Swift指针Unsafe Pointer
对照Objective-C
例子
Swift指针Unsafe Pointer如果不是只读，可以修改 （ 写入 ），就加一个 Mutable,
如果没有具体的类型（ 通过泛型的方式 ），就加一个 Raw,
如果不是一个单独的对象 （ 指向集合类型 ），就加上 buffer.
Unsafe [ Mutable ] [ Raw ] [ Buffer ] Pointer [ ]
苹果没有编译保护的 [ 可变的 ] [没有类型的] [ 是集合的 ] 指针 [< 具体的类型 >]
对照Objective-Cswift 的 unsafeMutablePointer<T>： OC 的 T *
swift 的 unsafePointer<T>： OC 的 const T *
swift 的 unsafeRawPointer： OC 的 const void *
swift 的 unsafeMutableRawPointer： OC 的 void *
例子例子 1， 无类型的指针
let count = 2
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * count
do {
print("Raw pointers")
let pointer = UnsafeMutableRawPointer.allocate(
byteCount: byteCount,
alignment: alignment)
// 指针的创建，与销毁
defer {
// 需要手动管理，指针的内存
pointer.deallocate()
}
// store 存值
pointer.storeBytes(of: 42, as: Int.self)
// 指针需要移动 stride，才能到达下一个指针
pointer.advanced(by: stride).storeBytes(of: 6, as: Int.self)
// (pointer+stride).storeBytes(of: 6, as: Int.self)， 这个是另一种方式
// load 取值
print(pointer.load(as: Int.self))
print(pointer.advanced(by: stride).load(as: Int.self))
// 集合的指针
let bufferPointer = UnsafeRawBufferPointer(start: pointer, count: byteCount)
for (index, byte) in bufferPointer.enumerated() {
print("byte \(index): \(byte)")
}
}
2， 具体类型的指针
具体类型的指针，可以通过指针的 pointee 属性，方便的操作 load 和 store
let count = 2
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * count
do {
print("Typed pointers")
let pointer = UnsafeMutablePointer<Int>.allocate(capacity: count)
pointer.initialize(repeating: 0, count: count)
// 与上面的一样，指针的内存，需要手动管理
defer {
pointer.deinitialize(count: count)
pointer.deallocate()
}
pointer.pointee = 42
// 因为编译器做了优化，指针到达下一个指针，不需要移动 stride
// 指针移动 1 ，就到了下一个指针
pointer.advanced(by: 1).pointee = 6
print( pointer.pointee )
print(pointer.advanced(by: 1).pointee)
let bufferPointer = UnsafeBufferPointer(start: pointer, count: count)
for (index, value) in bufferPointer.enumerated() {
print("value \(index): \(value)")
}
}
例子 3: 通过绑定内存，来做指针的转化
bindMemory
let count = 2
let stride = MemoryLayout<Int>.stride
let alignment = MemoryLayout<Int>.alignment
let byteCount = stride * count
do {
print("Converting raw pointers to typed pointers")
let rawPointer = UnsafeMutableRawPointer.allocate(
byteCount: byteCount,
alignment: alignment)
defer {
rawPointer.deallocate()
}
// 这一步，将任意指针，转化为类型指针
let typedPointer = rawPointer.bindMemory(to: Int.self, capacity: count)
typedPointer.initialize(repeating: 0, count: count)
defer {
typedPointer.deinitialize(count: count)
}
typedPointer.pointee = 42
typedPointer.advanced(by: 1).pointee = 6
// 看结果
print(typedPointer.pointee)
print(typedPointer.advanced(by: 1).pointee)
let bufferPointer = UnsafeBufferPointer(start: typedPointer, count: count)
for (index, value) in bufferPointer.enumerated() {
print("value \(index): \(value)")
}
}
例子 4， 查看指针的字节
struct Demo{
let number: UInt32
let flag: Bool
}
do {
print("Getting the bytes of an instance")
var one = Demo(number: 25, flag: true)
withUnsafeBytes(of: &one) { bytes in
for byte in bytes {
print(byte)
}
}
}
例子 4.1， 指针的字节, 算 check sum
struct Demo{
let number: UInt32
let flag: Bool
}
do {
print("Checksum the bytes of a struct")
var one = Demo(number: 25, flag: true)
let checksum = withUnsafeBytes(of: &one) { (bytes) -> UInt32 in
return ~bytes.reduce(UInt32(0)) { $0 + numericCast($1) }
}
print("checksum", checksum) // checksum 4294967269
}
checeSum 的使用，分为 checeSum 的计算与校验
本文简单描述 checeSum 的计算
数据块，分为 n 个包， size 相同
拿包的字节，计算 checkSum, checkSum 的大小限制在包的 size
例子 5, 获取变量的指针
var cat = "fly"
// 返回的是，闭包中的参数
// withUnsafePointer , 把闭包里面的结果，rethrow 出去 （ 相当于 return 出来 ）
let warrior = withUnsafePointer(to: &cat, { $0 })
print(warrior.pointee)
例子 6, 指向多个元素的指针
struct Cat{
var habit = "eat"
var paws = 6
var name = "load"
}
let ptr = UnsafeMutablePointer<Cat>.allocate(capacity: 2) // 指向两个 Cat 结构体
ptr.initialize(repeating: Cat(), count: 2)
defer{
ptr.deinitialize(count: 2)
ptr.deallocate()
}
var one = Cat()
one.paws = 8
ptr[1] = one
// 以下两个等价
print(ptr[0])
print(ptr.pointee)
// 下面 3 个等价
print(ptr[1])
print((ptr + 1).pointee)
print(ptr.successor().pointee)
例子 7: 元素组合的探索
var pair = (66, 666)
func test(ptr: UnsafePointer<Int>){
print(ptr.pointee)
print(ptr.successor().pointee)
}
withUnsafePointer(to: &pair) { (tuplePtr: UnsafePointer<(Int, Int)>) in
// 假定内存绑定，不需要经过内存检查
test(ptr: UnsafeRawPointer(tuplePtr).assumingMemoryBound(to:Int.self))
}
参考了 Unsafe Swift: Using Pointers and Interacting With C
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