Automatic Reference Counting (ARC)
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The object has a field that represents the object's count.
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Expensive.
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There are checks every time the object is modified.
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Cycles are problematic.
Mark and Sweep
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Has no problems with cycles.
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There's no need to check every time the object is modified, but the checks are done during the GC Pause, which is where the GC pauses the app to look for garbage.