package unsafe
//ArbitraryType仅用于文档目的，实际上并不是unsafe包的一部分,它表示任意Go表达式的类型。
type ArbitraryType int
//任意类型的指针，类似于C的*void
type Pointer *ArbitraryType
//确定结构在内存中占用的确切大小
func Sizeof(x ArbitraryType) uintptr
//返回结构体中某个field的偏移量
func Offsetof(x ArbitraryType) uintptr
//返回结构体中某个field的对其值（字节对齐的原因）
func Alignof(x ArbitraryType) uintptr

func main() {
  i := 10
  ip := &i

  fp := (*float64)(unsafe.Pointer(ip))

  *fp = *fp * 3

  fmt.Println(i)
}

// 结果： 30

func main() {
  i := 10
  ip := &i

  fp := (*string)(unsafe.Pointer(ip))

  *fp = "a"

  fmt.Println(i)
  // 结果：19678090
}

type Student struct {
  Name string
  Age int
}

func main() {
  s := Student{}
  s.Name = "Peter"
  s.Age = 33

  pStudent := unsafe.Pointer(&s)
  // 整个对象转换成指针，默认是获取第一个属性
  name := (*string)(unsafe.Pointer(pStudent))
  fmt.Println("name:", *name)
  // 利用Offsetof获取age属性的偏移量获取属性
  age := (*int)(unsafe.Pointer(uintptr(pStudent) + unsafe.Offsetof(s.Age)))
  fmt.Println("age:", *age)
  
  // 修改指针的值
  *name = "Mary"
  *age = 20
  fmt.Println(s)
}

type Teacher struct {
  name string
  age int
}

func main() {
  t := Teacher{"ttt", 20}

  pt := unsafe.Pointer(&t)
  name := (*string)(unsafe.Pointer(pt))
  fmt.Println("name:", *name)
}

func main() {
  array := []int{0, 1, -2, 3, 4}
  pointer := &array[0]
  fmt.Print(*pointer, " ")
  memoryAddress := uintptr(unsafe.Pointer(pointer)) + unsafe.Sizeof(array[0])
  for i := 0; i < len(array)-1; i++ {
    pointer = (*int)(unsafe.Pointer(memoryAddress))
    fmt.Print(*pointer, " ")
    memoryAddress = uintptr(unsafe.Pointer(pointer)) + unsafe.Sizeof(array[0])
  }
}