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| 4 | <title>Optional Reference Binding</title> | |
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| 24 | </div> | |
| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h3 class="title"> | |
| 27 | <a name="boost_optional.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 | </h3></div></div></div> | |
| 30 | <p> | |
| 31 | <a name="optional_reference_binding"></a>A number of compilers incorrectly | |
| 32 | treat const lvalues of integral type as rvalues, and create an illegal temporary | |
| 33 | when binding to an lvalue reference to const in some expressions. This could | |
| 34 | result in creating an optional lvalue reference that is in fact bound to | |
| 35 | an unexpected temporary rather than to the intended object. In order to prevent | |
| 36 | hard to find run-time bugs, this library performs compile-time checks to | |
| 37 | prevent expressions that would otherwise bind an optional reference to an | |
| 38 | unexpected temporary. As a consequence, on certain compilers certain pieces | |
| 39 | of functionality in optional references are missing. In order to maintain | |
| 40 | a portability of your code across diferent compilers, it is recommended that | |
| 41 | you only stick to the minimum portable interface of optional references: | |
| 42 | prefer direct-initialization and copy assignment of optional references to | |
| 43 | copy-initialization and assignment from <code class="computeroutput"><span class="identifier">T</span><span class="special">&</span></code>: | |
| 44 | </p> | |
| 45 | <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> | |
| 46 | <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> | |
| 47 | <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> | |
| 48 | <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> | |
| 49 | ||
| 50 | <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> | |
| 51 | <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> | |
| 52 | </pre> | |
| 53 | <p> | |
| 54 | Compilers known to have these deficiencies include GCC versions 4.2, 4.3, | |
| 55 | 4.4, 4.5, 5.1, 5.2; QCC 4.4.2; MSVC versions 8.0, 9.0, 10.0, 11.0, 12.0. | |
| 56 | In order to check if your compiler correctly implements reference binding | |
| 57 | use this test program. | |
| 58 | </p> | |
| 59 | <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">cassert</span><span class="special">></span> | |
| 60 | ||
| 61 | <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> | |
| 62 | ||
| 63 | <span class="keyword">struct</span> <span class="identifier">TestingReferenceBinding</span> | |
| 64 | <span class="special">{</span> | |
| 65 | <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> | |
| 66 | <span class="special">{</span> | |
| 67 | <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> | |
| 68 | <span class="special">}</span> | |
| 69 | ||
| 70 | <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> | |
| 71 | <span class="special">{</span> | |
| 72 | <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> | |
| 73 | <span class="special">}</span> | |
| 74 | ||
| 75 | <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> | |
| 76 | <span class="special">{</span> | |
| 77 | <span class="identifier">assert</span><span class="special">(</span><span class="keyword">false</span><span class="special">);</span> | |
| 78 | <span class="special">}</span> | |
| 79 | <span class="special">};</span> | |
| 80 | ||
| 81 | <span class="keyword">int</span> <span class="identifier">main</span><span class="special">()</span> | |
| 82 | <span class="special">{</span> | |
| 83 | <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> | |
| 84 | <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> | |
| 85 | ||
| 86 | <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span> | |
| 87 | <span class="identifier">ttt</span> <span class="special">=</span> <span class="identifier">global_i</span><span class="special">;</span> | |
| 88 | ||
| 89 | <span class="identifier">TestingReferenceBinding</span> <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span> | |
| 90 | <span class="identifier">ttt2</span> <span class="special">=</span> <span class="identifier">iref</span><span class="special">;</span> | |
| 91 | <span class="special">}</span> | |
| 92 | </pre> | |
| 93 | </div> | |
| 94 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 95 | <td align="left"></td> | |
| 96 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
| 97 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 98 | 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>) | |
| 99 | </p> | |
| 100 | </div></td> | |
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