转换结构失败：...不可转换为

Question

我有协议：

protocol Occurrence {
    var isEmpty: Bool { get }
    mutating func addOccurrence(occ: Occurrence) -> Occurrence
    mutating func removeOccurrence(occ: Occurrence) -> Occurrence
}

以及符合该协议的结构：

struct NonEmptyOccurrence: Occurrence, Printable {

    ...
    private var _occurrence: Int

    var isEmpty: Bool {
        get {
            return false
        }
    }

    ...

    mutating func addOccurrence(other: Occurrence) -> Occurrence {
        if other.isEmpty {
            return self
        } else {
            //error here: Occurrence is not convertible to NonEmptyOccurrence
            _occurrence = _occurrence + (other as NonEmptyOccurrence)._occurrence 
            return self
        }
    }
}

在addOccurrence方法中有一个错误，因为我把它放在评论中。我在这里错过了什么？为什么我不能将该实例转换为 NonEmptyOccurrence？

Answer 1

试试这样的东西：

mutating func addOccurrence(other: Occurrence) -> Occurrence {
    if !other.isEmpty, let otherAsNEO = (other as? NonEmptyOccurrence) {
        _occurrence = _occurrence + otherAsNEO._occurrence 
    }
    return self
}

Answer 2

事实证明，您可以在 Xcode 6 beta3 中转换结构，但在当前的稳定版本 (Xcobe 6.2) 中不能。

转换结构失败：...不可转换为

Casting struct fails with: ... is not convertible to

struct

casting

swift