What's the name for this data type or approach?

data-type.swift

enum S {
    case a, b
}

struct MapOfS<T> {
    private var a: T, b: T

    subscript(_ s: S) -> T {
        get {
            switch s {
            case .a: return a
            case .b: return b
            }
        }
        set {
            switch s {
            case .a: a = newValue
            case .b: b = newValue
            }
        }
    }
}