Swift extends determined generics type without using objc bindings

Question

I wanted to try implementing message pack in pure swift. Everything worked well until I came to Array.

The approach I had was to implement a protocol called MsgPackMarshable thats primitiv types extended :

 //Protocol
 protocol MsgPackMarshable{

    func msgpack_marshal() -> Array<Byte>
}

//Implementation for some types
extension UInt8 : MsgPackMarshable{
    func msgpack_marshal() -> Array<Byte>{

        return [0xcc, self]
    }
}

extension UInt16 : MsgPackMarshable{
    func msgpack_marshal() -> Array<Byte>{

        let bigEndian: UInt16 = self.bigEndian
        return [0xcd, Byte((bigEndian & 0xFF00) >> 8), Byte(bigEndian & 0x00FF)]
    }
}

extension UInt32 : MsgPackMarshable{
    func msgpack_marshal() -> Array<Byte>{

        let bigEndian: UInt32 = self.bigEndian
        return [0xce, Byte((bigEndian & 0xFF000000) >> 24), Byte((bigEndian & 0xFF0000) >> 16), Byte((bigEndian & 0xFF00) >> 8), Byte(bigEndian & 0x00FF)]
    }
}

I had some troubles extending Array. I wanted to verify dynamically that the type of the array were implementing the MsgPackMarshable protocol:

extension Array: MsgPackMarshable{
    func msgpack_marshal() -> Array<Byte>{

        for item in self{

           //How to check type here?
        }
                    return []
    }
}

Due to the fact that Array is a struct in swift, I wanted to avoid redefining a new type of Array<MsgPackMarshable> that embeds an Array.

Unrelated to your array problem, but note that for UInt16/32 you have to right-shift by 8, 16, or 24, not by 2, 3, or 4. — Martin R, Jul 14 '14 at 09:02

score 0 · Answer 1 · answered Jul 14 '14 at 11:54

0

The type parameter T from the type being extended (ie Array) is in scope and can be used to determine the type. Use the is operator to do type checking (T is yourtype).

answered Jul 14 '14 at 11:54

Jesper

7,477
4
40
57

Hm; [see this question](http://stackoverflow.com/questions/24047164/extension-of-constructed-generic-type-in-swift). – Jesper Jul 14 '14 at 12:10
1

I'm not allowed to use the `is` operator on a non @objc protocol. The error is : cannot downcast from 'T' to non-@objc protocol type 'MsgPackMarshable' – yageek Jul 14 '14 at 12:39
Very weird. I can't find anything that speaks directly to this without making a function that has an extra constraint where T is constrained to an additional protocol. Maybe you could use the `MsgPackMarshable` protocol. – Jesper Jul 14 '14 at 13:22
What do you mean by "Maybe you could use the MsgPackMarshable protocol?" – yageek Jul 14 '14 at 13:52
You're already making various objects conform to the `MsgPackMarshable` protocol. If you use a [type constraint](https://developer.apple.com/library/prerelease/ios/documentation/swift/conceptual/swift_programming_language/Generics.html#//apple_ref/doc/uid/TP40014097-CH26-XID_243) on the function (``) to say that T must conform to that protocol, you know that all the items conform to `MsgPackMarshable` and you have those methods available to you in the for loop. It doesn't help you in knowing exactly which type, but you can call the methods. – Jesper Jul 14 '14 at 13:56
That only seems to work if the function is global. I haven't been able to find a syntax that adds constraints to a generic type and says, in effect, this function only exists in the generic if this condition is met. A similar problem arises if you try to implement an extension on Array that adds a `uniqueItems` method, but only on Arrays where T: Comparable. – David Berry Jul 14 '14 at 15:34

Swift extends determined generics type without using objc bindings

1 Answers1