Declaring and using a bit field enum in Swift

Question

How should bit fields be declared and used in Swift?

Declaring an enum like this does work, but trying to OR 2 values together fails to compile:

enum         


        

Answer 1

Here's something I put together to try to make a Swift enum that resembles to some extent a C# flags-style enum. But I'm just learning Swift, so this should only be considered to be "proof of concept" code.

/// This EnumBitFlags protocol can be applied to a Swift enum definition, providing a small amount
/// of compatibility with the flags-style enums available in C#.
///
/// The enum should be defined as based on UInt, and enum values should be defined that are powers
/// of two (1, 2, 4, 8, ...). The value zero, if defined, should only be used to indicate a lack of
/// data or an error situation.
///
/// Note that with C# the enum may contain a value that does not correspond to the defined enum
/// constants. This is not possible with Swift, it enforces that only valid values can be set.
public protocol EnumBitFlags : RawRepresentable, BitwiseOperations {

   var rawValue : UInt { get }  // This provided automatically by enum

   static func createNew(_ rawValue : UInt) -> Self  // Must be defined as some boiler-plate code
}

/// Extension methods for enums that implement the EnumBitFlags protocol.
public extension EnumBitFlags {

   // Implement protocol BitwiseOperations. But note that some of these operators, especially ~, 
   // will almost certainly result in an invalid (nil) enum object, resulting in a crash.

   public static func & (leftSide: Self, rightSide: Self) -> Self {
      return self.createNew(leftSide.rawValue & rightSide.rawValue)
   }

   public static func | (leftSide: Self, rightSide: Self) -> Self {
      return self.createNew(leftSide.rawValue | rightSide.rawValue)
   }

   public static func ^ (leftSide: Self, rightSide: Self) -> Self {
      return self.createNew(leftSide.rawValue ^ rightSide.rawValue)
   }

   public static prefix func ~ (x: Self) -> Self {
      return self.createNew(~x.rawValue)
   }

   public static var allZeros: Self {
      get {
         return self.createNew(0)
      }
   }

   // Method hasFlag() for compatibility with C#
   func hasFlag<T : EnumBitFlags>(_ flagToTest : T) -> Bool {
      return (self.rawValue & flagToTest.rawValue) != 0
   }
}

This shows how it can be used:

class TestEnumBitFlags {

   // Flags-style enum specifying where to write the log messages
   public enum LogDestination : UInt, EnumBitFlags {
      case none = 0             // Error condition
      case systemOutput = 0b01  // Logging messages written to system output file
      case sdCard       = 0b10  // Logging messages written to SD card (or similar storage)
      case both         = 0b11  // Both of the above options

      // Implement EnumBitFlags protocol
      public static func createNew(_ rawValue : UInt) -> LogDestination {
         return LogDestination(rawValue: rawValue)!
      }
   }

   private var _logDestination : LogDestination = .none
   private var _anotherEnum : LogDestination = .none

   func doTest() {

      _logDestination = .systemOutput
      assert(_logDestination.hasFlag(LogDestination.systemOutput))
      assert(!_logDestination.hasFlag(LogDestination.sdCard))

      _anotherEnum = _logDestination
      assert(_logDestination == _anotherEnum)

      _logDestination = .systemOutput | .sdCard
      assert(_logDestination.hasFlag(LogDestination.systemOutput) &&
             _logDestination.hasFlag(LogDestination.sdCard))

      /* don't do this, it results in a crash
      _logDestination = _logDestination & ~.systemOutput
      assert(_logDestination == .sdCard)
      */

      _logDestination = .sdCard
      _logDestination |= .systemOutput
      assert(_logDestination == .both)
   }
}

Suggestions for improvement are welcome.

EDIT: I've given up on this technique myself, and therefore obviously can't recommend it anymore.

The big problem is that Swift demands that rawValue must match one of the defined enum values. This is OK if there are only 2 or 3 or maybe even 4 flag bits - just define all of the combination values in order to make Swift happy. But for 5 or more flag bits it becomes totally crazy.

I'll leave this posted in case someone finds it useful, or maybe as a warning of how NOT to do it.

My current solution to this situation is based on using a struct instead of enum, together with a protocol and some extension methods. This works much better. Maybe I'll post it someday when I'm more sure that that isn't also isn't going to backfire on me.

Answer 2

If you don't need to interoperate with Objective-C and just want the syntax of bit masks in Swift, I've written a simple "library" called BitwiseOptions that can do this with regular Swift enumerations, e.g.:

enum Animal: BitwiseOptionsType {
    case Chicken
    case Cow
    case Goat
    static let allOptions = [.Chicken, .Cow, .Goat]
}

var animals = Animal.Chicken | Animal.Goat
animals ^= .Goat
if animals & .Chicken == .Chicken {
    println("Chick-Fil-A!")
}

and so on. No actual bits are being flipped here. These are set operations on opaque values. You can find the gist here.

Answer 3

I'm taking a guess that something like this is how they are modeling enum options in Foundation:

struct TestOptions: RawOptionSet {

    // conform to RawOptionSet
    static func fromMask(raw: UInt) -> TestOptions {
        return TestOptions(raw)
    }

    // conform to LogicValue
    func getLogicValue() -> Bool {
        if contains([1, 2, 4], value) {
            return true
        }
        return false
    }

    // conform to RawRepresentable
    static func fromRaw(raw: UInt) -> TestOptions? {
        if contains([1, 2, 4], raw) {
            return TestOptions(raw)
        }
        return nil
    }
    func toRaw() -> UInt {
        return value
    }

    // options and value
    var value: UInt
    init(_ value: UInt) {
        self.value = value
    }

    static var OptionOne: TestOptions {
        return TestOptions(1)
    }
    static var OptionTwo: TestOptions {
        return TestOptions(2)
    }
    static var OptionThree: TestOptions {
        return TestOptions(4)
    }
}

let myOptions = TestOptions.OptionOne | TestOptions.OptionThree
println("myOptions: \(myOptions.toRaw())")
if (myOptions & TestOptions.OptionOne) {
    println("OPTION ONE is in there")
} else {
    println("nope, no ONE")
}
if (myOptions & TestOptions.OptionTwo) {
    println("OPTION TWO is in there")
} else {
    println("nope, no TWO")
}
if (myOptions & TestOptions.OptionThree) {
    println("OPTION THREE is in there")
} else {
    println("nope, no THREE")
}

let nextOptions = myOptions | TestOptions.OptionTwo
println("options: \(nextOptions.toRaw())")
if (nextOptions & TestOptions.OptionOne) {
    println("OPTION ONE is in there")
} else {
    println("nope, no ONE")
}
if (nextOptions & TestOptions.OptionTwo) {
    println("OPTION TWO is in there")
} else {
    println("nope, no TWO")
}
if (nextOptions & TestOptions.OptionThree) {
    println("OPTION THREE is in there")
} else {
    println("nope, no THREE")
}

...where myOptions and nextOptions are of type TestOptions - I'm not exactly sure how fromMask() and getLogicValue() are supposed to act here (I just took some best guesses), maybe somebody could pick this up and work it out?

Answer 4

I think maybe some of the answers here are outdated with overcomplicated solutions? This works fine for me..

enum MyEnum: Int  {

    case One = 0
    case Two = 1
    case Three = 2
    case Four = 4
    case Five = 8
    case Six = 16

}

let enumCombined = MyEnum.Five.rawValue | MyEnum.Six.rawValue

if enumCombined & MyEnum.Six.rawValue != 0 {
    println("yay") // prints
}

if enumCombined & MyEnum.Five.rawValue != 0 {
    println("yay again") // prints
}

if enumCombined & MyEnum.Two.rawValue != 0 {
    println("shouldn't print") // doesn't print
}

Answer 5

You have to use .toRaw() after each member:

let combined: Int = MyEnum.One.toRaw() | MyEnum.Four.toRaw()

will work. Because as it is you're just trying to assign "One" which is a MyEnum type, not an integer. As Apple's documentation says:

“Unlike C and Objective-C, Swift enumeration members are not assigned a default integer value when they are created. In the CompassPoints example, North, South, East and West do not implicitly equal 0, 1, 2 and 3. Instead, the different enumeration members are fully-fledged values in their own right, with an explicitly-defined type of CompassPoint.”

so you have to use raw values if you want the members to represent some other type, as described here:

Enumeration members can come prepopulated with default values (called raw values), which are all of the same type. The raw value for a particular enumeration member is always the same. Raw values can be strings, characters, or any of the integer or floating-point number types. Each raw value must be unique within its enumeration declaration. When integers are used for raw values, they auto-increment if no value is specified for some of the enumeration members. Access the raw value of an enumeration member with its toRaw method.

Answer 6

You can build a struct that conforms to the RawOptionSet protocol, and you'll be able to use it like the built-in enum type but with bitmask functionality as well. The answer here shows how: Swift NS_OPTIONS-style bitmask enumerations.