ceph/src/boost/libs/optional/doc/html/boost_optional/reference/dependencies_and_portability/optional_reference_binding.html

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  26 <div class="titlepage"><div><div><h4 class="title">
  27 <a name="boost_optional.reference.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="optional_reference_binding.html" title="Optional Reference Binding">Optional
  28         Reference Binding</a>
  29 </h4></div></div></div>
  30 <p>
  31           On compilers that do not conform to Standard C++ rules of reference binding,
  32           operations on optional references might give adverse results: rather than
  33           binding a reference to a designated object they may create an unexpected
  34           temporary and bind to it. Compilers known to have these deficiencies include
  35           GCC versions 4.2, 4.3, 4.4, 4.5; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0,
  36           11.0, 12.0. On these compilers prefer using direct-initialization and copy
  37           assignment of optional references to copy-initialization and assignment
  38           from <code class="computeroutput"><span class="identifier">T</span><span class="special">&amp;</span></code>:
  39         </p>
  40 <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>
  41 <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">or1</span><span class="special">;</span>
  42 <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">or2</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>  <span class="comment">// not portable</span>
  43 <span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span>                       <span class="comment">// not portable</span>
  44
  45 <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">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span>   <span class="comment">// portable</span>
  46 <span class="identifier">or1</span> <span class="special">=</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">i</span><span class="special">);</span> <span class="comment">// portable</span>
  47 </pre>
  48 <p>
  49           In order to check if your compiler correctly implements reference binding
  50           use this test program.
  51         </p>
  52 <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cassert</span><span class="special">&gt;</span>
  53
  54 <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>
  55
  56 <span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
  57 <span class="special">{</span>
  58   <span class="identifier">TestingReferenceBinding</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">ii</span><span class="special">)</span>
  59   <span class="special">{</span>
  60     <span class="identifier">assert</span><span class="special">(&amp;</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">global_i</span><span class="special">);</span>
  61   <span class="special">}</span>
  62
  63   <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">&amp;</span> <span class="identifier">ii</span><span class="special">)</span>
  64   <span class="special">{</span>
  65     <span class="identifier">assert</span><span class="special">(&amp;</span><span class="identifier">ii</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">global_i</span><span class="special">);</span>
  66   <span class="special">}</span>
  67
  68   <span class="keyword">void</span> <span class="keyword">operator</span><span class="special">=(</span><span class="keyword">int</span><span class="special">&amp;&amp;)</span> <span class="comment">// remove this if your compiler doesn't have rvalue refs</span>
  69   <span class="special">{</span>
  70     <span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
  71   <span class="special">}</span>
  72 <span class="special">};</span>
  73
  74 <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
  75 <span class="special">{</span>
  76   <span class="keyword">const</span> <span class="keyword">int</span><span class="special">&amp;</span> <span class="identifier">iref</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
  77   <span class="identifier">assert</span><span class="special">(&amp;</span><span class="identifier">iref</span> <span class="special">==</span> <span class="special">&amp;</span><span class="identifier">global_i</span><span class="special">);</span>
  78
  79   <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
  80   <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
  81
  82   <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
  83   <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
  84 <span class="special">}</span>
  85 </pre>
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  89 <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>
  90         Distributed under the Boost Software License, Version 1.0. (See accompanying
  91         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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