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relational_operators.html

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

  26. <div class="titlepage"><div><div><h3 class="title">
    27. 

  27. <a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
    28. 

  28.       operators</a>
    29. 

  29. </h3></div></div></div>
    30. 

  30. <p>
    31. 

  31.         Type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
    32. 

  32.         <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</span></code> is <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>. Two optional
    33. 

  33.         objects containing a value compare in the same way as their contained values.
    34. 

  34.         The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    35. 

  35.         is treated as a distinct value, equal to itself, and unequal to any value
    36. 

  36.         of type <code class="computeroutput"><span class="identifier">T</span></code>:
    37. 

  37.       </p>
    38. 

  38. <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><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">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">;</span>
    39. 

  39. <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">o0</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
    40. 

  40. <span class="identifier">boost</span><span class="special">::</span><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>
    41. 

  41. 

  42. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="identifier">o0</span><span class="special">);</span>
    43. 

  43. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">oN</span><span class="special">);</span>
    44. 

  44. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="identifier">o1</span><span class="special">);</span>
    45. 

  45. <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>
    46. 

  46. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="identifier">o0</span><span class="special">);</span>
    47. 

  47. </pre>
    48. 

  48. <p>
    49. 

  49.         The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
    50. 

  50.         as well as from <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code> implies the existence and semantics
    51. 

  51.         of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
    52. 

  52.         and <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> as
    53. 

  53.         well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
    54. 

  54.         <code class="computeroutput"><span class="identifier">optionl</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>:
    55. 

  55.       </p>
    56. 

  56. <pre class="programlisting"><span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">!=</span> <span class="number">0</span><span class="special">);</span>
    57. 

  57. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o1</span> <span class="special">!=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
    58. 

  58. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">!=</span> <span class="number">1</span><span class="special">);</span>
    59. 

  59. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">oN</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">);</span>
    60. 

  60. <span class="identifier">assert</span><span class="special">(</span><span class="identifier">o0</span> <span class="special">==</span> <span class="number">0</span><span class="special">);</span>
    61. 

  61. </pre>
    62. 

  62. <p>
    63. 

  63.         This mixed comparison has a practical interpretation, which is occasionally
    64. 

  64.         useful:
    65. 

  65.       </p>
    66. 

  66. <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">choice</span> <span class="special">=</span> <span class="identifier">ask_user</span><span class="special">();</span>
    67. 

  67. <span class="keyword">if</span> <span class="special">(</span><span class="identifier">choice</span> <span class="special">==</span> <span class="number">2</span><span class="special">)</span>
    68. 

  68.     <span class="identifier">start_procedure_2</span><span class="special">();</span>
    69. 

  69. </pre>
    70. 

  70. <p>
    71. 

  71.         In the above example, the meaning of the comparison is 'user chose number
    72. 

  72.         2'. If user chose nothing, he didn't choose number 2.
    73. 

  73.       </p>
    74. 

  74. <p>
    75. 

  75.         In case where <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> is
    76. 

  76.         compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
    77. 

  77.         required that <code class="computeroutput"><span class="identifier">T</span></code> be <a href="http://www.sgi.com/tech/stl/EqualityComparable.html" target="_top"><code class="computeroutput"><span class="identifier">EqualityComparable</span></code></a>.
    78. 

  78.       </p>
    79. 

  79. <p>
    80. 

  80.         In a similar manner, type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
    81. 

  81.         is <a href="http://www.sgi.com/tech/stl/LessThanComparable.html" target="_top"><code class="computeroutput"><span class="identifier">LessThanComparable</span></code></a> whenever <code class="computeroutput"><span class="identifier">T</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>. The optional
    82. 

  82.         object containing no value is compared less than any value of <code class="computeroutput"><span class="identifier">T</span></code>. To illustrate this, if the default ordering
    83. 

  83.         of <code class="computeroutput"><span class="identifier">size_t</span></code> is {<code class="computeroutput"><span class="number">0</span></code>, <code class="computeroutput"><span class="number">1</span></code>,
    84. 

  84.         <code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
    85. 

  85.         of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">size_t</span><span class="special">&gt;</span></code>
    86. 

  86.         is {<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>, <code class="computeroutput"><span class="number">0</span></code>,
    87. 

  87.         <code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
    88. 

  88.         ...}. This order does not have a practical interpretation. The goal is to
    89. 

  89.         have any semantically correct default ordering in order for <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> to
    90. 

  90.         be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
    91. 

  91.         is usable).
    92. 

  92.       </p>
    93. 

  93. <p>
    94. 

  94.         Mixed relational operators are the only case where the contained value of
    95. 

  95.         an optional object can be inspected without the usage of value accessing
    96. 

  96.         function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
    97. 

  97.         <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
    98. 

  98.       </p>
    99. 

  99. </div>
    100. 

  100. <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    101. 

  101. <td align="left"></td>
    102. 

  102. <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>
    103. 

  103.         Distributed under the Boost Software License, Version 1.0. (See accompanying
    104. 

  104.         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>)
    105. 

  105.       </p>
    106. 

  106. </div></td>
    107. 

  107. </tr></table>
    108. 

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    109. 

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