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Re: [Qemu-devel] [PATCH 3/3] qom: add unit test for Interfaces

From: Peter Crosthwaite
Subject: Re: [Qemu-devel] [PATCH 3/3] qom: add unit test for Interfaces
Date: Mon, 18 Jun 2012 23:46:06 +1000

>>> +#define HERBIVORE(obj) \
>>> +    INTERFACE_CHECK(Herbivore, (obj), TYPE_HERBIVORE)
>>> +
>>> +typedef struct Herbivore
>>> +{
>>> +    Object obj;
>>> +} Herbivore;
>> All this is doing is saying Herbivores are Objects right? A user cant
>> add anything to this struct given that interfaces are stateless so
>> could this be simplified to
>> typedef Object Herbivore;
> This is admittedly a little wierd...
> Interfaces don't exist as Objects in QOM.

Not in the sense that they used too, but all objects that implement an
interface are still OBJECTs just through the inheritance path of their
concrete class, which is the point of this struct yes?

 They are just classes.  But it's
> very handy to be able to have a Herbivore type that you can cast objects to.

Yes I agree in full, but the typedef definition is functionally
equivalent to what you have there, and removes the temptation to add
fields to the object type. Having a skeletal struct there gives the
illusion to readers that interface objects are in someway extensible.

> I probably need to respin this though.  INTERFACE_CHECK() asserts that an
> object implements the interface class and then just returns the obj and
> casts it to the dummy Interface object type.
> A better approach would be to just teach object_dynamic_cast to do this for
> anythign that's an interface type.  That would fix the link problem you
> pointed out too.  I'll spin a v2.

Please see my patch, I have a little 3 patch series up on the list.
that starts with your v1, does the axi-stream stuff then fixes that


> Regards,
> Anthony Liguori
>> ?
>>> +
>>> +typedef struct HerbivoreClass
>>> +{
>>> +    InterfaceClass parent;
>>> +
>>> +    void (*feed_greens)(Herbivore *obj);

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