Swift中和C语言数据类型转换

原文链接

另外一个swift指针介绍不错的文章

简介

[UInt8]是Swift中的数组类型，也是作用比较特殊的一种数组类型, 常用于底层交互与内存操作与管理。如: 字符串编解码, 充当字节缓冲区等等

1.字符串编解码

• [UInt8]（或[Int8]）转 String

/// "这是一个字符串" 的utf8编码
let bytes: [UInt8] = [232, 191, 153, 230, 152, 175, 228, 184, 128, 228, 184, 170, 229, 173, 151, 231, 172, 166, 228, 184, 178]
/// 转换成字符串
if let msg = String(bytes: bytes, encoding: .utf8) {
	print(msg)
}
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• String 转 [UInt8](或[Int8]）

let msg = "这是一个字符串"

if let data = msg.data(using: .utf8) {
  let bytes = [UInt8](data)
  print(bytes)
}
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2. Swift和C语言数据类型底层转换

• 转换到C语言中的const char*, char*, void*,const void*

• 几种特殊的指针类型
  UnsafeRawBufferPointer
  UnsafeMutableRawBufferPointer
  UnsafeBufferPointer
  UnsafeMutableBufferPointer

这些带有Buffer的指针类型， 可以理解为对应的不带Buffer的指针类型加上了缓冲区大小, 比如:
UnsafePointer 对应 const int *, 表示仅有地址
那UnsafeBufferPointer对应 const int * 加 size, 表示该地址与内存大小所对应的一块缓冲区

从[UInt8]或[Int8] 转换到 const unsigned char*或 const char *

[UInt8] -> UnsafeRawBufferPointer -> UnsafeBufferPointer -> UnsafePointer (Int8同理)

/// 0. 原始字节数据 8字节
let buffer = [UInt8](repeating: 0, count: 8) 

/// 1. [UInt8] -> UnsafeRawBufferPointer
let unsafeRawBufferPointer = buffer.withUnsafeBytes { $0 } 

/// 2. UnsafeRawBufferPointer -> UnsafeBufferPointer
let unsafeBufferPointer = unsafeRawBufferPointer.bindMemory(to: UInt8.self)

/// 3. UnsafeBufferPointer -> UnsafePointer
let unsafePointer = unsafeBufferPointer.baseAddress

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其中 步骤1和步骤2可以合并

/// 0. 原始字节数据 8字节
let buffer = [UInt8](repeating: 0, count: 8)

/// 1. [UInt8] -> UnsafeBufferPointer
let unsafeBufferPointer = buffer.withUnsafeBufferPointer {$0}

/// 2.. UnsafeBufferPointer -> UnsafePointer
let unsafePointer = unsafeBufferPointer.baseAddress
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从[UInt8]或[Int8]转换到 unsigned char *或 char *

[UInt8] -> UnsafeMutableRawBufferPointer -> UnsafeMutableBufferPointer -> UnsafeMutablePointer (Int8同理)

/// 0. 原始字节数据 8字节
var buffer = [UInt8](repeating: 0, count: 8)

/// 1. [UInt8] -> UnsafeMutableRawBufferPointer
let unsafeMutableRawBufferPointer = buffer.withUnsafeMutableBytes { $0 }

/// 2. UnsafeMutableRawBufferPointer -> UnsafeMutableBufferPointer
let unsafeMutableBufferPointer = unsafeMutableRawBufferPointer.bindMemory(to: UInt8.self)

/// 3. UnsafeMutableBufferPointer -> UnsafeMutablePointer
let unsafeMutablePointer = unsafeMutableBufferPointer.baseAddress
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其中的步骤1和步骤2也是可以合并的

/// [UInt8] -> UnsafeMutableBufferPointer
let unsafeMutableBufferPointer = buffer.withUnsafeMutableBufferPointer {$0 }
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• 从[UInt8]或[Int8]转换成const void *或者void *
  在上述的步骤里, 在步骤1得到的unsafeMutableRawBufferPointer就可以通过该对象的baseAddress字段就能获取到对应的void *

/// 获取 const void *
let unsafeRawPointer = unsafeRawBufferPointer.baseAddress

/// 获取void *
let unsafeMutableRawPointer = unsafeMutableRawBufferPointer.baseAddress
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2. 从C语言中的const char * , char *, const unsigned char *, unsigned char *, const void *, void *对应缓冲区的数据创建 Data

/// 这里是示例， 类型为 UnsafeMutablePointer， 也就是 char *
/// message可能来自任何C语言的接口
let message = strerror(errno) 

/// 通过指针和大小来构造 UnsafeBufferPointer
let unsafeBufferPointer = UnsafeBufferPointer(start: message, count: strlen(msg!))

/// 构造Data
let data = Data(buffer: unsafeBufferPointer)
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3. 从Data得到 [UInt8] 或者 [Int8]以及String

• Data其实内部就是[UInt8]，在大部分情况下都可以直接当成[UInt8]来使用, 字节的append，remove, insert, find等等都有相同的操作
• 但是如果需要拷贝出来一份 [UInt8]或者[Int8]，可以使用map函数

/// 缓冲区里的数据    
let data = Data(buffer: unsafeBufferPointer)

/// [UInt8]
let u8a = data.map {$0}

/// [Int8]
let s8a = data.map {Int8($0)}
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• String也自带从data数据的构造函数

let text = String(data: data, encoding: .utf8)
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数据类型转换方法

    // 将int转化为UInt8
    public static func toUInt(signed: Int) -> UInt {
        let unsigned = signed >= 0
                       ? UInt(signed)
                       : UInt(signed  - Int.min) + UInt(Int.max) + 1
        
        return unsigned
    }
    
    // 将UInt8 转化为 int
    public static func convertToInt(unsigned: UInt) -> Int {
        
        let signed = (unsigned <= UInt(Int.max))
                    ? Int(unsigned)
                    : Int(unsigned - UInt(Int.max) - 1) + Int.min
        
        return signed
    }
    
    //4bytes转Int
    public static func convert4BytesToInt(data: Data) -> Int {
        var value : UInt32 = 0
        let data = NSData(bytes: [UInt8](data), length: data.count)
        data.getBytes(&value, length: data.count)
        value = UInt32(bigEndian: value)
        return Int(value)
    }
    
    // MARK:- int 转成 4字节的bytes
    public static func convertIntTo4Bytes(value: Int) -> [UInt8] {
        if value < 0 || value > 65535 {
            return [UInt8]()
        }
        let UInt = UInt32.init(Double.init(value))
        return [UInt8(truncatingIfNeeded: UInt >> 24),
                UInt8(truncatingIfNeeded: UInt >> 16),
                UInt8(truncatingIfNeeded: UInt >> 8),
                UInt8(truncatingIfNeeded: UInt)]
    }
    
    
    /// UInt 16 转[UInt8]
    public static func convertUInt16To2Bytes(value: UInt16) -> [UInt8] {
        return [UInt8(truncatingIfNeeded: value >> 8), UInt8(truncatingIfNeeded: value)]
    }
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