On Jun 13, 2014, at 11:02 AM, Boris Zbarsky wrote:
> On 6/13/14, 1:51 PM, Allen Wirfs-Brock wrote:
>
>> The object you return is an instance of the superclass and not of the
>> subclass.
>
> Ah, right.
>
> So just to make sure, now that I think about it: in the @@create setup, where
> is the prototype of the object set? What is the prototype of the thing
> @@create returns?
yes.
The default @@create is approximately:
Function.prototype[Symbol.create] = function(I) {
return Object.create(this.prototype)
}
>
> Or put another way, is there really a difference, conceptually, between doing
> the "create an instance of the superclass and then change the prototype"
> thing and what @@create gives us?
No, except that changing the [[Prototype]] after the fact require a mutable
[[Prototype]] and I can imaging that some object representations might not want
to allow that.
>> The key thing that @@create is doing is separating determining the physical
>> characteristics of an object (making it exotic, branding it, using a custom
>> C struct as its representation, etc.) from logically initializing it at an
>> appropriate place within some class hierarchy
>
> Looking at what's involved in object allocation in at least SpiderMonkey, the
> following pieces of data are required to be known at object creation time:
>
> 1) The Realm to use.
normally that would be the same Realm as that of the @@@create method. But an
@@create method could decide otherwise.
> 2) Some metainformation about the exotic-or-not nature of the object,
> its internal layout, etc.
>
> These are effectively not mutable in SpiderMonkey.
Right, that is essentially the primary motivation for @@create. There are some
low level characteristics of some objects that must be fixed when the storage
of the object is allocated.
>
> In addition to those, it's good to have the prototype available at
> object-creation time as well, since later dynamic prototype mutation, while
> possible, leads to deoptimization in the JIT. This may be a
> SpiderMonkey-specific issue, of course.
+1
Allen
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