use __weak to local variable in ARC

Question

When you write code like below in ARC

__weak NSMutableArray* array = [[NSMutableArray alloc] init];

The compiler will show you a warning or error and say "Assigning retained object to weak variable. object will be released after assignment".

But if you write like this:

__weak NSMutableArray* array = [NSMutableArray array];

There is no error.

I'm wondering if the second usage is reasonable in some situations? What's the difference between these two codes from memory management perspective? How does it work?

Answer 1

There can be very subtle differences that the compiler cannot know.

Let's take an immutable version: [NSArray array] could return the very same static object every time. So your weak variable will point to a valid object, that has a strong reference somewhere else. Or think of the singleton pattern. On the other hand, when calling alloc/init, convention dictates that you're the only owner of that object, and that will typically leave you with nothing.

Also, the factory method certainly looks more like a (functional) method. In any case, it doesn't communicate ownership of what it returns very well. For an autoreleased object, it's not clear whether you're the the only owner or a shared owner.