Is it possible to pass interface obj to interface{} type?

Question

I'm new to golang and I want to implement an overloaded method something similar to C++ overloading, and my code looks something like this:

type someStruct struct {
    val  int
    some string
}

type object interface {
    toByte()
}

// someStruct implementing an object interface
func (s *someStruct) toByte() {
}

func overload(overLoadedObj interface{}) {

    switch str := overLoadedObj .(type) {
    case string:
        fmt.Println("string : ", str)
    case int:
        fmt.Println("int : ", str)
    case object: //* It doesn't come here at all*
        fmt.Println("interface obj", str)
    }
}

func main() {
    overload("hello")
    overload(5)
    overload(someStruct{val: 5, some: "say"})
}

So the question is:

How to make sure that whoever implements object interface will fall in at case object type ?

Thanks in advance.

Answer 1

The "problem" is that someStruct.toByte() has a pointer receiver. This means the method toByte() belongs to the type *someStruct and not to someStruct. So someStruct does not implement object, only *someStruct. And you pass a value of someStruct to overload().

Pass a value of *someStruct, and you'll get what you want:

overload(&someStruct{val: 5, some: "say"})

Output (try it on the Go Playground):

string :  hello
int :  5
interface obj &{5 say}

Relevant section from the spec: Method sets:

A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type *T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T).

Behind the scenes

Note that behind the scenes when you call overload() like this:

overload(&someStruct{val: 5, some: "say"})

This will wrap the *someStruct pointer value in an interface{} value (because overload() has a parameter of interface{} type), and not in an interface value of type object.

Inside overload(), the type switch will check types in the listed order. And when it reaches case object, it will see that the value wrapped in the overLoadedObj parameter does implement object so this case will be executed.