如何将字符串转换为UnsafePointer<UInt8>和长度

Question

如何将字符串转换为UnsafePointer<UInt8>和长度

30

当我使用NSOutputStream的write方法时

func write(_ buffer: UnsafePointer<UInt8>, maxLength length: Int) -> Int

我不知道如何将String转换为UnsafePointer<UInt8>并获取长度。

在Swift中，我该怎么做？

- Chen-Tsu Lin

10个回答

17

以下是如何在Swift 3中完成。在Swift 4中也能正常运行。

extension String {

  func toPointer() -> UnsafePointer<UInt8>? {
    guard let data = self.data(using: String.Encoding.utf8) else { return nil }

    let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)
    let stream = OutputStream(toBuffer: buffer, capacity: data.count)

    stream.open()
    data.withUnsafeBytes({ (p: UnsafePointer<UInt8>) -> Void in
      stream.write(p, maxLength: data.count)
    })

    stream.close()

    return UnsafePointer<UInt8>(buffer)
  }
}

将String转换为UnsafeMutablePointer<Int8>

let cString = strdup("Hello") // UnsafeMutablePointer<Int8>

将 UnsafeMutablePointer<Int8> 转换为 String

let string = String(cString: cString!) // String

- onmyway133

1

重要的是要记住，toPointer()函数返回指向在堆上分配的内存的指针，因此，在使用完毕后应该使用free()释放它，以避免内存泄漏。 - Nathan F.

1

我想转换为UnsafeMutablePointer<UInt8>，请帮忙。 - famfamfam

7

Swift 4

将String转换为NSString，然后使用NSString的方法。

let text = "Hello"
let pointer: UnsafePointer<Int8>? = NSString(string: text).utf8String
let length = NSString(string: text).length

- Jirui

6

现在给那些使用Swift 4的人一个答案。你不能再从Data对象中获取字节，你需要将它们复制到UnsafeMutablePointer中。

let helloWorld = "Hello World!"

let data = helloWorld.data(using: String.Encoding.utf8, allowLossyConversion: false)!
var dataMutablePointer = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)

//Copies the bytes to the Mutable Pointer
data.copyBytes(to: dataMutablePointer, count: data.count)

//Cast to regular UnsafePointer
let dataPointer = UnsafePointer<UInt8>(dataMutablePointer)

//Your stream
oStream.write(dataPointer, maxLength: data.count)

- David Hertel

6

你也可以让Swift来完成它！

import Foundation

// Example function:
func printUTF8Vals(_ ptr: UnsafePointer<UInt8>, _ len: Int) { 
    for i in 0..<len { 
        print(ptr[i]) 
    } 
} 

// Call it:
let str = "Hello"
printUTF8Vals(str, str.lengthOfBytes(using: String.Encoding.utf8))

// Prints:
// 72
// 101
// 108
// 108
// 111

- Lou Zell

为什么不直接使用 str.utf8.count 呢？ - Ilya

1

传递 len？是的，那也可以！我试图说明您可以将String类型传递给一个期望UnsafePointer<UInt8>的函数，而无需进行任何显式转换。 - Lou Zell

5

这里是一个针对 Swift 5 的字符串扩展，可以将字符串转换为 UnsafePointer<UInt8> 和 UnsafeMutablePointer<Int8>

extension String {
    func toUnsafePointer() -> UnsafePointer<UInt8>? {
        guard let data = self.data(using: .utf8) else {
            return nil
        }

        let buffer = UnsafeMutablePointer<UInt8>.allocate(capacity: data.count)
        let stream = OutputStream(toBuffer: buffer, capacity: data.count)
        stream.open()
        let value = data.withUnsafeBytes {
            $0.baseAddress?.assumingMemoryBound(to: UInt8.self)
        }
        guard let val = value else {
            return nil
        }
        stream.write(val, maxLength: data.count)
        stream.close()

        return UnsafePointer<UInt8>(buffer)
    }

    func toUnsafeMutablePointer() -> UnsafeMutablePointer<Int8>? {
        return strdup(self)
    }
}

将UnsafeMutablePointer<Int8>转换为String。

guard let mutablePointer = "test".toUnsafeMutablePointer() else {
    return
}

let str = String(cString: mutablePointer)

- abdullahselek

嗨，能帮我解决这个问题吗：https://stackoverflow.com/questions/67701338/swift-pjsip-cannot-convert-value-of-type-pj-str-t-to-expected-argument-type-u - famfamfam

4

迄今为止最简单的方法（在Swift 5中）：

let s = "hello, world"
let pointer = UnsafePointer(Array(s.utf8CString))

不确定这个向后兼容性有多强。

- funct7

这个失败了：无法将类型为“UnsafePointer<CChar>”（即“UnsafePointer<Int8>”）的返回表达式转换为返回类型“UnsafePointer<UInt8>”。 - yerlilbilgin

1

int8_t 实际上就是 char，而 UnsafePointer<CChar> 和 UnsafePointer<Int8> 是相同的，它们都是有符号的 int8。但 UInt8 是无符号的 char，所以你需要将有符号转换为无符号来解决它。 - Emre Tufekci

1

我看到有其他答案和一个被接受的答案，所以似乎你已经得到了你需要的。我来这里是因为我注意到 Swift 5 对于 withUnsafeMutableBytes 等的弃用警告，并开始测试 @abdullahselek 的答案，但我在 Swift 5 中（尚未验证它在之前的版本中是否有效）发现 String 可以直接转换为 UnsafePointer<UInt8>，因此您可以在需要 UnsafePointer<UInt8> 的地方使用它。如果需要看到另一个示例，这是我们旧的和更新的函数，发布在这里：

OLD

let derivationStatus = localDerivedKeyData.withUnsafeMutableBytes { derivedKeyBytes in
  salt.withUnsafeBytes { saltBytes in

    CCKeyDerivationPBKDF(
      CCPBKDFAlgorithm(kCCPBKDF2),
      password,
      passwordData.count,
      saltBytes,
      salt.count,
      algorithm,
      UInt32(rounds),
      derivedKeyBytes,
      derivedKeyData.count
    )
  }
}

新的。

let derivationStatus = localDerivedKeyData.withUnsafeMutableBytes { (outputBytes: UnsafeMutableRawBufferPointer) -> Int32 in
  let status = CCKeyDerivationPBKDF(
    CCPBKDFAlgorithm(kCCPBKDF2),
    password, // a String
    passwordData.count, // just the password String converted to Data
    String(data: salt, encoding: .utf8),  // converts salt (Data) to String
    salt.count,
    algorithm,
    UInt32(rounds),
    outputBytes.baseAddress?.assumingMemoryBound(to: UInt8.self),
    derivedKeyData.count
  )
  return status
}

话虽如此，你可以采用类似的方法获取你的流，具体如下：

let stream = OutputStream(toBuffer: UnsafeMutablePointer(mutating: someString), capacity: someString.data(using: .utf8)!.count)

（感叹号用于消除编译器错误，但应尽可能避免强制解包。）

- BJ Miller

嗨，localDerivedKeyData的类型是什么？在这段新代码中，哪个是输出变量？ - Satish Mavani

localDerivedKeyData 是 Data 类型。derivationStatus 是一个 Int32 类型，用于给出操作的输出状态。这个操作会逐字节地将数据写入到 localDerivedKeyData 中（假设它是一个字节数组（Data），具有适当的长度，但一开始为空）。 - BJ Miller

函数如果抛出任何错误/异常，你能分享完整的函数使用吗？ - Satish Mavani

1

我创建了一个Swift扩展，其中包括许多其他功能，用于从字符串生成指针。

它包括完整的测试套件，并支持：

- `myString.stackPointer() -> UnsafePointer?` - `myString.mutableStackPointer() -> UnsafeMutablePointer?` - `myString.withUnsignedStackPointer { (ptr: UnsafePointer?) in` - `myString.withUnsignedMutableStackPointer { (ptr: UnsafeMutablePointer?) in` - `myString.heapPointer() -> UnsafePointer?` - `myString.mutableHeapPointer() -> UnsafeMutablePointer?` - `myString.unsignedHeapPointer() -> UnsafePointer?` - `myString.unsignedMutableHeapPointer() -> UnsafeMutablePointer?`

https://gist.github.com/nathan-fiscaletti/892e074dc14e6707603414cd2d80c287

如果你想要测试它，你应该能够直接将它粘贴到 Swift Playground 中。

- Nathan F.

-1

file.cString（使用String.Encoding.utf8）

- kintan

1

添加一些描述，说明原代码存在的问题以及这个代码将如何进行更正。 - jjj

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- Martin R · Accepted Answer

你需要先将字符串转换为UTF-8数据

let string = "foo bar"
let data = string.dataUsingEncoding(NSUTF8StringEncoding, allowLossyConversion: false)!

然后将其写入输出流

let outputStream: NSOutputStream = ... // the stream that you want to write to
let bytesWritten = outputStream.write(UnsafePointer(data.bytes), maxLength: data.length)

UnsafePointer() 强制转换是必需的，因为 data.bytes 的类型为 UnsafePointer<Void>，而不是 UnsafePointer<UInt8>，这与 write() 方法所期望的类型不符。

Swift 3 更新：

let string = "foo bar"
// Conversion to UTF-8 data (cannot fail):
let data = string.data(using: String.Encoding.utf8)! 

// Write to output stream:
let outputStream: NSOutputStream = ... // the stream that you want to write to
let bytesWritten = data.withUnsafeBytes { outputStream.write($0, maxLength: data.count) }