Where is my copy?

Why does the copy disappear?

var src, dst []int
src = []int{1, 2, 3}
copy(dst, src) // Copy elements to dst from src.
fmt.Println("dst:", dst)

dst: []

Answer

The number of elements copied by the copy function is the minimum of len(dst) and len(src). To make a full copy, you must allocate a big enough destination slice.

var src, dst []int
src = []int{1, 2, 3}
dst = make([]int, len(src))
n := copy(dst, src)
fmt.Println("dst:", dst, "(copied", n, "numbers)")

dst: [1 2 3] (copied 3 numbers)

The return value of the copy function is the number of elements copied. See Copy function for more about the built-in copy function in Go.

Using append

You could also use the append function to make a copy by appending to a nil slice.

var src, dst []int
src = []int{1, 2, 3}
dst = append(dst, src...)
fmt.Println("dst:", dst)

dst: [1 2 3]

Note that the capacity of the slice allocated by append may be a bit larger than len(src).

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