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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>
  28 </h2></div></div></div>
  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>
  70 </ul></div>
  71 <div class="warning"><table border="0" summary="Warning">
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  73 <td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../doc/src/images/warning.png"></td>
  74 <th align="left">Warning</th>
  75 </tr>
  76 <tr><td align="left" valign="top"><p>
  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>
  83 </table></div>
  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>
 104         Distributed under the Boost Software License, Version 1.0. (See accompanying
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