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26 <div class=
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27 <a name=
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29 </h3></div></div></div>
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>:
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>
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>
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>:
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>
63 This mixed comparison has a practical interpretation, which is occasionally
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>
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.
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>.
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>
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>).
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 Krzemie
ński
<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>)
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