Hashable

Swift language Mid-level Swift 5.0+ iOS 17+ Reviewed 31 Oct 2026 By Alok Singh

Spot the bug

A Set ends up holding both "Swift" and "swift". Tap the line that breaks the Hashable contract.

struct Tag: Hashable {
let name: String
static func == (lhs: Tag, rhs: Tag) -> Bool {
lhs.name.lowercased() == rhs.name.lowercased()
}
func hash(into hasher: inout Hasher) {
hasher.combine(name)
}
}

Hint 1 of 1

Do equal values produce equal hashes here?

Choose an answer, then open Show answer below to check it.

Show answer

Answer: line 7 hasher.combine(name)

Line 7 hashes the original string, but == compares lowercased names. Equal tags then get different hashes. Hash name.lowercased() instead.

30-second answer

Hashable lets a type be used in sets and as dictionary keys. The rule: values that are equal must produce the same hash. Hash exactly the properties that == compares, in the same way. Swift synthesizes both when all stored properties are Hashable, which is usually best.

Practise saying it · 30 s. No recording, just a timer

Explain the Hashable contract.

Say your answer out loud, then check which of these points you covered:

Equal values, same hash
Hash what == compares
Prefer synthesis

Deep dive

Unequal values may share a hash (collisions are allowed).

Hash values are randomly seeded per process, so never persist them.

Changing a hashed property of an object inside a Set breaks the Set.

Identifiable is separate: identity for UI, not hashing.

Common mistakes

Comparing case-insensitively in == but hashing the raw string.
Saving hashValue to disk.

Senior take & follow-ups

Seniors state the equal-implies-same-hash rule and why mutation inside a Set is dangerous.

Follow-up questions

Think of your answer first, then open the hint.

Why are hash seeds random?
Show hint
To resist hash-flooding attacks.
What happens if equal values hash differently?
Show hint
Sets and dictionaries can hold duplicates and fail lookups.

Suggest a fix if something here is wrong or out of date.