Box

Box.swift

@propertyWrapper
struct Box<T> {
    class Storage {
        var value: T
        init(_ value: T) {
            self.value = value
        }
    }

    private let storage: Storage

    var wrappedValue: T {
        get { storage.value }
        nonmutating set { storage.value = newValue }
    }

    init(wrappedValue: T) {
        storage = Storage(wrappedValue)
    }
}