How to enumerate an enum with String type?

I made a utility function iterateEnum() for iterating cases for arbitrary enum types.

Here is the example usage:

enum Suit: String {
    case Spades = "♠"
    case Hearts = "♥"
    case Diamonds = "♦"
    case Clubs = "♣"
}

for f in iterateEnum(Suit) {
    println(f.rawValue)
}

Which outputs:

♠
♥
♦
♣

But, this is only for debug or test purposes: This relies on several undocumented Swift1.1 compiler behaviors, so, use it at your own risk.

Here is the code:

func iterateEnum<T: Hashable>(_: T.Type) -> GeneratorOf<T> {
    var cast: (Int -> T)!
    switch sizeof(T) {
        case 0: return GeneratorOf(GeneratorOfOne(unsafeBitCast((), T.self)))
        case 1: cast = { unsafeBitCast(UInt8(truncatingBitPattern: $0), T.self) }
        case 2: cast = { unsafeBitCast(UInt16(truncatingBitPattern: $0), T.self) }
        case 4: cast = { unsafeBitCast(UInt32(truncatingBitPattern: $0), T.self) }
        case 8: cast = { unsafeBitCast(UInt64($0), T.self) }
        default: fatalError("cannot be here")
    }

    var i = 0
    return GeneratorOf {
        let next = cast(i)
        return next.hashValue == i++ ? next : nil
    }
}

The underlying idea is:

• Memory representation of enum, excluding enums with associated types, is just an index of cases when the count of the cases is 2...256, it's identical to UInt8, when 257...65536, it's UInt16 and so on. So, it can be unsafeBitcast from corresponding unsigned integer types.
• .hashValue of enum values is the same as the index of the case.
• .hashValue of enum values bitcasted from invalid index is 0.

---

Revised for Swift2 and implemented casting ideas from @Kametrixom's answer:

func iterateEnum<T: Hashable>(_: T.Type) -> AnyGenerator<T> {
    var i = 0
    return anyGenerator {
        let next = withUnsafePointer(&i) { UnsafePointer<T>($0).memory }
        return next.hashValue == i++ ? next : nil
    }
}

---

Revised for Swift3:

func iterateEnum<T: Hashable>(_: T.Type) -> AnyIterator<T> {
    var i = 0
    return AnyIterator {
        let next = withUnsafePointer(to: &i) {
            $0.withMemoryRebound(to: T.self, capacity: 1) { $0.pointee }
        }
        if next.hashValue != i { return nil }
        i += 1
        return next
    }
}

---

Revised for Swift3.0.1:

func iterateEnum<T: Hashable>(_: T.Type) -> AnyIterator<T> {
    var i = 0
    return AnyIterator {
        let next = withUnsafeBytes(of: &i) { $0.load(as: T.self) }
        if next.hashValue != i { return nil }
        i += 1
        return next
    }
}

Swift 4.2+

Starting with Swift 4.2 (with Xcode 10), just add protocol conformance to CaseIterable to benefit from allCases. To add this protocol conformance, you simply need to write somewhere:

extension Suit: CaseIterable {}

If the enum is your own, you may specify the conformance directly in the declaration:

enum Suit: String, CaseIterable { case spades = "♠"; case hearts = "♥"; case diamonds = "♦"; case clubs = "♣" }

Then the following code will print all possible values:

Suit.allCases.forEach {
    print($0.rawValue)
}

---

Compatibility with earlier Swift versions (3.x and 4.x)

If you need to support Swift 3.x or 4.0, you may mimic the Swift 4.2 implementation by adding the following code:

#if !swift(>=4.2)
public protocol CaseIterable {
    associatedtype AllCases: Collection where AllCases.Element == Self
    static var allCases: AllCases { get }
}
extension CaseIterable where Self: Hashable {
    static var allCases: [Self] {
        return [Self](AnySequence { () -> AnyIterator<Self> in
            var raw = 0
            var first: Self?
            return AnyIterator {
                let current = withUnsafeBytes(of: &raw) { $0.load(as: Self.self) }
                if raw == 0 {
                    first = current
                } else if current == first {
                    return nil
                }
                raw += 1
                return current
            }
        })
    }
}
#endif

This post is relevant here https://www.swift-studies.com/blog/2014/6/10/enumerating-enums-in-swift

Essentially the proposed solution is

enum ProductCategory : String {
     case Washers = "washers", Dryers = "dryers", Toasters = "toasters"

     static let allValues = [Washers, Dryers, Toasters]
}

for category in ProductCategory.allValues{
     //Do something
}