接口与指针到接口的Golang反射

Question

In the example of gob usage http://golang.org/src/encoding/gob/example_interface_test.go they provide the following thesis: Pass pointer to interface so Encode sees (and hence sends) a value of interface type. If we passed p directly it would see the concrete type instead. See the blog post, "The Laws of Reflection" for background.

I've read The Laws of reflection twice, and a related Russ Cox article too. But I can't find a distinction between pointer-to-interface and interface there.

So why is it that through the pointer it sees a value of interface type, and with no pointer it sees (surprisingly to me) the concrete type?

Answer 1

It seems to me that the relevant part is this:

Continuing, we can do this:

var empty interface{}
empty = w

and our empty interface value empty will again contain that same pair, (tty, *os.File). That's handy: an empty interface can hold any value and contains all the information we could ever need about that value.

(emphasis added)

When you assign an interface value to a value of type interface{}, the "pointer to data" part of the new value doesn't point to the old value, but rather to the data old value was pointing to. We can prove that with a bit of unsafe code:

type iface struct {
    Type, Data unsafe.Pointer
}

var r io.Reader = &bytes.Buffer{}
var i interface{} = r

rr := *(*iface)(unsafe.Pointer(&r))
ii := *(*iface)(unsafe.Pointer(&i))

fmt.Printf("%v
", ii.Data == rr.Data) // Prints true.

On the other hand, if we use a pointer, it will point to the interface value itself. So now reflect can actually see, what interface exactly are we talking about. E.g.:

var i2 interface{} = &r
ii2 := *(*iface)(unsafe.Pointer(&i2))
fmt.Printf("%v
", ii2.Data == unsafe.Pointer(&r)) // Prints true.

Playground: http://play.golang.org/p/0ZEMdIFhIj