TSD.swift

protocol TSDKey {
    associatedtype Value
    static var defaultValue: Value { get }
}

struct TSD {
    private var storage: [ObjectIdentifier: Any] = [:]
    
    subscript<Key: TSDKey>(key: Key.Type) -> Key.Value {
        get {
            return storage[ObjectIdentifier(key)] as? Key.Value ?? Key.defaultValue
        }
        set {
            storage[ObjectIdentifier(key)] = newValue
        }
    }
}

enum Name: TSDKey {
    static var defaultValue = ""
}

enum Age: TSDKey {
    static var defaultValue: Int? = nil
}

var d = TSD()
d[Name.self] = "Thingy"
d[Name.self]
d[Age.self]
d[Age.self] = 42
d[Age.self]
d[Name.self].append("doodle")
d[Name.self]