带你深度了解 unsafe.Pointer

package main
import "fmt"
func double(x int) {  x += x}func main() {  var a = 3  double(a)  fmt.Println(a) // 3}

package main
import "fmt"
func double(x *int) {  *x += *x}func main() {  var a = 3  double(&a)  fmt.Println(a) // 6  p := &a  double(p)  fmt.Println(a, p == nil) // 12 false}

type Pointer *ArbitraryType
Pointer represents a pointer to an arbitrary type. There are four special operationsavailable for type Pointer that are not available for other types:    //  Pointer代表了一个任意类型的指针。Pointer类型有四种特殊的操作是其他类型不能使用的:   - A pointer value of any type can be converted to a Pointer.       //  任意类型的指针可以被转换为Pointer   - A Pointer can be converted to a pointer value of any type.       //  Pointer可以被转换为任务类型的值的指针   - A uintptr can be converted to a Pointer.                         //  uintptr可以被转换为Pointer   - A Pointer can be converted to a uintptr.                         //  Pointer可以被转换为uintptrPointer therefore allows a program to defeat the type system and read and writearbitrary memory. It should be used with extreme care.                //  因此Pointer允许程序不按类型系统的要求来读写任意的内存，应该非常小心地使用它。

// uintptr is an integer type that is large enough to hold the bit pattern of// any pointer.type uintptr uintptr

指针类型 *T <-> unsafe.Pointer <-> uintptr

package main
import (  "fmt"  "unsafe")
func main() {  i := 100  intI := &i  var floatI *float64  floatI = (*float64)(unsafe.Pointer(intI))  *floatI = *floatI * 3  fmt.Printf("%T\n", i)  fmt.Println(i)  fmt.Printf("%T\n", intI)  fmt.Printf("%T\n", floatI)}
// 输出int300*int*float64

type StringHeader struct {  Data uintptr  Len  int}
type SliceHeader struct {  Data uintptr  Len  int  Cap  int}

func string2bytes(s string) []byte {  stringHeader := (*reflect.StringHeader)(unsafe.Pointer(&s))
  bh := reflect.SliceHeader{    Data: stringHeader.Data,    Len:  stringHeader.Len,    Cap:  stringHeader.Len,  }
  return *(*[]byte)(unsafe.Pointer(&bh))}
func bytes2string(b []byte) string{  sliceHeader := (*reflect.SliceHeader)(unsafe.Pointer(&b))
  sh := reflect.StringHeader{    Data: sliceHeader.Data,    Len:  sliceHeader.Len,  }
  return *(*string)(unsafe.Pointer(&sh))}

package main
import (  "fmt"  "unsafe")
func main() {  arr := [3]int{1, 2, 3}  ap := &arr  arr0p := (*int)(unsafe.Pointer(ap))  arr1p := (*int)(unsafe.Pointer(uintptr(unsafe.Pointer(ap)) + unsafe.Sizeof(arr[0])))  *arr0p += 10  *arr1p += 20  fmt.Println(arr) // [11 22 3]}

package main
import (  "fmt"  "unsafe")
type user struct {  name string  age  int}
func main() {  u := new(user)  fmt.Println(*u) // { 0}
  pName := (*string)(unsafe.Pointer(u))  *pName = "张三"
  pAge := (*int)(unsafe.Pointer(uintptr(unsafe.Pointer(u)) + unsafe.Offsetof(u.age)))  *pAge = 20
  fmt.Println(*u) // {张三 20}}