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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_optional.quick_start.optional_automatic_variables"></a><a class="link" href="optional_automatic_variables.html" title="Optional automatic variables">Optional | |
| 28 | automatic variables</a> | |
| 29 | </h3></div></div></div> | |
| 30 | <p> | |
| 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: | |
| 33 | </p> | |
| 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> | |
| 41 | ||
| 42 | <span class="keyword">return</span> <span class="identifier">ans</span><span class="special">;</span> | |
| 43 | <span class="special">}</span> | |
| 44 | </pre> | |
| 45 | <p> | |
| 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 | |
| 59 | copy assignment. | |
| 60 | </p> | |
| 61 | </div> | |
| 62 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 63 | <td align="left"></td> | |
| 64 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
| 65 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 66 | 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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