Why Does json.Unmarshal Return a Map Instead of the Expected Struct?

In this playground example, json.Unmarshal returns a map instead of the expected struct: http://play.golang.org/p/dWku6SPqj5.

The issue arises because an interface{} parameter is passed to json.Unmarshal, and the library attempts to unmarshal it into a byte array. However, the library doesn't have a direct reference to the corresponding struct, even though it has a reflect.Type reference.

The problem lies in the following code:

var ping interface{} = Ping{}
deserialize([]byte(`{"id":42}`), &ping)
fmt.Println("DONE:", ping) // It's a simple map now, not a Ping. Why?

To resolve this issue, either pass a pointer to the Ping struct explicitly as an abstract interface:

var ping interface{} = &Ping{}
deserialize([]byte(`{"id":42}`), ping)
fmt.Println("DONE:", ping)

Alternatively, if a direct pointer is unavailable, create a new pointer using reflect, deserialize into it, and then copy the value back:

var ping interface{} = Ping{}
nptr := reflect.New(reflect.TypeOf(ping))
deserialize([]byte(`{"id":42}`), nptr.Interface())
ping = nptr.Interface()
fmt.Println("DONE:", ping)