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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="boost_optional.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional references</a>
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29<p>
30 This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
31 to be of reference type: <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span>
32 <span class="keyword">const</span><span class="special">&amp;</span></code>.
33 </p>
34<p>
35 However, since references are not real objects some restrictions apply and
36 some operations are not available in this case:
37 </p>
38<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
39<li class="listitem">
40 Converting constructors
41 </li>
42<li class="listitem">
43 Converting assignment
44 </li>
45<li class="listitem">
46 InPlace construction
47 </li>
48<li class="listitem">
49 InPlace assignment
50 </li>
51<li class="listitem">
52 Value-access via pointer
53 </li>
54</ul></div>
55<p>
56 Also, even though <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>
57 treats it wrapped pseudo-object much as a real value, a true real reference
58 is stored so aliasing will ocurr:
59 </p>
60<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
61<li class="listitem">
62 Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</span></code>
63 will copy the references but all these references will nonetheless refer
64 to the same object.
65 </li>
66<li class="listitem">
67 Value-access will actually provide access to the referenced object rather
68 than the reference itself.
69 </li>
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77 On compilers that do not conform to Standard C++ rules of reference binding,
78 operations on optional references might give adverse results: rather than
79 binding a reference to a designated object they may create an unexpected
80 temporary and bind to it. For more details see <a class="link" href="dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
81 and Portability section</a>.
82 </p></td></tr>
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84<h4>
85<a name="boost_optional.optional_references.h0"></a>
86 <span class="phrase"><a name="boost_optional.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.optional_references.rvalue_references">Rvalue
87 references</a>
88 </h4>
89<p>
90 Rvalue references and lvalue references to const have the ability in C++ to
91 extend the life time of a temporary they bind to. Optional references do not
92 have this capability, therefore to avoid surprising effects it is not possible
93 to initialize an optional references from a temporary. Optional rvalue references
94 are disabled altogether. Also, the initialization and assignment of an optional
95 reference to const from rvalue reference is disabled.
96 </p>
97<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// legal</span>
98<span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;&gt;</span> <span class="identifier">oi</span> <span class="special">=</span> <span class="number">1</span><span class="special">;</span> <span class="comment">// illegal</span>
99</pre>
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103<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014 Andrzej Krzemie&#324;ski<p>
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