Create instance of struct via reflection and set values - json

Question

what I try to do
I try to pass an instance of a struct - including json tags to a func, create a new instance, and set value on field
after this i try to serialize (JSON), but the values are empty
NOTICE: i looked up loads of articles on SO about setting values via reflection, but it seems i missed a little detail
struct definition
this part defines the struct with json and xml tags
type Person struct {
Name string `json:"Name" xml:"Person>FullName"`
Age int `json:"Age" xml:"Person>Age"`
}
create instance (+wrapping into empty interface)
afterwards I create an instance and store it in an interface{} - why? because in my production code this stuff will be done in a func which accepts a interface{}
var iFace interface{} = Person{
Name: "Test",
Age: 666,
}
creating a new instance of the struct and setting values via reflection
iFaceType := reflect.TypeOf(iFace)
item := reflect.New(iFaceType)
s := item.Elem()
if s.Kind() == reflect.Struct {
fName := s.FieldByName("Name")
if fName.IsValid() {
// A Value can be changed only if it is
// addressable and was not obtained by
// the use of unexported struct fields.
if fName.CanSet() {
// change value of N
switch fName.Kind() {
case reflect.String:
fName.SetString("reflectedNameValue")
fmt.Println("Name was set to reflectedNameValue")
}
}
}
fAge := s.FieldByName("Age")
if fAge.IsValid() {
// A Value can be changed only if it is
// addressable and was not obtained by
// the use of unexported struct fields.
if fAge.CanSet() {
// change value of N
switch fAge.Kind() {
case reflect.Int:
x := int64(42)
if !fAge.OverflowInt(x) {
fAge.SetInt(x)
fmt.Println("Age was set to", x)
}
}
}
}
}
Question
what am I doing wrong?
in production code I fill multiple copies with data and add it to a slice...
but this only makes sense if the json tags are kept in place and the stuff serializes just the same way.
code sample for play
package main
import (
"encoding/json"
"fmt"
"reflect"
)
func main() {
type Person struct {
Name string `json:"Name" xml:"Person>FullName"`
Age int `json:"Age" xml:"Person>Age"`
}
var iFace interface{} = Person{
Name: "Test",
Age: 666,
}
fmt.Println("normal: \n" + JSONify(iFace))
iFaceType := reflect.TypeOf(iFace)
item := reflect.New(iFaceType)
s := item.Elem()
if s.Kind() == reflect.Struct {
fName := s.FieldByName("Name")
if fName.IsValid() {
// A Value can be changed only if it is
// addressable and was not obtained by
// the use of unexported struct fields.
if fName.CanSet() {
// change value of N
switch fName.Kind() {
case reflect.String:
fName.SetString("reflectedNameValue")
fmt.Println("Name was set to reflectedNameValue")
}
}
}
fAge := s.FieldByName("Age")
if fAge.IsValid() {
// A Value can be changed only if it is
// addressable and was not obtained by
// the use of unexported struct fields.
if fAge.CanSet() {
// change value of N
switch fAge.Kind() {
case reflect.Int:
x := int64(42)
if !fAge.OverflowInt(x) {
fAge.SetInt(x)
fmt.Println("Age was set to", x)
}
}
}
}
}
fmt.Println("reflected: \n" + JSONify(item))
}
func JSONify(v interface{}) string {
var bytes []byte
bytes, _ = json.MarshalIndent(v, "", "\t")
return string(bytes)
}

Answer 1

Your item is of type reflect.Value. You have to call Value.Interface() to obtain the value wrapped in it:
fmt.Println("reflected: \n" + JSONify(item.Interface()))
With this change, output will be (try it on the Go Playground):
normal:
{
"Name": "Test",
"Age": 666
}
Name was set to reflectedNameValue
Age was set to 42
reflected:
{
"Name": "reflectedNameValue",
"Age": 42
}
reflect.Value itself is also a struct, but obviously trying to marshal it will not be identical to marshaling a Person struct value. reflect.Value does not implement marshaling the wrapped data to JSON.