| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>The semantics</title> |
| <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.78.1"> |
| <link rel="home" href="../../../index.html" title="Boost.Optional"> |
| <link rel="up" href="../design_overview.html" title="Design Overview"> |
| <link rel="prev" href="../design_overview.html" title="Design Overview"> |
| <link rel="next" href="the_interface.html" title="The Interface"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section"> |
| <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, 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>) |
| </p> |
| </div></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="../design_overview.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../design_overview.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="the_interface.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
