Re: [Qemu-devel] [RFC] Plan for moving forward with QOM

[Paolo Bonzini]

On 09/15/2011 04:18 PM, Anthony Liguori wrote:
> I think there are two real discussions happening here.  One is how to
> model and one is what is the right representation of that modelling.
>
> I think Paolo is strongly advocating a non-OO model that uses the
> command pattern to establish most of the relationships.  Maybe one level
> of inheritance is used just to provide the right infrastructure for the
> objects to have properties but that's inheritance for functionality
> verses modelling.
>
> I'm advocating a strict OO model.  I'm not normally a fan of OO based
> design but I think it fits a device model pretty well because it's such
> a concrete hierarchy.  All of the objects we create really correspond to
> real world objects and the inheritance relationships are pretty darn
> obvious.  When you're holding a PCI NE2000 card in your hand, there's
> simply no doubt that it is-a PciDevice.

I think it's in the eye of a beholder.  Hold a PCI NE2000 and E1000, 
they're clearly both PciDevices, but also they clearly both have 
PciConnectors.

Write a driver for a PCI and ISA NE2000, and then they're clearly both 
NE2000, but also they clearly both have an NE2K chip.

So, I don't find either model to be more or less OO.  It just boils down 
to what you pick as your primary choice for inheritance.  It is true 
that my model will produce a more shallow class hierarchies, but only 
because IMO hardware is more about composition than inheritance; that's 
why I am indeed advocating that relationships be established mostly by 
composition.  (I didn't understand your reference to the command 
pattern, but it's probably not so important).

Secondarily, I'd hate to force a particular inheritance model, because 
class inheritance is a very scarce resource---you can only inherit from 
one class at a time.

Paolo