1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>The semantics</title> 5<link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../index.html" title="Boost.Optional"> 8<link rel="up" href="../design_overview.html" title="Design Overview"> 9<link rel="prev" href="../design_overview.html" title="Design Overview"> 10<link rel="next" href="the_interface.html" title="The Interface"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../index.html">Home</a></td> 16<td align="center"><a href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<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> 24</div> 25<div class="section"> 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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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"><</span><span class="identifier">T</span><span class="special">></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 © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<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> 114</div></td> 115</tr></table> 116<hr> 117<div class="spirit-nav"> 118<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> 119</div> 120</body> 121</html> 122
