[hitsurume]

This is me right now. I get the basics of Go, but I don't understand when i'm suppose to have something be a pointer, and when i'm suppose to use * or & . Anyone who has a good explanation in a EL5 way, would help me a lot!!

[sacado2]

Imagine we're working together and I have some text document I want you to work on and update. If I send you the text by email, I send a copy of it (the original text is still on my computer). If you modify that copy and keep it for you, I won't ever know what you did. I just used a function

    func (c Coworker) SendForUpdates(d Document) {
        ...
    }

That wouldn't make sense. You worked hard and I don't even know what you did. So, what I would expect you to do is, once you made updates on the copy, to send me back that copy by email. That would be akin to

    func (c Coworker) SendForUpdates(d Document) Document {
        ...
        return d
    }

I sent you a copy, and you returned another updated copy. That is "pass-by-value", the default, no-pointer style.

Now, let's say I think those emails back and forth and boring. Rather than sending you a copy of the text each time, I could rather use Google Docs, and send you the link to that document. Its URL, rather than a copy of its content. Now, you can just go to that URL and do the updates on the document. You don't have to send me back the document: you're working on it, not on a copy of it! Well, that URL is a reference to the document rather than the document itself, or, if you prefer, a pointer to it. So, now, the function would be

    func (c Coworker) SendForUpdates(d *Document) {
        ...
    }

And we're done, no more back-and-forth dance now! That is "pass-by-reference".

You don't only use "pass-by-reference" just to be able to check updates on the document sent, by the way. If I want to send you some text just for your information and I don't expect any kind of update, I'll use pass-by-value (the very first function). But what if I want to send you a 3 GB video? I can't send that through e-mail! Sending a copy would be totally inefficient. Once again, I'll send you a pointer, an URL to download the video:

    func (c Coworker) InformText(d Document) // d is small: pass-by-value

    func (c Coworker) InformBigVideo(v *Video) // videos are huge: pass-by-reference

Why not use pointers everywhere by default, they seem easier, right? That's basically what java and python do. Well, they can be tricky too. I gave you the URL to the link and you could work on it. Once you're done, I don't want you to modify the document anymore. I want to send it to our boss. But, how could I know you didn't keep the URL somewhere in your bookmarks? How do I know, of all the coworkers I sent the URL to, one of them doesn't keep on updating that document even when I don't want to anymore? With copies, I'm safe, do whatever the hell you want with your copy, I don't care anymore. But a reference to the original document? That can be dangerous.

[SureshG]

>That's basically what java and python do. Well, they can be tricky too.

But,you can easily make the classes/collections immutable to avoid the issues you mentioned. I think in java records and many collections are immutable by default. Immutability is fundamental to functional style programming.

[olliepop]

This is a phenomenal explanation. Thank you - I've saved this for next time I need to explain to a junior.

[bpizzi]

You want a function that accepts a pointer when you want to modify an outside value within a function without returning the value. In that case, the func accepts a pointer (like in `func(p int)`) and you pass it either a pointer (a var declared as an {some_type}) or the address of a variable (with &variable).