CODEWITHSUNDEEP
iosgenericsinheritanceswift
Andrey Gordeev
Andrey Gordeev

Reputation: 32459

Generic class inheritance in Swift

I have the following class:

class BaseCache<T: Equatable>: NSObject {

    var allEntities = [T]()

    // MARK: - Append

    func appendEntities(newEntities: [T]) {
        ....
    }
}

Now I want to subclass it, but I get annoying error, that my type "does not conform to protocol 'Equatable'": enter image description here

It seems generics in Swift are real pain-in-the-ass.

Upvotes: 17

Views: 24175

Answers (2)

jimmy
jimmy

Reputation: 159

this should work: < swift 4 >

class TrackingCache<T: AftershipTracking>: BaseCache<T>

Another example:

protocol P {

}

class C: P {

}

class CS: C {

}

class L<T:P> {
    let c: T

    init(_ c: T) {
        self.c = c
    }
}

class LS<T:CS>:L<T> {

}

let i = LS(CS())

i.c

c is CS now.

Upvotes: 9

lassej
lassej

Reputation: 6494

Your class definition of TrackingCache is wrong. It repeats the generic parameter:

class TrackingCache<AftershipTracking>: BaseCache<AftershipTracking> { }

It should be left out:

class TrackingCache: BaseCache<AftershipTracking> { }

This triggers the underlying swift error Classes derived from generic classes must also be generic. You can work around this issue by specifying a type parameter that is required to be or inherit from AftershipTracking:

class TrackingCache<T: AftershipTracking>: BaseCache<AftershipTracking> { }

Full example:

class BaseCache<T: Equatable>: NSObject {
  var items: [T] = []

  func appendItems( items: [T]) {
    self.items += items
    didAppendItems()
  }

  func didAppendItems() {} // for overriding
}

class AftershipTracking: NSObject {
  var identifier: Int
  init( identifier: Int) {
    self.identifier = identifier
    super.init()
  }
}

extension AftershipTracking: Equatable { }

func ==( lhs: AftershipTracking, rhs: AftershipTracking) -> Bool {
  return lhs.identifier == rhs.identifier
}

class TrackingCache<T: AftershipTracking>: BaseCache<AftershipTracking> {
  override func didAppendItems() {
    // do something
  }
}

let a = TrackingCache<AftershipTracking>()
let b = TrackingCache<AftershipTracking>()

a.appendItems( [AftershipTracking( identifier: 1)])
b.appendItems( [AftershipTracking( identifier: 1)])

let result = a.items == b.items // true

Upvotes: 24

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