Referencing itself in a struct in Swift

It took a long time, I mean years, but it finally happened. I stumbled on a struct which had a property of the same type.

	struct Message {
	let id: Int
	// This is OK:
	let replies: [Message]
	// This is not OK
	// Value type 'Message' cannot have a stored property that recursively contains it
	let parent: Message?
	}

view raw Struct.swift hosted with ❤ by GitHub

At first, it is kind of interesting that the replies property compiles fine, although it is a collection of the same type. I guess it is so because array’s storage type is a reference type.

The simplest workaround is to use a closure for capturing the actual value.

	struct Message {
	let id: Int
	let replies: [Message]
	private let parentClosure: () -> Message?
	var parent: Message? { parentClosure() }
	}

view raw Struct2.swift hosted with ❤ by GitHub

Or we could go for using a boxed wrapper type.

	struct Message {
	let id: Int
	let replies: [Message]
	private let parentBoxed: Boxed<Message>?
	var parent: Message? { parentBoxed?.value}
	}
	class Boxed<T> {
	let value: T
	init(value: T) {
	self.value = value
	}
	}

view raw Struct3.swift hosted with ❤ by GitHub

Or if we prefer property wrappers, using that instead.

	struct Message {
	let id: Int
	let replies: [Message]
	@Boxed var parent: Message?
	}
	@propertyWrapper
	class Boxed<Value> {
	var value: Value
	init(wrappedValue: Value) {
	value = wrappedValue
	}
	var wrappedValue: Value {
	get { value }
	set { value = newValue }
	}
	}

view raw Struct4.swift hosted with ❤ by GitHub

Then there are also options like changing the struct into class instead, but that is something to consider. Or finally, creating a

All in all, it is fascinating how something simple like this actually has a pretty complex background.

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