copy-on-write

copy-on-write.swift

final class Content {
    var value: Int
    init(value: Int) { self.value = value }
    var debug: String { return String(describing: value) }
}

struct Wrapper {
    private var _content: Content
    var content: Content {
        mutating get {
            if !isKnownUniquelyReferenced(&_content) {
                print("copy")
                _content = Content(value: _content.value)
            }
            return _content
        }
    }

    init(content: Content) {
        self._content = content
    }

    var debug: String { return String(describing: _content.value) }
}

extension Wrapper: CustomStringConvertible {
    public var description: String {
        return self.debug
    }
}

extension Content: CustomStringConvertible {
    public var description: String {
        return self.debug
    }
}

var m = Wrapper(content: Content(value: 9))
var n = m

n.content.value = 11

n
m

// safe copy-on-write semantics implemented
// sources: http://chris.eidhof.nl/post/struct-semantics-in-swift/