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

[Slava Pestov]

> On Aug 10, 2017, at 8:23 PM, Slava Pestov via swift-users <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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> 
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