logger

fs := flag.NewFlagSet("", flag.ExitOnError)

fact := &Factory{
	CustomFlagHelper: func(d []string) string {
		return "choose a flag in : " + strings.Join(d, ", ")
	},
}

fact.Register(Stdout, NoLog)
fact.SetDefault(NoLog)

fs.SetOutput(os.Stdout)
fs.Var(fact, "output", fact.FlagHelper())

fs.PrintDefaults()
fs.Parse([]string{"-output=stdout"})

log := fact.LoggerFound()
log.Printf("hello %s\n", log)

Output:

-output value
    	choose a flag in : stdout, null (default null)
hello stdout

var t struct {
	Log *Factory `json:"output"`
}

data := []byte(`{"output":"stdout"}`)

if err := json.Unmarshal(data, &t); err != nil {
	log.Fatal(err)
}

if t.Log == nil {
	log.Fatal("no output parsed")
}

output := t.Log.LoggerFound()
if output == nil {
	log.Fatal("no output found")
}

output.Printf("hello %s\n", output)

Output:

hello stdout