How do I declare an array of weak references in Swift?

Create a generic wrapper as:

class Weak<T: AnyObject> {
  weak var value : T?
  init (value: T) {
    self.value = value
  }
}

Add instances of this class to your array.

class Stuff {}
var weakly : [Weak<Stuff>] = [Weak(value: Stuff()), Weak(value: Stuff())]

When defining Weak you can use either struct or class.

Also, to help with reaping array contents, you could do something along the lines of:

extension Array where Element:Weak<AnyObject> {
  mutating func reap () {
    self = self.filter { nil != $0.value }
  }
}

The use of AnyObject above should be replaced with T - but I don't think the current Swift language allows an extension defined as such.

A functional programming approach

No extra class needed.

Simply define an array of closures () -> Foo? and capture the foo instance as weak using [weak foo].

let foo = Foo()

var foos = [() -> Foo?]()
foos.append({ [weak foo] in return foo })

foos.forEach { $0()?.doSomething() }

You can use the NSHashTable with weakObjectsHashTable. NSHashTable<ObjectType>.weakObjectsHashTable()

For Swift 3: NSHashTable<ObjectType>.weakObjects()

NSHashTable Class Reference

Available in OS X v10.5 and later.

Available in iOS 6.0 and later.