golang fails to parse json for reflection created

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?