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POstrict aliasing and alignment
 

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    <p>I need a safe way to alias between arbitrary POD types, conforming to ISO-C++11 explicitly considering 3.10/10 and 3.11 of n3242 or later. There are a lot of questions about strict aliasing here, most of them regarding C and not C++. I found a "solution" for C which uses unions, probably using this section</p> <blockquote> <p>union type that includes one of the aforementioned types among its elements or nonstatic data members</p> </blockquote> <p>From that I built this.</p> <pre><code>#include &lt;iostream&gt; template &lt;typename T, typename U&gt; T&amp; access_as(U* p) { union dummy_union { U dummy; T destination; }; dummy_union* u = (dummy_union*)p; return u-&gt;destination; } struct test { short s; int i; }; int main() { int buf[2]; static_assert(sizeof(buf) &gt;= sizeof(double), ""); static_assert(sizeof(buf) &gt;= sizeof(test), ""); access_as&lt;double&gt;(buf) = 42.1337; std::cout &lt;&lt; access_as&lt;double&gt;(buf) &lt;&lt; '\n'; access_as&lt;test&gt;(buf).s = 42; access_as&lt;test&gt;(buf).i = 1234; std::cout &lt;&lt; access_as&lt;test&gt;(buf).s &lt;&lt; '\n'; std::cout &lt;&lt; access_as&lt;test&gt;(buf).i &lt;&lt; '\n'; } </code></pre> <p>My question is, just to be sure, is this program legal according to the standard?<sup>*</sup></p> <p>It doesn't give any warnings whatsoever and works fine when compiling with MinGW/GCC 4.6.2 using:</p> <pre><code>g++ -std=c++0x -Wall -Wextra -O3 -fstrict-aliasing -o alias.exe alias.cpp </code></pre> <p><sup>*</sup> Edit: And if not, how could one modify this to be legal?</p>
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    1. COI'm 99% sure that this isn't legal in C++ (you're reading an inactive union member). I'm not sure about C11, though, whose rules are more relaxed.
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    2. CO@KerrekSB he does not read an inactive union member. The access to the union member stays a write-access to an lvalue throughout the access.
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      1. USJohannes Schaub - litb
    3. COJust get rid of that `dummy_union` and there should be no problem, provided that the alignment of the types are compatible.
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