empty knoll
I have a Rust playground of a reduced example, showing what I want to do:
https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=58fbaffc5b8e0743cd315b3e24f20f81
Basically: how do I implement IntoIterator correctly for a Newtype(Cow<[T]>)?
I could just use std::vec::IntoIter for the associated type IntoIter, but then I'd have to (unnecessarily?) convert a potential &[T] into a Vec<T>, i.e. allocation.
lilac eagle
I could just use `std::vec::IntoIter` for the associated type `IntoIter`, but th...
if you have a Cow<[T]> and you consume it for the iterator, you cannot make one that returns &T
there is nowhere to borrow from
you can for example do this: ```rs
impl<'a> IntoIterator for &'a S<'a> {
type Item = &'a usize;
type IntoIter = std::slice::Iter<'a, usize>;
fn into_iter(self) -> Self::IntoIter {
self.0.iter()
}
}
this is also possible: ```rs
use either::Either;
use std::borrow::Cow;
use std::{iter, vec};
fn main() {
let s1 = S(Cow::Owned(vec![1, 2, 3]));
let s2 = S(Cow::Borrowed(&s1.0));
for i in s2 {
print!("{i} ");
}
println!();
for i in s1 {
print!("{i} ");
}
}
struct S<'a>(Cow<'a, [usize]>);
impl<'a> IntoIterator for S<'a> {
type Item = usize;
type IntoIter = Either<iter::Copied<std::slice::Iter<'a, usize>>, vec::IntoIter<usize>>;
fn into_iter(self) -> Self::IntoIter {
match self.0 {
Cow::Borrowed(s) => Either::Left(s.iter().copied()),
Cow::Owned(v) => Either::Right(v.into_iter()),
}
}
}
I think you misunderstood me. I'm not trying to return a &T. I'm just trying to implement the ordinary IntoIterator, but without uselessly allocating, since that's the whole purpose of Cow AFAIK.
lilac eagle
yes, I showed you in the second example
empty knoll
Ok, will try that out, 1 moment.
lilac eagle
the first one is also an "ordinary" IntoIterator
empty knoll
The 2nd example still seems to be copying the data though, right? Which to me seems just like calling Cow::Borrowed(slice) => slice.to_vec().into_iter(), and using type IntoIter = std::vec::IntoIter<Self::Item>;
In my real world project, my value isn't actually Copy. I thought this wouldn't matter so I left it out.
I would like to avoid cloning/copying if that's possible
empty knoll
Well, but Cow accepts a &[T] via Cow::Borrowed(slice)
lilac eagle
exactly
lilac eagle
The 2nd example still seems to be copying the data though, right? Which to me se...
it copies individual elements, but doesn't allocate a Vec
slice.to_vec() requires T: Clone too
empty knoll
Ok, whatever. It seems like my approach was fundamentally flawed for what I was trying to achieve...
Thanks for your help anyway! 🙂
acoustic badge
I have a Rust playground of a reduced example, showing what I want to do:
https:...
playground is dead, can you copy paste your code?
lilac eagle
wow it's that dead