Why can't I assign a *Struct to an *Interface?

Question

I'm just working through the Go tour, and I'm confused about pointers and interfaces. Why doesn't this Go code compile?

package main

type Interface interface {}

type Struct struct {}

func main() {
    var ps *Struct
    var pi *Interface
    pi = ps

    _, _ = pi, ps
}

i.e. if Struct is an Interface, why wouldn't a *Struct be a *Interface?

The error message I get is:

prog.go:10: cannot use ps (type *Struct) as type *Interface in assignment:
        *Interface is pointer to interface, not interface

Answer 1

When you have a struct implementing an interface, a pointer to that struct implements automatically that interface too. That's why you never have *SomeInterface in the prototype of functions, as this wouldn't add anything to SomeInterface, and you don't need such a type in variable declaration (see this related question).

An interface value isn't the value of the concrete struct (as it has a variable size, this wouldn't be possible), but it's a kind of pointer (to be more precise a pointer to the struct and a pointer to the type). Russ Cox describes it exactly here :

Interface values are represented as a two-word pair giving a pointer to information about the type stored in the interface and a pointer to the associated data.

This is why Interface, and not *Interface is the correct type to hold a pointer to a struct implementing Interface.

So you must simply use

var pi Interface

Answer 2

This is perhaps what you meant:

package main

type Interface interface{}

type Struct struct{}

func main() {
        var ps *Struct
        var pi *Interface
        pi = new(Interface)
        *pi = ps

        _, _ = pi, ps
}

Compiles OK. See also here.

Answer 3

Here's a very simple way of assigning a struct to an interface:

package main

type Interface interface{}

type Struct struct{}

func main() {
    ps := new(Struct)
    pi := Interface(ps)

    _, _ = pi, ps
}

https://play.golang.org/p/BRTaTA5AG0S