[移动开发]Swift中的内存访问冲突、指针、局部作用域

?内存访问冲突（Conflicting Access to Memory）

1、内存访问冲突会在两个访问满足以下条件时发生：

• 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）

//无内存访问冲突
func plus(_ num: inout Int) -> Int {
    num + 1
}
var number = 1
number = plus(&number)

//有内存访问冲突
var step = 1
func increment(_ num: inout Int) {
    num += step //error
}
increment(&step)

//解决内存访问冲突
var copyOfStep = step
increment(&copyOfStep)
step = copyOfStep

2、如果下面的条件可以满足 ，就说明重叠访问结构体的属性是安全的

• 你只访问实例存储属性，不是计算属性或者类属性
• 结构体是局部变量而非全局变量
• 结构体要么没有被闭包捕获要么只被非逃逸闭包捕获

指针

1、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中类型

• UnsafePointer<Pointee> 类似于const Pointee *
• UnsafeMutablePointer<Pointee> 类似于Pointee *
• UnsafeRawPointer 类似于const void *
• UnsafeMutableRawPointer 类似于 void *

var age = 10

func test1(_ ptr: UnsafeMutablePointer<Int>) {
    ptr.pointee = 20
    print("test1", ptr.pointee)
}

func test2(_ ptr: UnsafePointer<Int>) {
    print("test2", ptr.pointee);
}

func test4(_ ptr: UnsafeMutableRawPointer) {
    ptr.storeBytes(of: 30, as: Int.self)
}

func test5(_ ptr: UnsafeRawPointer) {
    print("test5", ptr.load(as: Int.self))
}

指针的应用示例?

//OC -> BOOL *
//Swift -> UnsafeMutablePointer<ObjCBool>

var arr = NSArray(objects: 11, 22, 33, 44)
arr.enumerateObjects { (element, idx, stop) in
    print(idx, element)
    if idx == 2 {
        stop.pointee = true
    }
}

获得指向某个变量的指针

var age = 10

var ptr1 = withUnsafeMutablePointer(to: &age) {
    $0
}

var ptr2 = withUnsafePointer(to: &age) {
    $0
}

ptr1.pointee = 22
print(ptr2.pointee) // 22
print(age) // 22

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

获得指向堆空间实例的指针

class Person {
    var age: Int
    
    init(age: Int) {
        self.age = age
    }
}

var person = Person(age: 21)

var ptr1 = withUnsafePointer(to: &person) {
    UnsafeRawPointer($0)
}
var personObjAddress = ptr1.load(as: UInt.self)
var ptr2 = UnsafeMutableRawPointer(bitPattern: personObjAddress)

//下面这种方式和上面的实现效果相同
//var ptr = unsafeBitCast(person, to: UnsafeRawPointer.self)

通过指针我们可以很容易的获取内存中变量的地址值，方便窥探底层做的事情。?

?创建指针

var ptr = malloc(16)

ptr?.storeBytes(of: 10, as: Int.self)
ptr?.storeBytes(of: 20, toByteOffset: 8, as: Int.self)

print(ptr?.load(as: Int.self)) // 10
print(ptr?.load(fromByteOffset: 8, as: Int.self)) // 20

free(ptr)

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)

ptr.storeBytes(of: 11, as: Int.self)
ptr.advanced(by: 8).storeBytes(of: 22, as: Int.self)

print(ptr.load(as: Int.self)) // 11
print(ptr.advanced(by: 8).load(as: Int.self)) // 22

ptr.deallocate()

var ptr = UnsafeMutablePointer<Int>.allocate(capacity: 2)
ptr.initialize(to: 10)
ptr.successor().initialize(to: 20)

print(ptr.pointee)
print((ptr + 1).pointee)

print(ptr.pointee)
print(ptr.successor().pointee)

print(ptr[0])
print(ptr[1])

ptr.deinitialize(count: 2)
ptr.deallocate()

测试内存的释放：

class Person {
    var age: Int
    var name: String
    
    init(age: Int, name: String) {
        self.age = age
        self.name = name
    }
    
    deinit {
        print(name, "deinit")
    }
}

var ptr = UnsafeMutablePointer<Person>.allocate(capacity: 3)
ptr.initialize(to: Person(age: 20, name: "Jack"))
(ptr + 1).initialize(to: Person(age: 21, name: "Rose"))
(ptr + 2).initialize(to: Person(age: 22, name: "Kate"))

ptr.deinitialize(count: 3)
ptr.deallocate()

?指针之间的转换

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1)

ptr.assumingMemoryBound(to: Int.self).pointee = 11
(ptr + 8).assumingMemoryBound(to: Double.self).pointee = 22.0

var ptr2 = unsafeBitCast(ptr, to: UnsafeMutablePointer<Int>.self)
ptr2.pointee = 11

ptr.deallocate()

1、unsafeBitCast是忽略数据类型的强制转换，不会因为数据类型的变化而改变原来的内存数据

• 类似于C++中的reinterpret_cast

局部作用域

class Dog {
    var age = 10
    func run() {}
}

do {
    let dog1 = Dog()
    dog1.age = 10
    dog1.run()
}

do {
    let dog2 = Dog()
    dog2.age = 10
    dog2.run()
}