Unfortunately the error messages you get with protocols are limited :). The
compiler is doing the right thing, no bug. The problem is that it is really
easy to mak
-- Howard.
> On 1 Jan 2018, at 3:40 pm, Toni Suter via swift-users <swift-users@swift.org>
> wrote:
>
> Hi Marc,
>
> There are several subtleties here, but I think the compiler is actually doing
> the right thing.
>
> The second class defines a static property that looks like it is 'overriding'
> the static property from
> the protocol extension, but since the types don't match (String vs. String?),
> it sort of 'overloads'
> the property (similar to function overloading). Nevertheless, the class still
> fulfills the requirements
> of the Trackable protocol, by inheriting the static property from the
> protocol extension.
>
> When you access analyticsID like a regular static property, the Swift
> compiler will choose the String property,
> because it shadows the String? property:
>
> let x = Something2.analyticsID
> print(x) // Wrong but compilers, returns
> wrong value
> print(type(of: x)) // String
>
> However, when the context of the expression Something2.analyticsID expects a
> String?, the Swift compiler will
> choose the String? property:
>
> let a: String? = Something2.analyticsID // explicit type
> annotation demands a String?
> print(a) // nil
> print(type(of: a)) // Optional<String>
>
> let b = Something2.analyticsID as String? // type cast demands a String?
> print(b) // nil
> print(type(of: b)) // Optional<String>
>
> A similar thing happens, when you write Something2.analyticsID ?? "nil". The
> nil coalescing operator ?? demands that the first parameter
> is an optional. Therefore, the Swift compiler will choose the String?
> property instead of the String property.
>
> I hope this helps!
>
> Best regards,
> Toni
>
>> Am 01.01.2018 um 18:29 schrieb Marc Palmer via swift-users
>> <swift-users@swift.org>:
>>
>> Hi,
>>
>> I hope everybody had a great New Year celebration.
>>
>> I was tracking down a weird bug in my Swift code today. A property defined
>> in a class in order to conform to a protocol was not being seen. A protocol
>> extension provided a default value of `nil` for this property, so I knew
>> where it was coming from. Turned out, in my class I had defined the property
>> with the correct name but incorrect type - I declared it as `String` instead
>> of `String?`.
>>
>> I isolated this behaviour in a playground, shown below, and it is pretty
>> weird behaviour.
>>
>> The output is:
>>
>> Something1 has id: nil
>> Something2 has id: nil
>> Something3 has id: Correct
>> -- Direct access--
>> Something1 - nil
>> Something2 - nil
>> Something2 with String(describing:) - Wrong but compiles, returns wrong value
>> Something3 - Correct
>>
>> The playground code:
>>
>> ======================
>>
>> protocol Trackable {
>> static var analyticsID: String? { get }
>> }
>>
>> extension Trackable {
>> static var analyticsID: String? { return nil }
>> }
>>
>> class Something1: Trackable {
>> }
>>
>> class Something2: Trackable {
>> static var analyticsID: String = "Wrong but compiles, returns wrong value"
>> }
>>
>> class Something3: Trackable {
>> static var analyticsID: String? = "Correct"
>> }
>>
>> func getID<T: Trackable>(_ trackable: T.Type) {
>> if let id = trackable.analyticsID {
>> print("\(trackable) has id: \(id)")
>> } else {
>> print("\(trackable) has id: nil")
>> }
>> }
>>
>> getID(Something1.self)
>> getID(Something2.self)
>> getID(Something3.self)
>>
>> print("-- Direct access--")
>> print("Something1 - \(Something1.self.analyticsID ?? "nil")")
>> print("Something2 A - \(Something2.self.analyticsID ?? "nil")")
>> print("Something2 with String(describing:) - \(String(describing:
>> Something2.self.analyticsID))")
>> print("Something3 - \(Something3.self.analyticsID ?? "nil")”)
>> ======================
>>
>> Thanks in advance for any information about my misinterpretations or
>> recommendations of what parts are actually undesirable so that I can raise
>> the JIRAs.
>>
>> Cheers
>>
>> —
>> Marc Palmer
>> Montana Floss Co. Ltd.
>>
>> Soundproof – Music Player for Practice
>> http://getsoundproof.com
>>
>>
>>
>> _______________________________________________
>> swift-users mailing list
>> swift-users@swift.org
>> https://lists.swift.org/mailman/listinfo/swift-users
>
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