Additionally, people comparing ARC to Objective-C's conservative GC in the replies don't seem to understand that (1) refcounting is a form of GC, often times inefficient compared to a mark-and-sweep GC, and (2) conservative GCs are quite limited and Apple's implementation was pretty bad compared to other implementations.
Objective-C objects are basically all a struct objc_class* under the hood, and conservative GCs in general cannot distinguish whether a given word is a pointer. Even worse, for a conservative GC to determine whether a word points into a heap-allocated block, it has to perform a lengthy, expensive scan of the entire heap. It also doesn't help that Apple decided to kickstart the GC if your messages began with "re" (the prefix for "retain" and "release" messages, which were used all the time before ARC came around). So at one point in time, you were able to marginally boost performance of a garbage collected Objective-C application by avoiding messages beginning with "re"!
[1] Figure 3 in https://iacoma.cs.uiuc.edu/iacoma-papers/pact18.pdf
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Additionally, people comparing ARC to Objective-C's conservative GC in the replies don't seem to understand that (1) refcounting is a form of GC, often times inefficient compared to a mark-and-sweep GC, and (2) conservative GCs are quite limited and Apple's implementation was pretty bad compared to other implementations.
Objective-C objects are basically all a struct objc_class* under the hood, and conservative GCs in general cannot distinguish whether a given word is a pointer. Even worse, for a conservative GC to determine whether a word points into a heap-allocated block, it has to perform a lengthy, expensive scan of the entire heap. It also doesn't help that Apple decided to kickstart the GC if your messages began with "re" (the prefix for "retain" and "release" messages, which were used all the time before ARC came around). So at one point in time, you were able to marginally boost performance of a garbage collected Objective-C application by avoiding messages beginning with "re"!