Re: [Qemu-devel] [PATCH 01/27] qom: add the base Object class

[Kevin O'Connor <kevin@koconnor.net>]

On Wed, Dec 21, 2011 at 08:35:16AM -0600, Anthony Liguori wrote:
> On 12/21/2011 07:35 AM, Paolo Bonzini wrote:
> >What's the need for "Type"? You can use simply the TypeImpl * and drop
> >type_get_instance. Outside object.h it can be an opaque pointer.
> 
> It's a bit nicer for type_register to return a handle that can later
> be unregistered (although that's not currently implemented).
> 
> You could have it return TypeImpl * of course.  GObject uses a
> simpler GType but they don't have the notion of a symbolic type
> name.
> 
> I used a symbolic type name to avoid the problem of dependencies.
> In order to create a type in gobject, you have to reference the
> parent's GType which usually means you have to call the _get_type()
> function which acts as a singleton which registers the type.
> 
> Since you have to specify the parent via a function call, you can't
> define the type in a unit-level static structure which I viewed as a
> critical requirement.

Why not declare types with something like the following:

TypeInfo my_device_info = {
    .name = "my-device",
    .parentinfo = &device_info,
    .instance_size = sizeof(MyDevice),
};

That is, instead of looking up the TypeImpl via a string, lookup the
TypeImpl via the address of the TypeInfo.  (Or possibly store a
pointer to TypeImpl in TypeInfo during registration.)

Module order shouldn't matter - all the info needed to register the
parent is there so it can be registered during first use.  Indeed,
pass a TypeInfo* to object_new() and one should be able to skip the
registration step - if the type hasn't been registered the code can
detect that and automatically register it.

-Kevin