Am 25. November 2015 01:02:25 MEZ, schrieb Gavin Lambert
On 2015-11-24 23:54, Agustín K-ballo Bergé wrote:
On 11/24/2015 5:20 PM, Andrey Semashev wrote:
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
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.
Best, Alexander