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

吴君恺 wjk_buaa at 163.com
Thu Aug 10 22:15:58 CDT 2017

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...

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