In Swift enumerations, what’s the difference between raw values and associated values?


Raw values are used to associate constant (literal) values to enum cases. The value type is part of the enum type, and each enum case must specify a unique raw value (duplicate values are not allowed).

The following example shows an enum with raw values of type Int:

enum IntEnum : Int {
    case ONE = 1
    case TWO = 2
    case THREE = 3

An enum value can be converted to its raw value by using the rawValue property:

var enumVar: IntEnum = IntEnum.TWO
var rawValue: Int = enumVar.rawValue

A raw value can be converted to an enum instance by using a dedicated initializer:

var enumVar: IntEnum? = IntEnum(rawValue: 1)

Associated values are used to associate arbitrary data to a specific enum case. Each enum case can have zero or more associated values, declared as a tuple in the case definition:

enum AssociatedEnum {
    case EMPTY
    case WITH_INT(value: Int)
    case WITH_TUPLE(value: Int, text: String, data: [Float])

Whereas the type(s) associated to a case are part of the enum declaration, the associated value(s) are instance specific, meaning that an enum case can have different associated values for different enum instances.


© 2017