How I can iterate over borrowed array?

Question

I thought that it should be something like this, but I cannot iterate over an borrowed array.

fn print_me<'a, I>(iter: &'a I) where I: Iterator<Item = i32> {
    for i in *iter {
        println!("{}", i);
    }
}

fn main() {
    let numbers = vec![1, 2, 3];
    //let numbers = &numbers;
    print_me(numbers.iter());
}

But Rust complains:

<anon>:15:12: 15:26 error: mismatched types:
 expected `&_`,
    found `core::slice::Iter<'_, _>`
(expected &-ptr,
    found struct `core::slice::Iter`) [E0308]
<anon>:15   print_me(numbers.iter());
                     ^~~~~~~~~~~~~~

Chris Morgan · Accepted Answer · 2015-05-28 23:12:40Z

4

An iterator is a regular object; you work with an iterator directly, not typically through references—and certainly not typically through immutable references, for taking the next value of an iterator takes &mut self. And a reference to an iterator is quite different from an iterator over references.

Fixing this part, you then have this:

fn print_me<I: Iterator<Item = i32>>(iter: I) {
    for i in iter {
        println!("{}", i);
    }
}

This doesn’t fix everything, however, because [T].iter() produces a type implementing Iterator<Item = &T>—it iterates over references to each item. The most common fix for that is cloning each value with the handy .cloned() method, which is equivalent to .map(|x| x.clone()).

Here, then, is the rest of what you end up with:

fn main() {
    let numbers = vec![1, 2, 3];
    print_me(numbers.iter().cloned());
}

answered May 28, 2015 at 23:12

Chris Morgan

91.6k28 gold badges217 silver badges220 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

lofcek Over a year ago

Thanks a lot. It does make sense. The other possibility is change a function as it use a reference.

Chris Morgan Over a year ago

@lofcek: it is not useful to take an iterator by mutable reference; if I implements Iterator, &mut I does too, so you can pass a mutable reference to an iterator in that way. There’s even the by_ref(&mut self) -> &mut Self method on Iterator for convenience of chaining.

Collectives™ on Stack Overflow

How I can iterate over borrowed array?

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related