Array of implicitly unwrapped optionals iterates forever in Xcode 8 beta 4

Question

My code (fragment below) causes the Simulator to Hang. What am I doing wrong?

To reproduce the problem, cut and paste into the Simulator.

class INK
{
    var test = 1
}

var array = [INK!](repeating: nil, count: 1)

for idx in 0..<array.count
{
    array[idx] = INK()
}

var idx2 = 0
for ink_item in array
{
    idx2 += 1
    print("idx2=\(idx2)")
}

Holohedral

You are adding items to the same array you are iterating over. Your question looks like you have not taken the time to debug it yourself. – Chlorobenzene

Thankyou for the feedback codingFriend. The issue that I have is that I want a lazily initiated array. But the problem that I am facing is that when an array has optional items within it .. the for loop Never exits. In the above code array.count =1 ... but the for loop iterates to infinity. – Holohedral

Answer 1

This is a known bug, see SR-1635. Since an IUO is no longer a distinct type, it shouldn't really be possible to have an array of them in the first place.

This is confirmed by the fact that the following code fails to compile:

// compiler error: Implicitly unwrapped optionals are only allowed at top level.
// and as function results
var array: [Ink!] = []

^{(note I renamed your class name to Ink to conform to Swift naming conventions)}

Depending on your situation, you may want to consider using a lazy property instead:

lazy var array : [Ink] = {

    var array = [Ink]()

    // populate array

    return array
}()

Or making the array itself an implicitly unwrapped optional (and defer both the allocation and initialisation of the array):

var array : [Ink]!

Although note that IUOs should always be a last resort due to their inherent unsafety.

Unspeakable

Thankyou Hamish for link the bug database. I had tried searches in Apple's bug tracker, but was unsuccessful (and it takes forever to return!). Thankyou also for the lazy suggestion. Glad that this is known issue. I was beginning to think that Swift3 conversion would required me revisiting all for loop to 'break' when 'nil' items were encountered. – Holohedral

@Holohedral Actually, I'm still able to reproduce this in Xcode 8 beta 5. – Unspeakable

@ Hamish. Agreed. This problem is still reproducible on Beta5. – Holohedral