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.