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I am trying to create a generic method in Go that will fill a struct using data from a map[string]interface{}. For example, the method signature and usage might look like:
func FillStruct(data map[string]interface{}, result interface{}) {
type MyStruct struct {
Name string
myData := make(map[string]interface{})
myData["Name"] = "Tony"
myData["Age"]
result := &MyStruct{}
FillStruct(myData, result)
// result now has Name set to "Tony" and Age set to 23
I know this can be done using JSON is there another more efficient way of doing this?
This is the same idea as Simon's answer, but with a little more error handling:
func SetField(obj interface{}, name string, value interface{}) error {
structValue := reflect.ValueOf(obj).Elem()
structFieldValue := structValue.FieldByName(name)
if !structFieldValue.IsValid() {
return fmt.Errorf("No such field: %s in obj", name)
if !structFieldValue.CanSet() {
return fmt.Errorf("Cannot set %s field value", name)
structFieldType := structFieldValue.Type()
val := reflect.ValueOf(value)
if structFieldType != val.Type() {
return errors.New("Provided value type didn't match obj field type")
structFieldValue.Set(val)
return nil
type MyStruct struct {
Name string
func (s *MyStruct) FillStruct(m map[string]interface{}) error {
for k, v := range m {
err := SetField(s, k, v)
if err != nil {
return err
return nil
func main() {
myData := make(map[string]interface{})
myData["Name"] = "Tony"
myData["Age"] = int64(23)
result := &MyStruct{}
err := result.FillStruct(myData)
if err != nil {
fmt.Println(err)
fmt.Println(result)
27.5k12658
Hashicorp's
library does this out of the box:
import "github.com/mitchellh/mapstructure"
mapstructure.Decode(myData, &result)
The second result parameter has to be an address of the struct.
You can do it ... it may get a bit ugly and you'll be faced with some trial and error in terms of mapping types .. but heres the basic gist of it:
func FillStruct(data map[string]interface{}, result interface{}) {
t := reflect.ValueOf(result).Elem()
for k, v := range data {
val := t.FieldByName(k)
val.Set(reflect.ValueOf(v))
Working sample:
I adapt dave's answer, and add a recursive feature. I'm still working on a more user friendly version. For example, a number string in the map should be able to be converted to int in the struct.
package main
func SetField(obj interface{}, name string, value interface{}) error {
structValue := reflect.ValueOf(obj).Elem()
fieldVal := structValue.FieldByName(name)
if !fieldVal.IsValid() {
return fmt.Errorf("No such field: %s in obj", name)
if !fieldVal.CanSet() {
return fmt.Errorf("Cannot set %s field value", name)
val := reflect.ValueOf(value)
if fieldVal.Type() != val.Type() {
if m,ok := value.(map[string]interface{}); ok {
// if field value is struct
if fieldVal.Kind() == reflect.Struct {
return FillStruct(m, fieldVal.Addr().Interface())
// if field value is a pointer to struct
if fieldVal.Kind()==reflect.Ptr && fieldVal.Type().Elem().Kind() == reflect.Struct {
if fieldVal.IsNil() {
fieldVal.Set(reflect.New(fieldVal.Type().Elem()))
// fmt.Printf("recursive: %v %v\n", m,fieldVal.Interface())
return FillStruct(m, fieldVal.Interface())
return fmt.Errorf("Provided value type didn't match obj field type")
fieldVal.Set(val)
return nil
func FillStruct(m map[string]interface{}, s interface{}) error {
for k, v := range m {
err := SetField(s, k, v)
if err != nil {
return err
return nil
type OtherStruct struct {
Name string
type MyStruct struct {
Name string
OtherStruct *OtherStruct
func main() {
myData := make(map[string]interface{})
myData["Name"]
myData["Age"]
= int64(23)
OtherStruct := make(map[string]interface{})
myData["OtherStruct"] = OtherStruct
OtherStruct["Name"]
OtherStruct["Age"]
= int64(23)
result := &MyStruct{}
err := FillStruct(myData,result)
fmt.Println(err)
fmt.Printf("%v %v\n",result,result.OtherStruct)
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