Re: [boost] [config] RFC PR 82

Subject: Re: [boost] [config] RFC PR 82
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2015-11-24 19:02:25

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

The only officially supported way to type-pun AFAIK is to cast via byte 
sequences (aka uint8_t * and friends), and even that can get you in 
trouble (on some platforms) if the alignments don't match.