Why cant i pass a struct pointer with *interface{} parameter?

Question

I can't find anything about *interface{} on google. So... The question is why these two approaches work differently?

package main

type MyStruct struct {}

func valI(x interface{}) {}
func pointI(x *interface{}) {}

func valS(s MyStruct) {}
func pointS(s *MyStruct) {}

func main() {
    s := MyStruct{}
    p := &s

    valI(s)
    valI(p) // Why? Success
    pointI(s) // Why?  Fail: cannot use s (type S) as type *interface {} in argument to point: *interface {} is pointer to interface, not interface
    pointI(p) // Why?  Fail: cannot use p (type *S) as type *interface {} in argument to point: *interface {} is pointer to interface, not interface

    valS(s)
    valS(p) // It's obvious to me why these two fail
    pointS(s) // -//-
    pointS(p)
}

Playground: https://play.golang.org/p/pio5vf-fBxH

I don't know if it's right to say that interfaces ARE pointers, but they certainly behave like it. — Corey Ogburn
– Corey Ogburn, Commented Jun 18, 2020 at 16:50
From golang.org/doc/faq#pointer_to_interface: "When should I use a pointer to an interface? Almost never." That is all to know. Pointer to interface is wrong in almost all cases. You really never need this (except when you are an expert and then it will be obvious why and how.) — Volker
– Volker, Commented Jun 18, 2020 at 19:46

Burak Serdar · Accepted Answer · 2020-06-18 16:50:54Z

3

An interface contains a pointer to the underlying data and type information. When you assign a non-interface value to an interface (or pass a non-interface value as an interface arg) the compiler generates code to pass the type and pointer to the underlying data. In a way, an interface is a struct:

type interface struct {
   Data pointer
   Type type
}

A pointer to an interface is simply a pointer to an instance of this struct.

All values satisfy the interface{} interface, so you can pass a struct or *struct where interface{} is required. *interface{} is a pointer to an interface, and only a pointer to an interface can be passed:

x:=interface{}(s)
pointI(&x)

answered Jun 18, 2020 at 16:50

Burak Serdar

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6 Comments

Stanislav Over a year ago

It's even more confusing now: fmt.Println( reflect.TypeOf(x), reflect.TypeOf(s)) // main.MyStruct main.MyStruct

Burak Serdar Over a year ago

TypeOf returns the concrete type of the passed value. TypeOf will not return an interface.

user5728991 Over a year ago

@Stanislav Use reflect.TypeOf(&x).Elem() to get the type of x.

Burak Serdar Over a year ago

@Stanislav note that TypeOf gets an interface{} argument, and it simply returns the type part of that arg. When you pass s, compiler constructs an interface containing the type of s. When you pass x, it is already an interface (containing s as its type), so it passes that directly. So for both cases, you get MyStruct.

Stanislav Over a year ago

Ok, I think I get the point. There is no difference between an empty interface and actual user-defined interfaces (like "MyInterface") in my example. I should look into interface pointers, not "interface{}" pointers. Ok, thanks

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