What criteria must be met for a value to be added to a set?

In order for a value to be added to a Set in iOS (Swift), it must conform to the Hashable protocol.

This means that:

• The value must have a hash value (hashValue), which is used for quick lookup in the set.
• An equality comparison function (Equatable) must be implemented to determine if two elements are the same.

Example:

struct Person: Hashable {
    var id: Int
    var name: String
}

var peopleSet = Set<Person>()
peopleSet.insert(Person(id: 1, name: "Ivan"))

If the type does not conform to Hashable, it cannot be added to a Set, as the set requires unique elements and quick existence checks.