If you have a property "value" in your subclass (using the term
'property' loosely, not in the Objective-C 2.0 sense), and Apple adds
the property "value" to the superclass, you're probably going to be
hosed in one way or another. For example, if Apple adds the _value
ivar, they're also likely to add -value and -setValue: methods, in
which case relying on KVC to find your ivar is going to fail when it
finds Apple's accessors. You could avoid that by implementing your
own accessors, but then Apple's code might contain calls to those
accessors that expect to find Apple's implementation (or a superclass-
aware subclass implementation) rather than your own superclass-
ignorant implementation.
Such are the hazards of subclassing, I think.
-Jeff
On Jul 10, 2008, at 11:19 AM, glenn andreas wrote:
The problem is that while when you recompile you'll see the
conflict, for existing users (who won't be compiling from source),
if they use KVC they'll have the problem.
@interface NSFoo {
id _reserved;
}
@end
@interface MyFoo: NSFoo {
float _value;
}
@end
And OS X+1 comes out with:
@interface NSFoo {
int _value;
}
@end
Any of your code that used _value (like, say, for bindings) could
suddenly break for users of OS X N+ 1 (since they won't be
compiling from source to see what is going on).
Granted, it's rare that this happens, but from time to time
reserved fields do get converted to something used. And lets face
it, if you hear "it works fine on OS X N, but breaks on OS X N+1",
"ivar collision" isn't the first thing you'll think about to try to
track down the problem...
Worse, this could actually happen on an OS X N+ 0.1 release and the
headers wouldn't get updated so you won't even have the safety net
of seeing compiler errors. Similarly, if you target the OS X N
SDK, you won't see the new headers either.
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