Union redux

[Andrei Alexandrescu via Digitalmars-d]

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