类型，接口和指针

Question

I have a simple code:

type Namer interface {
    PrintName()
}

type P struct {
    Name string
}

func (p *P) PrintName() {
    fmt.Printf("%s
", p.Name)
}

func main() {
    p := P{Name: "Name"}

    var namers []Namer
    namers = append(namers, &p)
    fmt.Println(reflect.TypeOf(namers[0]))

    on := &namers[0]
    fmt.Println(reflect.TypeOf(on))
    (*on).PrintName()
    (**on).Name = "EEEE"
    (*on).PrintName()
}

and bunch of questions :)

1. Why I cannot write: append(namers, p)? &p is a pointer to P, array namers is not a array of pointers
2. Why TypeOf(namers[0]) is *P and TypeOf(on) is *Namer? It does not make sense, TypeOf(&(*P)) should be **P
3. Why the last line print: "Name" instead of "EEE"?

Thanks for help!

Answer 1

1. P doesn't implement interface Namer, only *P does, as PrintName() is defined on *P.

2. As I understand it, TypeOf(namers[0]) is *P because you're inspecting the actual slice element itself, and TypeOf can see inside the interface. With TypeOf(&namers[0]) you're merely taking the address of the first element and inspecting its type (which is, of course, *Namer, as namers is a slice of Namer interface elements), without actually looking "inside" the element itself.

3. The last line doesn't print anything, because (**on).Name is not allowed. Change it to (*on).(*P).Name = "EEEE" and it works as intended.