golang中slice转化

1. 首先golang中确实没有提供引用的编程符号（c++ 中有&，建议可以先简单了解c++的引用）
   参考

2. 疑问1

c := &a
b := c[0:1] 
fmt.Printf("a:%p c:%p b:%p", &a, c, &b)

编译不通过，报错： cannot slice c (type *[]int32)

   s := make([]int, 20)
    ptr := unsafe.Pointer(&s[0])
    fmt.Printf("s:%p s type: %#v &s:%p ptr:%p &ptr:%p &s[0]:%p\n ", s, s, &s, ptr, &ptr, &s[0])


    ptr = unsafe.Pointer(ptr)
    s2t := ((*[20]int)(ptr))
    s2 := s2t[0:10]
    fmt.Printf("s2t: %p &s2t: %p  s2t type:%#v  s2: %p &s2: %p s2 type: %#v\n", s2t, &s2t, s2t, s2, &s2, s2)

编译通过，输出如下：

s:0xc420080000 s type: []int{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0} &s:0xc42000a060 ptr:0xc420080000 &ptr:0xc42000c028 &s[0]:0xc420080000

s2t: 0xc420080000 &s2t: 0xc42000c038  s2t type:&[20]int{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}  s2: 0xc420080000 &s2: 0xc42000a0c0 s2 type: []int{0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

大家是不是发现了 s2t type:&[20]int{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}这个&[20]int.. 不是[20]int，而代码强制转换是用的([20]int)

3、疑问2

    s := make([]int, 20)
    ptr := unsafe.Pointer(&s[0])
    var s1 = struct {
        addr uintptr
        len int
        cap int
    }{uintptr(ptr), 0, 10}
    s3t := (*[]int)(unsafe.Pointer(&s1))
    s3 := *s3t
    //s4 := s3t[0:10] //cannot slice s3t (type *[]int)
    st := &s

    //fmt.Printf("s1:%p  s3: %p  s3 type: %#v s3t:%p &s3t: %p s3t type:%#v s4:%p st:%#v\n", s1, s3, s3, s3t, &s3t, s3t, s4, st)
    fmt.Printf("s1:%p  s3: %p  s3 type: %#v s3t:%p &s3t: %p s3t type:%#v st:%#v\n", s1, s3, s3, s3t, &s3t, s3t, st)

输出如下：

s1:%!p(struct { addr uintptr; len int; cap int }={842350985216 0 10})  s3: 0xc420080000  s3 type: []int{} s3t:0xc420016180 &s3t: 0xc42000c028 s3t type:&[]int{} st:&[]int{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}

注释的编译不通过，报错cannot slice s3t (type *[]int)。报错原因：

s2t := ((*[20]int)(ptr))  ---  s3t := (*[]int)(unsafe.Pointer(&s1))

强转类型不一样，一个是20个元素的slice指针， 一个是slice指针， 所以就会出现报错。

type slice struct {
    array unsafe.Pointer
    len   int
    cap   int
}

slice的强转操作：第1种：将强转指针赋值到array字段了， 参数20作为len和cap进行设置。

第2种：可以理解为内存空间赋值（例如c语言(64位机器)： memcpy(s3t, &s1, 24)）

注： 本人能力有限，如有不正之处，欢迎指出