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Creating uninitialized arrays

Available from Swift 5.1

Paul Hudson      @twostraws

SE-0245 introduced a new initializer for arrays that doesn’t pre-fill values with a default. This was previously available as a private API, which meant Xcode wouldn’t list it in its code completion but you could still use it if you wanted – and if you were happy to take the risk that it wouldn’t be withdrawn in the future!

To use the initializer, tell it the capacity you want, then provide a closure to fill in the values however you need. Your closure will be given an unsafe mutable buffer pointer where you can write your values, as well as an inout second parameter that lets you report back how many values you actually used.

For example, we could make an array of 10 random integers like this:

let randomNumbers = Array<Int>(unsafeUninitializedCapacity: 10) { buffer, initializedCount in
    for x in 0..<10 {
        buffer[x] = Int.random(in: 0...10)
    }

    initializedCount = 10
}

There are some rules here:

  1. You don’t need to use all the capacity you ask for, but you can’t go over capacity. So, if you ask for a capacity of 10 you can set initializedCount to 0 through 10, but not 11.
  2. If you don’t initialize elements that end up being in your array – for example if you set initializedCount to 5 but don’t actually provide values for elements 0 through 4 – then they are likely to be filled with random data. This is A Bad Idea.
  3. If you don’t set initializedCount it will be 0, so any data you assigned will be lost.

Now, we could have rewritten the above code using map(), like this:

let randomNumbers2 = (0...9).map { _ in Int.random(in: 0...10) }

That’s certainly easier to read, but it’s less efficient: it creates a range, creates a new empty array, sizes it up to the correct amount, loops over the range, and calls the closure once for each range item.

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