rjmccall added a comment.

This isn't really an Objective-C user forum, but I'll try to summarize briefly. 
 `unsafe_unretained` is an unsafe version of `weak` — it's unsafe because it 
can be left dangling if the object is deallocated.  It's necessary for a small 
(and getting smaller every year) set of classes that don't support true weak 
references, and it can be useful as an optimization to avoid the performance 
overhead of reference counting.


Repository:
  rC Clang

https://reviews.llvm.org/D44539



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