NSNumber >= 13 won't retain. Everything else will.

Question

The code I'm currently working on requires adding an NSNumber object to an array. All of the NSNumbers with value 0-12 are added fine, but 13 onward causes a EXC_BAD_ACCESS. I turned on NSZombieEnabled and am now getting *** -[CFNumber retain]: message sent to deallocated instance 0x3c78420.

Here's the call stack:
#0 0x01eac3a7 in ___forwarding___
#1 0x01e886c2 in __forwarding_prep_0___
#2 0x01e3f988 in CFRetain
#3 0x01e4b586 in _CFArrayReplaceValues
#4 0x0002a2f9 in -[NSCFArray insertObject:atIndex:]
#5 0x0002a274 in -[NSCFArray addObject:]
#6 0x00010a3b in -[Faves addObject:] at Faves.m:24
#7 0x000062ff in -[ShowController processFave] at ShowController.m:458
#8 0x002af405 in -[UIApplication sendAction:to:from:forEvent:]
#9 0x00312b4e in -[UIControl sendAction:to:forEvent:]
#10 0x00314d6f in -[UIControl(Internal) _sendActionsForEvents:withEvent:]
#11 0x00313abb in -[UIControl touchesEnded:withEvent:]
#12 0x002c8ddf in -[UIWindow _sendTouchesForEvent:]
#13 0x002b27c8 in -[UIApplication sendEvent:]
#14 0x002b9061 in _UIApplicationHandleEvent
#15 0x02566d59 in PurpleEventCallback
#16 0x01e83b80 in CFRunLoopRunSpecific
#17 0x01e82c48 in CFRunLoopRunInMode
#18 0x02565615 in GSEventRunModal
#19 0x025656da in GSEventRun
#20 0x002b9faf in UIApplicationMain
#21 0x00002498 in main at main.m:14

If it wasn't isolated to NSNumbers of a certain range, I'd assume I screwed something up with my memory management, but I've just got no idea.

Any ideas?

Thanks,
Josh

Answer 1

Clearly NSNumbers >12 will retain. I suggest that you write a very small program that proves this to yourself. Then take that program, make it a function, and call it early in your program. Slowly move the function to later points in your program until the error appears. You will thus find your real bug.

Answer 2

The numbers 0 through 12 are special as I discovered when answering another question here. Keep in mind that this is an implementation detail, not a language specification thing.

Basically, numbers up to (and including) 12 give you a reference to an already existing NSNumber, something which is possible due to the fact that they're immutable. Investigation showed that numbers 13 or greater gave a separate instance.

So you probably have screwed up your memory management after all :-) It's just that the fact that numbers less than 13 are likely references to numbers already in existence that's saving your bacon in those cases. I suggest you post more code so that we can track down that specific problem.

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And based on your comment to another answer here:

I added a retain line into the code and everything works perfectly now. No idea why. I'm just going to roll with it. Thanks!

I think you'll find that the fact that NSNumbers less than 13 already have a retain count of 1 before you get your own (bumping the count up to 2) is why they're not causing the EXC_BAD_ACCESS. Obviously your code is losing all the numbers you allocate, but the system isn't freeing those under 13 since they're still in use (retain count of 1 or more).