On Jul 7, 2011, at 5:25 PM, Jens Alfke wrote: > On Jul 7, 2011, at 4:25 PM, Greg Parker wrote: >> This is less safe than you might like. On iOS and 64-bit Mac, you may end up >> with both copies of the class in use simultaneously. That means two separate >> invocations of +initialize, two separate locks for @synchronized([MyClass >> class]), etc. > > How’s that? I thought there would only ever be one instance of the class > loaded at a time.
No, not with the runtime formerly known as "new". Internally, ordinary clients of the class use a C symbol to reference the class. (This includes [MyClass message] callsites, and subclasses and categories of MyClass.) Thanks to two-level symbol linking, each client will cleanly link to one copy of the class, even if some other dylib loads another copy. The runtime allows any number of copies of the class to exist, but warns about the duplicates. One problem is that there is still only a single global namespace for Objective-C classes. Anybody who looks up a class by name may get a different copy of the class than the one they would have linked to. I forget whether lookup by name is first-copy-wins or last-copy-wins. -- Greg Parker gpar...@apple.com Runtime Wrangler _______________________________________________ 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 arch...@mail-archive.com
