On Mon, Jun 27, 2016 at 12:34 AM, Jan Mercl <0xj...@gmail.com> wrote: > On Mon, Jun 27, 2016 at 12:26 AM Peter Kleiweg <pklei...@xs4all.nl> wrote: > >> I don't know what I expected, but it's weird. Don't mess with slices. > > Well, working as expected, considering slice backing array is possibly > shared wrt the result of append. (Or other slice op, b/c slices _are_ > values.) > > However, it's easy to avoid the append sharing when/where not desirable: > https://play.golang.org/p/oIFHamYWB-
To unpack what Jan wrote a little: when slicing, you can specify a third integer which becomes the capacity of the new slice. So b := a[:len(a):len(a)] will create a new slice b with len(b) = len(a) and cap(b) = len(a). Without the third argument, cap(b) = cap(a). When cap(a) > len(a) you have the situation where append(a, 123) is quick: is can just insert the element into the underlying array without having to reallocate anything. So the line b := append(a[:len(a):len(a)], 3, 4) does two things: it slices a to obtain a new slice with a lower than normal capacity. Because the capacity exactly matches the length of the new slice, trying to append to it is guaranteed to reallocate the underlying array. So append makes a new, larger array and copies len(a) elements into it. It then writes 3 and 4 into the new array as well before returning a slice of the array. That slice becomes b. >From that point onwards, a and a are no longer connected via the underlying array. BTW, I was about to say that you could simplify the line one step further with b := append(a[::len(a)], 3, 4) but that gives a compilation error: prog.go:11: middle index required in 3-index slice I wonder what the rationale is for this? It seems inconsistent to me since the second (middle) index has a useful default (len(a)) that is used when there are only two indexes used. -- Martin Geisler -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
