Swift: How to assign a variable by reference, not by value?

Question

I\'m trying to get a reference to an Array and make modifications to it. Because Arrays in Swift are value types, instead of reference types, if I

Answer 1

Array is a struct, which means it's a value type in Swift. Because of this, arrays always behave according to value and not reference semantics. The problem here is that you're attempting to use mutable, reference based logic to operate on values types.

You don't want to rely on mutations occurring inside the function to propagate back to the caller. As you've found, this is only possible with inout parameters. What you should do instead is return the mutated array from the function back to the caller. The point of value oriented programming is that it shouldn't matter which array you have, but rather that any two equivalent arrays or values types are interchangeable.

It's slightly easier to imagine with another value type. Take an Int for example, and this function that does some math.

func addFive(int: Int) -> Int {
    return int + 5
}

Now consider a similar function, but written in the reference oriented style that you're attempting to use:

func addFive(inout int: Int) {
    int = int + 5
}

You can see it's simply not natural to operate on value types this way. Instead just return the updated value (the modified arrays) from your function and carry on from there.

Here is your function refactored with value semantics.

func move(source: [String], destination: [String], value:String) -> ([String], [String]) {

    var mutableSource = source
    var mutableDestination = destination

    mutableSource.removeAtIndex(source.indexOf(value)!)
    mutableDestination.append(value)

    return (mutableSource, mutableDestination)
}

let (updatedSource, updatedDestination) = move(odds, destination: evens, value:one)