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| 24 | </div> | |
| 25 | <div class="section"> | |
| 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"><</span><span class="identifier">T</span><span class="special">></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 way as their contained values. | |
| 34 | The uninitialized state of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="keyword">int</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> | |
| 39 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></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"><</span><span class="keyword">int</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="keyword">int</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">size_t</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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> | |
| 99 | </div> | |
| 100 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 101 | <td align="left"></td> | |
| 102 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
| 103 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 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 | </p> | |
| 106 | </div></td> | |
| 107 | </tr></table> | |
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