Abstract classes vs interfaces, casting from void*

[John Colvin]

On Saturday, 10 August 2019 at 10:11:15 UTC, Alex wrote:
> On Saturday, 10 August 2019 at 08:20:46 UTC, John Colvin wrote:
>> On Friday, 9 August 2019 at 13:39:53 UTC, Simen Kjærås wrote:
>>> <snip>
>>
>> Thanks for the extra detail.
>>
>> Is there a solid reason to ever use an interface over an 
>> abstract class? (Other than multiple inheritance).
>>
>> I'm such a noob at anything related to OO.
>
> The general question is tricky, as different languages differ 
> in details what is forced and what is allowed for abstract 
> classes and interfaces.
>
> But roughly speaking, my opinion is: if you can/want to provide 
> some default behavior than you are about to write an abstract 
> class.
> If you are about to provide information/restriction of 
> behavior, then this is more like an interface.

Ok. What would go wrong (in D) if I just replaced every interface 
with an abstract class?