Swift中指针UnsafePointer的常见用法

标签：UnsafeMutablePointer UnsafePointer 类型 let print Swift 指针

指针类型

//基本指针
UnsafePointer<T>                    const T *
UnsafeMutablePointer                T *
//集合指针
UnsafeBufferPointer                 const T * //指向一个连续已知类型区域，可以看成一个集合，并支持集合操作
UnsafeMutableBufferPointer          T * //指向一个连续已知类型区域，可以看成一个集合，并支持集合操作
//空指针
UnsafeRawPointer                    const void *
UnsafeMutableRawPointer             void *
UnsafeRawBufferPointer              const void * //指向一个连续未知类型区域
UnsafeMutableRawBufferPointer       void *  //指向一个连续未知类型区域

UnsafePointer 和 UnsafeMutablePointer UnsafePointer只读指针 UnsafeMutablePointer可修改指针
UnsafePointer只读指针 通过UnsafePointer<T>定义一个类型为T的指针，通过pointee成员即可获得T的值。

//UnsafePointer参数只能接收inout修饰的类型，而inout修饰的类型必然是可写的，所以参数只能用var定义
func call(_ p: UnsafePointer<Int>) {
    print("\(p.pointee)")
}
var a = 1234
//&a用于传递指针
call(&a) // 打印：1234

UnsafeMutablePointer可修改指针 可以通过设置p.pointee 修改指针的值。

func modify(_ p: UnsafeMutablePointer<Int>) {
    p.pointee = 5678
}
var a = 1234
modify(&a)
print("\(a)") // 打印：5678

UnsafeBufferPointer 集合指针类型 UnsafeBufferPointer指针实现了Collection协议,因此可以直接使用Collection中的各种方法来遍历操作数据,filter,map...等。 Swift的数组提供了函数withUnsafeBufferPointer，通过它我们可以方便的用指针来处理数组。下面通过withUnsafeBufferPointer获得变量p，p的类型为UnsafeBufferPointer<Int32>，它代表着一整块的连续内存，它是集合类型指针，并且它也支持大部分数组操作。

let a: [Int32] = [1, 2, -1, -2, 5, 6]
let p = a.withUnsafeBufferPointer { $0 }
print("\(p.count)") // 打印：6
print("\(p[3])") // 打印：-2

空指针：UnsafeRawPointer 就像C语言有void*（即空指针）一样，Swift也有自己的空指针，它通过类型UnsafeRawPointer来获得

let rp: UnsafeMutablePointer<CChar> = rRes.value
let strLength = rRes.length
//将UnsafeMutablePointer<CChar> C语言的char *指针转成 UnsafeMutableRawPointer 的void *任意指针。然后直接取出封装成String字符串
let strRes = String.init(bytesNoCopy: UnsafeMutableRawPointer(mutating: rp), length: Int(strLength), encoding: .utf8, freeWhenDone: false)

指针的类型转换

UnsafePointer 基本指针
//将指针的类型转换成一个临时的给定类型，并将指针传递给closure
func withMemoryRebound<T, Result>(to: T.Type, capacity: Int, (UnsafePointer<T>) -> Result) -> Result
//向下移动一位,并返回一个新的指针 
func successor() -> UnsafePointer<Pointee>
//向上移动一位,并返回一个新的指针
func predecessor() -> UnsafePointer<Pointee>

RawPointer 空指针
//转换给指定类型的指针
func assumingMemoryBound<T>(to: T.Type) -> UnsafeMutablePointer<T>
//转换成指定类型的指针,capacity指定了这个指针读取的T数据数量
func bindMemory<T>(to type: T.Type, capacity count: Int) -> UnsafeMutablePointer<T>

UnsafeBufferPointer 集合指针
let rpSub:UnsafeMutablePointer<UInt8> = rp.withMemoryRebound(to: UInt8.self, capacity: Int(strLength), { $0 })
let buffer = UnsafeBufferPointer<UInt8>.init(start: rpSub, count: Int(strLength))

指针的操作 指针的辅助函数，通过函数withUnsafePointer，获得指定类型的对应指针

//返回一个T类型指针。将第一个参数T以指针的形式传递给closure
func withUnsafePointer<T, Result>(to: T, (UnsafePointer<T>) -> Result) -> Result
func withUnsafePointer<T, Result>(to: inout T, (UnsafePointer<T>) -> Result) -> Result
func withUnsafeMutablePointer<T, Result>(to: inout T, (UnsafeMutablePointer<T>) -> Result) -> Result
func withUnsafePointer<T, Result>(to value: inout T, _ body: (UnsafePointer<T>) throws -> Result) rethrows -> Result

定义一个整形a，而p就是指向a的整形指针，它的类型会被自动转换为UnsafePointer<Int>，第二个参数被简化为了{ $0 }，代码块接收一个UnsafePointer<Int>参数，该参数即是a的地址，直接通过$0将它返回，即得到了a的指针，最终它被传给了p。

var a = 1234
let p = withUnsafePointer(to: &a) { $0 }
print("\(p.pointee)") // 打印：1234

获取指针并进行字节级操作 withUnsafeBytes 如果要对某块内存进行字节级编程。可以通过withUnsafeBytes得到某个类型的数据的字节指针，从而可以对它们进行字节级编程。因为withUnsafeBytes返回了一个类型UnsafeRawBufferPointer，该类型代表着一个字节级的内存块集合指针，所以以通过下标索引、for循环的方式来处理返回的对象。

var a: UInt32 = 0x12345678
let p = withUnsafeBytes(of: &a) { $0 }
var log = ""
for item in p {
    let hex = NSString(format: "%x", item)
    log += "\(hex)"
}
print("\(p.count)") // 打印：4
print("\(log)") // 对于小端机器会打印：78563412

通过指针动态创建、销毁内存 使用Swift可以直接操作内存，如可以开辟和管理一块内存，最后释放它，Swift提供了UnsafeMutablePointer的成员函数allocate来处理该工作。

let p = UnsafeMutablePointer<Int32>.allocate(capacity: 1)
p.initialize(to: 0) // 初始化
p.pointee = 32
print("\(p.pointee)") // 打印：32
p.deinitialize(count: 1) // 反初始化
p.deallocate()

C标准库函数的映射调用 Swift提供了大量的C标准库的桥接方法，我们可以像调用C语言库函数一样它们，如memcpy，strcpy等。

var n = 10086
// malloc
let p = malloc(MemoryLayout<Int32>.size)!
// memcpy
memcpy(p, &n, MemoryLayout<Int32>.size)
let p2 = p.assumingMemoryBound(to: Int32.self)
print("\(p2.pointee)") // 打印：10086
// strcpy
let str = "abc".cString(using: .ascii)!
if str.count != MemoryLayout<Int32>.size {
    return
}
let pstr = p.assumingMemoryBound(to: CChar.self)
strcpy(pstr, str)
print("\(String(cString: pstr))") // 打印：abc
// strlen
print("\(strlen(pstr))") // 打印： 3
// memset
memset(p, 0, MemoryLayout<Int32>.size)
print("\(p2.pointee)") // 打印：0
// strcat
strcat(pstr, "h".cString(using: .ascii)!)
strcat(pstr, "i".cString(using: .ascii)!)
print("\(String(cString: pstr))") // 打印：hi
// strstr
let s = strstr(pstr, "i")!
print("\(String(cString: s))") // 打印：i
// strcmp
print("\(strcmp(pstr, "hi".cString(using: .ascii)!))") // 打印：0
// free
free(p)

实际使用案例 1.将C函数库返回的char *指针，读取成字符串

let rRes = c_shmread()

let rp: UnsafeMutablePointer<CChar> = rRes.value
let strLength = rRes.length

//1.先将UnsafeMutablePointer<CChar> C语言的char *指针转成 UnsafeMutableRawPointer 的void *任意指针。然后直接取出封装成String字符串
if let strRes = String.init(bytesNoCopy: UnsafeMutableRawPointer(mutating: rp), length: Int(strLength), encoding: .utf8, freeWhenDone: false) {
    return strRes
} else {
    return ""
}

//2.使用withMemoryRebound转换指针类型从CChar转成UInt8类型；
let rpSub:UnsafeMutablePointer<UInt8> = rp.withMemoryRebound(to: UInt8.self, capacity: Int(strLength), { $0 })
//然后创建集合类型指针UnsafeBufferPointer;
let buffer = UnsafeBufferPointer<UInt8>.init(start: rpSub, count: Int(strLength))
buffer.forEach {
    print($0)
}
//然后创建Data, 再将data转成String
let data = Data(bytes: rpSub, count: Int(strLength))
let str = String(data: data, encoding: String.Encoding.utf8)

参考文章： https://juejin.cn/post/7030789069915291661#heading-6 https://www.jianshu.com/p/a9956ee1ab61

标签：UnsafeMutablePointer,UnsafePointer,类型,let,print,Swift,指针
From： https://www.cnblogs.com/zhou--fei/p/17652559.html

Swift中指针UnsafePointer的常见用法

相关文章

赞助商

阅读排行