Swift: inout with generic functions and constraints

Question

I am making my first steps in Swift and got along the first problem. I am trying to pass an array by reference using inout on an generic function with constraints.

First, my application start point:

import Foundation

let sort = Sort()
sort.sort(["A", "B", "C", "D"])

And here my class with the actual problem:

import Foundation

class Sort {
    func sort<T:Comparable>(items:[T]){
        let startIndex = 0
        let minIndex = 1
        exchange(&items, firstIndex: startIndex, secondIndex: minIndex)
    }

    func exchange<T:Comparable>(inout array:[T], firstIndex:Int, secondIndex:Int) {
        // do something with the array
    }
}

I am getting the following error in Xcode on the line calling exchange :

Cannot convert value of type '[T]' to expected argument type '[_]'

Am I missing here something?

Update: Added complete project code.

Answer 1

It works with following modifications:

• the array passed into it must be a var. As mentioned in the documentation , inouts must not be let or literals.

  You cannot pass a constant or a literal value as the argument, because constants and literals cannot be modified.

• items in declaration must also be inout to indicate that it must be var again

---

import Foundation


class Sort {
    func sort<T:Comparable>(inout items:[T]){
        let startIndex = 0
        let minIndex = 1
        exchange(&items, firstIndex: startIndex, secondIndex: minIndex)
    }

    func exchange<T:Comparable>(inout array:[T], firstIndex:Int, secondIndex:Int) {
        // do something with the array
    }
}


let sort = Sort()
var array = ["A", "B", "C", "D"]
sort.sort(&array)

Answer 2

You can "exchange" two values in an array by using the swift swap function.

eg

var a = [1, 2, 3, 4, 5]
swap(&a[0], &a[1])

would mean that a now is [2, 1, 3, 4, 5]