golang fails to parse json for reflection created object - json

Question

I try to write simple message protocol in go and i've encountered a problem. I have a lot of message types and i want to have a dictionary like this to manipulate with messages:
var dict map[reflect.Type]int = map[reflect.Type]int{
reflect.TypeOf(DataMessage{}): 1000,
reflect.TypeOf(TextMessage{}): 1001,
//....
}
func GetMessageTypeId(value interface{}) int {
if id, ok := dict[reflect.TypeOf(value)]; ok {
return id
} else {
return -1
}
}
func GetValueByTypeId(typeId int) interface{} {
for typeDec, id := range dict {
if id == typeId {
return reflect.Zero(typeDec).Interface()
}
}
fmt.Println("Unknown message type", typeId)
return nil
}
It works fine, but when i instantiate message with GetValueByTypeId and try to unmarshall json into it - i receive map[string]interface instead of my message.
I've made simple example to reproduce the problem:
http://play.golang.org/p/QEyDN9vztr

Answer 1

Please read this article: http://research.swtch.com/interfaces, especially the "Memory Optimizations".
The interface{} by definition consists of two pointers - to method table (e.g. type) and to data it holds. So for
var destination3 interface{} = reflect.Zero(reflect.TypeOf(Message{})).Interface()
it is empty method table (as interface{} has no methods) and reference to Message{}. Taking reference from it returns the reference to this struct so the unmarhal overwrites it with whatever matches interface{}.
If the data interface{} variable holds is a pointer itself, then it is optimized in a way that this pointer is used instead creating interface{} structure. So getting reference to it gives the reference to original variable.
http://play.golang.org/p/KsIS29rUAX
package main
import "fmt"
func main() {
var m1 struct{ Data string }
var m2 interface{}
var m3 interface{}
m2 = &m1
m3 = m1
fmt.Printf("&m1=%p m2=%p &m3=%p\n", &m1, m2, &m3)
}
In your case, using Zero is equivalent to m3 in the example above. Using New is equivalent to m2.

Answer 2

I've found the way how to do what i need
val := reflect.New(reflect.TypeOf(Message{}))
json.Unmarshal(data, val.Interface())
return val.Elem().Interface()
http://play.golang.org/p/8g9FSg3MSj
But was was wrong wit the first version???
It Looks like reflect.Zero(type) should be equivalent to reflect.New(type).Elem() - am i wrong?