On Dec 14, 2007, at 12:50 PM, P T Withington wrote:
On 2007-09-23, at 14:14 EDT, Brendan Eich wrote:
The reason for the original prototype-owned constructor was to afford
a back-pointer from prototype to constructor function without
imposing a per-instance property (which could be optimized to be
shared where possible, overridden where desired -- but at some cost
in implementation complexity).
I'm not convinced it's worth changing this for ES4. Anyway it is very
late to have a new proposal -- we are finalizing proposals next week
at the face-to-face meeting.
So, was nothing done about this? We're starting to work on an es4
back-end for our stuff and running into this issue. We can make sure
all our constructors give each instance a constructor slot (and we can
smash the class prototype.constructor to point to the superclass) as
we do for our es3 back end; but I was really hoping for a cleaner
solution in es4.
Nothing's changed, no proposal that avoids per-instance overhead or
more complicated implementation strategies to avoid that. There's
also an entrainment hazard, not huge but non-zero risk of introducing
leaks into existing web apps if we entrain the constructor where the
web app code intentionally clears prototype.constructor (which is
read/write).
The constructor property is and always was intended to be a property
of the prototype, not of each instance. Changing that now is hard.
The merits are not compelling enough IMHO.
Is there some other way in es4 that from an instance one can navigate
up the superclass chain? Can I say:
super.constructor
perhaps, to find my class's superclass?
For a class C, C.prototype.constructor is indeed C. But super does
not work that way -- it's used for invoking the super-class
constructor, rather:
>> class B { var x;function B(x) : x=x {} }
>> new B(2).x
2
>> class D extends B { var y; function D(x,y) : y=y, super(x) {} }
>> d = new D(3,4)
[object D]
>> d.x
3
>> d.y
4
To reflect on inheritance and other relations, use the meta-objects
interfaces:
http://wiki.ecmascript.org/doku.php?id=proposals:meta_objects
Given class C:
let Ctype = reflect::typeOf(C);
for (let Csup in Ctype.superTypes()) {
// Csup is a base class or interface of C here
}
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