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Date: Thu, 02 Feb 2012 13:36:14 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH 04/16] qom: add QObject-based property
 get/set wrappers
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org

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
>
>