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- <div class="titlepage"><div><div><h4 class="title">
- <a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
- semantics</a>
- </h4></div></div></div>
- <p>
- Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> are intended to be used in places where
- objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
- but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>'s purpose is to formalize the additional
- possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
- can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
- plus the additional semantics corresponding to this special state. As such,
- <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
- could be thought of as a <span class="emphasis"><em>supertype</em></span> of <code class="computeroutput"><span class="identifier">T</span></code>. Of course, we can't do that in C++,
- so we need to compose the desired semantics using a different mechanism.
- Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> a <span class="emphasis"><em>subtype</em></span> of
- <code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
- wrong but also impractical: it is not allowed to derive from a non-class
- type, such as a built-in type.
- </p>
- <p>
- We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code> the required basic semantics:
- </p>
- <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
- <li class="listitem">
- <span class="bold"><strong>Default Construction:</strong></span> To introduce
- a formally uninitialized wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Direct Value Construction via copy:</strong></span>
- To introduce a formally initialized wrapped object whose value is obtained
- as a copy of some object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
- a new yet equivalent wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
- To assign a value to the wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
- To initialize the wrapped object with a value obtained as a copy of
- some object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Assignment (upon initialized):</strong></span> To
- assign to the wrapped object the value of another wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
- initialize the wrapped object with value of another wrapped object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Deep Relational Operations (when supported by
- the type T):</strong></span> To compare wrapped object values taking into
- account the presence of uninitialized states.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
- object.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Initialization state query:</strong></span> To determine
- if the object is formally initialized or not.
- </li>
- <li class="listitem">
- <span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
- (with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
- </li>
- <li class="listitem">
- <span class="bold"><strong>De-initialization:</strong></span> To release the
- wrapped object (if any) and leave the wrapper in the uninitialized
- state.
- </li>
- </ul></div>
- <p>
- Additional operations are useful, such as converting constructors and converting
- assignments, in-place construction and assignment, and safe value access
- via a pointer to the wrapped object or null.
- </p>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 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>)
- </p>
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