Is it possible to implement support for Copy-on-write in a custom data structure?

Yes, support for Copy-on-write (COW) can be implemented in a custom data structure. The idea of COW is that multiple objects can share the same data until a modification is needed. When a modification is attempted, a copy of the data is created so that changes do not affect other objects.

In iOS and Swift, this is often implemented using reference counting and checking whether an object is unique before modifying it. It can be roughly done as follows:

class CowBuffer {
    private var data: [Int]
    private var isUniquelyReferenced: Bool {
        return isKnownUniquelyReferenced(&data)
    }

    init(data: [Int]) {
        self.data = data
    }

    func modify(at index: Int, value: Int) {
        if !isUniquelyReferenced {
            data = data.map { $0 } // create a copy
        }
        data[index] = value
    }
}

Thus, a copy of the data is only created when necessary, saving memory and improving performance.