> On Aug 10, 2017, at 8:23 PM, Slava Pestov via swift-users
> <[email protected]> wrote:
>
> Hi Lincoln,
>
> This is a known issue with ‘super’ method calls:
> https://bugs.swift.org/browse/SR-1736 <https://bugs.swift.org/browse/SR-1736>
>
> A workaround is to cast the result of the supermethod call to Self explicitly:
>
> let copied = super.copy() as! Self
I’m sorry, but this is nonsense. We don’t allow ‘Self’ to refer to the dynamic
Self type yet.
You could do something like this I guess,
func cast<T, U>(from: T, to: U.Type) -> U {
return from as! U
}
let copied = cast(from: super.copy(), to: type(of: self))
Slava
>
> Slava
>
>> On Aug 10, 2017, at 8:16 PM, 吴君恺 via swift-users <[email protected]
>> <mailto:[email protected]>> wrote:
>>
>> Hi all,
>>
>> here is the scenario, I want to implement a Copying protocol which can be
>> used homogeneously
>>
>> protocol Copying {
>> func copy() -> Self
>> }
>>
>> Because this protocol contains neither associated-type-requirements or
>> self-requirements(requiring a method returning Self is different), it can
>> surely be used homogeneously.
>>
>> let objects: [Copying] = ....
>> for copyable in objects {
>> .....
>> }
>>
>> It will work if I make one of my base class conform to Copying
>>
>> class Shape: Copying {
>> var color: UIColor?
>>
>> required init() {}
>>
>> func copy() -> Self {
>> let copied = type(of: self).init()
>> copied.color = color
>> return copied
>> }
>> }
>>
>> The implementation of `copy` above is forced by compiler to avoid any
>> explicit specification of type `Shape`, so that the returning value will
>> have a dynamic type, which is just what I want. Here, the type of `copied`
>> is `Self`
>>
>> However, if I try to make a subclass of Shape, I can't find a elegant way to
>> implement this `copy` method in that subclass, the following code will not
>> compile.
>>
>> class Circle: Shape {
>> var radius: Float = 5
>>
>> func copy() -> Self {
>> let copied = super.copy()
>> copied.radius = radius // compilation error
>> return copied
>> }
>> }
>>
>> The compiler will complain that `copied` has no property `radius`. It turns
>> out that calling copy() on super will yield a value of type Shape, rather
>> than Self.
>>
>> Swift now forbids explicit conversion to `Self` (I totally agree with this
>> rule), and will automatically allow `Self` to be treated as a specific type
>> in some circumstances. But for this case, I wonder whether this is the
>> correct behavior or a bug? Why calling `super.copy()` not be able to get a
>> `Self`?
>>
>> I did find a work-around for this problem afterwards, but this question
>> really haunts me...
>>
>> Cheers,
>> Lincoln.
>> _______________________________________________
>> swift-users mailing list
>> [email protected] <mailto:[email protected]>
>> https://lists.swift.org/mailman/listinfo/swift-users
>
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