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25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_optional.tutorial.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 | </h3></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 | |
33 | is 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 | |
45 | referenced 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 | <h5> | |
73 | <a name="boost_optional.tutorial.rebinding_semantics_for_assignment_of_optional_references.h0"></a> | |
74 | <span class="phrase"><a name="boost_optional.tutorial.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.rebinding_semantics_for_assignment_of_optional_references.rationale">Rationale</a> | |
75 | </h5> | |
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 | |
80 | states</strong></span> even at the expense of lack of consistency with the semantics | |
81 | 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> | |
82 | strives 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 | |
107 | <span class="emphasis"><em>initialized</em></span>? it would change the value of the referenced | |
108 | object (whatever that is); which is inconsistent with the other possible | |
109 | case. | |
110 | </p> | |
111 | <p> | |
112 | If <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> | |
113 | 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 | |
114 | Optional's assignment without explicitly handling the previous initialization | |
115 | state unless your code is capable of functioning whether after the assignment, | |
116 | <code class="computeroutput"><span class="identifier">a</span></code> aliases the same object | |
117 | as <code class="computeroutput"><span class="identifier">b</span></code> 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 | |
124 | very 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 | |
127 | precondition that the lvalue must be already initialized. If it isn't, then | |
128 | binding for the first time is OK while rebinding is not which is IMO very | |
129 | unlikely. In such a scenario, you can assign the value itself directly, as | |
130 | in: | |
131 | </p> | |
132 | <pre class="programlisting"><span class="identifier">assert</span><span class="special">(!!</span><span class="identifier">opt</span><span class="special">);</span> | |
133 | <span class="special">*</span><span class="identifier">opt</span><span class="special">=</span><span class="identifier">value</span><span class="special">;</span> | |
134 | </pre> | |
135 | </div> | |
136 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
137 | <td align="left"></td> | |
138 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
139 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
140 | 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>) | |
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