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POG++: Can __attribute__((__may_alias__)) be used for pointer to class instance rather than to class definition itself?
 

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  1. POG++: Can __attribute__((__may_alias__)) be used for pointer to class instance rather than to class definition itself?
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    <p>I'm looking to the answer to the following question: is <code>may_alias</code> suitable as attribute for pointer to an object of some class <code>Foo</code>? Or must it be used at class level only? </p> <p>Consider the following code(it is based on a real-world example which is more complex): </p> <pre><code>#include &lt;iostream&gt; using namespace std; #define alias_hack __attribute__((__may_alias__)) template &lt;typename T&gt; class Foo { private: /*alias_hack*/ char Data[sizeof (T)]; public: /*alias_hack*/ T&amp; GetT() { return *((/*alias_hack*/ T*)Data); } }; struct Bar { int Baz; Bar(int baz) : Baz(baz) {} } /*alias_hack*/; // &lt;- uncommeting this line apparently solves the problem, but does so on class-level(rather than pointer-level) // uncommenting previous alias_hack's doesn't help int main() { Foo&lt;Bar&gt; foo; foo.GetT().Baz = 42; cout &lt;&lt; foo.GetT().Baz &lt;&lt; endl; } </code></pre> <p>Is there any way to tell gcc that single pointer <code>may_alias</code> some another?</p> <p>BTW, please note that gcc detection mechanism of such problem is imperfect, so it is very easy to just make this warning go away without actually solving the problem.</p> <p>Consider the following snippet of code:</p> <pre><code>#include &lt;iostream&gt; using namespace std; int main() { long i = 42; long* iptr = &amp;i; //(*(short*)&amp;i) = 3; // with warning //(*(short*)iptr) = 3; // without warning cout &lt;&lt; i &lt;&lt; endl; } </code></pre> <p>Uncomment one of the lines to see the difference in compiler output.</p>
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