Union redux

[Timon Gehr via Digitalmars-d]

On 06/03/2015 02:51 AM, Andrei Alexandrescu wrote:
> On 6/2/15 4:14 PM, Timon Gehr wrote:
>> On 06/03/2015 01:07 AM, Andrei Alexandrescu wrote:
>>> On 6/2/15 3:42 PM, Steven Schveighoffer wrote:
>>>> On 6/2/15 6:14 PM, Andrei Alexandrescu wrote:
>>>>> On 6/2/15 2:57 PM, Timon Gehr wrote:
>>>>>> OK. What about copying in instead of out?
>>>>>
>>>>> If the appropriate type has already been constructed, assignment
>>>>> suffices. Otherwise, emplace(&target, source) is needed. -- Andrei
>>>>
>>>> My concern is if you copy the enclosing type. This means you have
>>>> already copied the bits, and the postblit of the enclosing type is
>>>> responsible for calling the postblit of the valid union member (as
>>>> defined by the semantics of the type). How does it do that?
>>>
>>> Copying unions as a whole won't do any miracles. Just memcpy. -- Andrei
>>>
>>
>> Copy is memcpy & postblit.
>>
>> If the bit-copy has already been done (e.g. execution is inside a
>> postblit) emplace(&f,f) seems wasteful.
>
> Oh, I only now understood the question. Yes, it's possible to only call
> postblit if defined:
>
> static if (hasElaborateCopyConstructor!T)
>      tvar.__postblit();
>
>
> Andrei

Indeed, but I think this facility is undocumented. Maybe it should be 
exposed via the standard library?