NSMutableSet class declares the programmatic interface to a mutable, unordered collection of distinct objects.
- iOS 8.0+
- macOS 10.10+
- tvOS 9.0+
- watchOS 2.0+
NSCountedSet class, which is a concrete subclass of
NSMutableSet, supports mutable sets that can contain multiple instances of the same element. The
NSSet class supports creating and managing immutable sets.
There should be little need of subclassing. If you need to customize behavior, it is often better to consider composition instead of subclassing.