Union redux

[deadalnix via Digitalmars-d]

On Monday, 1 June 2015 at 19:43:33 UTC, Andrei Alexandrescu wrote:
> Do not define constructors. They ostensibly imply commitment to 
> remembering to call the appropriate destructor. A union can 
> only be default constructed (with its init value as described) 
> and has no destructor.
>

This is implied in C++ because of manual memory management. I 
don't think it is in D.

>> 5/ union currently disallow members with postblit and/or 
>> destructor . It
>> seems that this was needed in C++ as per Andrei's comments. It 
>> seems to
>> me that, because of manual memory management, C++ would have a 
>> lot more
>> struct with copy constructor and/or destructor than in D, so 
>> I'm not
>> sure if this require change in spec. Andrei, can you give more 
>> details
>> on the C++ situation here ?
>
> That rule has hurt C++ everywhere, and the community credits 
> Lois Goldthwaite for pushing the current rule, which allows 
> types with cdtors in unions. Let's go the same way, unions are 
> all but useless otherwise.
>

Without knowing the rationale in C++, it is difficult to assert 
that this is also valid in D. You seems to be knowledgeable of 
the subject, so surely you can enlight us on that one.

>> 7/ struct with anonymous union in them have one entry per 
>> union field in
>> their tupleof. All the union field should be present as one 
>> entry in the
>> tupleof, with a compiler generated union type.
>
> Again the litmus test here is enabling introspection. Structs 
> with anonymous union members should have one member called e.g. 
> __anonymous__ in their tupleof, and in turn introspecting that 
> member should list its tuple.
>

Compiler generated tuples are auto flattening in D. That would 
not work unless the entry in the struct tuple is an (compiler 
generated) union.

>> 9/ union alignement is not specified. union align should be 
>> least common
>> multiple of union's fields.
>
> Yes. Since they're all powers of 2, taking the max is the same 
> thing. BTW, to my surprise on OSX real.alignof is 16, but the 
> alignof a union containing a real is 8. I think that's a bug. 
> Please verify and file.
>

Indeed, max is the same thing, but because of plateform specific 
characteristics. I don't think we should rely on them unless we 
need to (implementation can still use max).