Swift：将genericstypes转换为相同的genericstypes，但使用相关types的子类
考虑这些类：
struct OrderedSet<T: Hashable> {} class Exercise: Hashable {} class StrengthExercise: Exercise {} class CardioExercise: Exercise {} 我想要做以下事情：
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 } }
但是我得到这个错误：
Cannot assign value of type 'OrderedSet<StrengthExercise>' to type 'OrderedSet<Exercise>
我不太明白这一点，因为StrengthExercise是Exercise一个子类，并且拥有OrderedSet<Exercise>期望的一切。
问题（S）
- 为什么这个错误是必要的？
- 我如何写一些能够实现我要做的function？
雷达提交
rdar：// 23608799
关于协变和逆变的博客文章
https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html
恐怕从Swift 2.1开始，这是不可能的。 只支持以下转换
- build立在集合types是他们的元素types协变。
- 支持函数types之间的转换，performance函数结果types的协方差和函数参数types的相反性。 （参考Xcode 7.1发行说明 ）
由于Objective-C的generics支持types差异，并且考虑到Swift 2.1中函数types转换所取得的进展，我相信有理由相信未来types差异支持将被添加到Swift中。 同时，记得像jlieske那样提交雷达。
同时您将不得不复制集合或使用其中一种内置集合types。
自Swift以来的更新成为开源：我相信Swift 3.0 Dev Roadmap的Complete generics部分指出types差异将在3.0中解决。 虽然types差异没有明确地被调出，但是标准库中的特殊exception（包括types差异）是。
由于OrderedSet<StrengthExercise>具有特定的types，因此不能将其分配给更一般的OrderedSet<Exercise> 。 考虑一下，如果你在分配之后试图将有氧训练附加到OrderedSet，会发生什么情况。
答案可能是修改附加力量练习的内容而不是分配整个练习集。
这应该工作
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
…你的麻烦似乎在你的代码中的其他地方。 你能告诉我们你的generics结构OrderedSet的实现吗？ 好像你正在尝试做类似的事情
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
…这工作
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
….家伙，内置types或不
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
…如何处理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's 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's 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