On Oct 3, 2011, at 1:31 PM, Kyle Sluder wrote: >> 1. Apple reserves the underscore prefix for their own use, so you could, at >> least theoretically, clash with a superclass ivar this way, and > > [snip] > >> >> 3. If I use an ivar prefix that no one else uses (as far as I know), then I >> can make my class into a subclass of something from a publicly available >> framework without needing to worry about ivar name clashes. > > This only matters at compile time. If you have a class that subclasses > a framework class, and you add an ivar named "_foo", later releases of > the OS can add an ivar named "_foo" and your application will continue > to run on the latest release without issue. (Well, on 32-bit Mac OS X, > they can rename an existing ivar to "_foo" without issue; adding an > ivar will trigger the Fragile Base Class problem.) > > On 32-bit Mac OS X, ivar accesses get translated into an offset. > "self->_foo = 42" gets translated into something like "*(self + 18) = > 42". On 64-bit Mac OS X and on iOS, ivar access is done indirectly via > symbol. "self->_foo = 42" gets translated into something like "*(self > + OBJC_IVAR_$_MyClass__foo) = 18". Neither of these methods actually > cares what the ivar is named (the compiler could generate offset > symbols based on the days of the week if it wanted to).
While that’s mostly true, there are a few exceptions, such as object_[gs]etInstanceVariable and that KVO automatic ivar thing. If Apple’s actually using those things internally somewhere (as I learned in a recent thread here, they do use object_setInstanceVariable for nib outlets if they’re declared as ivars rather than KVO properties), then who knows what effect it could have. Besides, it’s always nice to save a few compile-time hassles, no? Charles _______________________________________________ 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
