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4 | <title>Optional references</title> | |
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24 | </div> | |
25 | <div class="section"> | |
26 | <div class="titlepage"><div><div><h3 class="title"> | |
27 | <a name="boost_optional.tutorial.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional | |
28 | references</a> | |
29 | </h3></div></div></div> | |
30 | <div class="section"> | |
31 | <div class="titlepage"><div><div><h4 class="title"> | |
32 | <a name="boost_optional.tutorial.optional_references.overview"></a><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview" title="Overview">Overview</a> | |
33 | </h4></div></div></div> | |
34 | <p> | |
35 | This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code> | |
36 | to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&</span></code>. | |
37 | </p> | |
38 | <p> | |
39 | However, since references are not real objects some restrictions apply | |
40 | and some operations are not available in this case: | |
41 | </p> | |
42 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
43 | <li class="listitem"> | |
44 | Converting constructors | |
45 | </li> | |
46 | <li class="listitem"> | |
47 | Converting assignment | |
48 | </li> | |
49 | <li class="listitem"> | |
50 | InPlace construction | |
51 | </li> | |
52 | <li class="listitem"> | |
53 | InPlace assignment | |
54 | </li> | |
55 | <li class="listitem"> | |
56 | Value-access via pointer | |
57 | </li> | |
58 | </ul></div> | |
59 | <p> | |
60 | Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> treats it wrapped pseudo-object | |
61 | much as a real value, a true real reference is stored so aliasing will | |
62 | ocurr: | |
63 | </p> | |
64 | <div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "> | |
65 | <li class="listitem"> | |
66 | Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code> will copy the references but | |
67 | all these references will nonetheless refer to the same object. | |
68 | </li> | |
69 | <li class="listitem"> | |
70 | Value-access will actually provide access to the referenced object | |
71 | rather than the reference itself. | |
72 | </li> | |
73 | </ul></div> | |
74 | <div class="caution"><table border="0" summary="Caution"> | |
75 | <tr> | |
76 | <td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td> | |
77 | <th align="left">Caution</th> | |
78 | </tr> | |
79 | <tr><td align="left" valign="top"><p> | |
80 | On compilers that do not conform to Standard C++ rules of reference binding, | |
81 | some operations on optional references are disabled in order to prevent | |
82 | subtle bugs. For more details see <a class="link" href="../dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies | |
83 | and Portability section</a>. | |
84 | </p></td></tr> | |
85 | </table></div> | |
86 | <h6> | |
87 | <a name="boost_optional.tutorial.optional_references.overview.h0"></a> | |
88 | <span class="phrase"><a name="boost_optional.tutorial.optional_references.overview.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.overview.rvalue_references">Rvalue | |
89 | references</a> | |
90 | </h6> | |
91 | <p> | |
92 | Rvalue references and lvalue references to const have the ability in C++ | |
93 | to extend the life time of a temporary they bind to. Optional references | |
94 | do not have this capability, therefore to avoid surprising effects it is | |
95 | not possible to initialize an optional references from a temporary. Optional | |
96 | rvalue references are disabled altogether. Also, the initialization and | |
97 | assignment of an optional reference to const from rvalue reference is disabled. | |
98 | </p> | |
99 | <pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span> | |
100 | <span class="identifier">optional</span><span class="special"><</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&></span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span> | |
101 | </pre> | |
102 | </div> | |
103 | </div> | |
104 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
105 | <td align="left"></td> | |
106 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
107 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
108 | 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>) | |
109 | </p> | |
110 | </div></td> | |
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