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| <div class="titlepage"><div><div><h3 class="title"> |
| <a name="boost_optional.tutorial.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional |
| references</a> |
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| <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> |
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| <th align="left">Warning</th> |
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| 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> |
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| <h5> |
| <a name="boost_optional.tutorial.optional_references.h0"></a> |
| <span class="phrase"><a name="boost_optional.tutorial.optional_references.rvalue_references"></a></span><a class="link" href="optional_references.html#boost_optional.tutorial.optional_references.rvalue_references">Rvalue |
| references</a> |
| </h5> |
| <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> |
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| <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014, 2015 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>) |
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