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

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  27 <a name="boost_optional.reference.header__boost_optional_optional_hpp_.detailed_semantics___free_functions"></a><a class="link" href="detailed_semantics___free_functions.html" title="Detailed Semantics - Free Functions">Detailed
  28         Semantics - Free Functions</a>
  29 </h4></div></div></div>
  30 <p>
  31           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  32         </p>
  33 <a name="reference_make_optional_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
  34             <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">make_optional</span><span class="special">(</span>
  35             <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>
  36           </p></blockquote></div>
  37 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  38 <li class="listitem">
  39               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
  40               type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
  41             </li>
  42 <li class="listitem">
  43               <span class="bold"><strong>Example:</strong></span>
  44 <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">void</span> <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">opt</span> <span class="special">)</span> <span class="special">;</span>
  45
  46 <span class="identifier">foo</span> <span class="special">(</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="number">1</span><span class="special">+</span><span class="number">1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span> <span class="comment">// Creates an optional&lt;int&gt;</span>
  47 </pre>
  48             </li>
  49 </ul></div>
  50 <p>
  51           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  52         </p>
  53 <a name="reference_make_optional_bool_value"></a><div class="blockquote"><blockquote class="blockquote"><p>
  54             <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">make_optional</span><span class="special">(</span>
  55             <span class="keyword">bool</span> <span class="identifier">condition</span><span class="special">,</span> <span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">v</span> <span class="special">)</span></code>
  56           </p></blockquote></div>
  57 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  58 <li class="listitem">
  59               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;(</span><span class="identifier">condition</span><span class="special">,</span> <span class="identifier">v</span><span class="special">)</span></code> for the <span class="emphasis"><em>deduced</em></span>
  60               type <code class="computeroutput"><span class="identifier">T</span></code> of <code class="computeroutput"><span class="identifier">v</span></code>.
  61             </li>
  62 <li class="listitem">
  63               <span class="bold"><strong>Example:</strong></span>
  64 <pre class="programlisting"><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">calculate_foo</span><span class="special">()</span>
  65 <span class="special">{</span>
  66   <span class="keyword">double</span> <span class="identifier">val</span> <span class="special">=</span> <span class="identifier">compute_foo</span><span class="special">();</span>
  67   <span class="keyword">return</span> <span class="identifier">make_optional</span><span class="special">(</span><span class="identifier">is_not_nan_and_finite</span><span class="special">(</span><span class="identifier">val</span><span class="special">),</span><span class="identifier">val</span><span class="special">);</span>
  68 <span class="special">}</span>
  69
  70 <span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">v</span> <span class="special">=</span> <span class="identifier">calculate_foo</span><span class="special">();</span>
  71 <span class="keyword">if</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">v</span> <span class="special">)</span>
  72   <span class="identifier">error</span><span class="special">(</span><span class="string">"foo wasn't computed"</span><span class="special">);</span>
  73 </pre>
  74             </li>
  75 </ul></div>
  76 <p>
  77           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
  78         </p>
  79 <a name="reference_operator_compare_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
  80             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
  81             <span class="special">==</span> <span class="special">(</span>
  82             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
  83           </p></blockquote></div>
  84 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  85 <li class="listitem">
  86               <span class="bold"><strong>Requires:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
  87               shall meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
  88             </li>
  89 <li class="listitem">
  90               <span class="bold"><strong>Returns:</strong></span> If both <code class="computeroutput"><span class="identifier">x</span></code>
  91               and <code class="computeroutput"><span class="identifier">y</span></code> are initialized,
  92               <code class="computeroutput"><span class="special">(*</span><span class="identifier">x</span>
  93               <span class="special">==</span> <span class="special">*</span><span class="identifier">y</span><span class="special">)</span></code>.
  94               If only <code class="computeroutput"><span class="identifier">x</span></code> or <code class="computeroutput"><span class="identifier">y</span></code> is initialized, <code class="computeroutput"><span class="keyword">false</span></code>.
  95               If both are uninitialized, <code class="computeroutput"><span class="keyword">true</span></code>.
  96             </li>
  97 <li class="listitem">
  98               <span class="bold"><strong>Notes:</strong></span> This definition guarantees
  99               that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 100               not containing a value is compared unequal to any <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> containing any value, and equal
 101               to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> not containing a value. Pointers
 102               have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
 103               has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">==</span></code> directly in generic code which expect
 104               to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#equal" target="_top"><code class="computeroutput"><span class="identifier">equal_pointees</span><span class="special">()</span></code></a>
 105               instead
 106             </li>
 107 <li class="listitem">
 108               <span class="bold"><strong>Example:</strong></span>
 109 <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">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
 110 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">)),</span> <span class="identifier">o1_</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
 111 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o2</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">2</span><span class="special">));</span>
 112
 113 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN</span> <span class="special">);</span>  <span class="comment">// Identity implies equality</span>
 114 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1</span> <span class="special">);</span>  <span class="comment">//</span>
 115
 116 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">oN_</span> <span class="special">);</span> <span class="comment">// Both uninitialized compare equal</span>
 117
 118 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o1</span> <span class="special">);</span>  <span class="comment">// Initialized unequal to initialized.</span>
 119
 120 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">==</span> <span class="identifier">o1_</span> <span class="special">);</span> <span class="comment">// Both initialized compare as (*lhs == *rhs)</span>
 121 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">o2</span> <span class="special">);</span>  <span class="comment">//</span>
 122 </pre>
 123             </li>
 124 </ul></div>
 125 <p>
 126           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 127         </p>
 128 <a name="reference_operator_compare_less_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 129             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 130             <span class="special">&lt;</span> <span class="special">(</span>
 131             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
 132           </p></blockquote></div>
 133 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 134 <li class="listitem">
 135               <span class="bold"><strong>Requires:</strong></span> Expression <code class="computeroutput"><span class="special">*</span><span class="identifier">x</span> <span class="special">&lt;</span> <span class="special">*</span><span class="identifier">y</span></code> shall be well-formed and its result
 136               shall be convertible to <code class="computeroutput"><span class="keyword">bool</span></code>.
 137             </li>
 138 <li class="listitem">
 139               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">(!</span><span class="identifier">y</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">false</span> <span class="special">:</span> <span class="special">(!</span><span class="identifier">x</span><span class="special">)</span> <span class="special">?</span> <span class="keyword">true</span> <span class="special">:</span> <span class="special">*</span><span class="identifier">x</span> <span class="special">&lt;</span>
 140               <span class="special">*</span><span class="identifier">y</span></code>.
 141             </li>
 142 <li class="listitem">
 143               <span class="bold"><strong>Notes:</strong></span> This definition guarantees
 144               that <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
 145               not containing a value is ordered as less than any <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> containing any value, and equivalent
 146               to any other <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> not containing a value. Pointers
 147               have shallow relational operators while <code class="computeroutput"><span class="identifier">optional</span></code>
 148               has deep relational operators. Do not use <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;</span></code> directly in generic code which
 149               expect to be given either an <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> or a pointer; use <a href="../../../../../../utility/OptionalPointee.html#less" target="_top"><code class="computeroutput"><span class="identifier">less_pointees</span><span class="special">()</span></code></a>
 150               instead. <code class="computeroutput"><span class="identifier">T</span></code> need not
 151               be <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>. Only
 152               single <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;</span></code>
 153               is required. Other relational operations are defined in terms of this
 154               one. If <code class="computeroutput"><span class="identifier">T</span></code>'s <code class="computeroutput"><span class="keyword">operator</span><span class="special">&lt;</span></code>
 155               satisfies the axioms of <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> (transitivity,
 156               antisymmetry and irreflexivity), <code class="computeroutput"><span class="identifier">optinal</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a>.
 157             </li>
 158 <li class="listitem">
 159               <span class="bold"><strong>Example:</strong></span>
 160 <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">oN</span><span class="special">,</span> <span class="identifier">oN_</span><span class="special">;</span>
 161 <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">T</span><span class="special">(</span><span class="number">0</span><span class="special">));</span>
 162 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">o1</span><span class="special">(</span><span class="identifier">T</span><span class="special">(</span><span class="number">1</span><span class="special">));</span>
 163
 164 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special">&lt;</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>  <span class="comment">// Identity implies equivalence</span>
 165 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o1</span> <span class="special">&lt;</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">);</span>
 166
 167 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN</span> <span class="special">&lt;</span> <span class="identifier">oN_</span><span class="special">)</span> <span class="special">);</span> <span class="comment">// Two uninitialized are equivalent</span>
 168 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">oN_</span> <span class="special">&lt;</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
 169
 170 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">oN</span> <span class="special">&lt;</span> <span class="identifier">o0</span> <span class="special">);</span>     <span class="comment">// Uninitialized is less than initialized</span>
 171 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o0</span> <span class="special">&lt;</span> <span class="identifier">oN</span><span class="special">)</span> <span class="special">);</span>
 172
 173 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">o1</span> <span class="special">&lt;</span> <span class="identifier">o2</span> <span class="special">)</span> <span class="special">;</span>    <span class="comment">// Two initialized compare as (*lhs &lt; *rhs)</span>
 174 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special">&lt;</span> <span class="identifier">o1</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
 175 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!(</span><span class="identifier">o2</span> <span class="special">&lt;</span> <span class="identifier">o2</span><span class="special">)</span> <span class="special">)</span> <span class="special">;</span>
 176 </pre>
 177             </li>
 178 </ul></div>
 179 <p>
 180           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 181         </p>
 182 <a name="reference_operator_compare_not_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 183             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 184             <span class="special">!=</span> <span class="special">(</span>
 185             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
 186           </p></blockquote></div>
 187 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 188               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
 189               <span class="identifier">x</span> <span class="special">==</span>
 190               <span class="identifier">y</span> <span class="special">);</span></code>
 191             </li></ul></div>
 192 <p>
 193           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 194         </p>
 195 <a name="reference_operator_compare_greater_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 196             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 197             <span class="special">&gt;</span> <span class="special">(</span>
 198             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
 199           </p></blockquote></div>
 200 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 201               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">(</span>
 202               <span class="identifier">y</span> <span class="special">&lt;</span>
 203               <span class="identifier">x</span> <span class="special">);</span></code>
 204             </li></ul></div>
 205 <p>
 206           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 207         </p>
 208 <a name="reference_operator_compare_less_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 209             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 210             <span class="special">&lt;=</span> <span class="special">(</span>
 211             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
 212           </p></blockquote></div>
 213 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 214               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
 215               <span class="identifier">y</span> <span class="special">&lt;</span>
 216               <span class="identifier">x</span> <span class="special">);</span></code>
 217             </li></ul></div>
 218 <p>
 219           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 220         </p>
 221 <a name="reference_operator_compare_greater_or_equal_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 222             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 223             <span class="special">&gt;=</span> <span class="special">(</span>
 224             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">y</span> <span class="special">);</span></code>
 225           </p></blockquote></div>
 226 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 227               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="special">!(</span>
 228               <span class="identifier">x</span> <span class="special">&lt;</span>
 229               <span class="identifier">y</span> <span class="special">);</span></code>
 230             </li></ul></div>
 231 <p>
 232           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 233         </p>
 234 <a name="reference_operator_compare_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
 235             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 236             <span class="special">==</span> <span class="special">(</span>
 237             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
 238             <span class="keyword">noexcept</span><span class="special">;</span></code>
 239           </p></blockquote></div>
 240 <div class="blockquote"><blockquote class="blockquote"><p>
 241             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 242             <span class="special">==</span> <span class="special">(</span>
 243             <span class="identifier">none_t</span><span class="special">,</span>
 244             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
 245           </p></blockquote></div>
 246 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 247 <li class="listitem">
 248               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="special">!</span><span class="identifier">x</span></code>.
 249             </li>
 250 <li class="listitem">
 251               <span class="bold"><strong>Notes:</strong></span> <code class="computeroutput"><span class="identifier">T</span></code>
 252               need not meet requirements of <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
 253             </li>
 254 </ul></div>
 255 <p>
 256           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 257         </p>
 258 <a name="reference_operator_compare_not_equal_optional_none"></a><div class="blockquote"><blockquote class="blockquote"><p>
 259             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 260             <span class="special">!=</span> <span class="special">(</span>
 261             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">none_t</span> <span class="special">)</span>
 262             <span class="keyword">noexcept</span><span class="special">;</span></code>
 263           </p></blockquote></div>
 264 <div class="blockquote"><blockquote class="blockquote"><p>
 265             <code class="computeroutput"><span class="keyword">bool</span> <span class="keyword">operator</span>
 266             <span class="special">!=</span> <span class="special">(</span>
 267             <span class="identifier">none_t</span><span class="special">,</span>
 268             <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span> <span class="keyword">noexcept</span><span class="special">;</span></code>
 269           </p></blockquote></div>
 270 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
 271               <span class="bold"><strong>Returns: </strong></span> <code class="computeroutput"><span class="keyword">bool</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span></code>
 272             </li></ul></div>
 273 <p>
 274           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 275         </p>
 276 <a name="reference_free_get_pointer"></a><div class="blockquote"><blockquote class="blockquote"><p>
 277             <code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
 278             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">o</span>
 279             <span class="special">)</span> <span class="special">-&gt;</span>
 280             <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">pointer_type</span>
 281             <span class="special">;</span></code>
 282           </p></blockquote></div>
 283 <div class="blockquote"><blockquote class="blockquote"><p>
 284             <code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_pointer</span>
 285             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">o</span> <span class="special">)</span> <span class="special">-&gt;</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">pointer_const_type</span>
 286             <span class="special">;</span></code>
 287           </p></blockquote></div>
 288 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 289 <li class="listitem">
 290               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_ptr</span><span class="special">()</span></code>.
 291             </li>
 292 <li class="listitem">
 293               <span class="bold"><strong>Throws:</strong></span> Nothing.
 294             </li>
 295 </ul></div>
 296 <p>
 297           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 298         </p>
 299 <a name="reference_free_get_value_or"></a><div class="blockquote"><blockquote class="blockquote"><p>
 300             <code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
 301             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reference_type</span> <span class="identifier">def</span>
 302             <span class="special">)</span> <span class="special">-&gt;</span>
 303             <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reference_type</span>
 304             <span class="special">;</span></code>
 305           </p></blockquote></div>
 306 <div class="blockquote"><blockquote class="blockquote"><p>
 307             <code class="computeroutput"><span class="keyword">auto</span> <span class="identifier">get_optional_value_or</span>
 308             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">o</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reference_const_type</span> <span class="identifier">def</span>
 309             <span class="special">)</span> <span class="special">-&gt;</span>
 310             <span class="keyword">typename</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;::</span><span class="identifier">reference_const_type</span>
 311             <span class="special">;</span></code>
 312           </p></blockquote></div>
 313 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 314 <li class="listitem">
 315               <span class="bold"><strong>Returns:</strong></span> <code class="computeroutput"><span class="identifier">o</span><span class="special">.</span><span class="identifier">get_value_or</span><span class="special">(</span><span class="identifier">def</span><span class="special">)</span></code>.
 316             </li>
 317 <li class="listitem">
 318               <span class="bold"><strong>Throws:</strong></span> Nothing.
 319             </li>
 320 <li class="listitem">
 321               <span class="bold"><strong>Remarks:</strong></span> This function is deprecated.
 322             </li>
 323 </ul></div>
 324 <p>
 325           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 326         </p>
 327 <a name="reference_swap_optional_optional"></a><div class="blockquote"><blockquote class="blockquote"><p>
 328             <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
 329             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;&amp;</span> <span class="identifier">y</span>
 330             <span class="special">)</span> <span class="special">;</span></code>
 331           </p></blockquote></div>
 332 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 333 <li class="listitem">
 334               <span class="bold"><strong>Requires:</strong></span> Lvalues of type <code class="computeroutput"><span class="identifier">T</span></code> shall be swappable and <code class="computeroutput"><span class="identifier">T</span></code> shall be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>.
 335             </li>
 336 <li class="listitem">
 337 <p class="simpara">
 338               <span class="bold"><strong>Effects:</strong></span>
 339             </p>
 340 <div class="informaltable"><table class="table">
 341 <colgroup>
 342 <col>
 343 <col>
 344 <col>
 345 </colgroup>
 346 <thead><tr></tr></thead>
 347 <tbody>
 348 <tr>
 349 <td>
 350                     </td>
 351 <td>
 352                       <p>
 353                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> contains a value</strong></span>
 354                       </p>
 355                     </td>
 356 <td>
 357                       <p>
 358                         <span class="bold"><strong><code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value</strong></span>
 359                       </p>
 360                     </td>
 361 </tr>
 362 <tr>
 363 <td>
 364                       <p>
 365                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 366                         contains a value</strong></span>
 367                       </p>
 368                     </td>
 369 <td>
 370                       <p>
 371                         calls <code class="computeroutput"><span class="identifier">swap</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">),</span> <span class="special">*</span><span class="identifier">rhs</span><span class="special">)</span></code>
 372                       </p>
 373                     </td>
 374 <td>
 375                       <p>
 376                         initializes the contained value of <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> as if direct-initializing
 377                         an object of type <code class="computeroutput"><span class="identifier">T</span></code>
 378                         with the expression <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*</span><span class="identifier">rhs</span><span class="special">)</span></code>, followed by <code class="computeroutput"><span class="identifier">rhs</span><span class="special">.</span><span class="identifier">val</span><span class="special">-&gt;</span><span class="identifier">T</span><span class="special">::~</span><span class="identifier">T</span><span class="special">()</span></code>,
 379                         <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code>
 380                         contains a value and <code class="computeroutput"><span class="identifier">rhs</span></code>
 381                         does not contain a value
 382                       </p>
 383                     </td>
 384 </tr>
 385 <tr>
 386 <td>
 387                       <p>
 388                         <span class="bold"><strong><code class="computeroutput"><span class="identifier">rhs</span></code>
 389                         does not contain a value</strong></span>
 390                       </p>
 391                     </td>
 392 <td>
 393                       <p>
 394                         initializes the contained value of <code class="computeroutput"><span class="identifier">rhs</span></code>
 395                         as if direct-initializing an object of type <code class="computeroutput"><span class="identifier">T</span></code> with the expression
 396                         <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">move</span><span class="special">(*(*</span><span class="keyword">this</span><span class="special">))</span></code>,
 397                         followed by <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>, <code class="computeroutput"><span class="special">*</span><span class="keyword">this</span></code> does not contain a value
 398                         and <code class="computeroutput"><span class="identifier">rhs</span></code> contains
 399                         a value
 400                       </p>
 401                     </td>
 402 <td>
 403                       <p>
 404                         no effect
 405                       </p>
 406                     </td>
 407 </tr>
 408 </tbody>
 409 </table></div>
 410 </li>
 411 <li class="listitem">
 412               <span class="bold"><strong>Postconditions:</strong></span> The states of <code class="computeroutput"><span class="identifier">x</span></code> and <code class="computeroutput"><span class="identifier">y</span></code>
 413               interchanged.
 414             </li>
 415 <li class="listitem">
 416               <span class="bold"><strong>Throws:</strong></span> If both are initialized, whatever
 417               <code class="computeroutput"><span class="identifier">swap</span><span class="special">(</span><span class="identifier">T</span><span class="special">&amp;,</span><span class="identifier">T</span><span class="special">&amp;)</span></code>
 418               throws. If only one is initialized, whatever <code class="computeroutput"><span class="identifier">T</span><span class="special">::</span><span class="identifier">T</span> <span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="special">)</span></code>
 419               throws.
 420             </li>
 421 <li class="listitem">
 422               <span class="bold"><strong>Example:</strong></span>
 423 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
 424 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
 425 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def0</span> <span class="special">;</span>
 426 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">def1</span> <span class="special">;</span>
 427 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optX</span><span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 428 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span> <span class="identifier">optY</span><span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
 429
 430 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">def1</span><span class="special">);</span> <span class="comment">// no-op</span>
 431
 432 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span>
 433 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">def0</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
 434 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
 435
 436 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">def0</span><span class="special">,</span><span class="identifier">optX</span><span class="special">);</span> <span class="comment">// Get back to original values</span>
 437
 438 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span><span class="identifier">optY</span><span class="special">);</span>
 439 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optX</span> <span class="special">==</span> <span class="identifier">y</span> <span class="special">);</span>
 440 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">*</span><span class="identifier">optY</span> <span class="special">==</span> <span class="identifier">x</span> <span class="special">);</span>
 441 </pre>
 442             </li>
 443 </ul></div>
 444 <p>
 445           <span class="inlinemediaobject"><img src="../../../images/space.png" alt="space"></span>
 446         </p>
 447 <a name="reference_swap_optional_reference"></a><div class="blockquote"><blockquote class="blockquote"><p>
 448             <code class="computeroutput"><span class="keyword">void</span> <span class="identifier">swap</span>
 449             <span class="special">(</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;&amp;</span> <span class="identifier">y</span>
 450             <span class="special">)</span> <span class="keyword">noexcept</span>
 451             <span class="special">;</span></code>
 452           </p></blockquote></div>
 453 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
 454 <li class="listitem">
 455               <span class="bold"><strong>Postconditions:</strong></span> <code class="computeroutput"><span class="identifier">x</span></code>
 456               refers to what <code class="computeroutput"><span class="identifier">y</span></code> refererred
 457               to before the swap (if anything). <code class="computeroutput"><span class="identifier">y</span></code>
 458               refers to whatever <code class="computeroutput"><span class="identifier">x</span></code>
 459               referred to before the swap.
 460             </li>
 461 <li class="listitem">
 462               <span class="bold"><strong>Example:</strong></span>
 463 <pre class="programlisting"><span class="identifier">T</span> <span class="identifier">x</span><span class="special">(</span><span class="number">12</span><span class="special">);</span>
 464 <span class="identifier">T</span> <span class="identifier">y</span><span class="special">(</span><span class="number">21</span><span class="special">);</span>
 465
 466 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">opt0</span><span class="special">;</span>
 467 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">optX</span> <span class="special">(</span><span class="identifier">x</span><span class="special">);</span>
 468 <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span> <span class="identifier">optY</span> <span class="special">(</span><span class="identifier">y</span><span class="special">);</span>
 469
 470 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">optX</span><span class="special">,</span> <span class="identifier">optY</span><span class="special">);</span>
 471 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optX</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
 472 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">optY</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">x</span><span class="special">));</span>
 473
 474 <span class="identifier">boost</span><span class="special">::</span><span class="identifier">swap</span><span class="special">(</span><span class="identifier">opt0</span><span class="special">,</span> <span class="identifier">optX</span><span class="special">);</span>
 475 <span class="identifier">assert</span> <span class="special">(</span> <span class="identifier">opt0</span> <span class="special">);</span>
 476 <span class="identifier">assert</span> <span class="special">(</span> <span class="special">!</span><span class="identifier">optX</span> <span class="special">);</span>
 477 <span class="identifier">assert</span> <span class="special">(</span><span class="identifier">addressof</span><span class="special">(*</span><span class="identifier">opt0</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">addressof</span><span class="special">(</span><span class="identifier">y</span><span class="special">));</span>
 478 </pre>
 479             </li>
 480 </ul></div>
 481 </div>
 482 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 483 <td align="left"></td>
 484 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
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