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- <div class="titlepage"><div><div><h3 class="title">
- <a name="boost_optional.dependencies_and_portability.optional_reference_binding"></a><a class="link" href="optional_reference_binding.html" title="Optional Reference Binding">Optional
- Reference Binding</a>
- </h3></div></div></div>
- <p>
- <a name="optional_reference_binding"></a>A number of compilers incorrectly
- treat const lvalues of integral type as rvalues, and create an illegal temporary
- when binding to an lvalue reference to const in some expressions. This could
- result in creating an optional lvalue reference that is in fact bound to
- an unexpected temporary rather than to the intended object. In order to prevent
- hard to find run-time bugs, this library performs compile-time checks to
- prevent expressions that would otherwise bind an optional reference to an
- unexpected temporary. As a consequence, on certain compilers certain pieces
- of functionality in optional references are missing. In order to maintain
- a portability of your code across diferent compilers, it is recommended that
- you only stick to the minimum portable interface of optional references:
- prefer direct-initialization and copy assignment of optional references to
- copy-initialization and assignment from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>:
- </p>
- <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>
- <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>
- <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">// caution: not portable</span>
- <span class="identifier">or1</span> <span class="special">=</span> <span class="identifier">i</span><span class="special">;</span> <span class="comment">// caution: not portable</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">or3</span><span class="special">(</span><span class="identifier">i</span><span class="special">);</span> <span class="comment">// portable</span>
- <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>
- </pre>
- <p>
- Compilers known to have these deficiencies include GCC versions 4.2, 4.3,
- 4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0.
- In order to check if your compiler correctly implements reference binding
- use this test program.
- </p>
- <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span>
- <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>
- <span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span>
- <span class="special">{</span>
- <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>
- <span class="special">{</span>
- <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>
- <span class="special">}</span>
- <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>
- <span class="special">{</span>
- <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>
- <span class="special">}</span>
- <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>
- <span class="special">{</span>
- <span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span>
- <span class="special">}</span>
- <span class="special">};</span>
- <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span>
- <span class="special">{</span>
- <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>
- <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>
- <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
- <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span>
- <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
- <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span>
- <span class="special">}</span>
- </pre>
- </div>
- <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
- <td align="left"></td>
- <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2018 Andrzej Krzemieński<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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