class="special">&amp;</span>
    668. 

  668.             <span class="identifier">rhs</span> <span class="special">)</span>
    669. 

  669.             <span class="special">;</span></code>
    670. 

  670.           </p></blockquote></div>
    671. 

  671. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    672. 

  672. <li class="listitem">
    673. 

  673.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    674. 

  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. 

  675.             </li>
    676. 

  676. <li class="listitem">
    677. 

  677. <p class="simpara">
    678. 

  678.               <span class="bold"><strong>Effects:</strong></span>
    679. 

  679.             </p>
    680. 

  680. <div class="informaltable"><table class="table">
    681. 

  681. <colgroup>
    682. 

  682. <col>
    683. 

  683. <col>
    684. 

  684. <col>
    685. 

  685. </colgroup>
    686. 

  686. <thead><tr></tr></thead>
    687. 

  687. <tbody>
    688. 

  688. <tr>
    689. 

  689. <td>
    690. 

  690.                     </td>
    691. 

  691. <td>
    692. 

  692.                       <p>
    693. 

  693.                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
    694. 

  694.                       </p>
    695. 

  695.                     </td>
    696. 

  696. <td>
    697. 

  697.                       <p>
    698. 

  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. 

  699.                       </p>
    700. 

  700.                     </td>
    701. 

  701. </tr>
    702. 

  702. <tr>
    703. 

  703. <td>
    704. 

  704.                       <p>
    705. 

  705.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    706. 

  706.                         contains a value</strong></span>
    707. 

  707.                       </p>
    708. 

  708.                     </td>
    709. 

  709. <td>
    710. 

  710.                       <p>
    711. 

  711.                         assigns <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
    712. 

  712.                         to the contained value
    713. 

  713.                       </p>
    714. 

  714.                     </td>
    715. 

  715. <td>
    716. 

  716.                       <p>
    717. 

  717.                         initializes the contained value as if direct-initializing
    718. 

  718.                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
    719. 

  719.                         with <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
    720. 

  720.                       </p>
    721. 

  721.                     </td>
    722. 

  722. </tr>
    723. 

  723. <tr>
    724. 

  724. <td>
    725. 

  725.                       <p>
    726. 

  726.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    727. 

  727.                         does not contain a value</strong></span>
    728. 

  728.                       </p>
    729. 

  729.                     </td>
    730. 

  730. <td>
    731. 

  731.                       <p>
    732. 

  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. 

  733.                       </p>
    734. 

  734.                     </td>
    735. 

  735. <td>
    736. 

  736.                       <p>
    737. 

  737.                         no effect
    738. 

  738.                       </p>
    739. 

  739.                     </td>
    740. 

  740. </tr>
    741. 

  741. </tbody>
    742. 

  742. </table></div>
    743. 

  743. </li>
    744. 

  744. <li class="listitem">
    745. 

  745.               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>;
    746. 

  746.             </li>
    747. 

  747. <li class="listitem">
    748. 

  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. 

  749.             </li>
    750. 

  750. <li class="listitem">
    751. 

  751.               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
    752. 

  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. 

  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. 

  754.               If an exception is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
    755. 

  755.               copy assignment, the state of its contained value is as defined by
    756. 

  756.               the exception safety guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s
    757. 

  757.               copy assignment.
    758. 

  758.             </li>
    759. 

  759. <li class="listitem">
    760. 

  760.               <span class="bold"><strong>Example:</strong></span>
    761. 

  761. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    762. 

  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. 

  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. 

  764. 

  765. <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
    766. 

  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. 

  767. <span class="comment">// previous value (copy of 'v') destroyed from within 'opt'.</span>
    768. 

  768. </pre>
    769. 

  769.             </li>
    770. 

  770. </ul></div>
    771. 

  771. <p>
    772. 

  772.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    773. 

  773.         </p>
    774. 

  774. <a name="reference_optional_operator_move_equal_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    775. 

  775.             <code class="computeroutput"><span class="identifier">optional</span><span class="special">&amp;</span>
    776. 

  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. 

  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. 

  778.           </p></blockquote></div>
    779. 

  779. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    780. 

  780. <li class="listitem">
    781. 

  781.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    782. 

  782.               is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
    783. 

  783.               and <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
    784. 

  784.             </li>
    785. 

  785. <li class="listitem">
    786. 

  786. <p class="simpara">
    787. 

  787.               <span class="bold"><strong>Effects:</strong></span>
    788. 

  788.             </p>
    789. 

  789. <div class="informaltable"><table class="table">
    790. 

  790. <colgroup>
    791. 

  791. <col>
    792. 

  792. <col>
    793. 

  793. <col>
    794. 

  794. </colgroup>
    795. 

  795. <thead><tr></tr></thead>
    796. 

  796. <tbody>
    797. 

  797. <tr>
    798. 

  798. <td>
    799. 

  799.                     </td>
    800. 

  800. <td>
    801. 

  801.                       <p>
    802. 

  802.                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
    803. 

  803.                       </p>
    804. 

  804.                     </td>
    805. 

  805. <td>
    806. 

  806.                       <p>
    807. 

  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. 

  808.                       </p>
    809. 

  809.                     </td>
    810. 

  810. </tr>
    811. 

  811. <tr>
    812. 

  812. <td>
    813. 

  813.                       <p>
    814. 

  814.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    815. 

  815.                         contains a value</strong></span>
    816. 

  816.                       </p>
    817. 

  817.                     </td>
    818. 

  818. <td>
    819. 

  819.                       <p>
    820. 

  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. 

  821.                       </p>
    822. 

  822.                     </td>
    823. 

  823. <td>
    824. 

  824.                       <p>
    825. 

  825.                         initializes the contained value as if direct-initializing
    826. 

  826.                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
    827. 

  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. 

  828.                       </p>
    829. 

  829.                     </td>
    830. 

  830. </tr>
    831. 

  831. <tr>
    832. 

  832. <td>
    833. 

  833.                       <p>
    834. 

  834.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    835. 

  835.                         does not contain a value</strong></span>
    836. 

  836.                       </p>
    837. 

  837.                     </td>
    838. 

  838. <td>
    839. 

  839.                       <p>
    840. 

  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. 

  841.                       </p>
    842. 

  842.                     </td>
    843. 

  843. <td>
    844. 

  844.                       <p>
    845. 

  845.                         no effect
    846. 

  846.                       </p>
    847. 

  847.                     </td>
    848. 

  848. </tr>
    849. 

  849. </tbody>
    850. 

  850. </table></div>
    851. 

  851. </li>
    852. 

  852. <li class="listitem">
    853. 

  853.               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>;
    854. 

  854.             </li>
    855. 

  855. <li class="listitem">
    856. 

  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. 

  857.             </li>
    858. 

  858. <li class="listitem">
    859. 

  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. 

  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. 

  861.             </li>
    862. 

  862. <li class="listitem">
    863. 

  863.               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
    864. 

  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. 

  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. 

  866.               <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
    867. 

  867.               is determined by the exception safety guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s
    868. 

  868.               move constructor. If an exception is thrown during the call to T's
    869. 

  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. 

  870.               safety guarantee of T's move assignment.
    871. 

  871.             </li>
    872. 

  872. <li class="listitem">
    873. 

  873.               <span class="bold"><strong>Example:</strong></span>
    874. 

  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. 

  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. 

  876. 

  877. <span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">def</span> <span class="special">;</span>
    878. 

  878. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">def</span> <span class="special">)</span> <span class="special">;</span>
    879. 

  879. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
    880. 

  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. 

  881. </pre>
    882. 

  882.             </li>
    883. 

  883. </ul></div>
    884. 

  884. <p>
    885. 

  885.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    886. 

  886.         </p>
    887. 

  887. <a name="reference_optional_operator_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    888. 

  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. 

  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. 

  890.             <span class="special">)</span> <span class="special">;</span></code>
    891. 

  891.           </p></blockquote></div>
    892. 

  892. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    893. 

  893. <li class="listitem">
    894. 

  894. <p class="simpara">
    895. 

  895.               <span class="bold"><strong>Effect:</strong></span>
    896. 

  896.             </p>
    897. 

  897. <div class="informaltable"><table class="table">
    898. 

  898. <colgroup>
    899. 

  899. <col>
    900. 

  900. <col>
    901. 

  901. <col>
    902. 

  902. </colgroup>
    903. 

  903. <thead><tr></tr></thead>
    904. 

  904. <tbody>
    905. 

  905. <tr>
    906. 

  906. <td>
    907. 

  907.                     </td>
    908. 

  908. <td>
    909. 

  909.                       <p>
    910. 

  910.                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
    911. 

  911.                       </p>
    912. 

  912.                     </td>
    913. 

  913. <td>
    914. 

  914.                       <p>
    915. 

  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. 

  916.                       </p>
    917. 

  917.                     </td>
    918. 

  918. </tr>
    919. 

  919. <tr>
    920. 

  920. <td>
    921. 

  921.                       <p>
    922. 

  922.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    923. 

  923.                         contains a value</strong></span>
    924. 

  924.                       </p>
    925. 

  925.                     </td>
    926. 

  926. <td>
    927. 

  927.                       <p>
    928. 

  928.                         assigns <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
    929. 

  929.                         to the contained value
    930. 

  930.                       </p>
    931. 

  931.                     </td>
    932. 

  932. <td>
    933. 

  933.                       <p>
    934. 

  934.                         initializes the contained value as if direct-initializing
    935. 

  935.                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
    936. 

  936.                         with <code class="computeroutput"><span class="special">*</span><span class="identifier">rhs</span></code>
    937. 

  937.                       </p>
    938. 

  938.                     </td>
    939. 

  939. </tr>
    940. 

  940. <tr>
    941. 

  941. <td>
    942. 

  942.                       <p>
    943. 

  943.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    944. 

  944.                         does not contain a value</strong></span>
    945. 

  945.                       </p>
    946. 

  946.                     </td>
    947. 

  947. <td>
    948. 

  948.                       <p>
    949. 

  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. 

  950.                       </p>
    951. 

  951.                     </td>
    952. 

  952. <td>
    953. 

  953.                       <p>
    954. 

  954.                         no effect
    955. 

  955.                       </p>
    956. 

  956.                     </td>
    957. 

  957. </tr>
    958. 

  958. </tbody>
    959. 

  959. </table></div>
    960. 

  960. </li>
    961. 

  961. <li class="listitem">
    962. 

  962.               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
    963. 

  963.             </li>
    964. 

  964. <li class="listitem">
    965. 

  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. 

  966.             </li>
    967. 

  967. <li class="listitem">
    968. 

  968.               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
    969. 

  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. 

  970.               is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
    971. 

  971.               constructor, no effect. If an exception is thrown during the call to
    972. 

  972.               <code class="computeroutput"><span class="identifier">T</span></code>'s assignment, the
    973. 

  973.               state of its contained value is as defined by the exception safety
    974. 

  974.               guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s copy
    975. 

  975.               assignment.
    976. 

  976.             </li>
    977. 

  977. <li class="listitem">
    978. 

  978.               <span class="bold"><strong>Example:</strong></span>
    979. 

  979. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    980. 

  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. 

  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. 

  982. 

  983. <span class="identifier">opt1</span> <span class="special">=</span> <span class="identifier">opt0</span> <span class="special">;</span>
    984. 

  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. 

  985. </pre>
    986. 

  986.             </li>
    987. 

  987. </ul></div>
    988. 

  988. <p>
    989. 

  989.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    990. 

  990.         </p>
    991. 

  991. <a name="reference_optional_operator_move_equal_other_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
    992. 

  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. 

  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. 

  994.             <span class="special">)</span> <span class="special">;</span></code>
    995. 

  995.           </p></blockquote></div>
    996. 

  996. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    997. 

  997. <li class="listitem">
    998. 

  998. <p class="simpara">
    999. 

  999.               <span class="bold"><strong>Effect:</strong></span>
    1000. 

  1000.             </p>
    1001. 

  1001. <div class="informaltable"><table class="table">
    1002. 

  1002. <colgroup>
    1003. 

  1003. <col>
    1004. 

  1004. <col>
    1005. 

  1005. <col>
    1006. 

  1006. </colgroup>
    1007. 

  1007. <thead><tr></tr></thead>
    1008. 

  1008. <tbody>
    1009. 

  1009. <tr>
    1010. 

  1010. <td>
    1011. 

  1011.                     </td>
    1012. 

  1012. <td>
    1013. 

  1013.                       <p>
    1014. 

  1014.                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
    1015. 

  1015.                       </p>
    1016. 

  1016.                     </td>
    1017. 

  1017. <td>
    1018. 

  1018.                       <p>
    1019. 

  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. 

  1020.                       </p>
    1021. 

  1021.                     </td>
    1022. 

  1022. </tr>
    1023. 

  1023. <tr>
    1024. 

  1024. <td>
    1025. 

  1025.                       <p>
    1026. 

  1026.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    1027. 

  1027.                         contains a value</strong></span>
    1028. 

  1028.                       </p>
    1029. 

  1029.                     </td>
    1030. 

  1030. <td>
    1031. 

  1031.                       <p>
    1032. 

  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. 

  1033.                       </p>
    1034. 

  1034.                     </td>
    1035. 

  1035. <td>
    1036. 

  1036.                       <p>
    1037. 

  1037.                         initializes the contained value as if direct-initializing
    1038. 

  1038.                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
    1039. 

  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. 

  1040.                       </p>
    1041. 

  1041.                     </td>
    1042. 

  1042. </tr>
    1043. 

  1043. <tr>
    1044. 

  1044. <td>
    1045. 

  1045.                       <p>
    1046. 

  1046.                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
    1047. 

  1047.                         does not contain a value</strong></span>
    1048. 

  1048.                       </p>
    1049. 

  1049.                     </td>
    1050. 

  1050. <td>
    1051. 

  1051.                       <p>
    1052. 

  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. 

  1053.                       </p>
    1054. 

  1054.                     </td>
    1055. 

  1055. <td>
    1056. 

  1056.                       <p>
    1057. 

  1057.                         no effect
    1058. 

  1058.                       </p>
    1059. 

  1059.                     </td>
    1060. 

  1060. </tr>
    1061. 

  1061. </tbody>
    1062. 

  1062. </table></div>
    1063. 

  1063. </li>
    1064. 

  1064. <li class="listitem">
    1065. 

  1065.               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>.
    1066. 

  1066.             </li>
    1067. 

  1067. <li class="listitem">
    1068. 

  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. 

  1069.             </li>
    1070. 

  1070. <li class="listitem">
    1071. 

  1071.               <span class="bold"><strong>Exception Safety:</strong></span> If any exception
    1072. 

  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. 

  1073.               is thrown during the call to <code class="computeroutput"><span class="identifier">T</span></code>'s
    1074. 

  1074.               constructor, no effect. If an exception is thrown during the call to
    1075. 

  1075.               <code class="computeroutput"><span class="identifier">T</span></code>'s assignment, the
    1076. 

  1076.               state of its contained value is as defined by the exception safety
    1077. 

  1077.               guarantee of <code class="computeroutput"><span class="identifier">T</span></code>'s copy
    1078. 

  1078.               assignment.
    1079. 

  1079.             </li>
    1080. 

  1080. <li class="listitem">
    1081. 

  1081.               <span class="bold"><strong>Example:</strong></span>
    1082. 

  1082. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    1083. 

  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. 

  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. 

  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. 

  1087. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
    1088. 

  1088. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt1</span> <span class="special">)</span>
    1089. 

  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. 

  1090. </pre>
    1091. 

  1091.             </li>
    1092. 

  1092. </ul></div>
    1093. 

  1093. <p>
    1094. 

  1094.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1095. 

  1095.         </p>
    1096. 

  1096. <a name="reference_optional_emplace"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1097. 

  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. 

  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. 

  1099.             <span class="special">);</span></code>
    1100. 

  1100.           </p></blockquote></div>
    1101. 

  1101. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1102. 

  1102. <li class="listitem">
    1103. 

  1103.               <span class="bold"><strong>Requires:</strong></span> The compiler supports rvalue
    1104. 

  1104.               references and variadic templates.
    1105. 

  1105.             </li>
    1106. 

  1106. <li class="listitem">
    1107. 

  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. 

  1108.               Then initializes in-place the contained value as if direct-initializing
    1109. 

  1109.               an object of type <code class="computeroutput"><span class="identifier">T</span></code>
    1110. 

  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. 

  1111.             </li>
    1112. 

  1112. <li class="listitem">
    1113. 

  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. 

  1114.             </li>
    1115. 

  1115. <li class="listitem">
    1116. 

  1116.               <span class="bold"><strong>Throws:</strong></span> Whatever the selected <code class="computeroutput"><span class="identifier">T</span></code>'s constructor throws.
    1117. 

  1117.             </li>
    1118. 

  1118. <li class="listitem">
    1119. 

  1119.               <span class="bold"><strong>Exception Safety:</strong></span> If an exception
    1120. 

  1120.               is thrown during the initialization of <code class="computeroutput"><span class="identifier">T</span></code>,
    1121. 

  1121.               <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
    1122. 

  1122.               is <span class="emphasis"><em>uninitialized</em></span>.
    1123. 

  1123.             </li>
    1124. 

  1124. <li class="listitem">
    1125. 

  1125.               <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    1126. 

  1126.               need not be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
    1127. 

  1127.               or <code class="computeroutput"><span class="identifier">MoveAssignable</span></code>.
    1128. 

  1128.               On compilers that do not suppor variadic templates or rvalue references,
    1129. 

  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. 

  1130.             </li>
    1131. 

  1131. <li class="listitem">
    1132. 

  1132.               <span class="bold"><strong>Example:</strong></span>
    1133. 

  1133. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    1134. 

  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. 

  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. 

  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. 

  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. 

  1138. </pre>
    1139. 

  1139.             </li>
    1140. 

  1140. </ul></div>
    1141. 

  1141. <p>
    1142. 

  1142.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1143. 

  1143.         </p>
    1144. 

  1144. <a name="reference_optional_operator_equal_factory"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1145. 

  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. 

  1146.             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span>
    1147. 

  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. 

  1148.           </p></blockquote></div>
    1149. 

  1149. <div class="blockquote"><blockquote class="blockquote"><p>
    1150. 

  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. 

  1151.             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span>
    1152. 

  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. 

  1153.           </p></blockquote></div>
    1154. 

  1154. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1155. 

  1155. <li class="listitem">
    1156. 

  1156.               <span class="bold"><strong>Effect:</strong></span> Assigns an <code class="computeroutput"><span class="identifier">optional</span></code>
    1157. 

  1157.               with a value of <code class="computeroutput"><span class="identifier">T</span></code> obtained
    1158. 

  1158.               from the factory.
    1159. 

  1159.             </li>
    1160. 

  1160. <li class="listitem">
    1161. 

  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. 

  1162.               and its value is <span class="emphasis"><em>directly given</em></span> from the factory
    1163. 

  1163.               <code class="computeroutput"><span class="identifier">f</span></code> (i.e., the value
    1164. 

  1164.               <span class="underline">is not copied</span>).
    1165. 

  1165.             </li>
    1166. 

  1166. <li class="listitem">
    1167. 

  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. 

  1168.               throws.
    1169. 

  1169.             </li>
    1170. 

  1170. <li class="listitem">
    1171. 

  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. 

  1172.               Factories</a>
    1173. 

  1173.             </li>
    1174. 

  1174. <li class="listitem">
    1175. 

  1175.               <span class="bold"><strong>Exception Safety:</strong></span> Exceptions can only
    1176. 

  1176.               be thrown during the call to the <code class="computeroutput"><span class="identifier">T</span></code>
    1177. 

  1177.               constructor used by the factory; in that case, the <code class="computeroutput"><span class="identifier">optional</span></code>
    1178. 

  1178.               object will be reset to be <span class="emphasis"><em>uninitialized</em></span>.
    1179. 

  1179.             </li>
    1180. 

  1180. </ul></div>
    1181. 

  1181. <p>
    1182. 

  1182.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1183. 

  1183.         </p>
    1184. 

  1184. <a name="reference_optional_reset_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1185. 

  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. 

  1186.           </p></blockquote></div>
    1187. 

  1187. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
    1188. 

  1188.               <span class="bold"><strong>Deprecated:</strong></span> same as <code class="computeroutput"><span class="keyword">operator</span><span class="special">=</span>
    1189. 

  1189.               <span class="special">(</span> <span class="identifier">T</span>
    1190. 

  1190.               <span class="keyword">const</span><span class="special">&amp;</span>
    1191. 

  1191.               <span class="identifier">v</span><span class="special">)</span>
    1192. 

  1192.               <span class="special">;</span></code>
    1193. 

  1193.             </li></ul></div>
    1194. 

  1194. <p>
    1195. 

  1195.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1196. 

  1196.         </p>
    1197. 

  1197. <a name="reference_optional_reset"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1198. 

  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. 

  1199.             <span class="special">;</span></code>
    1200. 

  1200.           </p></blockquote></div>
    1201. 

  1201. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
    1202. 

  1202.               <span class="bold"><strong>Effects:</strong></span> Same as <code class="computeroutput"><span class="keyword">operator</span><span class="special">=(</span> <span class="identifier">none_t</span>
    1203. 

  1203.               <span class="special">);</span></code>
    1204. 

  1204.             </li></ul></div>
    1205. 

  1205. <p>
    1206. 

  1206.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1207. 

  1207.         </p>
    1208. 

  1208. <a name="reference_optional_get"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1209. 

  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. 

  1210.           </p></blockquote></div>
    1211. 

  1211. <div class="blockquote"><blockquote class="blockquote"><p>
    1212. 

  1212.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
    1213. 

  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. 

  1214.           </p></blockquote></div>
    1215. 

  1215. <div class="blockquote"><blockquote class="blockquote"><p>
    1216. 

  1216.             <code class="computeroutput"><span class="keyword">inline</span> <span class="identifier">T</span>
    1217. 

  1217.             <span class="keyword">const</span><span class="special">&amp;</span>
    1218. 

  1218.             <span class="identifier">get</span> <span class="special">(</span>
    1219. 

  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. 

  1220.           </p></blockquote></div>
    1221. 

  1221. <div class="blockquote"><blockquote class="blockquote"><p>
    1222. 

  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. 

  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. 

  1224.             <span class="special">;</span></code>
    1225. 

  1225.           </p></blockquote></div>
    1226. 

  1226. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1227. 

  1227. <li class="listitem">
    1228. 

  1228.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
    1229. 

  1229.             </li>
    1230. 

  1230. <li class="listitem">
    1231. 

  1231.               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
    1232. 

  1232.               value
    1233. 

  1233.             </li>
    1234. 

  1234. <li class="listitem">
    1235. 

  1235.               <span class="bold"><strong>Throws:</strong></span> Nothing.
    1236. 

  1236.             </li>
    1237. 

  1237. <li class="listitem">
    1238. 

  1238.               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
    1239. 

  1239.               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
    1240. 

  1240.             </li>
    1241. 

  1241. </ul></div>
    1242. 

  1242. <p>
    1243. 

  1243.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1244. 

  1244.         </p>
    1245. 

  1245. <a name="reference_optional_operator_asterisk"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1246. 

  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. 

  1247.           </p></blockquote></div>
    1248. 

  1248. <div class="blockquote"><blockquote class="blockquote"><p>
    1249. 

  1249.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
    1250. 

  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. 

  1251.             <span class="special">&amp;;</span></code>
    1252. 

  1252.           </p></blockquote></div>
    1253. 

  1253. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1254. 

  1254. <li class="listitem">
    1255. 

  1255.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized
    1256. 

  1256.             </li>
    1257. 

  1257. <li class="listitem">
    1258. 

  1258.               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
    1259. 

  1259.               value
    1260. 

  1260.             </li>
    1261. 

  1261. <li class="listitem">
    1262. 

  1262.               <span class="bold"><strong>Throws:</strong></span> Nothing.
    1263. 

  1263.             </li>
    1264. 

  1264. <li class="listitem">
    1265. 

  1265.               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
    1266. 

  1266.               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
    1267. 

  1267.               On compilers that do not support ref-qualifiers on member functions
    1268. 

  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. 

  1269.               member functions.
    1270. 

  1270.             </li>
    1271. 

  1271. <li class="listitem">
    1272. 

  1272.               <span class="bold"><strong>Example:</strong></span>
    1273. 

  1273. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
    1274. 

  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. 

  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. 

  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. 

  1277. <span class="identifier">T</span> <span class="identifier">w</span> <span class="special">;</span>
    1278. 

  1278. <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">w</span> <span class="special">;</span>
    1279. 

  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. 

  1280. </pre>
    1281. 

  1281.             </li>
    1282. 

  1282. </ul></div>
    1283. 

  1283. <p>
    1284. 

  1284.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1285. 

  1285.         </p>
    1286. 

  1286. <a name="reference_optional_operator_asterisk_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1287. 

  1287.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
    1288. 

  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. 

  1289.             <span class="special">&amp;&amp;;</span></code>
    1290. 

  1290.           </p></blockquote></div>
    1291. 

  1291. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1292. 

  1292. <li class="listitem">
    1293. 

  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. 

  1294.             </li>
    1295. 

  1295. <li class="listitem">
    1296. 

  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. 

  1297.             </li>
    1298. 

  1298. <li class="listitem">
    1299. 

  1299.               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
    1300. 

  1300.               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
    1301. 

  1301.               On compilers that do not support ref-qualifiers on member functions
    1302. 

  1302.               this overload is not present.
    1303. 

  1303.             </li>
    1304. 

  1304. </ul></div>
    1305. 

  1305. <p>
    1306. 

  1306.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1307. 

  1307.         </p>
    1308. 

  1308. <a name="reference_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1309. 

  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. 

  1310.           </p></blockquote></div>
    1311. 

  1311. <div class="blockquote"><blockquote class="blockquote"><p>
    1312. 

  1312.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
    1313. 

  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. 

  1314.           </p></blockquote></div>
    1315. 

  1315. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1316. 

  1316. <li class="listitem">
    1317. 

  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. 

  1318.             </li>
    1319. 

  1319. <li class="listitem">
    1320. 

  1320.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1321. 

  1321.               ref-qualifiers on member functions these two overloads are replaced
    1322. 

  1322.               with the classical two: a <code class="computeroutput"><span class="keyword">const</span></code>
    1323. 

  1323.               and non-<code class="computeroutput"><span class="keyword">const</span></code> member functions.
    1324. 

  1324.             </li>
    1325. 

  1325. <li class="listitem">
    1326. 

  1326.               <span class="bold"><strong>Example:</strong></span>
    1327. 

  1327. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span> <span class="special">;</span>
    1328. 

  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. 

  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. 

  1330. 

  1331. <span class="keyword">try</span> <span class="special">{</span>
    1332. 

  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. 

  1333.   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">false</span> <span class="special">);</span>
    1334. 

  1334. <span class="special">}</span>
    1335. 

  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. 

  1336.   <span class="identifier">assert</span> <span class="special">(</span> <span class="keyword">true</span> <span class="special">);</span>
    1337. 

  1337. <span class="special">}</span>
    1338. 

  1338. </pre>
    1339. 

  1339.             </li>
    1340. 

  1340. </ul></div>
    1341. 

  1341. <p>
    1342. 

  1342.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1343. 

  1343.         </p>
    1344. 

  1344. <a name="reference_optional_value_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1345. 

  1345.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;&amp;</span>
    1346. 

  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. 

  1347.           </p></blockquote></div>
    1348. 

  1348. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1349. 

  1349. <li class="listitem">
    1350. 

  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. 

  1351.             </li>
    1352. 

  1352. <li class="listitem">
    1353. 

  1353.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1354. 

  1354.               ref-qualifiers on member functions this overload is not present.
    1355. 

  1355.             </li>
    1356. 

  1356. </ul></div>
    1357. 

  1357. <p>
    1358. 

  1358.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1359. 

  1359.         </p>
    1360. 

  1360. <a name="reference_optional_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1361. 

  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. 

  1362.             <span class="identifier">v</span><span class="special">)</span>
    1363. 

  1363.             <span class="keyword">const</span><span class="special">&amp;</span>
    1364. 

  1364.             <span class="special">;</span></code>
    1365. 

  1365.           </p></blockquote></div>
    1366. 

  1366. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1367. 

  1367. <li class="listitem">
    1368. 

  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. 

  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. 

  1370.             </li>
    1371. 

  1371. <li class="listitem">
    1372. 

  1372.               <span class="bold"><strong>Remarks:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code>
    1373. 

  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. 

  1374.               is not convertible to <code class="computeroutput"><span class="identifier">T</span></code>,
    1375. 

  1375.               the program is ill-formed.
    1376. 

  1376.             </li>
    1377. 

  1377. <li class="listitem">
    1378. 

  1378.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1379. 

  1379.               ref-qualifiers on member functions this overload is replaced with the
    1380. 

  1380.               <code class="computeroutput"><span class="keyword">const</span></code>-qualified member
    1381. 

  1381.               function. On compilers without rvalue reference support the type of
    1382. 

  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. 

  1383.             </li>
    1384. 

  1384. </ul></div>
    1385. 

  1385. <p>
    1386. 

  1386.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1387. 

  1387.         </p>
    1388. 

  1388. <a name="reference_optional_value_or_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1389. 

  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. 

  1390.             <span class="identifier">v</span><span class="special">)</span>
    1391. 

  1391.             <span class="special">&amp;&amp;</span> <span class="special">;</span></code>
    1392. 

  1392.           </p></blockquote></div>
    1393. 

  1393. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1394. 

  1394. <li class="listitem">
    1395. 

  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. 

  1396.             </li>
    1397. 

  1397. <li class="listitem">
    1398. 

  1398.               <span class="bold"><strong>Remarks:</strong></span> If <code class="computeroutput"><span class="identifier">T</span></code>
    1399. 

  1399.               is not <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
    1400. 

  1400.               or <code class="computeroutput"><span class="identifier">U</span> <span class="special">&amp;&amp;</span></code>
    1401. 

  1401.               is not convertible to <code class="computeroutput"><span class="identifier">T</span></code>,
    1402. 

  1402.               the program is ill-formed.
    1403. 

  1403.             </li>
    1404. 

  1404. <li class="listitem">
    1405. 

  1405.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1406. 

  1406.               ref-qualifiers on member functions this overload is not present.
    1407. 

  1407.             </li>
    1408. 

  1408. </ul></div>
    1409. 

  1409. <p>
    1410. 

  1410.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1411. 

  1411.         </p>
    1412. 

  1412. <a name="reference_optional_value_or_call"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1413. 

  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. 

  1414.           </p></blockquote></div>
    1415. 

  1415. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1416. 

  1416. <li class="listitem">
    1417. 

  1417.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    1418. 

  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. 

  1419.               is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
    1420. 

  1420.             </li>
    1421. 

  1421. <li class="listitem">
    1422. 

  1422.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1423. 

  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. 

  1424.             </li>
    1425. 

  1425. <li class="listitem">
    1426. 

  1426.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1427. 

  1427.               ref-qualifiers on member functions this overload is replaced with the
    1428. 

  1428.               <code class="computeroutput"><span class="keyword">const</span></code>-qualified member
    1429. 

  1429.               function.
    1430. 

  1430.             </li>
    1431. 

  1431. <li class="listitem">
    1432. 

  1432.               <span class="bold"><strong>Example:</strong></span>
    1433. 

  1433. <pre class="programlisting"><span class="keyword">int</span> <span class="identifier">complain_and_0</span><span class="special">()</span>
    1434. 

  1434. <span class="special">{</span>
    1435. 

  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. 

  1436.   <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
    1437. 

  1437. <span class="special">}</span>
    1438. 

  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. 

  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. 

  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. 

  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. 

  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. 

  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. 

  1447. </pre>
    1448. 

  1448.             </li>
    1449. 

  1449. </ul></div>
    1450. 

  1450. <p>
    1451. 

  1451.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1452. 

  1452.         </p>
    1453. 

  1453. <a name="reference_optional_value_or_call_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1454. 

  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. 

  1455.             <span class="special">;</span></code>
    1456. 

  1456.           </p></blockquote></div>
    1457. 

  1457. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1458. 

  1458. <li class="listitem">
    1459. 

  1459.               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
    1460. 

  1460.               is <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>
    1461. 

  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. 

  1462.               whose result type is convertible to <code class="computeroutput"><span class="identifier">T</span></code>.
    1463. 

  1463.             </li>
    1464. 

  1464. <li class="listitem">
    1465. 

  1465.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1466. 

  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. 

  1467.               <span class="identifier">f</span><span class="special">();</span></code>.
    1468. 

  1468.             </li>
    1469. 

  1469. <li class="listitem">
    1470. 

  1470.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1471. 

  1471.               ref-qualifiers on member functions this overload is not present.
    1472. 

  1472.             </li>
    1473. 

  1473. </ul></div>
    1474. 

  1474. <p>
    1475. 

  1475.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1476. 

  1476.         </p>
    1477. 

  1477. <a name="reference_optional_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1478. 

  1478.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">-&gt;</span>
    1479. 

  1479.             </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"> <span class="special">;</span></code>
    1480. 

  1480.           </p></blockquote></div>
    1481. 

  1481. <div class="blockquote"><blockquote class="blockquote"><p>
    1482. 

  1482.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&amp;</span> <span class="special">-&gt;</span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput">
    1483. 

  1483.             <span class="special">;</span></code>
    1484. 

  1484.           </p></blockquote></div>
    1485. 

  1485. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1486. 

  1486. <li class="listitem">
    1487. 

  1487.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1488. 

  1488.               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span>
    1489. 

  1489.               <span class="identifier">none</span><span class="special">;</span></code>
    1490. 

  1490.             </li>
    1491. 

  1491. <li class="listitem">
    1492. 

  1492.               <span class="bold"><strong>Notes:</strong></span> The return type of these overloads
    1493. 

  1493.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">))&gt;</span></code>.
    1494. 

  1494.               On compilers that do not support ref-qualifiers on member functions,
    1495. 

  1495.               these two (as well as the next one) overloads are replaced with good
    1496. 

  1496.               old const and non-const overloads.
    1497. 

  1497.             </li>
    1498. 

  1498. <li class="listitem">
    1499. 

  1499.               <span class="bold"><strong>Example:</strong></span>
    1500. 

  1500. <pre class="programlisting"><span class="keyword">auto</span> <span class="identifier">length</span> <span class="special">=</span> <span class="special">[](</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">s</span><span class="special">){</span> <span class="keyword">return</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">size</span><span class="special">();</span> <span class="special">};</span>
    1501. 

  1501. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">o1</span> <span class="special">{},</span> <span class="identifier">o2</span> <span class="special">{</span><span class="string">"cat"</span><span class="special">};</span>
    1502. 

  1502. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">size_t</span><span class="special">&gt;</span> <span class="identifier">os1</span> <span class="special">=</span> <span class="identifier">o1</span><span class="special">.</span><span class="identifier">map</span><span class="special">(</span><span class="identifier">length</span><span class="special">),</span> <span class="identifier">os2</span> <span class="special">=</span> <span class="identifier">o2</span><span class="special">.</span><span class="identifier">map</span><span class="special">(</span><span class="identifier">length</span><span class="special">);</span>
    1503. 

  1503. <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">os1</span> <span class="special">)</span> <span class="special">;</span>
    1504. 

  1504. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">os2</span> <span class="special">)</span> <span class="special">;</span>
    1505. 

  1505. <span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">os2</span> <span class="special">==</span> <span class="number">3</span> <span class="special">)</span> <span class="special">;</span>
    1506. 

  1506. </pre>
    1507. 

  1507.             </li>
    1508. 

  1508. </ul></div>
    1509. 

  1509. <p>
    1510. 

  1510.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1511. 

  1511.         </p>
    1512. 

  1512. <a name="reference_optional_map_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1513. 

  1513.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&amp;&amp;</span>
    1514. 

  1514.             <span class="special">-&gt;</span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput">
    1515. 

  1515.             <span class="special">;</span></code>
    1516. 

  1516.           </p></blockquote></div>
    1517. 

  1517. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1518. 

  1518. <li class="listitem">
    1519. 

  1519.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1520. 

  1520.               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="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>
    1521. 

  1521.               <span class="identifier">none</span><span class="special">;</span></code>
    1522. 

  1522.             </li>
    1523. 

  1523. <li class="listitem">
    1524. 

  1524.               <span class="bold"><strong>Notes:</strong></span> The return type of this overload
    1525. 

  1525.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">decltype</span><span class="special">(</span><span class="identifier">f</span><span class="special">(</span><span class="identifier">istd</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)))&gt;</span></code>.
    1526. 

  1526.             </li>
    1527. 

  1527. </ul></div>
    1528. 

  1528. <p>
    1529. 

  1529.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1530. 

  1530.         </p>
    1531. 

  1531. <a name="reference_optional_flat_map"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1532. 

  1532.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">flat_map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="special">-&gt;</span>
    1533. 

  1533.             </code><span class="emphasis"><em>see below</em></span><code class="computeroutput"> <span class="special">;</span></code>
    1534. 

  1534.           </p></blockquote></div>
    1535. 

  1535. <div class="blockquote"><blockquote class="blockquote"><p>
    1536. 

  1536.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">flat_map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&amp;</span> <span class="special">-&gt;</span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput">
    1537. 

  1537.             <span class="special">;</span></code>
    1538. 

  1538.           </p></blockquote></div>
    1539. 

  1539. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1540. 

  1540. <li class="listitem">
    1541. 

  1541.               <span class="bold"><strong>Requires:</strong></span> The return type of expression
    1542. 

  1542.               <code class="computeroutput"><span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">)</span></code>
    1543. 

  1543.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code>
    1544. 

  1544.               for some object or reference type <code class="computeroutput"><span class="identifier">U</span></code>.
    1545. 

  1545.             </li>
    1546. 

  1546. <li class="listitem">
    1547. 

  1547.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1548. 

  1548.               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(**</span><span class="keyword">this</span><span class="special">);</span> <span class="keyword">else</span> <span class="keyword">return</span>
    1549. 

  1549.               <span class="identifier">none</span><span class="special">;</span></code>
    1550. 

  1550.             </li>
    1551. 

  1551. <li class="listitem">
    1552. 

  1552.               <span class="bold"><strong>Notes:</strong></span> The return type of these overloads
    1553. 

  1553.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code>.
    1554. 

  1554.               On compilers that do not support ref-qualifiers on member functions,
    1555. 

  1555.               these two (as well as the next one) overloads are replaced with good
    1556. 

  1556.               old const and non-const overloads.
    1557. 

  1557.             </li>
    1558. 

  1558. <li class="listitem">
    1559. 

  1559.               <span class="bold"><strong>Example:</strong></span>
    1560. 

  1560. <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">first_char</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">string</span><span class="special">&amp;</span> <span class="identifier">s</span><span class="special">)</span> <span class="special">{</span>
    1561. 

  1561.   <span class="keyword">return</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">empty</span><span class="special">()</span> <span class="special">?</span> <span class="identifier">none</span> <span class="special">:</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;(</span><span class="identifier">s</span><span class="special">[</span><span class="number">0</span><span class="special">]);</span>
    1562. 

  1562. <span class="special">};</span>
    1563. 

  1563. <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">o1</span> <span class="special">{},</span> <span class="identifier">o2</span> <span class="special">{</span><span class="string">"cat"</span><span class="special">};</span>
    1564. 

  1564. <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">char</span><span class="special">&gt;</span> <span class="identifier">os1</span> <span class="special">=</span> <span class="identifier">o1</span><span class="special">.</span><span class="identifier">flat_map</span><span class="special">(</span><span class="identifier">first_char</span><span class="special">),</span> <span class="identifier">os2</span> <span class="special">=</span> <span class="identifier">o2</span><span class="special">.</span><span class="identifier">flat_map</span><span class="special">(</span><span class="identifier">first_char</span><span class="special">);</span>
    1565. 

  1565. <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">os1</span> <span class="special">)</span> <span class="special">;</span>
    1566. 

  1566. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">os2</span> <span class="special">)</span> <span class="special">;</span>
    1567. 

  1567. <span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">os2</span> <span class="special">==</span> <span class="char">'c'</span> <span class="special">)</span> <span class="special">;</span>
    1568. 

  1568. </pre>
    1569. 

  1569.               <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1570. 

  1570.             </li>
    1571. 

  1571. </ul></div>
    1572. 

  1572. <a name="reference_optional_flat_map_move"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1573. 

  1573.             <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="keyword">auto</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">flat_map</span><span class="special">(</span><span class="identifier">F</span> <span class="identifier">f</span><span class="special">)</span> <span class="special">&amp;&amp;</span>
    1574. 

  1574.             <span class="special">-&gt;</span> </code><span class="emphasis"><em>see below</em></span><code class="computeroutput">
    1575. 

  1575.             <span class="special">;</span></code>
    1576. 

  1576.           </p></blockquote></div>
    1577. 

  1577. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1578. 

  1578. <li class="listitem">
    1579. 

  1579.               <span class="bold"><strong>Requires:</strong></span> The return type of expression
    1580. 

  1580.               <code class="computeroutput"><span class="identifier">f</span><span class="special">(</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(**</span><span class="keyword">this</span><span class="special">))</span></code>
    1581. 

  1581.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code>
    1582. 

  1582.               for some object or reference type <code class="computeroutput"><span class="identifier">U</span></code>.
    1583. 

  1583.             </li>
    1584. 

  1584. <li class="listitem">
    1585. 

  1585.               <span class="bold"><strong>Effects:</strong></span> <code class="computeroutput"><span class="keyword">if</span>
    1586. 

  1586.               <span class="special">(*</span><span class="keyword">this</span><span class="special">)</span> <span class="keyword">return</span> <span class="identifier">f</span><span class="special">(</span><span class="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>
    1587. 

  1587.               <span class="identifier">none</span><span class="special">;</span></code>
    1588. 

  1588.             </li>
    1589. 

  1589. <li class="listitem">
    1590. 

  1590.               <span class="bold"><strong>Notes:</strong></span> The return type of this overload
    1591. 

  1591.               is <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">U</span><span class="special">&gt;</span></code>.
    1592. 

  1592.             </li>
    1593. 

  1593. </ul></div>
    1594. 

  1594. <p>
    1595. 

  1595.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1596. 

  1596.         </p>
    1597. 

  1597. <a name="reference_optional_get_value_or_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1598. 

  1598.             <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>
    1599. 

  1599.           </p></blockquote></div>
    1600. 

  1600. <div class="blockquote"><blockquote class="blockquote"><p>
    1601. 

  1601.             <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span>
    1602. 

  1602.             <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>
    1603. 

  1603.             <span class="identifier">T</span><span class="special">&amp;</span>
    1604. 

  1604.             <span class="keyword">default</span> <span class="special">)</span>
    1605. 

  1605.             <span class="special">;</span></code>
    1606. 

  1606.           </p></blockquote></div>
    1607. 

  1607. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1608. 

  1608. <li class="listitem">
    1609. 

  1609.               <span class="bold"><strong>Deprecated:</strong></span> Use <code class="computeroutput"><span class="identifier">value_or</span><span class="special">()</span></code> instead.
    1610. 

  1610.             </li>
    1611. 

  1611. <li class="listitem">
    1612. 

  1612.               <span class="bold"><strong>Returns:</strong></span> A reference to the contained
    1613. 

  1613.               value, if any, or <code class="computeroutput"><span class="keyword">default</span></code>.
    1614. 

  1614.             </li>
    1615. 

  1615. <li class="listitem">
    1616. 

  1616.               <span class="bold"><strong>Throws:</strong></span> Nothing.
    1617. 

  1617.             </li>
    1618. 

  1618. <li class="listitem">
    1619. 

  1619.               <span class="bold"><strong>Example:</strong></span>
    1620. 

  1620. <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>
    1621. 

  1621. <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>
    1622. 

  1622. <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>
    1623. 

  1623. <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>
    1624. 

  1624. 

  1625. <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>
    1626. 

  1626. <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>
    1627. 

  1627. <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>
    1628. 

  1628. <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>
    1629. 

  1629. </pre>
    1630. 

  1630.             </li>
    1631. 

  1631. </ul></div>
    1632. 

  1632. <p>
    1633. 

  1633.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1634. 

  1634.         </p>
    1635. 

  1635. <a name="reference_optional_get_ptr"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1636. 

  1636.             <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>
    1637. 

  1637.           </p></blockquote></div>
    1638. 

  1638. <div class="blockquote"><blockquote class="blockquote"><p>
    1639. 

  1639.             <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
    1640. 

  1640.             <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>
    1641. 

  1641.             <span class="special">;</span></code>
    1642. 

  1642.           </p></blockquote></div>
    1643. 

  1643. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1644. 

  1644. <li class="listitem">
    1645. 

  1645.               <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
    1646. 

  1646.               contained value; else <code class="computeroutput"><span class="number">0</span></code>
    1647. 

  1647.               (<span class="emphasis"><em>null</em></span>).
    1648. 

  1648.             </li>
    1649. 

  1649. <li class="listitem">
    1650. 

  1650.               <span class="bold"><strong>Throws:</strong></span> Nothing.
    1651. 

  1651.             </li>
    1652. 

  1652. <li class="listitem">
    1653. 

  1653.               <span class="bold"><strong>Notes:</strong></span> The contained value is permanently
    1654. 

  1654.               stored within <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>,
    1655. 

  1655.               so you should not hold nor delete this pointer
    1656. 

  1656.             </li>
    1657. 

  1657. <li class="listitem">
    1658. 

  1658.               <span class="bold"><strong>Example:</strong></span>
    1659. 

  1659. <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">v</span><span class="special">;</span>
    1660. 

  1660. <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>
    1661. 

  1661. <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>
    1662. 

  1662. <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>
    1663. 

  1663. <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>
    1664. 

  1664. <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>
    1665. 

  1665. <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>
    1666. 

  1666. </pre>
    1667. 

  1667.             </li>
    1668. 

  1668. </ul></div>
    1669. 

  1669. <p>
    1670. 

  1670.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1671. 

  1671.         </p>
    1672. 

  1672. <a name="reference_optional_operator_arrow"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1673. 

  1673.             <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>
    1674. 

  1674.             <span class="special">-&gt;()</span> <span class="keyword">const</span>
    1675. 

  1675.             <span class="special">;</span></code>
    1676. 

  1676.           </p></blockquote></div>
    1677. 

  1677. <div class="blockquote"><blockquote class="blockquote"><p>
    1678. 

  1678.             <code class="computeroutput"><span class="identifier">T</span><span class="special">*</span>
    1679. 

  1679.             <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>
    1680. 

  1680.             <span class="special">;</span></code>
    1681. 

  1681.           </p></blockquote></div>
    1682. 

  1682. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1683. 

  1683. <li class="listitem">
    1684. 

  1684.               <span class="bold"><strong>Requires: </strong></span> <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> is initialized.
    1685. 

  1685.             </li>
    1686. 

  1686. <li class="listitem">
    1687. 

  1687.               <span class="bold"><strong>Returns:</strong></span> A pointer to the contained
    1688. 

  1688.               value.
    1689. 

  1689.             </li>
    1690. 

  1690. <li class="listitem">
    1691. 

  1691.               <span class="bold"><strong>Throws:</strong></span> Nothing.
    1692. 

  1692.             </li>
    1693. 

  1693. <li class="listitem">
    1694. 

  1694.               <span class="bold"><strong>Notes:</strong></span> The requirement is asserted
    1695. 

  1695.               via <code class="computeroutput"><span class="identifier">BOOST_ASSERT</span><span class="special">()</span></code>.
    1696. 

  1696.             </li>
    1697. 

  1697. <li class="listitem">
    1698. 

  1698.               <span class="bold"><strong>Example:</strong></span>
    1699. 

  1699. <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>
    1700. 

  1700. <span class="identifier">X</span> <span class="identifier">x</span> <span class="special">;</span>
    1701. 

  1701. <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>
    1702. 

  1702. <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>
    1703. 

  1703. </pre>
    1704. 

  1704.             </li>
    1705. 

  1705. </ul></div>
    1706. 

  1706. <p>
    1707. 

  1707.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1708. 

  1708.         </p>
    1709. 

  1709. <a name="reference_optional_operator_bool"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1710. 

  1710.             <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>
    1711. 

  1711.             <span class="keyword">bool</span><span class="special">()</span>
    1712. 

  1712.             <span class="keyword">const</span> <span class="keyword">noexcept</span>
    1713. 

  1713.             <span class="special">;</span></code>
    1714. 

  1714.           </p></blockquote></div>
    1715. 

  1715. <div class="blockquote"><blockquote class="blockquote"><p>
    1716. 

  1716.             <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">has_value</span><span class="special">()</span> <span class="keyword">const</span> <span class="keyword">noexcept</span> <span class="special">;</span></code>
    1717. 

  1717.           </p></blockquote></div>
    1718. 

  1718. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1719. 

  1719. <li class="listitem">
    1720. 

  1720.               <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>.
    1721. 

  1721.             </li>
    1722. 

  1722. <li class="listitem">
    1723. 

  1723.               <span class="bold"><strong>Notes:</strong></span> On compilers that do not support
    1724. 

  1724.               explicit conversion operators this falls back to safe-bool idiom.
    1725. 

  1725.             </li>
    1726. 

  1726. <li class="listitem">
    1727. 

  1727.               <span class="bold"><strong>Example:</strong></span>
    1728. 

  1728. <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>
    1729. 

  1729. <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>
    1730. 

  1730. <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>
    1731. 

  1731. <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt</span> <span class="special">);</span>
    1732. 

  1732. <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>
    1733. 

  1733. </pre>
    1734. 

  1734.             </li>
    1735. 

  1735. </ul></div>
    1736. 

  1736. <p>
    1737. 

  1737.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1738. 

  1738.         </p>
    1739. 

  1739. <a name="reference_optional_operator_not"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1740. 

  1740.             <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>
    1741. 

  1741.             <span class="special">;</span></code>
    1742. 

  1742.           </p></blockquote></div>
    1743. 

  1743. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
    1744. 

  1744. <li class="listitem">
    1745. 

  1745.               <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>.
    1746. 

  1746.             </li>
    1747. 

  1747. <li class="listitem">
    1748. 

  1748.               <span class="bold"><strong>Notes:</strong></span> This operator is provided for
    1749. 

  1749.               those compilers which can't use the <span class="emphasis"><em>unspecified-bool-type
    1750. 

  1750.               operator</em></span> in certain boolean contexts.
    1751. 

  1751.             </li>
    1752. 

  1752. <li class="listitem">
    1753. 

  1753.               <span class="bold"><strong>Example:</strong></span>
    1754. 

  1754. <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>
    1755. 

  1755. <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">opt</span> <span class="special">);</span>
    1756. 

  1756. <span class="special">*</span><span class="identifier">opt</span> <span class="special">=</span> <span class="identifier">some_T</span> <span class="special">;</span>
    1757. 

  1757. 

  1758. <span class="comment">// Notice the "double-bang" idiom here.</span>
    1759. 

  1759. <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>
    1760. 

  1760. </pre>
    1761. 

  1761.             </li>
    1762. 

  1762. </ul></div>
    1763. 

  1763. <p>
    1764. 

  1764.           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
    1765. 

  1765.         </p>
    1766. 

  1766. <a name="reference_optional_is_initialized"></a><div class="blockquote"><blockquote class="blockquote"><p>
    1767. 

  1767.             <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>
    1768. 

  1768.           </p></blockquote></div>
    1769. 

  1769. <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
    1770. 

  1770.               <span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">explicit</span> <span class="keyword">operator</span>
    1771. 

  1771.               <span class="keyword">bool</span> <span class="special">()</span>
    1772. 

  1772.               <span class="special">;</span></code>
    1773. 

  1773.             </li></ul></div>
    1774. 

  1774. </div>
    1775. 

  1775. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    1776. 

  1776. <td align="left"></td>
    1777. 

  1777. <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<p>
    1778. 

  1778.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    1779. 

  1779.         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>)
    1780. 

  1780.       </p>
    1781. 

  1781. </div></td>
    1782. 

  1782. </tr></table>
    1783. 

  1783. <hr>
    1784. 

  1784. <div class="spirit-nav">
    1785. 

  1785. <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>
    1786. 

  1786. </div>
    1787. 

  1787. </body>
    1788. 

  1788. </html>
    1789.