Making custom type array convertible in Swift

Question

Let's assume we have a custom type given by

struct S {
    let value_1: UInt = 1
    let value_2: UInt = 2
}

and I want to create a Double array (e.g. [1.0, 2.0]) using a cast:

let s = S()
let a = s as Array<Double> // fails obviously

The final goal is to call

func aMethod(array: [Double]) { ... }

in a convient way:

let s = S()
aMethod(s)

My first idea was to use

extension S {
    // but how to continue here?
}

Any ideas of solving this in an elegant way without a static method?

Eric Aya · Accepted Answer · 2015-10-12 10:10:10Z

2

You could add an array property to the Struct, for example:

struct S {
    let value_1: UInt = 1
    let value_2: UInt = 2
    var array: [UInt] {
        return [value_1, value_2]
    }
}

let s = S()
let a = s.array  // [1, 2]

An extension also works:

struct S {
    let value_1: UInt = 1
    let value_2: UInt = 2
}

extension S {
    var array: [UInt] {
        return [value_1, value_2]
    }
}

let s = S()
let a = s.array  // [1, 2]

And why not a protocol:

protocol HasArray {
    var value_1: UInt { get }
    var value_2: UInt { get }
    var array: [UInt] { get }
}

extension HasArray {
    var array: [UInt] {
        return [value_1, value_2]
    }
}

struct S: HasArray {
    let value_1: UInt = 1
    let value_2: UInt = 2
}

let s = S()
let a = s.array  // [1, 2]

edited Oct 12, 2015 at 10:10

answered Oct 12, 2015 at 9:56

Eric Aya

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4 Comments

Michael Dorner Over a year ago

I assume there is no way for methodExpectingADoubleArray(s), because there is no implicit conversion in Swift, right?

Eric Aya Over a year ago

Mm, I'm not sure I understand your question. Do you mean duck typing like in Ruby?

Michael Dorner Over a year ago

Actually no, more in the direction of implicit conversion. But I think this is not possible, right?

Eric Aya Over a year ago

Indeed, I think doing it exactly how you want isn't possible in Swift.

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