Re: [Qemu-devel] [PATCH 04/16] qom: add QObject-based property get/set wrappers

[Anthony Liguori]

On 02/02/2012 01:24 PM, Paolo Bonzini wrote:
> On 02/02/2012 08:06 PM, Anthony Liguori wrote:
> > I don't want object.h to have a dependency on QObject. We need to phase
> > out QObject.
>
> The header doesn't.
>
> > Couple things:
> >
> > 1) We shouldn't use generic interfaces to read/write properties from
> > objects. We should use type-safe accessors provided by the types
> > themselves.
> >
> > 2) If we want to get fancy, we can add property_set_int, etc. and then
> > implement (1) via header files that just call these functions.
>
> That's what patch 5 does. But writing visitors in C is a royal PITA. The only
> sane way to do so is via QObject.

You just need a variant visitor.  It's pretty simple to do, essentially:

typedef struct VariantVisitor
{
    Visitor parent;
    enum { VV_INT, VV_STR } kind;
    union { int64_t v_int; char *v_str };
} VariantVisitor;

/* input */
static void visit_int(...)
{
   v->kind = TYPE_INT;
   v->v_int = *value;
}

/* output */
static void visit_int(...)
{
   assert(v->kind == TYPE_INT);
   *value = v->v_int;
}

void variant_visitor_set_int(VariantVisitor *v, int64_t value)
{
   v->kind = TYPE_INT;
   v->v_int = value;
}

The only types that matter are int and string so the variant visitor is pretty 
simple.

Regards,

Anthony Liguori

> Paolo