.NET's hashing protocol is weird and arguably it's some awful baggage carried over from its Java influences. All instances, value or reference type, have GetHashCode. For a given type there is the 'default' implementation, or you can provide a specific one. This enables anything to be used as a key in a container like a dictionary/map.
For reference types, GetHashCode's default implementation assigns an object a semi-unique value that persists for the entire lifetime of the instance. So in a non-moving GC you could use the pointer address, but in a moving GC the runtime is basically assigning it a permanent identifier that sticks with the instance somewhere. For value types, the default hashing implementation basically walks over the whole type and hashes the fields to create a hash for the value as a whole. I'm surprised to hear that JS runtimes don't necessarily have ways to 'hash' a given JS value, but it makes sense. I can see how that is a great reason for 'get me a hash for this value' to never actually exist in the API, even if it's unfortunate that I have to recreate that facility myself in runtimes that do have it. -kg On 4 December 2014 at 21:24, Steve Fink <[email protected]> wrote: > On 12/04/2014 08:00 PM, Katelyn Gadd wrote: >> I do still use WeakMap in a few other places, for example to implement >> Object.GetHashCode. This is a case where the transposed representation >> is likely optimal - though in practice, I shouldn't need any sort of >> container here, if only the hashing mechanisms clearly built into the >> VM were exposed to user JS. > > If I am understanding correctly, I don't think there is any such hashing > mechanism in the Spidermonkey VM. We hash on an object's pointer > address, which can change during a moving GC. (We update any hashtables > that incorporate an object's numeric address into their hash key > computations.) > > I'm a little curious what you're generating the hashcode from. Is this > mimicking a value object? If the contents of the object change, would > you want the hashcode to change? Or are the "hashcodes" just > incrementing numerical object ids? > > (Sorry for the tangent to the current thread.) > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
