Using weak

Using weak will ensure that we never create a strong reference in ref, but will not retain the objects, either. If the object passed in ref is never retained by any other object, it will automatically be deallocated. This will lead to unexpected behavior, as the chain will be deallocated, and only a returned object will be kept in the memory:

class MemoryLeak {
    weak var ref: MemoryLeak?
    init(ref: MemoryLeak) {
        self.ref = ref
    }
    init() {
        ref = self
    }
}

func test() -> MemoryLeak {
    let a = MemoryLeak()
    let b = MemoryLeak(ref: a)
    let c = MemoryLeak(ref: b)
    a.ref = c
    return a
}

let result = test()
assert(result.ref != nil)

In the preceding code, we changed the MemoryLeak class, in order to keep a weak reference in ref. Unfortunately, the program will crash at the assertion line, as the ref property will be deallocated.