class Buffer {
    var value: Int
    init(value: Int = 0) { self.value = value }
}

struct COWStruct {
    
    private var buffer = Buffer()
    
    mutating func change() {
        if isKnownUniquelyReferenced(&buffer) {
            buffer.value += 1
            print("No copy")
        } else {
            buffer = Buffer(value: buffer.value + 1)
            print("Copy")
        }
    }
}

var dict = ["key": COWStruct()]

if let index = dict.index(forKey: "key") {
    dict.values[index].change() // Copy
}