Why can't I duplicate a slice with `copy()`?

Question

I need to make a copy of a slice in Go and reading the docs there is a copy function at my disposal.

The copy built-in function copies elements from a source slice into a destination slice. (As a special case, it also will copy bytes from a string to a slice of bytes.) The source and destination may overlap. Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).

But when I do:

arr := []int{1, 2, 3}
tmp := []int{}
copy(tmp, arr)
fmt.Println(tmp)
fmt.Println(arr)

My tmp is empty as it was before (I even tried to use arr, tmp):

[]
[1 2 3]

You can check it on go playground. So why can not I copy a slice?

Answer 1

The builtin copy(dst, src) copies min(len(dst), len(src)) elements.

So if your dst is empty (len(dst) == 0), nothing will be copied.

Try tmp := make([]int, len(arr)) (Go Playground):

arr := []int{1, 2, 3}
tmp := make([]int, len(arr))
copy(tmp, arr)
fmt.Println(tmp)
fmt.Println(arr)

Output (as expected):

[1 2 3]
[1 2 3]

Unfortunately this is not documented in the builtin package, but it is documented in the Go Language Specification: Appending to and copying slices:

The number of elements copied is the minimum of len(src) and len(dst).

Edit:

Finally the documentation of copy() has been updated and it now contains the fact that the minimum length of source and destination will be copied:

Copy returns the number of elements copied, which will be the minimum of len(src) and len(dst).