Swift 里字符串（二）创建



最终都要走到__StringStorage 的 create(realCodeUnitCapacity,countAndFlags) 方法里去。

默认实现是 UTF8

  internal static func _fromCodeUnits<
    Input: Collection,
    Encoding: Unicode.Encoding
  >(
    _ input: Input,
    encoding: Encoding.Type,
    repair: Bool
  ) -> (String, repairsMade: Bool)?
  where Input.Element == Encoding.CodeUnit {
    // TODO(String Performance): Attempt to form smol strings

    // TODO(String performance): Skip intermediary array, transcode directly
    // into a StringStorage space.
    var contents: [UInt8] = []
    contents.reserveCapacity(input.underestimatedCount)
    let repaired = transcode(
      input.makeIterator(),
      from: Encoding.self,
      to: UTF8.self,
      stoppingOnError: false,
      into: { contents.append($0) })
    guard repair || !repaired else { return nil }

    let str = contents.withUnsafeBufferPointer { String._uncheckedFromUTF8($0) }
    return (str, repaired)
  }

可以看到，把其他的编码方式，都转为了UTF8，然后编码后的缓存序列去创建了一个__StringStorage

  @usableFromInline
  internal static func _uncheckedFromUTF8(
    _ input: UnsafeBufferPointer<UInt8>, asciiPreScanResult: Bool
  ) -> String {
    if let smol = _SmallString(input) {
      return String(_StringGuts(smol))
    }

    let isASCII = asciiPreScanResult
    let storage = __StringStorage.create(
      initializingFrom: input, isASCII: isASCII)
    return storage.asString
  }

在创建过程中，又判断是否是_SmallString。