Copying field/value from src to dest object - go

Question

I am trying to copy the fields from one struct value to another, where they have the same field definitions. I have this program:
package main
import (
"log"
"reflect"
)
func setExistingFields(src interface{}, dst interface{}) {
fields := reflect.TypeOf(src)
values := reflect.ValueOf(src)
num := fields.NumField()
s := reflect.ValueOf(src).Elem()
d := reflect.ValueOf(dst).Elem()
for i := 0; i < num; i++ {
field := fields.Field(i)
value := values.Field(i)
fsrc := s.FieldByName(field.Name)
fdest := d.FieldByName(field.Name)
if fdest.IsValid() && fsrc.IsValid() {
if fdest.CanSet() && fsrc.CanSet() {
fdest.Set(value)
}
}
}
}
// and then we main:
func main() {
src := struct {
Foo string
Bar string
}{
"dog",
"pony",
}
dest := struct{ Foo string; Bar string }{}
setExistingFields(&src, &dest)
log.Println("dest.Foo", dest.Foo)
}
I run that, but I get an error:
reflect: NumField of non-struct type
I can't figure out what that's about.
Here's a playground link:
https://play.golang.org/p/TsHTfAaeKhc

Answer 1

Try this out:
func setExistingFields(src interface{}, dst interface{}) {
srcFields := reflect.TypeOf(src).Elem()
srcValues := reflect.ValueOf(src).Elem()
dstValues := reflect.ValueOf(dst).Elem()
for i := 0; i < srcFields.NumField(); i++ {
srcField := srcFields.Field(i)
srcValue := srcValues.Field(i)
dstValue := dstValues.FieldByName(srcField.Name)
if dstValue.IsValid() {
if dstValue.CanSet() {
dstValue.Set(srcValue)
}
}
}
}
Note that you need to do additional checking if src field value is assignable to dst field type.
Edit: The reason why you are getting that error is because fields at that point is a pointer to a struct. You need to get the actual struct value by using Elem().

Answer 2

This won't work: A struct always gets its "schema" (eg. its fields) during compile time... You cannot add more fields during runtime.
I don't see what your exact use case is, but consider something like map[string]string or even map[string]interface{} to be able to "extend" the content/fields of the thing you are passing around...