Fun with extern(C++)

[Steven Schveighoffer via Digitalmars-d]

On 1/26/16 8:58 AM, Jacob Carlborg wrote:
> On 2016-01-26 13:59, Manu via Digitalmars-d wrote:
>
>> Hmmm. I wonder if this will cause problems...
>> Is it impossible to support a C++ class implementing a D interface?
>
> No that I know how the C++ compatibility works but I would guess the
> compiler needs to know at compile time how to call a method. If it would
> be possible to implement a D interface as either a D class or C++ class.
> The compiler cannot know which runtime type a variable which is declared
> as an interface can hold.
>
> interface A { void foo(); }
> class B : A { void foo(); }
> extern(C++) class C : A { void foo(); } // assuming this works
>
> A a = new B;
> A b = new C; // assuming this works
>
> a.foo();
> b.foo();
>
> When the compiler sees "a" or "b", how should it know it should call
> "foo" as a D method or C++ method? It only knows about the static type
> which is A, a D interface.
>

The short answer to this question is, no you cannot. The layouts are 
different. It's why I have always said, D interfaces should implicitly 
convert to Object and support all Object methods. Only D classes can 
implement D interfaces.

-Steve