CODEWITHSUNDEEP
pointersgostruct
gempir
gempir

Reputation: 1911

Structs in go and working with pointers

I am trying my hand on my first go program which is supposed to be a very simple IRC bot.

I have the part of the connections etc done but I am confused with structs and pointers such. The structs are new to me coming from languages that use classes.

I have this struct and the constructor for it:

type Bot struct {
    server  string
    port    string
    nick    string
    channel string
    pass    string
    conn    net.Conn
}

// NewBot main config
func NewBot() *Bot {
    return &Bot{
        server:  "irc.twitch.tv",
        port:    "6667",
        nick:    "username",
        channel: "#channel",
        pass:    "password123",
        conn:    nil,
    }
}

My connect() method looks like this 

func (bot *Bot) Connect() (conn net.Conn, err error) {
    ircbot := NewBot()
    conn, err = net.Dial("tcp", bot.server+":"+bot.port)
    // irc connection...
    return bot.conn, nil
}

Everything of that works fine the problem I am having is with another method to my struct named Message. It's just supposed to send a message. Looks like this: 

// Message to send a message
func (bot *Bot) Message(message string) {
    if message == "" {
        return
    }
    fmt.Printf("Bot: " + message + "\n")
    fmt.Fprintf(bot.conn, "PRIVMSG "+bot.channel+" :"+message+"\r\n")
}

everytime when I then try to use this function I get this error and the program crashes

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x20 pc=0x463d73]

I'm unsure about how to use the & and * signs correctly to achieve what I want to do.

I thought a goroutine is something that is for multithreading and is done by saying "go [do something]" but I don't use that anywhere.

Edit: Solution

Thanks I found the solution! For those interested: I created a new instance of Bot in the place where I called the Message() function which resulted in an empty conn.

This was the important bit, I stupidly didn't post here. handle() wasn't even a method of Bot which is even more stupid of me.

func handle(line string) {
    ircbot := NewBot();
    // get username, message etc...
    ircbot .CmdInterpreter(username[1], usermessage)
}

and this is the correct way:

func (bot *Bot) handle(line string) {
    // get username, message etc...
    bot.CmdInterpreter(username[1], usermessage)
}

Upvotes: 0

Views: 78

Answers (1)

Herman Schaaf
Herman Schaaf

Reputation: 48475

The issue here seems to be with the Connect method, and not really with pointers in particular:

func (bot *Bot) Connect() (conn net.Conn, err error) {
    ircbot := NewBot()
    conn, err = net.Dial("tcp", bot.server+":"+bot.port)
    // irc connection...
    return bot.conn, nil
}

The method is defined to be on (a pointer to the) Bot struct, but it creates a new Bot, on this line:

ircbot := NewBot()

and then proceeds to use the one the method is defined on (called bot, not ircbot). If you choose to keep it so that there are separate NewBot and Connect functions (which is fine), then you should change it so that Connect actually uses an instantiated *Bot:

func (bot *Bot) Connect() (conn net.Conn, err error) {
    conn, err = net.Dial("tcp", bot.server+":"+bot.port)
    // irc connection...
    return bot.conn, nil
}

and call it with something like this:

bot := NewBot()
conn, err := bot.Connect()

The error,

panic: runtime error: invalid memory address or nil pointer dereference

is probably because you are accessing bot in the Connect method (e.g. in bot.server) but it has not been defined, and the pointer is nil.

Upvotes: 4

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