How get pointer of struct's member from interface{}

Question

I want to pass struct's pointer to function that expect interface{}. Then get (through reflection) the pointer to the struct's member and then modify it using this pointer. I've read a much of Q&A and tried much of variations, but still I can get it work.

Let's consider example below:

type Robot struct {
    Id int
}
f := func(i interface {}) {
    v :=  reflect.ValueOf(i).Elem().FieldByName("Id")
    ptr := v.Addr().Pointer()
    *ptr = 100
    //^ it needs to me for functions expecting the pointer: Scan(&pointerToValue)
}

robot := &Robot{}
f(robot)
println(robot.Id) //I want to get here 100

I think the problem in poor understanding what actually do Addr() and Pointer() methods of reflect package..

Answer 1

Here's a working version of function f:

func f(i interface{}) {
  v := reflect.ValueOf(i).Elem().FieldByName("Id")
  ptr := v.Addr().Interface().(*int)
  *ptr = 100
}

playground example

The conversion to integer pointer goes as follows:

• v is a reflect.Value representing the int field.
• v.Addr() is a relfect.Value representing a pointer to the int field.
• v.Addr().Interface() is an interface{} containing the int pointer.
• v.Addr().Interface().(*int) type asserts the interface{} to a *int

You can set the field directly without getting a pointer:

func f(i interface{}) {
  v := reflect.ValueOf(i).Elem().FieldByName("Id")
  v.SetInt(100)
}

playground example

If you are passing the value along to something expecting interface{} (like the db/sql Scan methods), then you can remove the type assertion:

func f(i interface{}) {
  v := reflect.ValueOf(i).Elem().FieldByName("Id")
  scan(v.Addr().Interface())
}

playground example