Swift: enter a generic type into the same generic type, but with a subclass of the associated type

Question

Consider these classes:

struct OrderedSet<T: Hashable> {} class Exercise: Hashable {} class StrengthExercise: Exercise {} class CardioExercise: Exercise {} 

I would like to do the following:

 var displayedExercises = OrderedSet<Exercise>() { didSet { self.tableView.reloadData() } } var cardioExercises = OrderedSet<CardioExercise>() var strengthExercises = OrderedSet<StrengthExercise>() @IBAction func segmentControlChanged(segmentControl: UISegmentedControl) { switch segmentControl.selectedSegmentIndex { case 0: self.displayedExercises = self.strengthExercises case 1: self.displayedExercises = self.cardioExercises default: break } } 

But I get this error:

 Cannot assign value of type 'OrderedSet<StrengthExercise>' to type 'OrderedSet<Exercise> 

I do not quite understand this, since StrengthExercise is a subclass of Exercise and will have everything that the OrderedSet<Exercise> expects.

Question (s)

• Why is this error needed?
• How do I write something that achieves the functionality I'm going to?

Radar file
rdar: // 23608799

Blog post on covariance and contravariance
https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html

Answer 1

I am afraid that this is not possible at present compared to Swift 2.1. Only the following conversions are supported

• Built-in collection types are covariant in type of elements.
• Conversions between types of functions are supported, showing covariance in the types of function results and contravariance in the types of function parameters. (See Xcode 7.1 Release Notes )

How variance of the Objective-C generics type supports the display type, and given the progress made in converting function types to Swift 2.1, I believe there is reason to believe that support for type variations will be added to Swift in the future. At the same time, be sure to record a radar such as jlieske .

At the same time, you will have to copy the collection or use one of the built-in collection types.

Update since the opening of Swift, open source: I believe that the full section of the Swift 3.0 generic sections Dev Roadmap indicates that type dispersion will be covered in 3.0. While type variance is not specifically invoked, special exceptions with casing are used in the standard library (including type variance).

Answer 2

Since an OrderedSet<StrengthExercise> is a specific type, it cannot be assigned to the more general OrderedSet<Exercise> . Think about what happens if you try to add the Cardio exercise to this OrderedSet after you assign it.

The answer may be to modify the addition of the contents of the strength exercises that are set to the set of exercises, rather than assigning the entire typed set.

Answer 3

it should work

 class Base {} class A: Base {} class B: Base {} var arrBase: Array<Base> = [] var arrA: Array<A> = [] arrA.append(A()) var arrB: Array<B> = [] arrB.append(B()) arrBase = arrA // no error arrBase = arrB // no error 

... your problem seems to be somewhere else in your code. Can you show us your general OrderedSet structure? it looks like you are trying to do something like

 class Base {} class A: Base {} let base = Base() let a = A() struct S<T:Base> { var t: T } var s = S(t: base) let sa = S(t: a) //s = sa // error: cannot assign value of type 'S<A>' to type 'S<Base>' let sb = S(t: a as Base) s = sb 

... it works

 protocol P { func whoAmI()->Void } class Base:P { func whoAmI() { print("I am Base") } } class A: Base { override func whoAmI() { print("I am A") } } let base = Base() let a = A() struct S<T: Base> { var t: Base } var s = S(t: base) let sa = S(t: a) s = sa stwhoAmI() // I am A 

.... guys, built-in type or not

 import Foundation // Int and Double conforms to Hashable protocol var a: Set<Int> = [] var b: Set<Double> = [] a = b // IMPOSSIBLE eventhough Set<T:Hashable> is build-in Swift type 

... how to work with OrderedSet

 import Foundation class Exercise: Hashable { var name: String = "" var hashValue: Int { return name.hashValue } } func ==(lhs: Exercise, rhs: Exercise) -> Bool { return lhs.name == rhs.name } class StrengthExercise: Exercise {} class CardioExercise: Exercise {} var displayedExercises = Set<Exercise>() let strengthExercises = Set<StrengthExercise>() let cardioExercises = Set<CardioExercise>() displayedExercises = strengthExercises // OK, the question is how to implement OrderedSet<T:Hashable> // ------------------------------------------------------------------------------------------ // // OrderedSet.swift // Weebly // // Created by James Richard on 10/22/14. // Copyright (c) 2014 Weebly. All rights reserved. // // Slightly modified by user3441734 on 11/18/15 // // original code OrderedSet is available under the MIT license /// An ordered, unique collection of objects. public struct OrderedSet<T: Hashable> { private var contents = [T: Index]() // Needs to have a value of Index instead of Void for fast removals private var sequencedContents = Array<UnsafeMutablePointer<T>>() /** Inititalizes an empty ordered set. :return: An empty ordered set. */ public init() { } /** Initializes a new ordered set with the order and contents of sequence. If an object appears more than once in the sequence it will only appear once in the ordered set, at the position of its first occurance. :param: sequence The sequence to initialize the ordered set with. :return: An initialized ordered set with the contents of sequence. */ public init<S: SequenceType where S.Generator.Element == T>(sequence: S) { // FIXME: For some reason, Swift gives the error "Cannot convert the expression type 'S' to type 'S'" with a regular for-in, so this is a hack to fix that. var gen = sequence.generate() while let object: T = gen.next() { if contents[object] == nil { contents[object] = contents.count let pointer = UnsafeMutablePointer<T>.alloc(1) pointer.initialize(object) sequencedContents.append(pointer) } } } /** Replace, remove, or retrieve an object in the ordered set. When setting an index to nil the object will be removed. If it is not the last object in the set, all subsequent objects will be shifted down one position. When setting an index to another object, the existing object at that index will be removed. If you attempt to set an index that does not currently have an object, this is a no-op. :param: index The index to retrieve or set. :return: On get operations, the object at the specified index, or nil if no object exists at that index. */ public subscript(index: Index) -> T { get { return sequencedContents[index].memory } set { contents[sequencedContents[index].memory] = nil contents[newValue] = index sequencedContents[index].memory = newValue } } /** Locate the index of an object in the ordered set. It is preferable to use this method over the global find() for performance reasons. :param: object The object to find the index for. :return: The index of the object, or nil if the object is not in the ordered set. */ public func indexOfObject(object: T) -> Index? { if let index = contents[object] { return index } return nil } /// The number of objects contained in the ordered set. public var count: Int { return contents.count } /// Whether the ordered set has any objects or not. public var isEmpty: Bool { return count == 0 } /** Tests if the ordered set contains an object or not. :param: object The object to search for. :return: true if the object exists in the ordered set, otherwise false. */ public func contains(object: T) -> Bool { return contents[object] != nil } /** Appends an object to the end of the ordered set. :param: object The object to be appended. */ mutating public func append(object: T) { if contents[object] != nil { return } contents[object] = contents.count let pointer = UnsafeMutablePointer<T>.alloc(1) pointer.initialize(object) sequencedContents.append(pointer) } /** Appends a sequence of objects to the end of the ordered set. :param: objects The objects to be appended. */ mutating public func appendObjects<S: SequenceType where S.Generator.Element == T>(objects: S) { var gen = objects.generate() while let object: T = gen.next() { append(object) } } /** Removes an object from the ordered set. If the object exists in the ordered set, it will be removed. If it is not the last object in the ordered set, subsequent objects will be shifted down one position. :param: object The object to be removed. */ mutating public func remove(object: T) { if let index = contents[object] { contents[object] = nil sequencedContents[index].dealloc(1) sequencedContents.removeAtIndex(index) for (object, i) in contents { if i < index { continue } contents[object] = i - 1 } } } /** Removes the given objects from the ordered set. :param: objects The objects to be removed. */ mutating public func removeObjects<S: SequenceType where S.Generator.Element == T>(objects: S) { var gen = objects.generate() while let object: T = gen.next() { remove(object) } } /** Removes an object at a given index. This method will cause a fatal error if you attempt to move an object to an index that is out of bounds. :param: index The index of the object to be removed. */ mutating public func removeObjectAtIndex(index: Index) { if index < 0 || index >= count { fatalError("Attempting to remove an object at an index that does not exist") } remove(sequencedContents[index].memory) } /** Removes all objects in the ordered set. */ mutating public func removeAllObjects() { contents.removeAll() sequencedContents.removeAll() } /** Return an OrderedSet containing the results of calling `transform(x)` on each element `x` of `self` :param: transform A closure that is called for each element in the ordered set. The result of the closure is appended to the new ordered set. :result: An ordered set containing the result of `transform(x)` on each element. */ public func map<U: Hashable>(transform: (T) -> U) -> OrderedSet<U> { var result = OrderedSet<U>() for object in self { result.append(transform(object)) } return result } /// The first object in the ordered set, or nil if it is empty. public var first: T? { return count > 0 ? self[0] : nil } /// The last object in the ordered set, or nil if it is empty. public var last: T? { return count > 0 ? self[count - 1] : nil } /** Swaps two objects contained within the ordered set. Both objects must exist within the set, or the swap will not occur. :param: first The first object to be swapped. :param: second The second object to be swapped. */ mutating public func swapObject(first: T, withObject second: T) { if let firstPosition = contents[first] { if let secondPosition = contents[second] { contents[first] = secondPosition contents[second] = firstPosition sequencedContents[firstPosition].memory = second sequencedContents[secondPosition].memory = first } } } /** Tests if the ordered set contains any objects within a sequence. :param: sequence The sequence to look for the intersection in. :return: Returns true if the sequence and set contain any equal objects, otherwise false. */ public func intersectsSequence<S: SequenceType where S.Generator.Element == T>(sequence: S) -> Bool { var gen = sequence.generate() while let object: T = gen.next() { if contains(object) { return true } } return false } /** Tests if a the ordered set is a subset of another sequence. :param: sequence The sequence to check. :return: true if the sequence contains all objects contained in the receiver, otherwise false. */ public func isSubsetOfSequence<S: SequenceType where S.Generator.Element == T>(sequence: S) -> Bool { for (object, _) in contents { if !sequence.contains(object) { return false } } return true } /** Moves an object to a different index, shifting all objects in between the movement. This method is a no-op if the object doesn't exist in the set or the index is the same that the object is currently at. This method will cause a fatal error if you attempt to move an object to an index that is out of bounds. :param: object The object to be moved :param: index The index that the object should be moved to. */ mutating public func moveObject(object: T, toIndex index: Index) { if index < 0 || index >= count { fatalError("Attempting to move an object at an index that does not exist") } if let position = contents[object] { // Return if the client attempted to move to the current index if position == index { return } let adjustment = position < index ? -1 : 1 let range = index < position ? index..<position : position..<index for (object, i) in contents { // Skip items not within the range of movement if i < range.startIndex || i > range.endIndex || i == position { continue } let originalIndex = contents[object]! let newIndex = i + adjustment let firstObject = sequencedContents[originalIndex].memory let secondObject = sequencedContents[newIndex].memory sequencedContents[originalIndex].memory = secondObject sequencedContents[newIndex].memory = firstObject contents[object] = newIndex } contents[object] = index } } /** Moves an object from one index to a different index, shifting all objects in between the movement. This method is a no-op if the index is the same that the object is currently at. This method will cause a fatal error if you attempt to move an object fro man index that is out of bounds or to an index that is out of bounds. :param: index The index of the object to be moved. :param: toIndex The index that the object should be moved to. */ mutating public func moveObjectAtIndex(index: Index, toIndex: Index) { if ((index < 0 || index >= count) || (toIndex < 0 || toIndex >= count)) { fatalError("Attempting to move an object at or to an index that does not exist") } moveObject(self[index], toIndex: toIndex) } /** Inserts an object at a given index, shifting all objects above it up one. This method will cause a fatal error if you attempt to insert the object out of bounds. If the object already exists in the OrderedSet, this operation is a no-op. :param: object The object to be inserted. :param: atIndex The index to be inserted at. */ mutating public func insertObject(object: T, atIndex index: Index) { if index > count || index < 0 { fatalError("Attempting to insert an object at an index that does not exist") } if contents[object] != nil { return } // Append our object, then swap them until its at the end. append(object) for i in Range(start: index, end: count-1) { swapObject(self[i], withObject: self[i+1]) } } /** Inserts objects at a given index, shifting all objects above it up one. This method will cause a fatal error if you attempt to insert the objects out of bounds. If an object in objects already exists in the OrderedSet it will not be added. Objects that occur twice in the sequence will only be added once. :param: objects The objects to be inserted. :param: atIndex The index to be inserted at. */ mutating public func insertObjects<S: SequenceType where S.Generator.Element == T>(objects: S, atIndex index: Index) { if index > count || index < 0 { fatalError("Attempting to insert an object at an index that does not exist") } var addedObjectCount = 0 // FIXME: For some reason, Swift gives the error "Cannot convert the expression type 'S' to type 'S'" with a regular for-in, so this is a hack to fix that. var gen = objects.generate() // This loop will make use of our sequncedContents array to update the contents dictionary's // values. During this loop there will be duplicate values in the dictionary. while let object: T = gen.next() { if contents[object] == nil { let seqIdx = index + addedObjectCount let element = UnsafeMutablePointer<T>.alloc(1) element.initialize(object) sequencedContents.insert(element, atIndex: seqIdx) contents[object] = seqIdx addedObjectCount++ } } // Now we'll remove duplicates and update the shifted objects position in the contents // dictionary. for i in index + addedObjectCount..<count { contents[sequencedContents[i].memory] = i } } } extension OrderedSet: MutableCollectionType { public typealias Index = Int public typealias _Element = T public typealias Generator = OrderedSetGenerator<T> public func generate() -> Generator { return OrderedSetGenerator(set: self) } public var startIndex: Int { return 0 } public var endIndex: Int { return count } } public struct OrderedSetGenerator<T: Hashable>: GeneratorType { public typealias Element = T private var generator: IndexingGenerator<Array<UnsafeMutablePointer<T>>> public init(set: OrderedSet<T>) { generator = set.sequencedContents.generate() } mutating public func next() -> Element? { return generator.next()?.memory } } public func +<T: Hashable, S: SequenceType where S.Generator.Element == T> (lhs: OrderedSet<T>, rhs: S) -> OrderedSet<T> { var joinedSet = lhs joinedSet.appendObjects(rhs) return joinedSet } public func +=<T: Hashable, S: SequenceType where S.Generator.Element == T> (inout lhs: OrderedSet<T>, rhs: S) { lhs.appendObjects(rhs) } public func -<T: Hashable, S: SequenceType where S.Generator.Element == T> (lhs: OrderedSet<T>, rhs: S) -> OrderedSet<T> { var purgedSet = lhs purgedSet.removeObjects(rhs) return purgedSet } public func -=<T: Hashable, S: SequenceType where S.Generator.Element == T> (inout lhs: OrderedSet<T>, rhs: S) { lhs.removeObjects(rhs) } extension OrderedSet: Equatable { } public func ==<T: Hashable> (lhs: OrderedSet<T>, rhs: OrderedSet<T>) -> Bool { if lhs.count != rhs.count { return false } for object in lhs { if lhs.contents[object] != rhs.contents[object] { return false } } return true } // ------------------------------------------------------------------------------------------ // finaly what do you want var displayedExercises1 = OrderedSet<Exercise>() let strengthExercises1 = OrderedSet<StrengthExercise>() let cardioExercises1 = OrderedSet<CardioExercise>() displayedExercises = strengthExercises