Struct inheritance

[Richard (Rikki) Andrew Cattermole]

On 24/09/2024 2:52 AM, Ogion wrote:
> On Thursday, 19 September 2024 at 15:56:06 UTC, Richard (Rikki) Andrew 
> Cattermole wrote:
>> …
>>
> 
> Would be nice to have proper struct inheritance but there are a lot of 
> things we have to consider.
> 
> What about slicing?
> 
> ```D
> struct A {
>      int x;
> }
> 
> struct B : A {
>      int y;
>      this(int x, int y) {
>          super(x);
>          this.y = y;
>      }
> }
> 
> void main() {
>      B b1 = B(1, 2), b2 = B(3, 4);
>      A* p = &b2;
>      *p = b1;
>      writeln(b2); // B(1, 4)
> }
> ```
> 
> If we want something better than `alias this`, we have to prohibit 
> implicit partial assignment via pointer/reference. When it’s necessary, 
> it would still be possible with `*p = cast(A)b1;`.

It is probably better to completely disable casting up the hierarchy in 
``@safe`` code. If you need that, use ``opCast``.

It is not like with classes, where its guaranteed to have an allocation 
containing the entire thing, with a vtable.

Only concern is when you've got a pointer to a struct, and you cast up. 
``opCast!(A*)`` support is likely what we want I think.

I'm not sure how we'd do that one. Maybe an attribute?