swift - 无法为值添加下标

Question

我想设置一个下标，这样我总是能够在 swift 中安全地使用数组

let array = ["a", "b", "c", "d"]
let sliceOfArray = array[1...3]

extension Collection {
    subscript(test: Index) -> Element? {
        return indices.contains(test) ? self[test] : nil
    }
}

sliceOfArray[test: 0]

这给了我错误 Cannot subscript a value of type 'ArraySlice<String>' with an index of type '(test: Int)'

那么我怎样才能让这个下标运行呢？

Answer 1

Swift likes to be safe, but one problematic area can be reading from arrays and dictionaries. In the case of dictionaries, reading a missing key will return nil rather than the value you might have expected, but in the case of arrays it’s worse: your app will crash. More details

尝试这样

extension Collection  {
    subscript(safe test: Index) -> Element? {
        return indices.contains(test) ? self[test] : nil
    }
}