How to add a struct to an array of structs in Go

Question

In golang I'm trying to make an array of messages, and the ability to easily add a new "object" to the array.

type Message struct {
    Name string
    Content string
}

var Messages = []Message{
    {
        Name: "Alice",
        Content: "Hello Universe",
    },{
        Name: "Bob",
        Content: "Hello World",
    },
}

func addMessage(m string) {
    var msg = new(Message)
    msg.Name = "Carol"
    msg.Content = m
    Messages = append(Messages, msg)
}

When building I get an error that says:

cannot use msg (type *Message) as type Message in append

Why is append() not working (as I might expect from JavaScript's array.concat()), or is new() not working?

Any other tips on how to improve this code are welcome since I'm obviously new to Go.

See if adding ... after msg helps (stackoverflow.com/questions/8461462/…) — VortixDev
– VortixDev, Commented Jul 19, 2016 at 5:01

sberry · Accepted Answer · 2016-07-19 06:06:55Z

8

Change these 3 lines

var msg = new(Message)
msg.Name = "Carol"
msg.Content = m

to

msg := Message{
    Name:    "Carol",
    Content: m,
}

and everything should work. new creates a pointer to Message. Your slice is not a slice of Message pointers, but a slice of Messages.

answered Jul 19, 2016 at 6:06

sberry

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Comments

slm · Accepted Answer · 2021-06-17 03:27:05Z

In your code, Messages is a slice of Message type, and you are trying to append a pointer of Message type (*Message) to it.

You can fix your program by doing the following:

func addMessage(m string) {
    var msg = new(Message) // return a pointer to msg (type *msg)
    msg.Name = "Carol"
    msg.Content = m
    Messages = append(Messages, *msg) // use the value pointed by msg
}

Alternatively, you can declare Messages as a slice of *Message:

var Messages = []*Message{
    &Message{ // Now each entry must be an address to a Message struct
        Name: "Alice",
        Content: "Hello Universe",
    },
    &Message{
        Name: "Bob",
        Content: "Hello World",
    },
}

In above case, addMessage wouldn't require any changes. But you'll have to modify Messages access everywhere else.

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