MatterGoal MatterGoal - 1 year ago 91
Swift Question

Optional chaining and Array in swift

Let's take these two simple classes to show my problem:

class Object{
var name:String?
// keep it simple... and useless

class TestClass {
var objects:AnyObject[]?

func initializeObjects (){
objects?.insert(Object(), atIndex:0) // Error
objects?.insert(Object(), atIndex:1) // Error
objects?.insert(Object(), atIndex:2) // Error

With this implementation I get 3 errors
Could not find member 'insert'
where I try to add object into the

Now, if I remove the optional from
definition and the optional chain in
it works with no problem (here the working code)

class Object{
var name:String?

class TestClass {
var objects:AnyObject[] = AnyObject[]() // REMOVE optional and initialize an empty array

func initializeObjects (){
objects.insert(Object(), atIndex:0) // Remove Opt chaining
objects.insert(Object(), atIndex:1) // Remove Opt chaining
objects.insert(Object(), atIndex:2) // Remove Opt chaining

I can't understand what is wrong in the first implementation.
I thought it checks with
is not
and at this point it adds an element using
. But I'm probably wrong -.-

Answer Source

Arrays in Swift are structs and structs are value types.
Optionals in Swift are actually enums (Optional<T> or ImplicitlyUnwrappedOptional<T>).

When you are unwrapping an optional (implicitly or explicitly) of a value type, what you get is actually a constant copy of the struct. And you can't call mutating methods on a constant struct.

Executing objects?.insert(Object(), atIndex:0) basically means this:

if let tmp = objects {
    tmp.insert(Object(), atIndex:0)

As a workaround, you need to assign the unwrapped value to a variable and then assign the variable back to your optional property. That's how value types work.

This is reproducible for any struct, not only Arrays:

struct S {
    var value: Int = 0

var varS: S = S()
varS.value = 10 //can be called

let constS: S = S()
constS.value = 10 //cannot be called - constant!

var optionalS: S? = S()
optionalS?.value = 10 //cannot be called, unwrapping makes a constant copy!

if optionalS {
    var tmpS = optionalS!
    tmpS.value = 10
    optionalS = tmpS

Some relevant discussion here:

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