16.深入了解Swift底层

MemoryLayout

可以使用MemoryLayout获取数据类型占用的内存大小

enum Password {
    case number(Int, Int, Int, Int) // 32
    case other
}

MemoryLayout<Password>.size // 33
MemoryLayout<Password>.stride // 40
MemoryLayout<Password>.alignment // 8

inout传递参数是传递地址

var number = 10
 
 func test(_ num: Int) {
 
 }
 
 test(number)
 
 0x100000f5e <+78>: movq   -0x30(%rbp), %rdi
 0x100000f62 <+82>: callq  0x100000f70               ; TestSwift.test(Swift.Int) -> () at main.swift:24
 
 var number = 10
 
 func test(_ num: inout Int) {
 
 }
 
 test(&number)
 
 0x100000f47 <+55>: leaq   0x10ca(%rip), %rdi        ; TestSwift.number : Swift.Int
 0x100000f4e <+62>: callq  0x100000f70               ; TestSwift.test(inout Swift.Int) -> () at main.swift:24

1. 如果实参有物理内存地址，且没有设置属性观察器
   直接将实参的内存地址传入函数(实参进行引用传递)
2. 如果实参是计算属性 或者 设置了属性观察器
   采取了Copy In Copy Out的做法

• 调用该函数时，先复制实参的值，产生副本【get】
• 将副本的内存地址传入函数(副本进行引用传递)，在函数内部可以修改副本的值
• 函数返回后，再将副本的值覆盖实参的值【set】
  总结:inout的本质就是引用传递(地址传递)

枚举关联值存储在内存中

func testEnum() {
    enum TestEnum {
        case test1(Int, Int, Int)
        case test2(Int, Int)
        case test3(Int)
        case test4(Bool)
        case test5
    }
    
    // 1个字节存储成员值
    // N个字节存储关联值（N取占用内存最大的关联值），任何一个case的关联值都共用这N个字节
    // 共用体
    
    // 小端：高高低低
    // 01 00 00 00 00 00 00 00
    // 02 00 00 00 00 00 00 00
    // 03 00 00 00 00 00 00 00
    // 00
    // 00 00 00 00 00 00 00
    var e = TestEnum.test1(1, 2, 3)
    print(Mems.ptr(ofVal: &e))
    
    // 04 00 00 00 00 00 00 00
    // 05 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 01
    // 00 00 00 00 00 00 00
    e = .test2(4, 5)
    print(Mems.memStr(ofVal: &e))
    
    // 06 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 02
    // 00 00 00 00 00 00 00
    e = .test3(6)
    
    // 01 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 03
    // 00 00 00 00 00 00 00
    e = .test4(true)
    
    // 00 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 00 00 00 00 00 00 00 00
    // 04
    // 00 00 00 00 00 00 00
    e = .test5
}

对象的堆空间申请过程

• Class.__allocationg_init()
• libswiftCore.dylib: swift_allocObject
• libswiftCore.dylib: swift_slowAlloc
• libsystem_mallic.dylib: malloc

字符串的内存

// 字符串长度 <= 0xF，字符串内容直接存放在str1变量的内存中
 var str1 = "0123456789"
 
 // 字符串长度 > 0xF，字符串内容存放在__TEXT.cstring中（常量区）
 // 字符串的地址值信息存放在str2变量的后8个字节中
 var str2 = "0123456789ABCDEF"
 
 
 // 由于字符串长度 <= 0xF，所以字符串内容依然存放在str1变量的内存中
 str1.append("ABCDE")
 // 开辟堆空间
 str1.append("F")
 
 // 开辟堆空间
 str2.append("G")

Array的内存

var arr = [1, 2, 3, 4]

01

指针

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 *
• 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 += 10 }
func test2(_ ptr: UnsafePointer<Int>) { print(ptr.pointee)
}
test1(&age)
test2(&age) // 20
print(age) // 20

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

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
     }
}

获取某个变量的指针

var age = 11
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

获取指向堆空间实例的指针

class Person {}
var person = Person()
var ptr = withUnsafePointer(to: &person) { UnsafeRawPointer($0) } var heapPtr = UnsafeRawPointer(bitPattern: ptr.load(as: UInt.self)) print(heapPtr!)

创建指针

 var ptr = UnsafeRawPointer(bitPattern: 0x100001234)
 // 创建
var ptr = malloc(16)
// 存
ptr?.storeBytes(of: 11, as: Int.self)
ptr?.storeBytes(of: 22, toByteOffset: 8, as: Int.self)
// 取
print((ptr?.load(as: Int.self))!) // 11 
print((ptr?.load(fromByteOffset: 8, as: Int.self))!) // 22 // 销毁
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: 3) 
ptr.initialize(to: 11)
ptr.successor().initialize(to: 22) 
ptr.successor().successor().initialize(to: 33)
print(ptr.pointee) // 11 
print((ptr + 1).pointee) // 22 
print((ptr + 2).pointee) // 33
print(ptr[0]) // 11
print(ptr[1]) // 22
print(ptr[2]) // 33
ptr.deinitialize(count: 3) 
ptr.deallocate()

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: 10, name: "Jack"))
(ptr + 1).initialize(to: Person(age: 11, name: "Rose")) 
(ptr + 2).initialize(to: Person(age: 12, name: "Kate"))
// Jack deinit
// Rose deinit
// Kate deinit 
ptr.deinitialize(count: 3) 
ptr.deallocate()

指针之间的切换

var ptr = UnsafeMutableRawPointer.allocate(byteCount: 16, alignment: 1) 
ptr.assumingMemoryBound(to: Int.self).pointee = 11
(ptr + 8).assumingMemoryBound(to: Double.self).pointee = 22.0 print(unsafeBitCast(ptr, to: UnsafePointer<Int>.self).pointee) // 11
print(unsafeBitCast(ptr + 8, to: UnsafePointer<Double>.self).pointee) 
// 22.0 
ptr.deallocate()

• 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

class Person {}
var person = Person()
var ptr = unsafeBitCast(person, to: UnsafeRawPointer.self) 
print(ptr)