D should have a root interface

[Frank Benoit]

Neil Vice schrieb:
> Robert Fraser wrote:
>> Frank Benoit Wrote:
>>
>>> // user lib
>>> interface List{
>>>     void add(Object);
>>> }
>>> interface ArrayList : List{
>>>     void add(Object){}
>>> }
>>>
>>> // calling code
>>> List list = ...
>>> IFile file = ... // interface type
>>> list.add( file ); // compile error
>>> list.add( cast(Object) file );
>>>
>>> This makes problems when one want to add() an interface instance. A 
>>> cast to Object is necessary. This is so ugly.
>>>
>>> If there would be at least an empty root interface for all D 
>>> interfaces, this can be used for method signatures.
>>>
>>> // D runtime and used by compiler
>>> interface IObject {
>>> }
>>>
>>> // user lib
>>> interface List{
>>>     void add(Object);
>>>     void add(IObject);
>>> }
>>> interface ArrayList : List{
>>>     void add(Object){ ... }
>>>     void add(IObject io){ add(cast(Object)io); }
>>> }
>>>
>>> This would not break any code and helps to make the calling code less 
>>> casty.
>>
>> Or, you know, just make all instances of interfaces that don't extend 
>> IUnknown implicitly castable to object.
> 
> Can not structs implement interfaces? Should they be implicitly castable 
> to Object?

structs are plain data. They do not have a virtual function table. So 
they are neither compatible to interfaces and not to classes.