[swift-users] Protocol extension code executed instead of class code

[Diego Sánchez]

That would be great, thanks for the heads up!

2016-02-20 23:22 GMT+00:00 Joe Groff <jgroff at apple.com>:

>
> > On Feb 20, 2016, at 4:41 AM, Diego Sánchez via swift-users <
> swift-users at swift.org> wrote:
> >
> > Consider the following code
> >
> > protocol MyProtocol {
> >     func doSomething()
> > }
> >
> > extension MyProtocol {
> >     func doSomething() {
> >         print("default impl")
> >     }
> > }
> >
> > class A: MyProtocol {}
> >
> > class B: A {
> >     func doSomething() {
> >         print("B impl")
> >     }
> > }
> >
> > let a: MyProtocol = A()
> > a.doSomething() // Prints "default impl"
> > let b: MyProtocol = B()
> > b.doSomething() // Prints "default impl" instead of "B impl"!
> >
> > Now let's override doSomething in A...
> >
> > class A: MyProtocol {
> >     func doSomething() {
> >         print("A impl")
> >     }
> > }
> >
> > class B: A {
> >      override func doSomething() {
> >         print("B impl")
> >     }
> > }
> >
> > let a: MyProtocol = A()
> > a.doSomething() // Now it prints "A impl"
> > let b: MyProtocol = B()
> > b.doSomething() // Now it prints "B impl"
> >
> > That's clearly inconsistent. I would expect to print "B impl" in the
> first case; or maybe always "default impl" (I highly prefer the first
> option)
>
> If you don't say 'override', you're not overriding anything, but defining
> a logically independent method. In the first example, `doSomething` isn't
> really a member of `A`, so it's not overrideable by subclasses, and the
> declaration of `B.doSomething` just shadows the protocol extension
> implementation instead of overriding it. This is admittedly weird, so we do
> have some proposals floating to improve things by implicitly mirroring
> protocol methods as class methods when a class conforms to a protocol.
>
> -Joe
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