Question

我想将Collection的SubSequence转换为String。例如，此函数收集集合的前两个字节并将其转换为String。

func convert<T: Collection>(_ c: T) -> String 
    where T.Iterator.Element == UInt8 
{
    let start = c.startIndex
    let end = c.index(after: start)
    return String(bytes: c[start ... end], encoding: String.Encoding.utf8)!
}

这会导致此错误：

error: ambiguous reference to member 'subscript'
        return String(bytes: c[start ... end], encoding: String.Encoding.utf8)!
                             ~^~~~~~~~~~~~~~~
Swift.Collection:167:12: note: found this candidate
    public subscript(position: Self.Index) -> Self.Iterator.Element { get }
           ^
Swift.Collection:189:12: note: found this candidate
    public subscript(bounds: Range<Self.Index>) -> Self.SubSequence { get }
           ^
Swift.Collection:25:12: note: found this candidate
    public subscript(bounds: Range<Self.Index>) -> Slice<Self> { get }
           ^
Swift.IndexableBase:54:12: note: found this candidate
    public subscript(position: Self.Index) -> Self._Element { get }
           ^
Swift.IndexableBase:63:12: note: found this candidate
    public subscript(bounds: Range<Self.Index>) -> Self.SubSequence { get }
           ^
Swift.Indexable:23:12: note: found this candidate
    public subscript(bounds: ClosedRange<Self.Index>) -> Self.SubSequence { get }
           ^
Swift.Indexable:23:12: note: found this candidate
    public subscript(bounds: CountableRange<Self.Index>) -> Self.SubSequence { get }
           ^
Swift.Indexable:45:12: note: found this candidate
    public subscript(bounds: CountableClosedRange<Self.Index>) -> Self.SubSequence { get }

我在这里缺少什么？： - ）

Answer 1

目前，Collection的{{1}}不能保证与集合本身具有相同的元素类型，这是由于SubSequence的限制。

事实上，SE-0142: Permit where clauses to constrain associated types的一个动机是允许约束associatedtype，这将强制实现这种关系。

虽然在这种特殊情况下，由于您首先没有使用associatedtype SubSequence : Sequence where SubSequence.Iterator.Element == Iterator.Element约束，因此您可以约束T.Iterator.Element == UInt8（感谢@MartinR）：

T.SubSequence.Iterator.Element

（在一般情况下，您还需要func convert<T: Collection>(_ c: T) -> String where T.SubSequence.Iterator.Element == UInt8 { let start = c.startIndex let end = c.index(after: start) // please consider handling the case where String(bytes:encoding:) returns nil. return String(bytes: c[start ... end], encoding: String.Encoding.utf8)! }约束到给定类型，您可能希望添加T.Iterator.Element作为附加约束。）

将Collection的子序列转换为String

1 个答案: