Golang flag.FlagSet.Var() function example

21st June 2015

package flag

Var defines a flag with the specified name(2nd parameter) and usage string(3rd parameter). The type and value of the flag are represented by the first argument, of type Value (1st parameter), which typically holds a user-defined implementation of Value. For instance, the caller could create a flag that turns a comma-separated string into a slice of strings by giving the slice the methods of Value; in particular, Set would decompose the comma-separated string into the slice.

Golang flag.FlagSet.Var() function usage example

 package main

 import (
 "flag"
 "fmt"
 )


 type flagStr struct {
 value *string
  }

  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 (
 flagSet *flag.FlagSet

 appFlags struct {
 appID flagStr
 appSecret flagStr
 }
  )



 func main() {

 flagSet = flag.NewFlagSet("example", flag.ContinueOnError)

 flagSet.Var(&appFlags.appID, "FBAppID", "Facebook Application ID") // <-- here
 appFlags.appID.Set("1234567890")

 err := flagSet.Parse([]string{"FBAppID"})

 if err != nil {
 fmt.Println(err)
 }

 fmt.Println(flagSet.Lookup("FBAppID"))

 fmt.Println(&appFlags.appID)
 }

Output :