JSON Unmarshal Requires a Pointer

Why This Happens

json.Unmarshal needs to modify the target variable, so it requires a pointer. If you pass a value, it cannot write the decoded data back. This applies to json.Unmarshal, json.NewDecoder().Decode(), and similar functions.

The Problem

type Config struct {
    Port int `json:"port"`
}

func main() {
    var config Config
    data := []byte(`{"port": 8080}`)
    json.Unmarshal(data, config) // needs pointer
}

The Fix

func main() {
    var config Config
    data := []byte(`{"port": 8080}`)
    err := json.Unmarshal(data, &config)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Println(config.Port)
}

Step-by-Step Fix

1. 1

   Identify the unmarshal call

   Find the json.Unmarshal or Decode call that is not receiving a pointer.

2. 2

   Add the address operator

   Add & before the target variable to pass a pointer.

3. 3

   Check the error

   Always check the error return value from unmarshal operations.