COM/OLE advanced questions

[Guillaume Piolat]

Answering my own questions:

On Thursday, 2 November 2017 at 14:22:56 UTC, Guillaume Piolat 
wrote:
> Question 1. Is it mandatory to inherit from 
> core.sys.windows.unknwn.IUnknown, or just having an interface 
> named "IUnknown" validate it for being a COM interface?

Deriving from an interface named "IUnknown", in whatever module, 
with whatever actual methods, make the compiler consider it a 
"COM interface", which applies semantic restrictions.

The actual layout is _separated_ from the compiler considering it 
as a "COM interface", and one can achieve the exact desired 
layout with a IUnknown-like interface named in other way. One can 
test this with `delete interfaceInstance`.

The only layout effect of having the compiler consider it like 
COM interface, is that derived class will become "COM classes" 
whose only effects seems to be extern(System) as default linkage 
type.

So: you don't need the compiler to follow COM ABI, it just makes 
it easier.


> Question 2. If this fails, may I emulate COM vtable layout with 
> extern(C++) class? I wonder what the exact differences are 
> anyway between extern(C++) and the special IUnknown.

extern(C++) is indeed very flexible, with no virtual methods, 
whereas COM objects will have at least 8.
However this doesn't seem needed.


> Question 3. It seems I can inherit both from A D object and a 
> COM interface. What will be the choosen layout?

Should not be a problem.
The problem I had was my inability to override a base class 
methods since I forgot extern(D). This problem is talked about in 
the spec: https://dlang.org/spec/interface.html#com-interfaces