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27 <a name=
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28 relational comparisons
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29 </h4></div></div></div>
31 Because
<code class=
"computeroutput"><span class=
"identifier">T
</span></code> is convertible
32 to
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><</span><span class=
"identifier">T
</span><span class=
"special">></span></code>
33 and because
<code class=
"computeroutput"><span class=
"identifier">opiotnal
</span><span class=
"special"><</span><span class=
"identifier">T
</span><span class=
"special">></span></code>
34 is
<a href=
"http://www.sgi.com/tech/stl/LessThanComparable.html" target=
"_top"><code class=
"computeroutput"><span class=
"identifier">LessThanComparable
</span></code></a> when
<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>, you can sometimes
35 get an unexpected runtime result where you would rather expect a compiler
38 <pre class=
"programlisting"><span class=
"identifier">optional
</span><span class=
"special"><</span><span class=
"keyword">double
</span><span class=
"special">></span> <span class=
"identifier">Flight_plan
</span><span class=
"special">::
</span><span class=
"identifier">weight
</span><span class=
"special">();
</span> <span class=
"comment">// sometimes no weight can be returned
</span>
40 <span class=
"keyword">bool
</span> <span class=
"identifier">is_aircraft_too_heavy
</span><span class=
"special">(
</span><span class=
"identifier">Flight_plan
</span> <span class=
"keyword">const
</span><span class=
"special">&</span> <span class=
"identifier">p
</span><span class=
"special">)
</span>
41 <span class=
"special">{
</span>
42 <span class=
"keyword">return
</span> <span class=
"identifier">p
</span><span class=
"special">.
</span><span class=
"identifier">weight
</span><span class=
"special">()
</span> <span class=
"special">></span> <span class=
"identifier">p
</span><span class=
"special">.
</span><span class=
"identifier">aircraft
</span><span class=
"special">().
</span><span class=
"identifier">max_weight
</span><span class=
"special">();
</span> <span class=
"comment">// compiles!
</span>
43 <span class=
"special">}
</span> <span class=
"comment">// returns false when the optional contains no value
</span>
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© 2003-
2007 Fernando Luis Cacciola Carballal
<br>Copyright
© 2014-
2016 Andrzej Krzemie
ński
<p>
49 Distributed under the Boost Software License, Version
1.0. (See accompanying
50 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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