Are D classes proper reference types?

[Ola Fosheim Grøstad]

On Saturday, 26 June 2021 at 07:39:44 UTC, kinke wrote:
> I'm pretty sure I haven't seen a weak pointer in C++ yet. I 
> don't look at C++ much anymore, but I suspect they are even a 
> lot rarer than shared_ptr.

Weak pointers are usually not needed, but sometimes you do need 
them and then not having them becomes a big weakness. Usually 
ones strives to have ownership defined in a way that makes it 
possible to dismantle a graph in a structured way from a single 
thread, and then weak pointers are not needed at all.

> Wrt. mixed class hierarchies, being able to inherit from and 
> instantiate C++ classes in D is of some priority and mostly 
> works today. (Let's not get into discussing multiple 
> inheritance here, it's hardly a show-stopper for most use 
> cases.)

Is it possible to inherit from a C++ class and get a D subclass, 
and is it possible to inherit from a D class and get a C++ class?

> Is Swift a thing outside the Apple universe (I admittedly 
> despise ;))? It's surely much better than their Objective-C 
> crap, but still...

The Apple universe is pretty big, we can dislike that, but that 
does not change the market…

So the goal would be to put the ref-count on a negative offset so 
that the layout can match up with C++ or Swift. I don't think 
that implies any big disadvantages, but I could be wrong. The 
only way to find out is to hash out the alternatives.

> So for rare use cases like shared_ptr/weak pointer interop, a 
> library solution (just like they are in C++) is IMO enough.

But the best solution is to get to a place where you can hand 
D-objects to other languages with ease without doing a runtime 
conversion from one layout to another.