Swift 通用问题

Question

我正在关注Apple 的Generic 教程https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Generics.html .

但是在教程的最后。我遇到了一些问题:

var myStack = Stack<String>()
myStack.push("a")
myStack.push("b")
myStack.push("c")

var arrayOfStrings = ["a", "b", "c"]

if allItemsMatch(myStack, arrayOfStrings) {
    print("All items match.")
} else {
    print("Not all items match.")
}

在 if allItemsMatch(myStack, arrayOfStrings) 这行，它说:

Cannot invoke 'allItemsMatch' with an argument list of type '(Stack< String>, [String])'

这是我的代码:

import UIKit

struct Stack<Element> {
    var items = [Element]()
    mutating func push(item: Element) {
        items.append(item)
    }
    mutating func pop() -> Element {
        return items.removeLast()
    }
}

extension Stack {
    var topItem: Element? {
        return items.isEmpty ? nil : items[items.count - 1]
    }
    mutating func append(item: Element) {
        self.push(item)
    }
    var count: Int {
        return items.count
    }
    subscript(i: Int) -> Element {
        return items[i]
    }
}


func findIndex<T: Equatable>(array: [T], _ valueToFind: T) -> Int? {
    for (index, value) in array.enumerate() {
        if value == valueToFind {
            return index
        }
    }
    return nil
}

let doubleIndex = findIndex([3.14159, 0.1, 0.25], 9.3)
let stringIndex = findIndex(["Mike", "Malcolm", "Andrea"], "Andrea")

protocol Container {
    associatedtype ItemType
    mutating func append(item: ItemType)
    var count: Int { get }
    subscript(i: Int) -> ItemType { get }
}

extension Array: Container {}

func allItemsMatch< C1: Container, C2: Container where C1.ItemType == C2.ItemType, C1.ItemType: Equatable> (someContainer: C1, _ anotherContainer: C2) -> Bool {
    if someContainer.count != anotherContainer.count {
        return false
    }
    for i in 0..<someContainer.count {
        if someContainer[i] != anotherContainer[i] {
            return false
        }
    }
    return true

}

var myStack = Stack<String>()
myStack.push("a")
myStack.push("b")
myStack.push("c")

var arrayOfStrings = ["a", "b", "c"]

if allItemsMatch(myStack, arrayOfStrings) {
    print("All items match.")
} else {
    print("Not all items match.")
}

我是不是漏了什么地方？

Answer 1

您永远不会明确符合您的 Stack<Element>结构到你的 Container协议(protocol)。因此，Swift 严格的类型安全将阻止您将它传递给期望符合 Container 的参数。协议(protocol)(即使它隐含地符合)。

您可以明确符合您的 Stack至 Container通过扩展。例如:

extension Stack:Container {}