What requirements must be considered when creating objects that support hashing?

When creating objects that support hashing (for example, for use in dictionaries or sets), the following requirements must be considered:

1. Defined and stable hash code:

   • The hash code calculation method should return the same value for the same object throughout its lifetime.
   • Changing fields involved in hash calculation can lead to errors when searching in hash tables.

2. Correct overriding of hash and equals methods (or their equivalents):

   • If two objects are considered equal (equals returns true), their hash codes must match.
   • Not following this rule breaks the operation of hash data structures.

3. Immutability of key fields:

   • Fields involved in hash calculation and comparison should ideally be immutable.

Example in Swift:

struct Person: Hashable {
    let id: Int
    let name: String

    func hash(into hasher: inout Hasher) {
        hasher.combine(id)
    }

    static func == (lhs: Person, rhs: Person) -> Bool {
        return lhs.id == rhs.id
    }
}

Here, id is used for hashing and comparison, ensuring correct operation in hash tables.