Generically modify struct fields using reflection in golang - go

Question

Below is a method which uses reflect package to modify fields of a struct ,this works for a specific struct type
func modify(obj Car) interface{} {
ty := reflect.TypeOf(obj)
for i := 0; i < ty.NumField(); i++ {
rval := reflect.Indirect(reflect.ValueOf(&obj))
field := rval.Field(i)
fieldType := field.Kind()
switch fieldType {
case reflect.String:
field.SetString("")
case reflect.Int:
field.SetInt(0)
case reflect.Ptr:
field.Set(reflect.ValueOf(nil))
}
}
return obj
}
modifying the signature to
func modify(obj interface{}) interface{} {
results in
panic: reflect: call of reflect.Value.Field on interface Value
at line
field := rval.Field(i)
https://go.dev/play/p/pGfKtIg5RUp
It works with the signature
func modify(obj Car) interface{} {
https://go.dev/play/p/31Oh6WLmlGP
Why is the compile time type modifying the behaviour ?
The goal here is to mask certain fields based on struct tags .It could wrap an endpoint and the input and output to the method being wrapped could be struct or pointer so in above case both calls should work
modify(car)
modify(&car)

Answer 1

This is how it works for both value and pointer types
func modify(obj interface{}) interface{} {
rv := reflect.ValueOf(obj)
trv := reflect.TypeOf(obj)
value := reflect.New(rv.Type())
if rv.Kind() == reflect.Pointer {
rv = reflect.ValueOf(obj).Elem()
trv = reflect.TypeOf(obj).Elem()
value = reflect.New(rv.Type())
}
for i := 0; i < rv.NumField(); i++ {
field := rv.Field(i)
fieldType := field.Kind()
v := value.Elem().Field(i)
tag, _ := trv.Field(i).Tag.Lookup("es")
if len(tag) != 0 {
switch fieldType {
case reflect.String:
v.SetString(tag)
case reflect.Int:
v.SetInt(0)
case reflect.Ptr:
v.Set(reflect.ValueOf(nil))
}
} else {
v.Set(field)
}
}
return value
}
https://go.dev/play/p/C1pqw_UbPcG