And... is this all OK? No one think this is a broken design?
So, in addition to the example with getInt() (when no method declaration
exists in protocol itself, but just in extension and in type), we have this:
protocol A {
func f()
func x()
}
extension A {
func x() {print("a-x")}
}
class B: A {
func f() {}
}
class C: B {
func x(){print("c-x")}
}
var c : A = C()
c.x() // a-x. but C() *implements* x() that *is* defined in protocol
I don't know how this could be called 'expected' and 'wanted' behavior.
Who was talking here regarding the main target of Swift to be readable and
understandable even by 'poor beginners' ? ;-)
On 19.05.2016 20:12, Goffredo Marocchi via swift-evolution wrote:
Hello Jeremy,
It seems it is a little bit more complex than this:
http://allblue.me/swift/2016/01/23/swift-method-dispatch-with-protocol-extension-protocol-extension-and-subclass/
diagram
* IF the inferred type of a variable is the protocol:
o IF the method is in a class conform directly to the original protocol:
+ AND the method is defined in the original protocol
# THEN the runtime type’s implementation is called,
irrespective of whether there is a default implementation
in the extension.
+ AND the method is not defined in the original protocol,
# THEN the default implementation is called.
o ELSE IF the method is in a subclass of another class who conform to
the original protocol
+ THEN the default implementation of protocol is called.
* ELSE IF the inferred type of the variable is the type
o THEN the type’s implementation is called.
On Thu, May 19, 2016 at 5:24 PM, Jeremy Pereira via swift-evolution
<[email protected] <mailto:[email protected]>> wrote:
> On 19 May 2016, at 13:30, Krystof Vasa via swift-evolution
<[email protected] <mailto:[email protected]>> wrote:
>
> See this example that demonstrates how it's pretty much unusable (IMHO),
since whenever you refer to the instance as to the protocol, the default
implementation gets invoked:
>
> protocol MyProtocol { }
>
> extension MyProtocol {
> func getInt() -> Int {
> return 0
> }
> }
>
> class MyClass: MyProtocol {
> func getInt() -> Int {
> return 1
> }
> }
>
>
> let instance = MyClass()
> instance.getInt() // 1
>
> var anyInstance: MyProtocol = instance
> anyInstance.getInt() // 0 !!!!!!!
>
>
> Since anyInstance is of MyProtocol type, you get the default
implementation (no dynamic dispatch).
That’s because the only information that the compiler has about
anyInstance is that it conforms to MyProtocol which has no methods so
it doesn’t know that it can dispatch getInt() to the implementation in
MyClass. Change the protocol to
protocol MyProtocol {
func getInt() -> Int
}
and it will work as expected.
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