Gob decode cannot decode interface after register type

Question

I have these types defined:

func init() {
    gob.RegisterName("MyMessageHeader", MyMessageHeader{})
    gob.RegisterName("OtherMsg", OtherMsg{})
}

//
// Messages
//

type MyMessageHeader struct {
    MessageId InstanceIdType
    OtherId   uint64
}

type MyMessage interface {
    Id() *MyMessageHeader
}

type otherStruct struct {
    X uint8
    Y uint8
}
type OtherMsg struct {
    MyMessageHeader
    OtherField *otherStruct
}


func (m *OtherMsg) Id() *MyMessageHeader {
    return &m.MyMessageHeader
}

func MarshalMessage(m MyMessage) ([]byte, error) {
    var buff bytes.Buffer
    encoder := gob.NewEncoder(&buff)
    if err := encoder.Encode(&m); err != nil {
        return nil, errors.Newf("Could not marshal message: %v", err)
    }
    return buff.Bytes(), nil
}

func UnmarshalMessage(buf []byte) (MyMessage, error) {
    var msg MyMessage
    decoder := gob.NewDecoder(bytes.NewReader(buf))
    if err := decoder.Decode(&msg); err != nil {
        return nil, errors.Newf("Could not decode my message: %v", err)
    }
    return msg, nil
}

When I try to use UnmarshalMessage I got the error gob: OtherMsg is not assignable to type MyMessage. I have no difference with the usage in the go examples for encoding and decoding an interface. What am I doing wrong?

Answer 1

The error message says:

gob: OtherMsg is not assignable to type MyMessage

Which is true. MyMessage is an interface which requires implemener types to have a method:

Id() *MyMessageHeader

A value of type OtherMsg does not have such method in its method set, only a pointer to it, that is a value of type *OtherMsg (because OtherMsg.Id() has pointer receiver).

To make it work, register a value of type *OtherMsg:

gob.RegisterName("OtherMsg", &OtherMsg{})

Or simply:

gob.Register(&OtherMsg{})

See a working example on the Go Playground:

func init() {
    gob.Register(&OtherMsg{})
}

func main() {
    var m MyMessage = &OtherMsg{
        MyMessageHeader: MyMessageHeader{
            MessageId: 1,
            OtherId:   2,
        },
        OtherField: "othervalue",
    }

    data, err := MarshalMessage(m)
    if err != nil {
        panic(err)
    }

    m2, err := UnmarshalMessage(data)
    if err != nil {
        panic(err)
    }
    fmt.Printf("%+v\n", m2)
}

Output (try it on the Go Playground):

&{MyMessageHeader:{MessageId:1 OtherId:2} OtherField:othervalue}