在函数中更改全局切片

Question

I've never worked with C/C++, so I'm a bit stuck with pointers in go. The problem is: I have a map[string][]InteractiveItems for each "room" and I wanna change slice of it in a function. Here it is:

func (r *room) getItem(arg string) InteractiveItem {
for i, val := range r.interactiveItems {
    for _, item := range val {
        if item.getName() == arg {
            var idxToDelete int
            for idx := range val {
                if val[idx].getName() == arg {
                    idxToDelete = idx
                    break
                }
                if len(val) == 0 {
                    delete(r.interactiveItems, i)
                }
            }
            val = append(val[:idxToDelete], val[idxToDelete+1:]...)
            return item
        }
    }
}
return nil

Obviously, val is changing inside the function, but room's map is not. How should I deal with pointers to delete the element of a slice?

Answer 1

This is because everything in Go is passed by value.

What this means for your case is that when you are iterating over the map, on each iteration the val variable is assigned a copy of the corresponding slice value. So re-slicing the val value inside the loop has no effect on the original slice inside the map, since one is a copy of, not a reference to, the other.

To change the slice that's inside the map you can re-assign the result of the append operation to the corresponding map key.

r.interactiveItems[i] = append(val[:idxToDelete], val[idxToDelete+1:]...)

Keep in mind that when a slice is copied it does not copy the data that the slice points to, it copies only the slice's "descriptor".