Is it possible to make an Array extension in Swift that is restricted to one class?

Question

Can I make an Array extension that applies to, for instance, just Strings?

Answer 1

As of Swift 2, this can now be achieved with protocol extensions, which provide method and property implementations to conforming types (optionally restricted by additional constraints).

A simple example: Define a method for all types conforming to SequenceType (such as Array) where the sequence element is a String:

extension SequenceType where Generator.Element == String {
    func joined() -> String {
        return "".join(self)
    }
}

let a = ["foo", "bar"].joined()
print(a) // foobar

The extension method cannot be defined for struct Array directly, but only for all types conforming to some protocol (with optional constraints). So one has to find a protocol to which Array conforms and which provides all the necessary methods. In the above example, that is SequenceType.

Another example (a variation of How do I insert an element at the correct position into a sorted array in Swift?):

extension CollectionType where Generator.Element : Comparable, Index : RandomAccessIndexType {
    typealias T = Generator.Element
    func insertionIndexOf(elem: T) -> Index {
        var lo = self.startIndex
        var hi = self.endIndex
        while lo != hi {
            // mid = lo + (hi - 1 - lo)/2
            let mid = lo.advancedBy(lo.distanceTo(hi.predecessor())/2)
            if self[mid] < elem {
                lo = mid + 1
            } else if elem < self[mid] {
                hi = mid
            } else {
                return mid // found at position `mid`
            }
        }
        return lo // not found, would be inserted at position `lo`
    }
}

let ar = [1, 3, 5, 7]
let pos = ar.insertionIndexOf(6)
print(pos) // 3

Here the method is defined as an extension to CollectionType because subscript access to the elements is needed, and the elements are required to be Comparable.