Fun with extern(C++)

[Manu via Digitalmars-d]

On 27 January 2016 at 01:38, Steven Schveighoffer via Digitalmars-d
<digitalmars-d at puremagic.com> wrote:
> 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.

Interface is just a vtable... what makes the layout different?
Surely any class can take a vtable definition and populate it with
some function pointers.