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docs: described optional reference binding issues
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Andrzej Krzemienski
@ -32,7 +32,7 @@
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be used with special caution and consideration.
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</p>
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<p>
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First, it is functionally similar to a tristate boolean (false,maybe,true)
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First, it is functionally similar to a tristate boolean (false, maybe, true)
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—such as <a href="../../../../../doc/html/tribool.html" target="_top">boost::tribool</a>—
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except that in a tristate boolean, the maybe state <span class="underline">represents
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a valid value</span>, unlike the corresponding state of an uninitialized
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@ -42,10 +42,11 @@
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needed.
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</p>
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<p>
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Second, <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
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provides an implicit conversion to <code class="computeroutput"><span class="keyword">bool</span></code>.
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This conversion refers to the initialization state and not to the contained
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value. Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
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Second, although <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>
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provides a contextual conversion to <code class="computeroutput"><span class="keyword">bool</span></code>
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in C++11, this falls back to an implicit conversion on older compilers. This
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conversion refers to the initialization state and not to the contained value.
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Using <code class="computeroutput"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span></code> can
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lead to subtle errors due to the implicit <code class="computeroutput"><span class="keyword">bool</span></code>
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conversion:
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</p>
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@ -67,6 +68,29 @@
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takes an <code class="computeroutput"><span class="keyword">int</span></code> instead, it won't
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compile).
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</p>
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<p>
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Third, mixed comparisons with <code class="computeroutput"><span class="keyword">bool</span></code>
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work differently than similar mixed comparisons between pointers and <code class="computeroutput"><span class="keyword">bool</span></code>, so the results might surprise you:
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</p>
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<pre class="programlisting"><span class="identifier">optional</span><span class="special"><</span><span class="keyword">bool</span><span class="special">></span> <span class="identifier">oEmpty</span><span class="special">(</span><span class="identifier">none</span><span class="special">),</span> <span class="identifier">oTrue</span><span class="special">(</span><span class="keyword">true</span><span class="special">),</span> <span class="identifier">oFalse</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders true</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false!</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oEmpty</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false!</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders true!</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oFalse</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders false</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="identifier">none</span><span class="special">);</span> <span class="comment">// renders false</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">);</span> <span class="comment">// renders false</span>
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<span class="keyword">if</span> <span class="special">(</span><span class="identifier">oTrue</span> <span class="special">==</span> <span class="keyword">true</span><span class="special">);</span> <span class="comment">// renders true</span>
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</pre>
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<p>
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In other words, for <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code>, the following assertion does not hold:
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</p>
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<pre class="programlisting"><span class="identifier">assert</span><span class="special">((</span><span class="identifier">opt</span> <span class="special">==</span> <span class="keyword">false</span><span class="special">)</span> <span class="special">==</span> <span class="special">(!</span><span class="identifier">opt</span><span class="special">));</span>
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</pre>
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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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@ -27,10 +27,74 @@
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<a name="boost_optional.dependencies_and_portability"></a><a class="link" href="dependencies_and_portability.html" title="Dependencies and Portability">Dependencies
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and Portability</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc"><dt><span class="section"><a href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.optional_reference_binding">Optional
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Reference Binding</a></span></dt></dl></div>
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<p>
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The implementation uses <code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">alignment_of</span><span class="special">.</span><span class="identifier">hpp</span></code> and
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<code class="computeroutput"><span class="identifier">type_traits</span><span class="special">/</span><span class="identifier">type_with_alignment</span><span class="special">.</span><span class="identifier">hpp</span></code>
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</p>
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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.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="dependencies_and_portability.html#boost_optional.dependencies_and_portability.optional_reference_binding" title="Optional Reference Binding">Optional
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Reference Binding</a>
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</h3></div></div></div>
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<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. Compilers known to have these deficiencies include
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GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0,
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11.0, 12.0. On these compilers prefer using direct-initialization and copy
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assignment of optional references to copy-initialization and assignment from
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<code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
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</p>
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<pre class="programlisting"><span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</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">or1</span><span class="special">;</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">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</span>
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<span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// not portable</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">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
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<span class="identifier">or1</span> <span class="special">=</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">i</span><span class="special">);</span> <span class="comment">// portable</span>
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</pre>
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<p>
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In order to check if your compiler correctly implements reference binding
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use this test program.
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</p>
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<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
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<span class="keyword">const</span> <span class="keyword">int</span> <span class="identifier">global_i</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</span>
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<span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
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<span class="special">{</span>
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<span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">ii</span><span class="special">)</span>
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<span class="special">{</span>
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<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&&)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
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<span class="special">{</span>
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<span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
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<span class="special">}</span>
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<span class="special">};</span>
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<span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
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<span class="special">{</span>
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<span class="keyword">const</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
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<span class="identifier">assert</span><span class="special">(&</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&</span><span class="identifier">global_i</span><span class="special">);</span>
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<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
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<span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
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<span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
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<span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
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<span class="special">}</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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@ -1446,9 +1446,9 @@
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<span class="keyword">const</span> <span class="special">;</span></code>
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</p></blockquote></div>
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<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
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<span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">operator</span>
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</code><span class="emphasis"><em>unspecified-bool-type</em></span><code class="computeroutput"><span class="special">()</span>
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<span class="special">;</span></code>
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<span class="bold"><strong>Deprecated:</strong></span> Same as <code class="computeroutput"><span class="keyword">explicit</span>
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<span class="keyword">operator</span> <span class="keyword">bool</span>
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<span class="special">()</span> <span class="special">;</span></code>
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</li></ul></div>
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<p>
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<span class="inlinemediaobject"><img src="../images/space.png" alt="space"></span>
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@ -76,11 +76,9 @@
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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 a temporary and
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bind to it. Compilers known to have these deficiencies include GCC versions
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4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0.
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On these compilers prefer using direct-initialization and copy assignment
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of optional references to copy-initialization and assignment from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>.
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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.html#boost_optional.dependencies_and_portability.optional_reference_binding" 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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<h4>
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