Marshal map[int]int in Golang

Date: 24 May 2015
Author: Erik Dubbelboer

The Golang JSON module doesn’t support marshaling maps with keys that are anything else than a string. I needed to serialize a map with integers as keys, so I wrote one. I’m just posting this here in case it’s useful for anyone else:

type Intmap map[int]int

func (i Intmap) MarshalJSON() ([]byte, error) {
  x := make(map[string]int)
  for k, v := range i {
    x[strconv.FormatInt(int64(k), 10)] = v
  }
  return json.Marshal(x)
}

func (i *Intmap) UnmarshalJSON(b []byte) error {
  x := make(map[string]int)
  if err := json.Unmarshal(b, &x); err != nil {
    return err
  }
  *i = make(Intmap, len(x))
  for k, v := range x {
    if ki, err := strconv.ParseInt(k, 10, 32); err != nil {
      return err
    } else {
      (*i)[int(ki)] = v
    }
  }
  return nil
}

It still serializes the keys to strings seeing as only strings are supported by the JSON standard. But it correctly handles the unserializing from the string keys back to integers.

Erik's Code

Marshal map[int]int in Golang