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

[Andreas Färber]

Am 18.06.2012 15:46, schrieb Peter Crosthwaite:
>>>> +#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.

Why have a typedef at all then? You can just use Object directly. If
that ever changes it leads to all kinds of problems (well, necessary
adjustments), as seen in the pci_host series.

Andreas

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