in Swift: Difference between Array VS NSArray VS [AnyObject]

Question

As the title says, what\'s the difference between Array vs NSArray vs [AnyObject]?

Also, what is most recommended way of approaching this. What i mean recommended is

Answer 1

Array is a Swift construct, and generic struct, which means that it can be an array of any specific type (Int, String, AnyObject, etc.)

[T] is syntactic sugar for Array<T>

AnyObject is an object of any class, including Objective-C classes.

NSArray is an Objective-C construct that can hold any Objective-C object and is transparently mapped to and from Array<AnyObject>

Answer 2

Array is a struct, therefore it is a value type in Swift. NSArray is an immutable Objective C class, therefore it is a reference type in Swift and it is bridged to Array<AnyObject>. NSMutableArray is the mutable subclass of NSArray.

var arr : NSMutableArray = ["Pencil", "Eraser", "Notebook"]
var barr = ["Pencil", "Eraser", "Notebook"]

func foo (var a : Array<String>)
{
    a[2] = "Pen"
}

func bar (a : NSMutableArray)
{
    a[2] = "Pen"
}

foo(barr)
bar(arr)

println (arr)
println (barr)

Prints:

(
    Pencil,
    Eraser,
    Pen
)
[Pencil, Eraser, Notebook]

Because foo changes the local value of a and bar changes the reference. It will also work if you do let arr instead of var as with other reference types.

Answer 3

Adding to @Krzak's excellent answer, this is why

print(NSArray().object(at: 1))   // Triggers an UnmanagedException

2018-11-09 11:38:08.798088-0600 AppName[38786:10497909] * Terminating app due to uncaught exception 'NSRangeException', reason: '* -[__NSArray0 objectAtIndex:]: index 1 beyond bounds for empty NSArray'

and

print(Array<Int>()[1])  // Halts with "Thread 1: Fatal error: Index out of range"

This different handling of the error helped me understanding the difference..... e

Answer 4

Using the Krzak answer, here is a practical example:

// Let´s create an Array as a struct showing alternative ways
    var arrStruct = ["Pencil", "Eraser", "Notebook"]
    // or var arrStruct: [String] = ["Pencil", "Eraser", "Notebook"]
    // or var arrStruct: Array = ["Pencil", "Eraser", "Notebook"]
    // or var arrStruct = Array(["Pencil", "Eraser", "Notebook"])
    // All this alternative ways create an array as struct

    // Now let´s create a function that modifies this array struct
    func modifyArr(alternativeArr: [String]) 
    // or func modify(alternativeArr: Array<String>)
    {
        alternativeArr[2] = "Pen" // compilation error
        // This won´t work. In swift >= 3.0 all func parametes are a let variable, 
        // this means alternativeArr is defined as a let. What one has to do is 
        // create a local variable and copy the value.

        var localAlternativeArr = alternativeArr
        // or var localAlternativeArr: [String] = alternativeArr
        // or var localAlternativeArr: Array = alternativeArr

        // now we can change it.
        localAlternativeArr[2] = "Pen"
        print(localAlternativeArr) // ["Pencil", "Eraser", "Pen"]
        print(alternativeArr) // ["Pencil", "Eraser", "Notebook"]
    }

    modifyArr(alternativeArr: arrStruct)
    print(arrStruct) // ["Pencil", "Eraser", "Notebook"]

    // Since the arrStruct is a struct every time we assign to another variable or 
    // pass it as a func argument a copy is made.



// Now let´s create as an NSMutableArray
    var arrClass: NSMutableArray = ["Pencil", "Eraser", "Notebook"]
    // or var arrStruct = NSMutableArray(array: ["Pencil", "Eraser", "Notebook"])
    // All this create an NSMutableArray as a class

    // Now let´s create a function that modifies this array struct
    func modifyArr(alternativeArr: NSMutableArray)
    {
        alternativeArr[2] = "Pen"
        print(alternativeArr)
        // (
        //   Pencil,
        //   Eraser,
        //   Pen
        // )
    }

    modifyArr(alternativeArr: arrClass)
    print(arrClass)
    // (
    //   Pencil,
    //   Eraser,
    //   Pen
    // )

    // Since the arrClass is a class everytime we assign to another variable or 
    // pass it as a func argument is passed by reference. Means that any change
    // inside modifyArr is going to change the arrClass outside. The change 
    // is made in the same pointer.