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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>
</h3></div></div></div>
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<div class="titlepage"><div><div><h4 class="title">
<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>
</h4></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">&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>.
        </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">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</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">&lt;</span><span class="identifier">T</span><span class="special">&amp;&gt;</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>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
<th align="left">Caution</th>
</tr>
<tr><td align="left" valign="top"><p>
            On compilers that do not conform to Standard C++ rules of reference binding,
            some operations on optional references are disabled in order to prevent
            subtle bugs. 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>
<h6>
<a name="boost_optional.tutorial.optional_references.overview.h0"></a>
          <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
          references</a>
        </h6>
<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">&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>
<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>
</pre>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright &#169; 2014-2018 Andrzej Krzemie&#324;ski<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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