Immutable/Mutable Collections in Swift

Question

I was referring to Apple\'s Swift programming guide for understanding creation of Mutable/ immutable objects(Array, Dictionary, Sets, Data) in Swift language. But I could\'t

Answer 1

Swift does not have any drop in replacement for NSArray or the other collection classes in Objective-C.

There are array and dictionary classes, but it should be noted these are "value" types, compared to NSArray and NSDictionary which are "object" types.

The difference is subtle but can be very important to avoid edge case bugs.

In swift, you create an "immutable" array with:

let hello = ["a", "b", "c"]

And a "mutable" array with:

var hello = ["a", "b", "c"]

Mutable arrays can be modified just like NSMutableArray:

var myArray = ["a", "b", "c"]

myArray.append("d") // ["a", "b", "c", "d"]

However you can't pass a mutable array to a function:

var myArray = ["a", "b", "c"]

func addToArray(myArray: [String]) {
  myArray.append("d") // compile error
}

But the above code does work with an NSMutableArray:

var myArray = ["a", "b", "c"] as NSMutableArray

func addToArray(myArray: NSMutableArray) {
  myArray.addObject("d")
}

addToArray(myArray)

myArray // ["a", "b", "c", "d"]

You can achieve NSMutableArray's behaviour by using an inout method parameter:

var myArray = ["a", "b", "c"]

func addToArray(inout myArray: [String]) {
  myArray.append("d")
}

addToArray(&myArray)

myArray // ["a", "b", "c", "d"]

Re-wrote this answer 2015-08-10 to reflect the current Swift behaviour.