C++ interface vs D and com

[Adam Sansier via Digitalmars-d-learn]

So, com throughs me a interface ptr and I need to map it to an 
interface. When I do, I get an access violation.

I have an (com) ptr and an interface. How do I link them up so I 
can call the functions?

I marked the interface extern(C++) so it's a C++ style interface. 
 The first field of a COM object is a pointer to its vtable.  
This is still true in extern(C++) D, right? The calling 
convention is thiscall.

https://dlang.org/spec/cpp_interface.html