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. 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/archive%40mail-archive.com This email sent to arch...@mail-archive.com
