Swift: What does it mean when you combine NSMutable- with let?

Question

In Swift, the let keyword denotes immutability. What does it mean to the compiler why you combine let and NSMutable?

e.g.

let nsArray: NSMutableArray = ["a", "b"];
nsArray.addObject("c")  // Still works

Answer 1

NSMutableArray is a class so it's passed by reference, not by value: here what is constant is your nsArray object, not the mutable array it contains.

So you can do:

nsArray.addObject("c")

But you can't do:

nsArray = ["d", "e"]

Answer 2

Interesting question. This is because you're not assigning the array itself. If you instead did:

nsArray = ["a", "sb"]

Then you would get a compiler error. This is somewhat related to the discussion here: let Non-mutable array in swift