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- <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>
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