Swift String(validatingUTF8：) 和 String(utf8String：) 之间有区别吗

发布于12月25日

这两个函数似乎有非常接近的签名和非常接近的描述:

https://developer.apple.com/documentation/swift/string/init(utf8string:)-3mcco个

String.init(utf8String:)
Creates a string by copying the data from a given null-terminated C array of UTF8-encoded bytes.

https://developer.apple.com/documentation/swift/string/init(validatingutf8:)-208fn个

String.init(validatingUTF8:)
Creates a new string by copying and validating the null-terminated UTF-8 data referenced by the given pointer.

由于两者都是可以为空的初始值设定项，它们的实际区别是什么？是否存在一个可能的输入， for each 输入提供不同的输出？如果它们在行为上相同，那么推荐使用哪一个？

推荐答案

String.init(validatingUTF8:)是一种来自SWIFT标准库的方法，实现在CString.swift:

  public init?(validatingUTF8 cString: UnsafePointer<CChar>) {
    let len = UTF8._nullCodeUnitOffset(in: cString)
    guard let str = cString.withMemoryRebound(to: UInt8.self, capacity: len, {
      String._tryFromUTF8(UnsafeBufferPointer(start: $0, count: len))
    })
    else { return nil }

    self = str
  }

String.init(utf8String:)在NSStringAPI.swift:

  /// Creates a string by copying the data from a given
  /// C array of UTF8-encoded bytes.
  public init?(utf8String bytes: UnsafePointer<CChar>) {
    if let str = String(validatingUTF8: bytes) {
      self = str
      return
    }
    if let ns = NSString(utf8String: bytes) {
      self = String._unconditionallyBridgeFromObjectiveC(ns)
    } else {
      return nil
    }
  }

是Foundation NSString初始化式的SWIFT覆盖

- (nullable instancetype)initWithUTF8String:(const char *)nullTerminatedCString

它又在swift-corelibs-foundation/Sources/Foundation/NSString.swift个版本中实现了非苹果平台

public convenience init?(utf8String nullTerminatedCString: UnsafePointer<Int8>) {
    guard let str = String(validatingUTF8: nullTerminatedCString) else { return nil }
    self.init(str)
}

因此，这两种方法在转换C字符串的方式上没有区别，但是String.init(utf8String:)需要import Foundation，而String.init(validatingUTF8:)不需要额外的导入.