ceph/src/boost/libs/optional/doc/html/boost_optional/tutorial/design_overview/the_semantics.html

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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_optional.tutorial.design_overview.the_semantics"></a><a class="link" href="the_semantics.html" title="The semantics">The
  28         semantics</a>
  29 </h4></div></div></div>
  30 <p>
  31           Objects of type <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> are intended to be used in places where
  32           objects of type <code class="computeroutput"><span class="identifier">T</span></code> would
  33           but which might be uninitialized. Hence, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>'s purpose is to formalize the additional
  34           possibly uninitialized state. From the perspective of this role, <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  35           can have the same operational semantics of <code class="computeroutput"><span class="identifier">T</span></code>
  36           plus the additional semantics corresponding to this special state. As such,
  37           <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code>
  38           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++,
  39           so we need to compose the desired semantics using a different mechanism.
  40           Doing it the other way around, that is, making <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> a <span class="emphasis"><em>subtype</em></span> of
  41           <code class="computeroutput"><span class="identifier">T</span></code> is not only conceptually
  42           wrong but also impractical: it is not allowed to derive from a non-class
  43           type, such as a built-in type.
  44         </p>
  45 <p>
  46           We can draw from the purpose of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&gt;</span></code> the required basic semantics:
  47         </p>
  48 <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
  49 <li class="listitem">
  50               <span class="bold"><strong>Default Construction:</strong></span> To introduce
  51               a formally uninitialized wrapped object.
  52             </li>
  53 <li class="listitem">
  54               <span class="bold"><strong>Direct Value Construction via copy:</strong></span>
  55               To introduce a formally initialized wrapped object whose value is obtained
  56               as a copy of some object.
  57             </li>
  58 <li class="listitem">
  59               <span class="bold"><strong>Deep Copy Construction:</strong></span> To obtain
  60               a new yet equivalent wrapped object.
  61             </li>
  62 <li class="listitem">
  63               <span class="bold"><strong>Direct Value Assignment (upon initialized):</strong></span>
  64               To assign a value to the wrapped object.
  65             </li>
  66 <li class="listitem">
  67               <span class="bold"><strong>Direct Value Assignment (upon uninitialized):</strong></span>
  68               To initialize the wrapped object with a value obtained as a copy of
  69               some object.
  70             </li>
  71 <li class="listitem">
  72               <span class="bold"><strong>Assignment (upon initialized):</strong></span> To
  73               assign to the wrapped object the value of another wrapped object.
  74             </li>
  75 <li class="listitem">
  76               <span class="bold"><strong>Assignment (upon uninitialized):</strong></span> To
  77               initialize the wrapped object with value of another wrapped object.
  78             </li>
  79 <li class="listitem">
  80               <span class="bold"><strong>Deep Relational Operations (when supported by
  81               the type T):</strong></span> To compare wrapped object values taking into
  82               account the presence of uninitialized states.
  83             </li>
  84 <li class="listitem">
  85               <span class="bold"><strong>Value access:</strong></span> To unwrap the wrapped
  86               object.
  87             </li>
  88 <li class="listitem">
  89               <span class="bold"><strong>Initialization state query:</strong></span> To determine
  90               if the object is formally initialized or not.
  91             </li>
  92 <li class="listitem">
  93               <span class="bold"><strong>Swap:</strong></span> To exchange wrapped objects.
  94               (with whatever exception safety guarantees are provided by <code class="computeroutput"><span class="identifier">T</span></code>'s swap).
  95             </li>
  96 <li class="listitem">
  97               <span class="bold"><strong>De-initialization:</strong></span> To release the
  98               wrapped object (if any) and leave the wrapper in the uninitialized
  99               state.
 100             </li>
 101 </ul></div>
 102 <p>
 103           Additional operations are useful, such as converting constructors and converting
 104           assignments, in-place construction and assignment, and safe value access
 105           via a pointer to the wrapped object or null.
 106         </p>
 107 </div>
 108 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
 109 <td align="left"></td>
 110 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<p>
 111         Distributed under the Boost Software License, Version 1.0. (See accompanying
 112         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>)
 113       </p>
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