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26 <div class=
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27 <a name=
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28 automatic variables
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29 </h3></div></div></div>
31 We could write function
<code class=
"computeroutput"><span class=
"identifier">convert
</span></code>
32 in a slightly different manner, so that it has a single
<code class=
"computeroutput"><span class=
"keyword">return
</span></code>-statement:
34 <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">convert
</span><span class=
"special">(
</span><span class=
"keyword">const
</span> <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">string
</span><span class=
"special">&</span> <span class=
"identifier">text
</span><span class=
"special">)
</span>
35 <span class=
"special">{
</span>
36 <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">ans
</span><span class=
"special">;
</span>
37 <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">stringstream
</span> <span class=
"identifier">s
</span><span class=
"special">(
</span><span class=
"identifier">text
</span><span class=
"special">);
</span>
38 <span class=
"keyword">int
</span> <span class=
"identifier">i
</span><span class=
"special">;
</span>
39 <span class=
"keyword">if
</span> <span class=
"special">((
</span><span class=
"identifier">s
</span> <span class=
"special">>></span> <span class=
"identifier">i
</span><span class=
"special">)
</span> <span class=
"special">&&</span> <span class=
"identifier">s
</span><span class=
"special">.
</span><span class=
"identifier">get
</span><span class=
"special">()
</span> <span class=
"special">==
</span> <span class=
"identifier">std
</span><span class=
"special">::
</span><span class=
"identifier">char_traits
</span><span class=
"special"><</span><span class=
"keyword">char
</span><span class=
"special">>::
</span><span class=
"identifier">eof
</span><span class=
"special">())
</span>
40 <span class=
"identifier">ans
</span> <span class=
"special">=
</span> <span class=
"identifier">i
</span><span class=
"special">;
</span>
42 <span class=
"keyword">return
</span> <span class=
"identifier">ans
</span><span class=
"special">;
</span>
43 <span class=
"special">}
</span>
46 The default constructor of
<code class=
"computeroutput"><span class=
"identifier">optional
</span></code>
47 creates an unitialized optional object. Unlike with
<code class=
"computeroutput"><span class=
"keyword">int
</span></code>s
48 you cannot have an
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><</span><span class=
"keyword">int
</span><span class=
"special">></span></code>
49 in an indeterminate state. Its state is always well defined. Instruction
50 <code class=
"computeroutput"><span class=
"identifier">ans
</span> <span class=
"special">=
</span>
51 <span class=
"identifier">i
</span></code> initializes the optional object.
52 It uses the 'mixed' assignment from
<code class=
"computeroutput"><span class=
"keyword">int
</span></code>.
53 In general, for
<code class=
"computeroutput"><span class=
"identifier">optional
</span><span class=
"special"><</span><span class=
"identifier">T
</span><span class=
"special">></span></code>,
54 when an assignment from
<code class=
"computeroutput"><span class=
"identifier">T
</span></code>
55 is invoked, it can do two things. If the optional object is not initialized
56 (our case here), it initializes the contained value using
<code class=
"computeroutput"><span class=
"identifier">T
</span></code>'s
57 copy constructor. If the optional object is already initialized, it assigns
58 the new value to it using
<code class=
"computeroutput"><span class=
"identifier">T
</span></code>'s
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© 2003-
2007 Fernando Luis Cacciola Carballal
<br>Copyright
© 2014-
2016 Andrzej Krzemie
ński
<p>
65 Distributed under the Boost Software License, Version
1.0. (See accompanying
66 file LICENSE_1_0.txt or copy at
<a href=
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</a>)
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