Inheritance and arrays

[FeepingCreature]

On Monday, 3 July 2023 at 09:50:20 UTC, Arafel wrote:
> Hi!
>
> I am a D user coming from java, rather than from C/C++ 
> (although obviously also have some exposure to them), and thus 
> apparently one of the few people here who likes OO (within 
> reason, of course).
>
> So while I appreciate the fact that D closely follows java's 
> design, I wonder why there is no implicit inheritance for 
> arrays (also the same applies to AAs):
>
> ```d
> interface I {}
> class C : I {}
>
> void main() {
>     I i;
>     C c = null;
>     i = c; // Works
>
>     I[] ii;
>     C[] cc = null;
>     // ii = cc; // Doesn't work: Error: cannot implicitly 
> convert expression `cc` of type `C[]` to `I[]`
>     ii = cast (I[]) cc; // Works, but why do I need to cast?
> }
> ```
>
The `cast` version "works", but will crash at runtime.

In D, as opposed to Java, a reference to an object has a 
*different pointer value* than a reference to the interface-typed 
version of that object. This is necessary for efficient compiled 
virtual method calls on the interface. But for the same reason, 
you cannot reinterpret an array of objects to an array of 
interfaces; even if you can implicitly convert each object to 
that interface, there's a difference between automatically 
rewriting a value and automatically rewriting every element of an 
array: one is O(1), the other is O(n) and incurs a GC allocation.