Set pointer to struct passed in via interface{} to nil using reflect in Golang?

Question

I am trying to use reflect to set a pointer to a struct to nil passed in through an interface{} in Go.

I have this example program but it always prints false as a is not set to nil. What am I doing wrong?

package main

import "reflect"

type MyStruct struct {}

func main() {

    a := &MyStruct{}

    wipePassed(a)
    println(a == nil)

}

func wipePassed(r interface{}){
    v := reflect.ValueOf(&r)
    p := v.Elem()
    p.Set(reflect.Zero(p.Type()))
}

Answer 1

You can't with your current code. The pointer a is passed (as if) by value even though it's being wrapped in an interface, and there's no way to zero the original copy. This is just the same as the case when a is a non-pointer type (for example an int); there's no function f that can change the value of a in the code a := 42; f(a) -- you have to pass a pointer.

Your code can work if you pass in the address of the pointer a:

package main

import "reflect"

type MyStruct struct{}

func main() {
    a := &MyStruct{}
    wipePassed(&a)
    println(a == nil)
}

func wipePassed(r interface{}) {
    v := reflect.ValueOf(r)
    p := v.Elem()
    p.Set(reflect.Zero(p.Type()))
}