Re: [boost] [config] RFC PR 82

On 2015-11-25 18:32, Alexander Lauser wrote:

Am 25. November 2015 01:02:25 MEZ, schrieb Gavin Lambert :

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On 2015-11-24 23:54, Agustín K-ballo Bergé wrote:

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On 11/24/2015 5:20 PM, Andrey Semashev wrote:

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> Ditto BOOST_HAS_UNION_TYPE_PUNNING_TRICK (doesn't any > compiler support > this?).

'I'm all with you on this one' but since 'it is not in the standard' language purists will probably complain if it is used unconditionally...

To some extent this is guaranteed by [class.union]/1 in C++11.

No, it isn't.

Why? Reading different members of the standard layout union within

On 25/11/2015 10:03, Andrey Semashev wrote: the

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common initial sequence is enough to implement a bitwise_cast.

I don't have a standard reference handy, but I'm pretty sure that reading from a different member of a union than was written to is still

explicitly UB -- although for practical reasons *most* compilers will generate the expected result provided that the two types have the same initial alignment.

http://en.cppreference.com/w/cpp/language/union have an example that explicitly states what Gavin claimed. Not sure about the reliability of that site, though.

That sounds like self contradiction to me. The page says it's well defined to examine the common subsequence of standard-layout union members but at the same time it's UB to read from them. What's the difference?