1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Rebinding semantics for assignment of optional references</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="../optional_references.html" title="Optional references"> 9<link rel="prev" href="../optional_references.html" title="Optional references"> 10<link rel="next" href="../in_place_factories.html" title="In-Place Factories"> 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="../optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../optional_references.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="../in_place_factories.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.optional_references.rebinding_semantics_for_assignment_of_optional_references"></a><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html" title="Rebinding semantics for assignment of optional references">Rebinding 28 semantics for assignment of optional references</a> 29</h4></div></div></div> 30<p> 31 If you assign to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> the effect is to bind (for the 32 first time) to the object. Clearly, there is no other choice. 33 </p> 34<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> 35<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span> 36<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span> <span class="special">;</span> 37<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">x</span><span class="special">)</span> <span class="special">;</span> 38<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// now 'ora' is bound to 'x' through 'rx'</span> 39<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> <span class="comment">// Changes value of 'x' through 'ora'</span> 40<span class="identifier">assert</span><span class="special">(</span><span class="identifier">x</span><span class="special">==</span><span class="number">2</span><span class="special">);</span> 41</pre> 42<p> 43 If you assign to a bare C++ reference, the assignment is forwarded to the 44 referenced object; its value changes but the reference is never rebound. 45 </p> 46<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> 47<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> 48<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> 49<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> 50<span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">rb</span> <span class="special">;</span> <span class="comment">// Changes the value of 'a' to 'b'</span> 51<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="identifier">b</span><span class="special">);</span> 52<span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> 53<span class="identifier">assert</span><span class="special">(</span><span class="identifier">ra</span><span class="special">!=</span><span class="identifier">b</span><span class="special">);</span> <span class="comment">// 'ra' is not rebound to 'b'</span> 54</pre> 55<p> 56 Now, if you assign to an <span class="emphasis"><em>initialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>, 57 the effect is to <span class="bold"><strong>rebind</strong></span> to the new object 58 instead of assigning the referee. This is unlike bare C++ references. 59 </p> 60<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">a</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> 61<span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> 62<span class="keyword">int</span><span class="special">&</span> <span class="identifier">ra</span> <span class="special">=</span> <span class="identifier">a</span> <span class="special">;</span> 63<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rb</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> 64<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">ora</span><span class="special">(</span><span class="identifier">ra</span><span class="special">)</span> <span class="special">;</span> 65<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">orb</span><span class="special">(</span><span class="identifier">rb</span><span class="special">)</span> <span class="special">;</span> 66<span class="identifier">ora</span> <span class="special">=</span> <span class="identifier">orb</span> <span class="special">;</span> <span class="comment">// 'ora' is rebound to 'b'</span> 67<span class="special">*</span><span class="identifier">ora</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> <span class="comment">// Changes value of 'b' (not 'a')</span> 68<span class="identifier">assert</span><span class="special">(</span><span class="identifier">a</span><span class="special">==</span><span class="number">1</span><span class="special">);</span> 69<span class="identifier">assert</span><span class="special">(</span><span class="identifier">b</span><span class="special">==</span><span class="number">3</span><span class="special">);</span> 70</pre> 71<h6> 72<a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.h0"></a> 73 <span class="phrase"><a name="boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.tutorial.optional_references.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a> 74 </h6> 75<p> 76 Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span> 77 <code class="computeroutput"><span class="identifier">optional</span></code> references has 78 been chosen to provide <span class="bold"><strong>consistency among initialization 79 states</strong></span> even at the expense of lack of consistency with the semantics 80 of bare C++ references. It is true that <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">U</span><span class="special">></span></code> strives to behave as much as possible 81 as <code class="computeroutput"><span class="identifier">U</span></code> does whenever it is 82 initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code> 83 is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, 84 doing so would result in inconsistent behavior w.r.t to the lvalue initialization 85 state. 86 </p> 87<p> 88 Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> 89 forwarding assignment to the referenced object (thus changing the referenced 90 object value but not rebinding), and consider the following code: 91 </p> 92<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">get</span><span class="special">();</span> 93<span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> 94<span class="keyword">int</span><span class="special">&</span> <span class="identifier">rx</span> <span class="special">=</span> <span class="identifier">x</span> <span class="special">;</span> 95<span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">&></span> <span class="identifier">b</span><span class="special">(</span><span class="identifier">rx</span><span class="special">);</span> 96<span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> 97</pre> 98<p> 99 What does the assignment do? 100 </p> 101<p> 102 If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>, 103 the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code> 104 (we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>). 105 But what if <code class="computeroutput"><span class="identifier">a</span></code> is already 106 <span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced 107 object (whatever that is); which is inconsistent with the other possible 108 case. 109 </p> 110<p> 111 If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> 112 would assign just like <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code> does, you would never be able to use 113 Optional's assignment without explicitly handling the previous initialization 114 state unless your code is capable of functioning whether after the assignment, 115 <code class="computeroutput"><span class="identifier">a</span></code> aliases the same object 116 as <code class="computeroutput"><span class="identifier">b</span></code> or not. 117 </p> 118<p> 119 That is, you would have to discriminate in order to be consistent. 120 </p> 121<p> 122 If in your code rebinding to another object is not an option, then it is 123 very likely that binding for the first time isn't either. In such case, 124 assignment to an <span class="emphasis"><em>uninitialized </em></span> <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> shall be prohibited. It is quite 125 possible that in such a scenario it is a precondition that the lvalue must 126 be already initialized. If it isn't, then binding for the first time is 127 OK while rebinding is not which is IMO very unlikely. In such a scenario, 128 you can assign the value itself directly, as in: 129 </p> 130<pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span> 131<span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span> 132</pre> 133</div> 134<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 135<td align="left"></td> 136<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<p> 137 Distributed under the Boost Software License, Version 1.0. (See accompanying 138 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>) 139 </p> 140</div></td> 141</tr></table> 142<hr> 143<div class="spirit-nav"> 144<a accesskey="p" href="../optional_references.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../optional_references.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="../in_place_factories.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 145</div> 146</body> 147</html> 148
