What would be the consequence of implementing interfaces as fat pointers ?

[Orvid King]

On Sun, 30 Mar 2014 00:11:50 -0500, Walter Bright  
<newshound2 at digitalmars.com> wrote:

> On 3/29/2014 8:38 PM, Daniel Murphy wrote:
>> "Walter Bright"  wrote in message news:lh83dr$6m1$1 at digitalmars.com...
>>
>>> On 3/29/2014 8:26 PM, Daniel Murphy wrote:
>>> > It could, but we don't have a stable ABI
>>>
>>> ? The ABI for interfaces hasn't changed in many years. In fact I don't  
>>> even
>>> remember changes in it.
>>
>> I meant stable in the sense that we _can't_ break it, not that we  
>> _haven't_
>> broken it recently.
>>
>> ie we don't guarantee binary compatibility across releases.
>
> ok.

Actually, there is one big thing that this would break; C++ interop. With  
the way interfaces currently work, you can define an interface with  
virtual methods, and you can call those methods on that interface, and, if  
you've properly overlayed it over a C++ class instance, you will be  
calling a virtual method defined on that C++ class. Fat interfaces would  
break this capability, and would actually break some of my existing code,  
due to the fact I use that exact capability. I do however mark my  
interfaces as extern(C++), so perhaps they would have to be an exception  
to the ABI if this change were made?