Swift十六(1: 内存访问冲突,2: 指针)

1 内存访问冲突
2 swift指针

一: 内存访问冲突

内存访问冲突会在两个访问满足下列条件时发生：
至少一个是写入操作
它们访问的是同一块内存
它们的访问时间重叠（比如在同一个函数内）

// 不存在内存访问冲突
func plus(_num: inoutInt) -> Int {num + 1}
var number = 1  
number = plus(&number) // 正常输入输出参数, 做+ 1 操作

// 存在访问冲突
// Simultaneous accesses to 0x0, but modification requires exclusive access 
// 同时访问0x0，但修改需要独占访问
var step=1 
func increment(_num: inoutInt){num += step}
increment(&step)
// num += step 同时调用了, step变量的setter方法 和getter方法 访问冲突

// 解决内存访问冲突
var copyOfStep = step 
increment(&copyOfStep) // 修改copyOfStep 变量
step = copyOfStep // 重新赋值step

二: 指针类型

定义:
Swift中也有专门的指针类型，这些都被定性为“Unsafe”（不安全的），常见的有以下4种类型
UnsafePointer<Pointee> 类似于 const Pointee * p UnsafeMutablePointer<Pointee> 类似于 Pointee * p
UnsafeRawPointer 类似于 const void * p
UnsafeMutableRawPointer 类似于 void *

1 UnsafePointer & UnsafeMutablePointer

var age = 10
func test1(_ ptr: UnsafeMutablePointer<Int>) { // 代表需要传个地址, 读写
   ptr.pointee += 10     // 效果类似inout输入输出参数
}
func test2(_ ptr: UnsafePointer<Int>) { // 代表需要传个地址, 只读
   print(ptr.pointee)
}
test1(&age)
test2(&age) // 20
print(age) // 20

2 UnsafeRawPointer & UnsafeMutableRawPointer

var age = 10
func test3(_ ptr: UnsafeMutableRawPointer) {
   ptr.storeBytes(of: 20, as: Int.self) // 类似setter 写入
}
func test4(_ ptr: UnsafeRawPointer) {
   print(ptr.load(as: Int.self)) // 类似getter读取
}
test3(&age)
test4(&age) // 20
print(age) // 20

3 指针的应用示例 stop.pointee对比break

var arr = NSArray(objects: 11, 22, 33, 44)
arr.enumerateObjects { (obj, idx, stop) in
print(idx, obj)
if idx == 2 { // 下标为2就停止遍历
    stop.pointee = true
} }

var arr = NSArray(objects: 11, 22, 33, 44)
for (idx, obj) in arr.enumerated() {
print(idx, obj)
if idx == 2 {
   break
} }

4 获得指向某个变量的指针

var age = 11
var ptr1 = withUnsafeMutablePointer(to: &age) { $0 } // 读写
var ptr2 = withUnsafePointer(to: &age) { $0 } // 只读
ptr1.pointee = 22 // 操作指针修改内存
print(ptr2.pointee) // 22
print(age) // 22

let ptr6 = withUnsafeMutablePointer(to: &age) { (p) -> UnsafeMutablePointer in
    return   p // ptr1 原版写法, 返回值p的类型和参数闭包返回值类型要一致
}
ptr6.pointee = 40
print(age)
// 返回 withUnsafeMutablePointer / withUnsafePointer类型

var ptr3 = withUnsafeMutablePointer(to: &age) { UnsafeMutableRawPointer($0) }
var ptr4 = withUnsafePointer(to: &age) { UnsafeRawPointer($0) }
ptr3.storeBytes(of: 33, as: Int.self)
print(ptr4.load(as: Int.self)) // 33
print(age) // 33
// 返回 UnsafeMutableRawPointer / UnsafeRawPointer类型