No, it should not operate that way.
Remember that all function calls in Go pass arguments by value. A slice is
actually a value somewhat analogous to
type SliceOfT struct {
Data *T
Len int
Cap int
}
When you pass a slice to a function, it’s this struct that gets passed in.
Notice that the length and capacity are values, but the data pointer is a
pointer. This is why you can change the contents of a slice from within a
function, but not its length or capacity.
func modify(x []int) {
x[0] = 1
x = append(x, 4)
}
func main() {
x := []int{2, 3}
modify(x)
fmt.Println(x) // prints [1 3]
}
copy() is a function just like any other Go function; just because it’s built
in does not mean it follows any special semantics.
The current behavior of copy() allows you to do interesting things with copy();
for example:
type buffer struct {
data []byte
pos int
}
// …
func (b *buffer) append(val …byte) {
for len(val) > 0 {
n := copy(b.data[b.pos:], val)
b.pos += n
if b.pos == len(b.data) {
b.flush()
b.pos = 0
}
val = val[n:]
}
}
No matter what the value of b.pos is, the copy() will still copy in at least 0
elements into b.data without altering b.data itself. If copy() altered b.data
somehow, this loop would not work at all and would need to be rewritten somehow.
As another example:
a := make([]byte, 50)
b := make([]byte, 32)
copy(b[:16], a[4:])
copy(b[16:], a[25:])
to fill in b with different fragments of a.
> On Oct 2, 2016, at 1:11 AM, 高橋誠二 <timaki...@gmail.com> wrote:
>
> As official doc explains, copy to zero length slice results to be empty.
>
> package main
>
> import "fmt"
>
> func main() {
> src := []int{1, 2, 3}
> dst := []int{}
> fmt.Println(len(dst))
> copy(dst, src)
> fmt.Println(src)
> fmt.Println(dst)
> // [1 2 3]
> // []
>
> src2 := []int{1, 2, 3}
> dst2 := make([]int, len(src))
> copy(dst2, src2)
> fmt.Println(src2)
> fmt.Println(dst2)
> // [1 2 3]
> // [1 2 3]
> }
>
>
> But it seems unusual, is it should be like following?
> package main
>
> import "fmt"
>
> func main() {
> src := []int{1, 2, 3}
> dst := []int{}
> fmt.Println(len(dst))
> copy(dst, src)
> fmt.Println(src)
> fmt.Println(dst)
> // [1 2 3]
> // [1 2 3]
> }
>
>
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