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<title>The semantics</title>
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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>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 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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