On Fri, Apr 11, 2008 at 12:53 PM, Alastair Houghton <[EMAIL PROTECTED]> wrote: > On 11 Apr 2008, at 16:07, Michael Ash wrote: > > On Fri, Apr 11, 2008 at 6:04 AM, Alastair Houghton > > <[EMAIL PROTECTED]> wrote: > > > > > Under GC, these kinds of tricks are no longer possible because there is > no > > > -retain call any more, so no way to tell if it is safe to re-initialise > and > > > return the same wrapper object. > > > > > > Aside from re-implementing the data structure itself in ObjC and making > all > > > of its nodes objects, which might not be possible in some cases, has > anyone > > > thought of an efficient solution to this problem when running under GC? > > > > > > > The obvious technique would be to stop trying to make the optimization > > implicit and to make it explicit and managed by the caller instead. In > > other words, you write your API so that it explicitly returns the same > > object every time you call -nextObject, just with a new state. Then > > you have a copy method which the caller can use to get a new > > unchanging object for the case where he wants to keep one around. > > > > Yes. It's a shame though because that doesn't really fit with the way > people expect NSEnumerator to behave.
Honestly the original one doesn't fit either. I expect this code to work: id a = [enumerator nextObject]; id b = [enumerator nextObject]; assert(a != b); (Except, of course, for cases where the collection actually does hold two copies of the same object.) The very nature of this optimization is such that it's going to require the calling code to conform to it. The nature of that conformance varies slightly under GC and under the traditional rules, but it's there in both cases. Technically the above may not be legal under the traditional rules either, depending on how you interpret them. The scenario is similar to the getter/setter problem of whether an object returned from a getter is still valid if you then call the setter. But I think the expectation here is that the object should remain valid even if it may be technically legal to destroy it. > I was really just wondering if someone else had had some stroke of genius > and come up with a way to do this kind of thing under the GC. Or if, > perhaps, it would be worth asking for some new feature as part of the GC to > help with this kind of problem. I think it's inherently not possible to do without destroying the speed you're trying to gain. Due to how Apple's collector is implemented, not generating write barriers for stack values, seeing if an object has been stored requires re-scanning all dirty parts of the stack. Mike _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to [EMAIL PROTECTED]
