Why Does JSON Unmarshal Return a Map Instead of a Struct in Go When Working with Interfaces?

Mystery Behind JSON Unmarshal Returning a Map Instead of the Intended Struct

JSON unmarshaling in Go can pose challenges when dealing with interfaces and structs. Developers may encounter situations where the unmarshal process yields a map instead of the expected struct, as is the case in the given snippet:

<code class="go">import "encoding/json"
import "fmt"
import "reflect"

func main() {
    type Ping struct {
        ID int `json:"id"`
    }
    var ping interface{} = Ping{}
    if err := json.Unmarshal([]byte(`{"id":42}`), &ping); err != nil {
        panic(err)
    }
    fmt.Println("Unexpected:", ping) // Result: map[id:42]
}</code>

The underlying reason for this behavior lies in the abstract nature of interfaces. When JSON unmarshaling is performed on an interface, the result is a map representing the underlying type's fields. In the example above, the interface Ping holds a map with a single key-value pair: {"id":42}.

To rectify this issue and obtain the desired struct, it is crucial to pass a pointer to the specific struct type:

<code class="go">type Ping struct {
    ID int `json:"id"`
}
func main() {
    var ping Ping
    if err := json.Unmarshal([]byte(`{"id":42}`), &ping); err != nil {
        panic(err)
    }
    fmt.Println("Success:", ping) // Result: {42}
}</code>

By passing the pointer to Ping, the JSON unmarshal process is directed to create an instance of the struct and populate its fields rather than creating a map.

Alternatively, if a pointer to the concrete struct is unavailable, you can employ reflection to dynamically create the pointer and subsequently assign its value to the interface:

<code class="go">import "reflect"

func main() {
    var ping interface{} = Ping{}
    nptr := reflect.New(reflect.TypeOf(ping))
    if err := json.Unmarshal([]byte(`{"id":42}`), nptr.Interface()); err != nil {
        panic(err)
    }
    ping = nptr.Elem().Interface()
    fmt.Println("Reflect-Based:", ping) // Result: {42}
}</code>

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