[swift-users] Ambiguity passing `Any?` to generic constructor

[Zhao Xin]

You can just remove

    public init(_ a: T) {
        actual = a
    }

It is redundant. Below code works.

public struct check<T> {

    let actual: T?

//

//    public init(_ a: T) {

//        actual = a

//    }



    public init(_ a: T?) {

        actual = a

    }



    public var isNil: Bool {

        return actual == nil

    }

}


let str: String = "Hi"

check(str).isNil // false


let maybe: String? = nil

check(maybe).isNil // true


let hmm: Any? = nil

check(hmm).isNil // true


Zhaoxin

On Mon, Aug 29, 2016 at 11:39 PM, Jens Alfke via swift-users <
swift-users at swift.org> wrote:

> I’m making a little utility struct that needs to be able to wrap an
> arbitrary value, which may be an optional, and be able to tell whether it’s
> nil. So I declared it like this (I’ve removed the interesting/useful parts
> of the code leaving just the constructors and one property):
>
> public struct check<T> {
>     let actual: T?
>
>     public init(_ a: T) {
>         actual = a
>     }
>
>     public init(_ a: T?) {
>         actual = a
>     }
>
>     public var isNil: Bool {
>         return actual == nil
>     }
> }
>
>
> This works fine, except when I try to use it with a value of type `Any?`:
>
> let str: String = "Hi"
> check(str).isNil // false
>
> let maybe: String? = nil
> check(maybe).isNil // true
>
> let hmm: Any? = nil
> check(hmm).isNil // Error: “Ambiguous use of ‘init'"
>
>
> (I agree that `Any?` is sort of a weird type; in my code it represents a
> value that might be a JSON object. NSJSONSerialization parses to type
> `Any`, but there might not be a value, so it becomes an Optional<Any> or
> Any?.)
>
> This worked in Swift 2.2, but after upgrading to Swift 3 [in the latest
> Xcode 8 beta] the compiler started complaining.
>
> How can I fix the `check` struct's constructor(s) to make this work?
>
> —Jens
>
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
>
>
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