Question

Under ARC,
if I define a ivar like this:

@interface ObjectA : NSObject {
    id A;
}

@property(nonatomic,unsafe_unretained) id A;

What’s the property of A? strong or unsafe_unretained?

Answer 1

It depends on how you implement it. There are two ways you can go about doing it:

• Using @synthesize A; or @synthesize A = __A; In this case it’s __unsafe_unretained, because the ownership spec of the property trumps the ownership spec of the ivar¹.
• Manually writing A and setA. In this case, the result will depend on the kind of backing variable that you choose for your implementation.²

Here is a link to the documentation that spells out the rules of how the ownership of properties is determined. See Section 4.1.1.

¹ If the associated instance variable already exists, then its ownership qualification must equal the ownership of the property; otherwise, the instance variable is created with that ownership qualification [of the property].

² A property’s specified ownership is preserved in its metadata, but otherwise the meaning is purely conventional unless the property is synthesized.