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| 4 | <title>False positive with -Wmaybe-uninitialized</title> | |
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| 25 | <div class="section"> | |
| 26 | <div class="titlepage"><div><div><h4 class="title"> | |
| 27 | <a name="boost_optional.tutorial.gotchas.false_positive_with__wmaybe_uninitialized"></a><a class="link" href="false_positive_with__wmaybe_uninitialized.html" title="False positive with -Wmaybe-uninitialized">False | |
| 28 | positive with -Wmaybe-uninitialized</a> | |
| 29 | </h4></div></div></div> | |
| 30 | <p> | |
| 31 | Sometimes on GCC compilers below version 5.1 you may get an -Wmaybe-uninitialized | |
| 32 | warning when copiling with option -02 on a perfectly valid <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span></code> usage. For instance in this | |
| 33 | program: | |
| 34 | </p> | |
| 35 | <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">optional</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> | |
| 36 | ||
| 37 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">getitem</span><span class="special">();</span> | |
| 38 | ||
| 39 | <span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="keyword">int</span> <span class="identifier">argc</span><span class="special">,</span> <span class="keyword">const</span> <span class="keyword">char</span> <span class="special">*[])</span> | |
| 40 | <span class="special">{</span> | |
| 41 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">a</span> <span class="special">=</span> <span class="identifier">getitem</span><span class="special">();</span> | |
| 42 | <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span><span class="special">;</span> | |
| 43 | ||
| 44 | <span class="keyword">if</span> <span class="special">(</span><span class="identifier">argc</span> <span class="special">></span> <span class="number">0</span><span class="special">)</span> | |
| 45 | <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">argc</span><span class="special">;</span> | |
| 46 | ||
| 47 | <span class="keyword">if</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">!=</span> <span class="identifier">b</span><span class="special">)</span> | |
| 48 | <span class="keyword">return</span> <span class="number">1</span><span class="special">;</span> | |
| 49 | ||
| 50 | <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span> | |
| 51 | <span class="special">}</span> | |
| 52 | </pre> | |
| 53 | <p> | |
| 54 | This is a bug in the compiler. As a workaround (provided in <a href="http://stackoverflow.com/questions/21755206/how-to-get-around-gcc-void-b-4-may-be-used-uninitialized-in-this-funct" target="_top">this | |
| 55 | Stack Overflow question</a>) use the following way of initializing | |
| 56 | an optional containing no value: | |
| 57 | </p> | |
| 58 | <pre class="programlisting"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span> <span class="identifier">b</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">make_optional</span><span class="special">(</span><span class="keyword">false</span><span class="special">,</span> <span class="keyword">int</span><span class="special">());</span> | |
| 59 | </pre> | |
| 60 | <p> | |
| 61 | This is obviously redundant, but makes the warning disappear. | |
| 62 | </p> | |
| 63 | </div> | |
| 64 | <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> | |
| 65 | <td align="left"></td> | |
| 66 | <td align="right"><div class="copyright-footer">Copyright © 2003-2007 Fernando Luis Cacciola Carballal<br>Copyright © 2014-2016 Andrzej Krzemieński<p> | |
| 67 | Distributed under the Boost Software License, Version 1.0. (See accompanying | |
| 68 | 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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