How to get a pointer to a variable that's masked as an interface?

Question

I prefer not to dive into the rationale of the situation below. It has to do with unmarshaling an serialized object that can be any of a fixed set of types, but you don't know which type.

I have the following types:

type I interface {
    Do()
}

type someI struct {}
func (i *someI) Do() {}

type otherI struct {}
func (i *otherI) Do() {}

So, two structs of which the pointers implement interface I.

Now I have this method that wants to return a value of type I:

func GetSomeI(marshalled []byte) (I, error) {
    var obj interface{}
    // The following method magically puts an instance
    // of either someI or otherI into obj.
    magicUnmarshall(marshalled, obj)
    // The problem now is that we cannot return obj,
    // because the raw structs don't implement I.

    // One solution would be to do a type switch like this:
    switch obj.(type) {
    case someI:
        i := obj.(someI)
        return &i, nil
    case otherI:
        i := obj.(otherI)
        return &i, nil
    default:
        return nil, errors.New("marschalled object was not of type I")
    }

    // But now consider the case that there are quite some
    // different implementations of I.
    // We would prefer to have a general way of getting
    // a reference to obj.
}

Answer 1

To tell if a value wrapped in an interface{} implements some other interface (I), you may simply use a type assertion.

Note that you must pass the address of the variable you want results unmarshaled to.

For demonstration purposes, let's use the following magicUnmarshal() function:

func magicUnmarshal(what int, obj interface{}) {
    v := reflect.ValueOf(obj).Elem()
    switch what {
    case 0:
        v.Set(reflect.ValueOf(&someI{}))
    case 1:
        v.Set(reflect.ValueOf(&otherI{}))
    case 2:
        v.Set(reflect.ValueOf("just a string"))
    case 3:
        v.Set(reflect.ValueOf(someI{}))
    case 4:
        v.Set(reflect.ValueOf(otherI{}))
    }
}

Note that case 3 and case 4 are returning non-pointers.

Your GetSomeI() implementation can be:

func GetSomeI(what int) (I, error) {
    var obj interface{}
    magicUnmarshal(what, &obj)

    // Try the value as-is:
    if i, ok := obj.(I); ok {
        return i, nil
    }

    // No success. Try a pointer to the value:
    v := reflect.Indirect(reflect.New(reflect.TypeOf(obj)))
    v.Set(reflect.ValueOf(obj))
    pobj := v.Addr().Interface()
    if i, ok := pobj.(I); ok {
        return i, nil
    }

    return nil, fmt.Errorf("%T does not implement I!", obj)
}

First GeSomeI() tests if the value got form magicUnmarshal() implements I, and if so, it is used as-is. If not, we construct a new using reflection, and get its address (a pointer to a value), and we test that. If that pointer implements I, we return it.

Testing it:

func main() {
    for what := 0; what < 5; what++ {
        i, err := GetSomeI(what)
        fmt.Printf("%T %v\n", i, err)
    }
}

And the output is (try it on the Go Playground):

*main.someI <nil>
*main.otherI <nil>
<nil> string does not implement I!
*main.someI <nil>
*main.otherI <nil>