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I am developing an iPhone app with ARC option enabled. i am creating IBOutlets in .h files which are connected from file owners icon to .xib elements.for eg.

IBOutlet UIButton *bt;
@property(nonatomic,retain)IBOutlet UIButton *bt;

in .m file, i am doing

@synthesize bt;

Is there a need to explicitly set bt to nil in viewDidUnload method? i.e. self.bt = nil; in viewDidUnload?

Also, do I need to write dealloc method with ARC option enabled? When should I make IBOutlets elements as strong and weak references with ARC enabled?

abhishek behl
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2 Answers2

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There's a difference between the need to put your outlets to nil in viewDidUnload and dealloc

Having ARC means you don't need to write that in your dealloc method (it gets done automatically), however the viewDidUnload method serves another purpose, and it is to free memory that the application isn't using when a memory warning occurs. dealloc is still needed in some cases, for example, when your class is registered for a notification, or when your class is someone else's delegate and you don't want some glitchy callback giving you a bad access

When you get a memory warning, all the UIViewControllers not being displayed will unload their view and call that method to free up memory. If you are still retaining outlets (like buttons, tables and etc) they will not get released, thus, killing the purpose of the viewDidUnload method.

Ismael
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    "he viewDidUnload method serves another purpose, and it is to free memory that the application isn't using when a memory warning occurs" NOPE. For that purpose there's didReceiveMemoryWarning. –  Aug 30 '12 at 19:40
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    They both get called on memory warning, though the correct place to set your outlets to nil is in viewDidUnload ... it's even noted on the method when you create a new UIViewController subclass!! – Ismael Aug 30 '12 at 19:44
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When using ARC there is no need to use modifiers like retain or copy, for example. That kind of memory managment is done automatically using the strong and weak modifiers.

You also don't have to worry about writing dealloc methods.

strong is kind of the equivalent of retain, so you should mark your outlets with it

@property(nonatomic, strong) IBOutlet UIButton *bt;

That's the way interface builder creates them by default.

I won't go into detail about their semantic differences, but you should really have a look at Apple's guide on transitioning to ARC if you want to know what's going on and read about the specifics of strong and weak modifiers.

EDIT: Sorry, interface builder creates outlets with weak by default.

EDIT 2: strong and retain are indeed 100% identical. (thanks to @Adam)

EDIT 3: You set your pointers to nil to avoid getting any message sent to deallocated instance or BAD_ACCESS_EXCEPTION errors.

If you are actually using ARC, you should make your outlets (nonatomic, weak) instead of (nonatomic, strong). By using the weak zeroing pointers, what the compiler does is automatically set your outlets to nil when nothing else references them.

So, summing up, if you don't use weak properties, you should set your pointers to nil.

Community
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jere
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  • thanks for your answer..one thing more..is it against the apple guidelines if i use retain instead of strong with ARC options enabled?because i have already used @property(nonatomic,retain) at many places. – abhishek behl Aug 30 '12 at 19:45
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    i'm pretty sure `retain` is 100% the same as `strong`. so there should be no problem using it – jere Aug 30 '12 at 19:50
  • ya its not creating any problem but i am unsure whether using it ARC option makes it against apple guidelines or not?? – abhishek behl Aug 30 '12 at 19:52
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    i don't think it's against any guideline. if it were the compiler would bitch about it. but if you really want to be consistent with conventions and best practises, you should change it to `strong`. after all, is just replacing a few words. have a look at this http://stackoverflow.com/questions/7796476/property-definitions-with-arc-strong-or-retain – jere Aug 30 '12 at 19:54
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    @abhishekbehl Core Data autogenerated classes have properties declared with `retain` even if your project uses ARC. So no, it is not against anything. [Apple says they are synonyms](http://developer.apple.com/library/ios/#releasenotes/ObjectiveC/RN-TransitioningToARC/Introduction/Introduction.html). – Adam Aug 30 '12 at 19:55
  • @Adam..thnks..even i think their shoudnt be a problem as apple says they are synonyms. – abhishek behl Aug 30 '12 at 20:02
  • @abhishekbehl Glad to help :) I have submitted a couple of iOS apps to the App Store. They all use ARC and Core Data. I have never changed `retain` to `strong` in the generated code and never got any of them rejected. – Adam Aug 30 '12 at 20:17
  • @Adam..hey thts gr8..whch all apps do u have??and can u tell me the list of points which i need to keep in mind while developing an app with ARC..?? – abhishek behl Aug 31 '12 at 03:32
  • @Adam..in my .xib files..i have created IBoutlets with @property(nonatomic,retain) and ARC enabled..so do i need to set them to nil in viewDidUnload method??Any sort of help would be great..:) – abhishek behl Aug 31 '12 at 03:50