How do I write an iterator that returns references to itself?

Question

I am having trouble expressing the lifetime of the return value of an Iterator implementation. How can I compile this code without changing the return value of

Answer 1

As mentioned in other answers, this is called a streaming iterator and it requires different guarantees from Rust's Iterator. One crate that provides such functionality is aptly called streaming-iterator and it provides the StreamingIterator trait.

Here is one example of implementing the trait:

extern crate streaming_iterator;

use streaming_iterator::StreamingIterator;

struct Demonstration {
    scores: Vec,
    position: usize,
}

// Since `StreamingIterator` requires that we be able to call
// `advance` before `get`, we have to start "before" the first
// element. We assume that there will never be the maximum number of
// entries in the `Vec`, so we use `usize::MAX` as our sentinel value.
impl Demonstration {
    fn new() -> Self {
        Demonstration {
            scores: vec![1, 2, 3],
            position: std::usize::MAX,
        }
    }

    fn reset(&mut self) {
        self.position = std::usize::MAX;
    }
}

impl StreamingIterator for Demonstration {
    type Item = i32;

    fn advance(&mut self) {
        self.position = self.position.wrapping_add(1);
    }

    fn get(&self) -> Option<&Self::Item> {
        self.scores.get(self.position)
    }
}

fn main() {
    let mut example = Demonstration::new();

    loop {
        example.advance();
        match example.get() {
            Some(v) => {
                println!("v: {}", v);
            }
            None => break,
        }
    }

    example.reset();

    loop {
        example.advance();
        match example.get() {
            Some(v) => {
                println!("v: {}", v);
            }
            None => break,
        }
    }
}

Unfortunately, streaming iterators will be limited until generic associated types (GATs) from RFC 1598 are implemented.