Golang : does not implement flag.Value (missing Set method)

Tags : golang method flag

Keep getting this error message : does not implement flag.Value (missing Set method) while trying to write example for flag.Var() function today. Apparently, to assign value with flag.Var() function to a variable in a struct. You need to implement Set() and Get() method. It is helpful to have String() method as well.

To fix this error, all you need to do is the add the Set method.

For example :

 package main

 import (

 type flagStr struct {
 value *string

 // compiler will throw out missing Set method
 // if this method below is ... well.. missing
 func (f *flagStr) Set(value string) error {
 f.value = &value
 return nil

 func (f *flagStr) Get() *string {
 return f.value

 func (f *flagStr) String() string {
 if f.value != nil {
 return *f.value
 return ""

 var (
  appFlags struct {
 appID flagStr
 appSecret flagStr

 func main() {

 flag.Var(&appFlags.appID, "FBAppID", "Facebook Application ID")

 flag.Var(&appFlags.appSecret, "FBAppSecret", "Facebook Application Secret")


 fmt.Println(flag.Lookup("FBAppID")) // print the Flag struct



Tags : golang method flag

By Adam Ng

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