forked from boostorg/optional
Fix: prevented the binding illegal temporary to optional<const int&>
Older MSVC versions add illegal temporary when you want to assign from const integral value.
This commit is contained in:
Andrzej Krzemienski
@ -7,7 +7,7 @@
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<a name="boost_optional.tutorial.optional_references"></a><a class="link" href="optional_references.html" title="Optional references">Optional
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references</a>
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</h3></div></div></div>
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<div class="section">
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<div class="titlepage"><div><div><h4 class="title">
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<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>
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</h4></div></div></div>
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<p>
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This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
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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>
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<span class="keyword">const</span><span class="special">&</span></code>.
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</p>
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This library allows the template parameter <code class="computeroutput"><span class="identifier">T</span></code>
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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>.
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</p>
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<p>
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However, since references are not real objects some restrictions apply and
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some operations are not available in this case:
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</p>
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However, since references are not real objects some restrictions apply
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and some operations are not available in this case:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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Converting constructors
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</li>
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Converting constructors
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</li>
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<li class="listitem">
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Converting assignment
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</li>
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Converting assignment
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</li>
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<li class="listitem">
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InPlace construction
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</li>
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InPlace construction
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</li>
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<li class="listitem">
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InPlace assignment
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</li>
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InPlace assignment
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</li>
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<li class="listitem">
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Value-access via pointer
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</li>
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Value-access via pointer
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</li>
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</ul></div>
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<p>
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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>
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treats it wrapped pseudo-object much as a real value, a true real reference
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is stored so aliasing will ocurr:
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</p>
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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
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much as a real value, a true real reference is stored so aliasing will
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ocurr:
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</p>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
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<li class="listitem">
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Copies of <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="identifier">T</span><span class="special">&></span></code>
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will copy the references but all these references will nonetheless refer
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to the same object.
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</li>
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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
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all these references will nonetheless refer to the same object.
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</li>
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<li class="listitem">
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Value-access will actually provide access to the referenced object rather
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than the reference itself.
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</li>
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Value-access will actually provide access to the referenced object
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rather than the reference itself.
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</li>
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</ul></div>
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<div class="warning"><table border="0" summary="Warning">
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<div class="caution"><table border="0" summary="Caution">
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<tr>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../../../../../doc/src/images/warning.png"></td>
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<th align="left">Warning</th>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Caution]" src="../../../../../../doc/src/images/caution.png"></td>
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<th align="left">Caution</th>
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</tr>
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<tr><td align="left" valign="top"><p>
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On compilers that do not conform to Standard C++ rules of reference binding,
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operations on optional references might give adverse results: rather than
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binding a reference to a designated object they may create an unexpected
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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
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and Portability section</a>.
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</p></td></tr>
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On compilers that do not conform to Standard C++ rules of reference binding,
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some operations on optional references are disabled in order to prevent
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subtle bugs. For more details see <a class="link" href="../dependencies_and_portability/optional_reference_binding.html" title="Optional Reference Binding">Dependencies
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and Portability section</a>.
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</p></td></tr>
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</table></div>
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<h5>
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<a name="boost_optional.tutorial.optional_references.h0"></a>
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<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
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references</a>
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</h5>
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<h6>
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<a name="boost_optional.tutorial.optional_references.overview.h0"></a>
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<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
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references</a>
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</h6>
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<p>
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Rvalue references and lvalue references to const have the ability in C++
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to extend the life time of a temporary they bind to. Optional references
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do not have this capability, therefore to avoid surprising effects it is
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not possible to initialize an optional references from a temporary. Optional
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rvalue references are disabled altogether. Also, the initialization and assignment
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of an optional reference to const from rvalue reference is disabled.
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</p>
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Rvalue references and lvalue references to const have the ability in C++
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to extend the life time of a temporary they bind to. Optional references
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do not have this capability, therefore to avoid surprising effects it is
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not possible to initialize an optional references from a temporary. Optional
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rvalue references are disabled altogether. Also, the initialization and
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assignment of an optional reference to const from rvalue reference is disabled.
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</p>
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<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>
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<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>
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</pre>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p>
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@ -109,7 +111,7 @@
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</tr></table>
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<hr>
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<div class="spirit-nav">
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