Hi, I think the core reasoning is that once a slice is created, its capacity is fixed – you can’t really change it without allocating a new backing array and copying elements over. You *can* reduce the length easily (s = s[:n]), but not the capacity.
```a := make([]int, 0, 10) b := make([]int, 10) copy(a, b) // nothing happens – a's len is 0 a = a[:10] // extend length to allow copy copy(a, b)``` Sometimes in low-level or performance-critical code, a slice is reused as a shared buffer across function calls to avoid allocating every time. You just reset the length as needed: ``` buf := make([]byte, 0, 4096) // reuse buffer buf = buf[:0] // zero-length, same capacity ``` So len becomes the contract for how much is valid at any moment, and cap just tells how many elements you in theory could add, if len is adjusted. Apart from the shared buffer case, there are probably other use cases for this, but that's the one that comes to mind. Maybe someone else has another reason why it’s designed this way? I hope this still helps. Am Mi., 28. Mai 2025 um 18:41 Uhr schrieb Сергей Пилипенко < territhi...@gmail.com>: > Hello, gophers! > > I have attempted to use the copy function for slices and found it’s API a > bit strange. Suppose I have two slices: one of size 0 and capacity 10, the > other of size 10 and capacity 10. When I copy the second one into the first > one nothing happens, because the length of the first size is 0, although it > could hold the elements just fine. > > So, I am wondering why isn’t copying limited by the capacity of the > destination slice instead? It doesn’t look natural for me to have to pad > the slice with the default values of the type, as they will be overwritten > by the copy anyway. > > Thank you for the time! > > -- > 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. > To view this discussion visit > https://groups.google.com/d/msgid/golang-nuts/0447e9ef-2daa-4fe5-b1b6-b05a5caeafeb%40Spark > <https://groups.google.com/d/msgid/golang-nuts/0447e9ef-2daa-4fe5-b1b6-b05a5caeafeb%40Spark?utm_medium=email&utm_source=footer> > . > -- 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. To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/CAC4E5ZkwRXK-kspT2S53vxrD%2Bhy6uJKKs7h_wV3Pu809KXnjSw%40mail.gmail.com.
