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+<html>
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
+<title>Optional references</title>
+<link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css">
+<meta name="generator" content="DocBook XSL Stylesheets V1.78.1">
+<link rel="home" href="../index.html" title="Chapter 1. Boost.Optional">
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+</div>
+<div class="section">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="boost_optional.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional references</a>
+</h2></div></div></div>
+<p>
+ This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
+ 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>.
+ </p>
+<p>
+ However, since references are not real objects some restrictions apply and
+ some operations are not available in this case:
+ </p>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ Converting constructors
+ </li>
+<li class="listitem">
+ Converting assignment
+ </li>
+<li class="listitem">
+ InPlace construction
+ </li>
+<li class="listitem">
+ InPlace assignment
+ </li>
+<li class="listitem">
+ Value-access via pointer
+ </li>
+</ul></div>
+<p>
+ 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 much as a real value, a true real reference
+ is stored so aliasing will ocurr:
+ </p>
+<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
+<li class="listitem">
+ 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 all these references will nonetheless refer
+ to the same object.
+ </li>
+<li class="listitem">
+ Value-access will actually provide access to the referenced object rather
+ than the reference itself.
+ </li>
+</ul></div>
+<div class="warning"><table border="0" summary="Warning">
+<tr>
+<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
+<th align="left">Warning</th>
+</tr>
+<tr><td align="left" valign="top"><p>
+ On compilers that do not conform to Standard C++ rules of reference binding,
+ operations on optional references might give adverse results: rather than
+ binding a reference to a designated object they may create an unexpected
+ 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
+ and Portability section</a>.
+ </p></td></tr>
+</table></div>
+<h4>
+<a name="boost_optional.optional_references.h0"></a>
+ <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
+ references</a>
+ </h4>
+<p>
+ Rvalue references and lvalue references to const have the ability in C++ to
+ extend the life time of a temporary they bind to. Optional references do not
+ have this capability, therefore to avoid surprising effects it is not possible
+ to initialize an optional references from a temporary. Optional rvalue references
+ are disabled altogether. Also, the initialization and assignment of an optional
+ reference to const from rvalue reference is disabled.
+ </p>
+<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>
+<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>
+</pre>
+</div>
+<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
+<td align="left"></td>
+<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014 Andrzej Krzemieński<p>
+ Distributed under the Boost Software License, Version 1.0. (See accompanying
+ 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>)
+ </p>
+</div></td>
+</tr></table>
+<hr>
+<div class="spirit-nav">
+<a accesskey="p" href="detailed_semantics.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.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="rebinding_semantics_for_assignment_of_optional_references.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a>
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