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?

On 9/23/2024 7:52 AM, Ogion via dip.ideas 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.

Careful with the term 'proper'.  It doesn't add clarity.  It does implicitly divide everything else into some vague other category of improper, but without actually helping describe any actual particulars.