Re: [boost] [config] RFC PR 82

Subject: Re: [boost] [config] RFC PR 82
From: Andrey Semashev (andrey.semashev_at_[hidden])
Date: 2015-11-24 16:44:05

On 2015-11-25 00:21, Agustín K-ballo Bergé wrote:
> On 11/24/2015 6:03 PM, 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.
>
> That would be 9.2 [class.mem]/19, notes are not normative.
>
> I can't tell from the context whether this would be enough for a
> `bitwise_cast` (what is it supposed to do?). The OP talks about "union
> type punning trick", which is a fine practice in C11 but undefined
> behavior in C++. Some compilers choose to offer this as a conforming
> extension.

I'm not sure I understand. This definitely was UB in C++03, but that 
addition you pointed to makes it defined behavior in C++11.

By bitwise_cast I meant something along these lines:

   template< typename To, typename From >
   To bitwise_cast(From from)
   {
     union
     {
       From as_from;
       To as_to;
     }
     caster = { from };
     return caster.as_to;
   }

With certain restrictions on From and To types I think this code has a 
well defined behavior in C++11.