What is the difference between weak, unsafe_unretained, and assign in Objective-C?

weak auto-nils, unsafe_unretained does not, assign is for primitives.

weak references are zeroed out when object deallocates. unsafe_unretained behaves like a dangling pointer if object is freed. assign is used for primitive types. unsafe_unretained is risky but sometimes used in performance-critical or legacy C-bridged code.