Re: [Qemu-devel] Issues around TYPE_INTERFACE

[Paolo Bonzini]

On 03/04/19 10:53, Markus Armbruster wrote:
> Why (1) our interfaces can't have state:
> 
> Since interfaces are abstract, there's no such thing as an interface
> instance.

Well, there is.  Since all class instances are Objects, and interfaces
are applied to classes, interface instances must be Object.  In fact,
INTERFACE_CHECK() expects that to be the case, since it needs to go
spelunking into the class.  However, it is an implementation detail as
far as the interface _user_ is concerned.

> When @obj points to an instance of MyFrob, then we can safely cast it to
> any supertype of MyFrob.
> 
> MyIface is not a supertype of MyFrob, it's an interface.  WTF?
> 
> Turns out we only ever define MyIface in one of two ways:
> 
>     typedef struct MyIface MyIface;     // incomplete
> 
>     typedef struct MyIface {            // fake subtype of Object
>         Object parent;
>     } MyIface;
> 
> If we somehow know that only subtypes of SuperFrob have interface
> MyIface, then we can exploit that and safely cast to SuperFrob * or any
> of its supertypes.

I'm not sure we want to know that.  That said, the incomplete type is
much nicer.  Anybody wants to add a BiteSizedTask.

Paolo