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  26 <div class="titlepage"><div><div><h3 class="title">
  27 <a name="boost_optional.tutorial.relational_operators"></a><a class="link" href="relational_operators.html" title="Relational operators">Relational
  28       operators</a>
  29 </h3></div></div></div>
  30 <p>
  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         <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         objects containing a value compare in the same as their contained values.
  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         is treated as a distinct value, equal to itself, and unequal to any value
  36         of type <code class="computeroutput"><span class="identifier">T</span></code>:
  37       </p>
  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 <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 <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
  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 <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 <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 <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 <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 </pre>
  48 <p>
  49         The converting constructor from <code class="computeroutput"><span class="identifier">T</span></code>
  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         of the mixed comparison between <code class="computeroutput"><span class="identifier">T</span></code>
  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         well as between <code class="computeroutput"><span class="identifier">none_t</span></code> and
  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       </p>
  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 <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 <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 <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 <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 </pre>
  62 <p>
  63         This mixed comparison has a practical interpretation, which is occasionally
  64         useful:
  65       </p>
  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 <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     <span class="identifier">start_procedure_2</span><span class="special">();</span>
  69 </pre>
  70 <p>
  71         In the above example, the meaning of the comparison is 'user chose number
  72         2'. If user chose nothing, he didn't choose number 2.
  73       </p>
  74 <p>
  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         compared to <code class="computeroutput"><span class="identifier">none</span></code>, it is not
  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       </p>
  79 <p>
  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         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         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         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         <code class="computeroutput"><span class="number">2</span></code>, ...}, the default ordering
  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         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         <code class="computeroutput"><span class="number">1</span></code>, <code class="computeroutput"><span class="number">2</span></code>,
  88         ...}. This order does not have a practical interpretation. The goal is to
  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         be usable in ordered associative containers (wherever <code class="computeroutput"><span class="identifier">T</span></code>
  91         is usable).
  92       </p>
  93 <p>
  94         Mixed relational operators are the only case where the contained value of
  95         an optional object can be inspected without the usage of value accessing
  96         function (<code class="computeroutput"><span class="keyword">operator</span><span class="special">*</span></code>,
  97         <code class="computeroutput"><span class="identifier">value</span></code>, <code class="computeroutput"><span class="identifier">value_or</span></code>).
  98       </p>
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 102 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<p>
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