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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.quick_start.optional_data_members"></a><a class="link" href="optional_data_members.html" title="Optional data members">Optional |
| data members</a> |
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| <p> |
| Suppose we want to implement a <span class="emphasis"><em>lazy load</em></span> optimization. |
| This is because we do not want to perform an expensive initialization of |
| our <code class="computeroutput"><span class="identifier">Resource</span></code> until (if at |
| all) it is really used. We can do it this way: |
| </p> |
| <pre class="programlisting"><span class="keyword">class</span> <span class="identifier">Widget</span> |
| <span class="special">{</span> |
| <span class="keyword">mutable</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">></span> <span class="identifier">resource_</span><span class="special">;</span> |
| |
| <span class="keyword">public</span><span class="special">:</span> |
| <span class="identifier">Widget</span><span class="special">()</span> <span class="special">{}</span> |
| |
| <span class="keyword">const</span> <span class="identifier">Resource</span><span class="special">&</span> <span class="identifier">getResource</span><span class="special">()</span> <span class="keyword">const</span> <span class="comment">// not thread-safe</span> |
| <span class="special">{</span> |
| <span class="keyword">if</span> <span class="special">(</span><span class="identifier">resource_</span> <span class="special">==</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span><span class="special">)</span> |
| <span class="identifier">resource_</span><span class="special">.</span><span class="identifier">emplace</span><span class="special">(</span><span class="string">"resource"</span><span class="special">,</span> <span class="string">"arguments"</span><span class="special">);</span> |
| |
| <span class="keyword">return</span> <span class="special">*</span><span class="identifier">resource_</span><span class="special">;</span> |
| <span class="special">}</span> |
| <span class="special">};</span> |
| </pre> |
| <p> |
| <code class="computeroutput"><span class="identifier">optional</span></code>'s default constructor |
| creates an uninitialized optional. No call to <code class="computeroutput"><span class="identifier">Resource</span></code>'s |
| default constructor is attempted. <code class="computeroutput"><span class="identifier">Resource</span></code> |
| doesn't have to be <a href="http://www.sgi.com/tech/stl/DefaultConstructible.html" target="_top"><code class="computeroutput"><span class="identifier">DefaultConstructible</span></code></a>. In function |
| <code class="computeroutput"><span class="identifier">getResource</span></code> we first check |
| if <code class="computeroutput"><span class="identifier">resource_</span></code> is initialized. |
| This time we do not use the contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>, |
| but a comparison with <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">none</span></code>. |
| These two ways are equivalent. Function <code class="computeroutput"><span class="identifier">emplace</span></code> |
| initializes the optional in-place by perfect-forwarding the arguments to |
| the constructor of <code class="computeroutput"><span class="identifier">Resource</span></code>. |
| No copy- or move-construction is involved here. <code class="computeroutput"><span class="identifier">Resource</span></code> |
| doesn't even have to be <code class="computeroutput"><span class="identifier">MoveConstructible</span></code>. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../../../doc/src/images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Function <code class="computeroutput"><span class="identifier">emplace</span></code> is only |
| available on compilers that support rvalue references and variadic templates. |
| If your compiler does not support these features and you still need to |
| avoid any move-constructions, use <a class="link" href="../tutorial/in_place_factories.html" title="In-Place Factories">In-Place |
| Factories</a>. |
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| <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 Andrzej Krzemieński<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| 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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