ceph/src/boost/libs/optional/doc/html/boost_optional/tutorial/optional_references.html

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  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">&amp;</span></code>, and to some extent, <code class="computeroutput"><span class="identifier">T</span> <span class="keyword">const</span><span class="special">&amp;</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">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</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">&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>
 100 <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>
 101 </pre>
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 106 <td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2016 Andrzej Krzemie&#324;ski<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>)
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