Sometimes, in Swift — especially for debugging — you will want to see if 2 objects references you are holding are actually referring to the same object. The ObjectIdentifier struct is useful for this.

var o1 = NSObject()
var o2 = o1
var o3 = NSObject()
print("o1's \(ObjectIdentifier(o1).debugDescription)")
print("o2's \(ObjectIdentifier(o2).debugDescription)")
print("o3's \(ObjectIdentifier(o3).debugDescription)")

You'll get something a unique identifier for each object instance like this and you can see that o1 and o2 refers to the same instance.

o1's ObjectIdentifier(0x0000608000001620)
o2's ObjectIdentifier(0x0000608000001620)
o3's ObjectIdentifier(0x0000610000001530)

Note that ObjectIdentifier doesn't work for value types like structs and enums.