The motivation for the use of length in copy function instead of the capacity
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Сергей Пилипенко
May 28, 2025, 7:41:58 PM (10 days ago) May 28
to golan...@googlegroups.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!
Alexander Ertli
May 28, 2025, 8:21:35 PM (10 days ago) May 28
to Сергей Пилипенко, golan...@googlegroups.com
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
lenbecomes the contract for how much is valid at any moment, andcapjust 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.
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burak serdar
May 28, 2025, 8:29:19 PM (10 days ago) May 28
to Сергей Пилипенко, golan...@googlegroups.com
because it is a constant time operation to reslice within the slice
capacity. So:
slice:=make([]byte,0,10)
copy(slice[:5],source)
is valid and efficient. You don't need to pad the slice, you can just
slice the existing slice to any length within the capacity.
If slice capacity was the choice instead, you would have to allocate
the correct length slice whenever there is size mismatch,
>
> Thank you for the time!
>
Def Ceb
May 28, 2025, 8:35:27 PM (10 days ago) May 28
to golang-nuts
You can, in fact, reduce the capacity.
The "full form" for slices is not a frequently used feature though.
This can also be used to reslic a slice to access the data between the length and capacity.
Jason E. Aten
May 29, 2025, 12:16:54 AM (10 days ago) May 29
to golang-nuts
Hi Сергей
It sounds like you are coming from a C/low level optimization point of view. Which
is great. But there are things about Go that might you surprise you then. For example,
the backing array for the slice is always fully zero-value initialized anyway. So changing
its length within its capacity doesn't do any _extra_ zero-ing, like you suggest. It was already done.
This question might also be learning-path dependent. When I learned Go, and still to this
day, I rarely if ever think about capacity, just the length.
I just allocate the length I want/need, and go from there. From that point of view,
copy makes perfect sense. Really cap is an optimization for when you're
mostly done and want to squeeze out the last 1% by staring at the profiler for a day.
I suggest that you can, and perhaps should, just ignore cap
when you are learning how Go works or is typically used.
The only time I even think about capacity is when I'm passing a temporary workspace slice to
a serialization or encryption function. This is a common pattern in the crypto libraries.
You have a big re-usable slice, but you set its length to zero when passing it
to the encryptor/serializer subroutine, leaving its capacity large. In turn, the subroutine can
re-use that memory, communicating back how much it used by returning a slice pointing
to the same backing array. Since encryption is typically 1-1 your program can be
made to do additional allocation after the setup.
Conversely, if the provided buffer was not big enough, the subroutine must allocate,
and it will return the bigger workspace, which can still be re-used
next time. BUT, the subroutine doesn't even really have to do anything to make this happen. Nice! What?
How can that be?
How this works: the subroutine just takes a slice as an input argument, appends blindly, and returns the slice in a return value.
The slice will automagically get reallocated --but only if there wasn't enough space to handle all the appending.
Hope this helps. You don't really have to think about cap until later in your journey, and it will
all work just fine.
Best wishes.
Jason
Message has been deleted
Keith Randall
May 29, 2025, 2:12:39 AM (10 days ago) May 29
to golang-nuts
Think of `copy` as a safe `memmove`. There's nothing more to it. It does not modify the length or capacity of anything. (And it can't. `copy(a,b)` cannot modify either the length or capacity of `a` or `b`. Slices, and particularly the slice's length and capacity, are passed by value.)
```
func f(a []int) {
a = a[1:2:3] // set a's length to 1, capacity to 2
}
a := make([]int, 5, 10)
f(a)
// a's length is 5 and capacity is 10 here.
// f only modified its local variable. It did not modify its caller's local variable.
```
This is why the results of `append` need to be assigned somewhere. The complete statement `append(a, 9)` writes 9 to a[len(a)], but doesn't (and can't) modify a. You have to use the statement `a = append(a, 9)` to get a's length updated.
Keith Randall
May 29, 2025, 2:18:29 AM (10 days ago) May 29
to golang-nuts
> 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.
This is exactly what `append` is for. If you're overwriting existing entries (those between 0 and length), use `copy`. If you want to write new entries (between length and capacity), use `append`.
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