Le 13 août 2012 à 17:56, Kyle Sluder <k...@ksluder.com> a écrit :
> On Aug 13, 2012, at 8:42 AM, Ben <ben_cocoa_dev_l...@yahoo.co.uk> wrote: > >> I see in the documentation - and from a compiler error - that some classes >> are not compatible with weak references. >> >> What makes these classes incompatible? > > They have custom implementations of -retain and -release. > > When NextSTEP was first released, there was no refcounting, just +new and > +free. That's why NSObject (actually Object at the time) has only one ivar: > isa, a pointer to the object's class. > > Some time later -retain and -release came into being, but the storage for > refcount couldn't be added to Object because of the fragile base class > problem—all existing code expected sizeof(Object)==sizeof(id), so they > hardcoded offsets for accessing subclass ivars. I don't get it. retain/release has been introduced by NSObject. As all classed that use ref counting are based on NSObject and not Object, it was perfectly possible to declare 2 ivars in NSObject without breaking anything when it was first introduced. > This means these new, super-common operations had to access memory stored > somewhere else than the object they were operating on. And to be thread-safe, > these accesses had to be protected by synchronization primitives. > > Blowing out your processor's cache every time you issue a -retain sucks, as > does taking a spinlock on an object that should only ever be accessed from > the main thread. So some classes that had some spare bits overrode -retain > and -release to store the retain count in the object's instance data and > possibly not protect it with synchronization. > > For example, either NSView or NSControl has such a retain count in one of its > bitfields. We also have a common base class, OFObject, that has an inline > retain count that is manipulated with atomic lockless increments/decremnts. > But with the advent of ARC, and the concurrent improvement of the data > structure that stores object retain counts, we're going to eventually > eliminate OFObject. > > Hopefully once Apple removes support for 32-bit OS X they will be able to > move the retain count storage into NSObject. > > --Kyle Sluder > _______________________________________________ > > 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: > https://lists.apple.com/mailman/options/cocoa-dev/devlists%40shadowlab.org > > This email sent to devli...@shadowlab.org -- Jean-Daniel _______________________________________________ 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: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
