// 按照上图定义的数据结构
type Slice struct {
  ptr  unsafe.Pointer    // Array pointer
  len  int        // slice length
  cap   int        // slice capacity
}

package main

import (
  "fmt"
  "unsafe"
)

// 按照上图定义的数据结构
type Slice struct {
  ptr unsafe.Pointer // Array pointer
  len int      // slice length
  cap int      // slice capacity
}

// 因为需要指针计算，所以需要获取int的长度
// 32位 int length = 4
// 64位 int length = 8
var intLen = int(unsafe.Sizeof(int(0)))

func main() {
  s := make([]int, 10, 20)

  // 利用指针读取 slice memory 的数据
  if intLen == 4 { // 32位
    m := *(*[4 + 4*2]byte)(unsafe.Pointer(&s))
    fmt.Println("slice memory:", m)
  } else { // 64 位
    m := *(*[8 + 8*2]byte)(unsafe.Pointer(&s))
    fmt.Println("slice memory:", m)
  }

  // 把slice转换成自定义的 Slice struct
  slice := (*Slice)(unsafe.Pointer(&s))
  fmt.Println("slice struct:", slice)
  fmt.Printf("ptr:%v len:%v cap:%v \n", slice.ptr, slice.len, slice.cap)
  fmt.Printf("golang slice len:%v cap:%v \n", len(s), cap(s))

  s[0] = 0
  s[1] = 1
  s[2] = 2

  // 转成数组输出
  arr := *(*[3]int)(unsafe.Pointer(slice.ptr))
  fmt.Println("array values:", arr)

  // 修改 slice 的 len
  slice.len = 15
  fmt.Println("Slice len: ", slice.len)
  fmt.Println("golang slice len: ", len(s))
}

$ go run slice.go

slice memory: [0 64 6 32 200 0 0 0 10 0 0 0 0 0 0 0 20 0 0 0 0 0 0 0]
slice struct: &{0xc820064000 10 20}
ptr:0xc820064000 len:10 cap:20
golang slice len:10 cap:20
array values: [0 1 2]
Slice len: 15
golang slice len: 15

// 每次cap改变，指向array的ptr就会变化一次
s := make([]int, 1)

fmt.Printf("len:%d cap: %d array ptr: %v \n", len(s), cap(s), *(*unsafe.Pointer)(unsafe.Pointer(&s)))

for i := 0; i < 5; i++ {
  s = append(s, i)
  fmt.Printf("len:%d cap: %d array ptr: %v \n", len(s), cap(s), *(*unsafe.Pointer)(unsafe.Pointer(&s)))
}

fmt.Println("Array:", s)

len:1 cap: 1 array ptr: 0xc8200640f0
len:2 cap: 2 array ptr: 0xc820064110
len:3 cap: 4 array ptr: 0xc8200680c0
len:4 cap: 4 array ptr: 0xc8200680c0
len:5 cap: 8 array ptr: 0xc82006c080
len:6 cap: 8 array ptr: 0xc82006c080
Array: [0 0 1 2 3 4]

s := []int{1, 2, 3, 4, 5}
fmt.Printf("len:%d cap: %d array ptr: %v \n", len(s), cap(s), *(*unsafe.Pointer)(unsafe.Pointer(&s)))
fmt.Println("Array:", s)

s1 := s[1:3]
fmt.Printf("len:%d cap: %d array ptr: %v \n", len(s1), cap(s1), *(*unsafe.Pointer)(unsafe.Pointer(&s1)))
fmt.Println("Array", s1)

len:5 cap: 5 array ptr: 0xc820012210
Array: [1 2 3 4 5]
len:2 cap: 4 array ptr: 0xc820012218
Array [2 3]