swift - can I call a struct default memberwise init from my custom init method?

Question

If I create a swift struct with no init, then I can call the compiler-generated default memberwise initialiser, like so:

struct OrderFill {
    let price:Int
    let qty: Int
    let timeStamp: NSDate
}
let o = OrderFill(price: 2, qty: 1, timeStamp: someDate)

What I'd like to do is create a convenience init method to deserialise from a dictionary, which then chains to the default memberwise init. Something like

struct OrderFill {
    let price:Int
    let qty: Int
    let timeStamp: NSDate

    init(dict:[String:AnyObject]) throws {
        self.init(
            price: dict["price"] as! Int
            qty: dict["qty"] as! Int
            timeStamp: try parseDate(dict["ts"] as! String)
    }
}
let o = OrderFill(someDict)

When I try write this code though, the compiler (Xcode 7.2) gives me the error "Extra argument 'qty' in call" as though it doesn't see the default memberwise and is trying to recursively call init(dictionary)

I can write my own memberwise init, or I can simply assign the properties directly from my init(dictionary), but it'd be nice if I could chain the call. Is there any way to do this in swift?

question from:https://stackoverflow.com/questions/34728518/swift-can-i-call-a-struct-default-memberwise-init-from-my-custom-init-method

Answer 1

Add your own initializer as an extension to your struct. Extensions cannot remove existing functionalities, so it will preserve struct's default initializer.

struct OrderFill {
    let price: Int
    let qty: Int
    let timeStamp: NSDate
}

extension OrderFill {

    init(dict: [String: AnyObject]) throws {
        self.init(
            price: dict["price"] as! Int,
            qty: dict["qty"] as! Int,
            timeStamp: try parseDate(dict["ts"] as! String)
        )
    }
}

let o = OrderFill(someDict)