Pointer to pointer parameter in Swift function

Question

How do you do pointers to pointers in Swift? In Objective-C I had a function which I would call recursively so that I could keep track of the number of recursions, but I'm stumped as to how to achieve this in Swift.

NSNumber *recursionCount = [NSNumber numberWithInteger:-1];
[self doRecursion:&recursionCount];

- (void)doRecursion:(NSNumber **)recursionCount {
    // I'm sure this is a terrible way to increment, but I have limited Objective-C experience
    // and this worked, so I moved on after failing to find a decent var++ equivalent :-(
    int addThis = (int)i + 1;
    *recursionCount = [NSNumber numberWithInt:[*recursionCount intValue] + addThis];
    [self doRecursion:recursionCount];
}

In the process of cutting this sample down for this post, I've ended up creating a never-ending loop, but you get the idea on how I'm remembering the value with each recursion.

Does anybody know how to do this in Swift? Thanks.

Antonio · Accepted Answer · 2014-09-10 12:12:44Z

10

Usage of pointers in swift is highly discouraged.

To change a variable passed as argument to a function, you have to pass it by reference (similar to passing its pointer) using the inout modifier. I've changed the logic of your function to stop after 10 iterations:

func doRecursion(inout recursionCount: Int) -> Int {
    if (recursionCount < 10) {
        ++recursionCount
         doRecursion(&recursionCount)
    }

    return recursionCount
}

Then you can call using:

var counter: Int = -1
let x = doRecursion(&counter) // Prints 10 in playground

1 Comment

Rainier Wolfcastle Over a year ago

Thanks. I must have missed or skimmed the section on inout parameters. I even tried AutoreleasingUnsafeMutablePointer<Int> and UnsafeMutablePointer<Int>, but that led me down a path of sorrow :)

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