切片指针和循环[重复]

Question

This question already has an answer here:

• Using Pointers in a for Loop - Golang 1 answer

I think this question where asked several times but I still confused:

I have following code:

type obj struct {
    s *string
}

var cmdsP = []*string {
    stringPointer("create"),
    stringPointer("delete"),
    stringPointer("update"),
}

var cmds = []string {
    "create",
    "delete",
    "update",
}

// []*string
func loop1() {
    slice := make([]obj, 0, 0)

    for _, cmd := range cmdsP {
        slice = append(slice, obj{cmd})
    }
    for _, o := range slice {
        fmt.Println(*o.s)
    }
}

// []string
func loop2() {
    slice := make([]obj, 0, 0)
    for _, cmd := range cmds {
        slice = append(slice, obj{&cmd})
    }
    for _, o := range slice {
        fmt.Println(*o.s)
    }
}

func stringPointer(v string) *string {
    return &v
}

https://play.golang.org/p/65Le_8Pi3Mi

The only difference is in slice semantic []*string and []string how does it change behavior of cmd variable? Could you please draw or explain in details what happens in memory during iteration through two loops?

</div>

Answer 1

When you call range on a collection, the go runtime initialises 2 memory locations; one for the index (in this case _), and one for the value cmd.

What range then does, is take each of the items in the collection and copy them into the memory location that it created when you called range.

This means that each of the items in the slice get put into that memory location one by one.

When you do &cmd you are taking a pointer. This pointer points to the shared memory location that each of the slice items are being copied into.

All of your pointers created using &cmd all point to the same memory location.

This means that after the range is done, the only value left in that memory location that your pointers are pointing to is the last value from the range iteration.

This is why you get the output

update
update
update