考虑这些课程：
struct OrderedSet<T: Hashable> {}
class Exercise: Hashable {}
class StrengthExercise: Exercise {}
class CardioExercise: Exercise {}
我想做以下事情：
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
}
}
但是我收到了这个错误：
Cannot assign value of type 'OrderedSet<StrengthExercise>' to type 'OrderedSet<Exercise>
我不太明白，因为StrengthExercise是Exercise的子类，并且会包含OrderedSet<Exercise>期望的所有内容。
问题
雷达提交
rdar://23608799
关于协方差和逆变的博客文章
https://www.mikeash.com/pyblog/friday-qa-2015-11-20-covariance-and-contravariance.html
答案 0 :(得分：7)
我担心Swift 2.1目前无法实现这一目标。仅支持以下转换
作为Objective-C的泛型支持类型方差，并且考虑到Swift 2.1中函数类型转换的进展，我相信有理由相信将来会向Swift添加类型差异支持。与此同时，请记住提交雷达，例如jlieske has。
与此同时，您必须复制集合或使用其中一种内置集合类型。
自Swift成为开源以来的更新： 我相信Swift 3.0 Dev Roadmap的 Complete generics 部分表示类型差异将在3.0中解决。虽然未明确调出类型方差，但标准库中的特殊外壳异常（包括类型方差）是。
答案 1 :(得分：2)
由于OrderedSet<StrengthExercise>属于该特定类型，因此无法将其分配给更一般的OrderedSet<Exercise>。想一想如果你在分配后将Cardio练习附加到OrderedSet会发生什么。
答案可能是修改将力量练习集的内容附加到练习集而不是分配整个类型集。
答案 2 :(得分：1)
这应该有效
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。你能告诉我们你的通用结构OrderedSet实现吗？看起来你正试图做像
这样的事情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
......这是有效的
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
s.t.whoAmI() // I am A
....伙伴们，内置类型与否
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