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Supposedly weak is like assign or unsafe_unretained, except that a weak variable is “zeroed” when the object that it points to is dealloc‘ed. But ARC is handled by the compiler, so why is weak not supported on iOS 4 and Mac OS X 10.6?
(for this question please don’t give a guess as an answer, but something more supported by facts/reference/docs)
Since
__weakrequires zeroing out one or more pointers in places other than the current object, additional data structures are required to keep track of weak references. This is in contrast to__strong,__unsafe_unretained, and__autoreleasing, which do not require additional tracking. The tracking structures and programs for manipulating them are built into the OS; the compiler inserts the code to perform the calls, but the OS support needs to be there in order for the compiled code to work.