Why is my setter not working on an anonymous struct field?

Question

I'm just learning Go, and here's some behaviour I can't quite get my head around:

package main

import "fmt"

type Message interface {
    SetSender(sender string)
}

type message struct {
    sender string
}

type Join struct {
    message
    Channel string
}

func (m message) SetSender(sender string) {
    m.sender = sender
}

func main() {
    var msg Message
    msg = Join{}
    msg.SetSender("Jim")
    fmt.Printf("%s", msg)
}

This prints {{} }, so the SetSender call doesn't seem to take effect. I suspect that it is somehow operating on a copy of the message, but I can't figure out where such a copy would come from. I tried changing SetSender to operate on *message but that won't compile.

Playground: http://play.golang.org/p/yNdnM1bfSG

Suggestions for alternative design patterns are also welcome. I have several types of messages (only Join is shown here), but they all have some fields in common (only sender is shown here). I'd like to avoid having to write and export a separate constructor for each message type, so it would be great if I can just keep exporting the types themselves.

Answer 1

You should use pointer to message, not a message.

func (m *message) SetSender(sender string) {
    m.sender = sender
}

func main() {
    var msg Message
    msg = new(Join)
    msg.SetSender("Jim")
    fmt.Printf("%s", msg)
}

It will change the structure. Function on message changes copy of the object, function on pointer to message changes the object.

http://golangtutorials.blogspot.com/2011/06/methods-on-structs.html