Retained Core Foundation Property

Question

(Xcode 4.2, iOS 5, ARC)

I have some properties of Core Foundation (/Graphics) objects that should take ownership of their objects. Now in these Appl

Answer 1

Now my question is, do I still need to explicitly set those properties to nil in my dealloc?

Yes, you do, with the declaration you showed.

In the Property declarations section of the ARC specification, it says:

Applying __attribute__((NSObject)) to a property not of retainable object pointer type has the same behavior it does outside of ARC: it requires the property type to be some sort of pointer and permits the use of modifiers other than assign. These modifiers only affect the synthesized getter and setter; direct accesses to the ivar (even if synthesized) still have primitive semantics, and the value in the ivar will not be automatically released during deallocation.

(emphasis on last part added by me)

In other words, applying __attribute__((NSObject)) and strong on a property with Core Foundation type makes the getters and setters work correctly, but it will not make ARC manage the underlying instance variable (because the instance variable's type will still be the Core Foundation type), and thus ARC will not release the instance variable upon dealloc if it is non-nil. With such a property declaration, you will have to nil it in dealloc or cause a leak.

However, there is a way to make ARC manage the variable itself. Since the variable has a Core Foundation type, you can make it an ARC-managed type by wrapping it in a typedef with __attribute__((NSObject)).

The Retainable object pointers section of the ARC specification says:

There are three kinds of retainable object pointer types:

• block pointers (formed by applying the caret (^) declarator sigil to a function type)
• Objective-C object pointers (id, Class, NSFoo*, etc.)
• typedefs marked with __attribute__((NSObject))

(emphasis on last item added by me)

So if you make a typedef like this:

typedef __attribute__((NSObject)) CGImageRef MyImageRef;

and declare your property like this:

@property (nonatomic, strong) MyImageRef loupeImage;

the underlying variable will be managed by ARC (because the underlying variable will have the typedef'd type, which is an ARC-managed type) and you will not have to nil it in dealloc.