[swift-users] Question with calling a method which returns `Self` on super

[吴君恺]

Thank you, Slava.

I will use the trick you provided.
And knowing that this bug has already been under tracking is a great comfort to me, really :)

Lincoln

> 在 2017年8月11日，上午11:29，Slava Pestov <spestov at apple.com> 写道：
> 
> 
>> On Aug 10, 2017, at 8:23 PM, Slava Pestov via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> 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 <swift-users at swift.org <mailto:swift-users at swift.org>> 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.
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>>> swift-users at swift.org <mailto:swift-users at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-users <https://lists.swift.org/mailman/listinfo/swift-users>
>> 
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> 

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