pointer to interface is not an interface error?

Question

The following code works:

type Brace interface {}

type Round struct {
    prev_ Brace
}

type Square struct {}

func main() {
    var r Round
    var s Square
    r.prev_ = s
}

Is it true that r.prev_ is now a copy of s? How can I change it that Round will contain a pointer to Brace? This code doesn't work:

type Brace interface {}

type Round struct {
    prev_ *Brace
}

type Square struct {}

func main() {
    var r Round
    var s Square
    r.prev_ = &s
}

because of the error:

cannot use &s (type *Square) as type *Brace in assignment: *Brace is pointer to interface, not interface

Forget about trying traditional, inheritance based OOP in Go: It does not work, no matter how hard you try. Really. Redesign. Doing (or even mimicking) traditional OOP in Go will just lead to bugs, problems and pain. You gain nothing by doing it this way. — Volker
– Volker, Commented Jul 31, 2018 at 10:36
As @Volker says Golang not works like object oriented technology. There is a difference on how interface works in go. — Himanshu
– Himanshu, Commented Jul 31, 2018 at 10:40

Himanshu · Accepted Answer · 2018-08-21 08:27:31Z

13

As the error says:

cannot use &s (type *Square) as type *Brace in assignment: *Brace is pointer to interface, not interface

An interface can wrap any type of value. But you are wraping the struct inside pointer type interface not an interface. That's not how interface works in golang.

If you wants to pass the address of an struct. An interface can wrap the pointer to struct directly. There is no need of creating a pointer to an interface to achieve that.

type Brace interface {}

type Round struct {
    prev_ Brace
}

type Square struct {}

func main() {
    var r Round
    var s Square
    r.prev_ = &s
    fmt.Printf("%#v", r)
}

Playground Example

In Golang you should avoid passing a pointer to interface as:

The compiler will complain about this error but the situation can still be confusing, because sometimes a pointer is necessary to satisfy an interface. The insight is that although a pointer to a concrete type can satisfy an interface, with one exception a pointer to an interface can never satisfy an interface.

Consider the variable declaration,

var w io.Writer

The printing function fmt.Fprintf takes as its first argument a value that satisfies io.Writer—something that implements the canonical Write method. Thus we can write

fmt.Fprintf(w, "hello, world\n")

If however we pass the address of w, the program will not compile.

fmt.Fprintf(&w, "hello, world\n") // Compile-time error.

The one exception is that any value, even a pointer to an interface, can be assigned to a variable of empty interface type (interface{}). Even so, it's almost certainly a mistake if the value is a pointer to an interface; the result can be confusing.

Check it on Go playground. You will find the same error you are getting in your code snippet when trying to pass pointer to interface.

edited Aug 21, 2018 at 8:27

answered Jul 31, 2018 at 10:30

Himanshu

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3 Comments

InFamous X Over a year ago

Thank you! But this way Type(r.prev_) == Brace, not Square. In my first block of code am I right that r.prev_ is now a copy of s?

Himanshu Over a year ago

@InFamousX yes In go lang every thing is passed by value. So you are passing the copy of struct not struct itself.

Himanshu Over a year ago

@InFamousX To compare the underlying values of interface you should use type assertion

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