Можете ли вы описать, каким образом вы реализовали механизм Copy-on-Write в своей практике?

The Copy-on-Write (COW) mechanism in iOS can be implemented to optimize memory usage when multiple objects share the same data until one of them attempts to modify that data.

In practice, I have used COW, for example, when working with large data structures or collections. In Swift, standard collections (Array, Dictionary, Set) already implement COW under the hood.

If you implement your own COW mechanism, usually a wrapper class is created to store the data, and the structure contains a reference to this class. When attempting to modify the data, it checks whether the reference is unique (using isKnownUniquelyReferenced), and if not, a copy of the data is made.

Example:

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

struct MyArray {
    private var storage: DataStorage

    init(_ data: [Int]) {
        storage = DataStorage(data: data)
    }

    mutating func append(_ value: Int) {
        if !isKnownUniquelyReferenced(&storage) {
            storage = DataStorage(data: storage.data) // copying data
        }
        storage.data.append(value)
    }

    var data: [Int] {
        return storage.data
    }
}

var a = MyArray([1, 2, 3])
var b = a // b and a share the same data
b.append(4) // when modifying b, a copy is created
print(a.data) // [1, 2, 3]
print(b.data) // [1, 2, 3, 4]