Object and interface compatibility

[Frits van Bommel]

Jarrett Billingsley wrote:
> "Frank Benoit (keinfarbton)" <benoit at tionex.removethispart.de> wrote in 
> message news:eqkuqa$3v1$1 at digitaldaemon.com...
>> Interfaces are NOT compatible to Object.
>>
>> interface I{
>> void func();
>> }
>> class C : I {
>> void func(){}
>> }
>>
>> void main(){
>> I i = new C;
>> i.func();                 // OK
>> i.toHash();               // error !!
>> (cast(C)i).toHash();      // OK
>> (cast(Object)i).toHash(); // OK
>> }
>>
>> I think this is a failure in the language design. What do you think?
> 
> I think it's because an interface may or may not be pointing to a D object. 
> If you create a COM interface, for example, there's no associated D object, 
> so there's no way for D to create a hash for it.  It's basically just a 
> pointer.

But isn't this only true for COM interfaces? The compiler knows when 
compiling "i.toHash();" whether or not typeof(i) is an interface 
inheriting from IUnknown, doesn't it? So it could make this work for all 
other interfaces...

> This could be solved by having a "cominterface" type which behaves like 
> interfaces currently do, and change interfaces to be more integrated into D. 
> Probably never going to happen. 

It's much easier solved by checking if "IUnknown" is a base of the 
interface in question.