Golang fill slice in function not working

Question

I am trying to add stuff to my slice but somehow the slice is never updated.

endpointsList := make([]string, 3)

for _, route := range routes {
    if len(route.Endpoints) > 0 {
        waitGroup.Add(1)
        go endpointRoutine(route, template, route.Protected, &waitGroup, &endpointsList)
    }
}

I pass the endpointsList by reference, meaning I should be able to assign new things to its memory location I think.

In the function endpointRoutine I do this:

list := make([]string, 3)

for _, r := range route.Endpoints {
    list = append(list, "some data comes here...")
}

endpointsList = &list

When I do a printr after this (below my first bit of code and AFTER the waitGroup.Wait() part) the slice is still empty.

Obviously, I am overwriting the slice now and my final goal is to ADD to the slice. But when I try to add with this code:

endpointsList = append(endpointsList, "fdssdfsdfsdf")

It gives me the error:

cannot use endpointsList (type *[]string) as []Type

Can someone please explain to me what might be wrong?

"I pass the endpointsList by reference" -- Not technically true. [Go doesn't allow pass by reference at all](https://stackoverflow.com/q/47296325/13860). — Jonathan Hall, Sep 28 '20 at 16:31
You're changing a copy of a pointer. That change does not make it back to your main code. — colm.anseo, Sep 28 '20 at 16:31
Also concurrently appending to a slice is not goroutine-safe. — colm.anseo, Sep 28 '20 at 16:34

score 1 · Accepted Answer · answered Sep 28 '20 at 16:30

1

With endpointsList = &list, you are assigning the pointer pointing to the slice to some other slice. To set the slice, do this instead:

*endpointsList=list

answered Sep 28 '20 at 16:30

Burak Serdar

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Good one, I didnt even notice that mistake, thanks so much :) this got me a bit further towards my goal! anyways, I am overwriting the slice now and I want to add (doh)... endpointsList = append(endpointsList, "fdssdfsdfsdf") this results in an error... any idea? – Sep 28 '20 at 16:37
Had to use (obviously) *endpointsList=list and now it is working succesfully! thanks a lot! – Sep 28 '20 at 16:59
@Ometecuthli while this solves the surface problem, concurrent updates of slices without synchronizing updates [is a bad idea](https://medium.com/@cep21/gos-append-is-not-always-thread-safe-a3034db7975). – colm.anseo Sep 28 '20 at 17:56

Golang fill slice in function not working

1 Answers1