Prototype of a callback function used to retain a value being added to a set.
- iOS 2.0+
- macOS 10.0+
- tvOS 9.0+
- watchOS 2.0+
- Core Foundation
The set’s allocator.
The value being added to the set.
The value to store in the set, which is usually the
value parameter passed to this callback, but may be a different value if a different value should be stored in the collection.