Есть ли возможность реализовать собственный механизм Copy-on-Write в проекте?

Yes, in iOS it is possible to implement a custom Copy-on-Write (CoW) mechanism to optimize memory usage and improve performance when working with mutable data structures.

The idea of CoW is that when copying an object, only a reference to the original data is actually created, and the real data is only copied when an attempt is made to modify the object (i.e., during mutation).

An example implementation of CoW for a structure with an array:

struct MyData {
    private class Storage {
        var array: [Int]
        init(array: [Int]) {
            self.array = array
        }
        func copy() -> Storage {
            return Storage(array: array)
        }
    }

    private var storage: Storage

    init(array: [Int]) {
        storage = Storage(array: array)
    }

    var array: [Int] {
        get { storage.array }
        set {
            if !isKnownUniquelyReferenced(&storage) {
                storage = storage.copy()
            }
            storage.array = newValue
        }
    }
}

Here, isKnownUniquelyReferenced checks that the storage object is not shared with other instances, and if necessary, a copy is made before modification.