Iterator trait

Build your own iterator by implementing the Iterator trait.

The Iterator trait

All iterators in Rust implement a single trait. The only required method is next; dozens of other methods (map, filter, sum...) come by default:

trait Iterator {
    type Item;
    fn next(&mut self) -> Option<Self::Item>;
    // map, filter, sum, collect ... default methods
}

type Item is an associated type: it says what the iterator produces. next returns the next element inside Some, and None when finished.

Your own iterator

Implementing Iterator for a struct gives you all iterator methods for free:

struct Geri {
    n: u32,
}

impl Iterator for Geri {
    type Item = u32;
    fn next(&mut self) -> Option<u32> {
        if self.n == 0 {
            return None;
        }
        self.n -= 1;
        Some(self.n + 1)
    }
}

let v: Vec<u32> = Geri { n: 3 }.collect();   // [3, 2, 1]

Exercise Countdown

Write the Geri iterator above. Collect the elements of Geri { n: 3 } into a Vec<u32> and print it with {:?}.

Show example solution

struct Geri {
    n: u32,
}

impl Iterator for Geri {
    type Item = u32;
    fn next(&mut self) -> Option<u32> {
        if self.n == 0 {
            return None;
        }
        self.n -= 1;
        Some(self.n + 1)
    }
}

fn main() {
    let v: Vec<u32> = Geri { n: 3 }.collect();
    println!("{:?}", v);
}

Test yourself

Why does <code>next</code> take <code>&mut self</code>?
- To delete elements
- It's a convention; not needed
- For thread safety
- The iterator advances its internal position on each call
Show answer

The iterator advances its internal position on each call — An iterator keeps state (where it is); next changes that state.

Item type

impl Iterator for Geri {
    type ____ = u32;
    fn next(&mut self) -> Option<u32> { .. }
}

Show answer

impl Iterator for Geri {
    type Item = u32;
    fn next(&mut self) -> Option<u32> { .. }
}

Piece and meaning

type Item

Produced type
fn next

Required method
Some(x)

Next element
None

Finished

Next lesson: next method

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