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| 4 | <title>Rebinding semantics for assignment of optional references</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h2 class="title" style="clear: both"> | |
| 27 | <a name="boost_optional.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 | </h2></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> | |
| 32 | the effect is to bind (for the first time) to the object. Clearly, there is | |
| 33 | no other choice. | |
| 34 | </p> | |
| 35 | <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> | |
| 36 | <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> | |
| 37 | <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> | |
| 38 | <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> | |
| 39 | <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> | |
| 40 | <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> | |
| 41 | <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> | |
| 42 | </pre> | |
| 43 | <p> | |
| 44 | If you assign to a bare C++ reference, the assignment is forwarded to the referenced | |
| 45 | object; its value changes but the reference is never rebound. | |
| 46 | </p> | |
| 47 | <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> | |
| 48 | <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> | |
| 49 | <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> | |
| 50 | <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> | |
| 51 | <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> | |
| 52 | <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> | |
| 53 | <span class="identifier">b</span> <span class="special">=</span> <span class="number">3</span> <span class="special">;</span> | |
| 54 | <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> | |
| 55 | </pre> | |
| 56 | <p> | |
| 57 | 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>, | |
| 58 | the effect is to <span class="bold"><strong>rebind</strong></span> to the new object | |
| 59 | instead of assigning the referee. This is unlike bare C++ references. | |
| 60 | </p> | |
| 61 | <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> | |
| 62 | <span class="keyword">int</span> <span class="identifier">b</span> <span class="special">=</span> <span class="number">2</span> <span class="special">;</span> | |
| 63 | <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> | |
| 64 | <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> | |
| 65 | <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> | |
| 66 | <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> | |
| 67 | <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> | |
| 68 | <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> | |
| 69 | <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> | |
| 70 | <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> | |
| 71 | </pre> | |
| 72 | <h4> | |
| 73 | <a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.h0"></a> | |
| 74 | <span class="phrase"><a name="boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale"></a></span><a class="link" href="rebinding_semantics_for_assignment_of_optional_references.html#boost_optional.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a> | |
| 75 | </h4> | |
| 76 | <p> | |
| 77 | Rebinding semantics for the assignment of <span class="emphasis"><em>initialized </em></span> | |
| 78 | <code class="computeroutput"><span class="identifier">optional</span></code> references has been | |
| 79 | chosen to provide <span class="bold"><strong>consistency among initialization states</strong></span> | |
| 80 | even at the expense of lack of consistency with the semantics of bare C++ references. | |
| 81 | 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 | |
| 82 | to behave as much as possible as <code class="computeroutput"><span class="identifier">U</span></code> | |
| 83 | does whenever it is initialized; but in the case when <code class="computeroutput"><span class="identifier">U</span></code> | |
| 84 | is <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, | |
| 85 | doing so would result in inconsistent behavior w.r.t to the lvalue initialization | |
| 86 | state. | |
| 87 | </p> | |
| 88 | <p> | |
| 89 | Imagine <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> | |
| 90 | forwarding assignment to the referenced object (thus changing the referenced | |
| 91 | object value but not rebinding), and consider the following code: | |
| 92 | </p> | |
| 93 | <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> | |
| 94 | <span class="keyword">int</span> <span class="identifier">x</span> <span class="special">=</span> <span class="number">1</span> <span class="special">;</span> | |
| 95 | <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> | |
| 96 | <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> | |
| 97 | <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">b</span> <span class="special">;</span> | |
| 98 | </pre> | |
| 99 | <p> | |
| 100 | What does the assignment do? | |
| 101 | </p> | |
| 102 | <p> | |
| 103 | If <code class="computeroutput"><span class="identifier">a</span></code> is <span class="emphasis"><em>uninitialized</em></span>, | |
| 104 | the answer is clear: it binds to <code class="computeroutput"><span class="identifier">x</span></code> | |
| 105 | (we now have another reference to <code class="computeroutput"><span class="identifier">x</span></code>). | |
| 106 | But what if <code class="computeroutput"><span class="identifier">a</span></code> is already <span class="emphasis"><em>initialized</em></span>? | |
| 107 | it would change the value of the referenced object (whatever that is); which | |
| 108 | is inconsistent with the other possible 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> | |
| 113 | does, you would never be able to use Optional's assignment without explicitly | |
| 114 | handling the previous initialization state unless your code is capable of functioning | |
| 115 | whether after the assignment, <code class="computeroutput"><span class="identifier">a</span></code> | |
| 116 | aliases the same object as <code class="computeroutput"><span class="identifier">b</span></code> | |
| 117 | or not. | |
| 118 | </p> | |
| 119 | <p> | |
| 120 | That is, you would have to discriminate in order to be consistent. | |
| 121 | </p> | |
| 122 | <p> | |
| 123 | If in your code rebinding to another object is not an option, then it is very | |
| 124 | likely that binding for the first time isn't either. In such case, assignment | |
| 125 | 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> | |
| 126 | shall be prohibited. It is quite possible that in such a scenario it is a precondition | |
| 127 | that the lvalue must be already initialized. If it isn't, then binding for | |
| 128 | the first time is OK while rebinding is not which is IMO very unlikely. In | |
| 129 | such a scenario, you can assign the value itself directly, as in: | |
| 130 | </p> | |
| 131 | <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span> | |
| 132 | <span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span> | |
| 133 | </pre> | |
| 134 | </div> | |
| 135 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 136 | <td align="left"></td> | |
| 137 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p> | |
| 138 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 139 | 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>) | |
| 140 | </p> | |
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