swift - 错误 : Cannot subscript a value of type 'X' with . ..'

Question

错误:无法使用类型为“(safe: Int)”的索引为类型为“[CustomClass]”的值下标

class CustomClass {
  let value: String
  init(value: String) {
    self.value = value
  }
}

extension Collection {

  subscript(safe: Int) -> Element? {
    if safe > count-1 {
      return nil
    }
    return self[safe]
  }
}

let steps: [CustomClass] = []

if let step = steps[safe: 4] { // error here
}

为什么会这样？

Answer 1

请注意，除了@Hamish 的评论中已经提到的下标参数问题之外，您的代码中还有一些其他问题:ArraySlice 也符合 RandomAccessCollection 所以只需检查数组计数不保证它是一个安全的索引。您应该添加一个 guard 语句来检查 indices 属性是否包含 Index。您还应该将下标参数更改为 Index 而不是 Int:

class CustomClass {
    let value: Int
    init(value: Int) {
        self.value = value
    }
}

extension Collection  {
    subscript(safe index: Index) -> Element? {
        guard indices.contains(index) else {
            return nil
        }
        return self[index]
        // or simply
        // return indices.contains(index) ? self[index] : nil
    }
}

---

Playground 测试:

let steps = [CustomClass(value: 0),CustomClass(value: 1),CustomClass(value: 2),CustomClass(value: 3),CustomClass(value: 4),CustomClass(value: 5),CustomClass(value: 6)]

if let step6 = steps[safe: 6] {
    print(step6.value)  // 6
}

let stepsSlice = steps[0...4]
let step6 = stepsSlice[safe: 6]
print(step6?.value)   // nil