Is it possible to implement a data structure using weak references?

Yes, in iOS (Swift/Objective-C) it is possible to implement data structures using weak references. This is especially useful for preventing cyclic references and memory leaks.

For example, when implementing a doubly linked list or a graph, where objects refer to each other, weak references allow one side not to hold the object in memory if there are no strong references to it.

Example in Swift:

class Node {
    var value: Int
    weak var parent: Node? // weak reference
    var children: [Node] = []

    init(value: Int) {
        self.value = value
    }
}

Here, parent is a weak reference to avoid a cycle between parent and children.