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IntoIterator

The Iterator trait tells you how to iterate once you have created an iterator. The related trait IntoIterator defines how to create an iterator for a type. It is used automatically by the for loop.

// Copyright 2023 Google LLC
// SPDX-License-Identifier: Apache-2.0

struct Grid {
    x_coords: Vec<u32>,
    y_coords: Vec<u32>,
}

impl IntoIterator for Grid {
    type Item = (u32, u32);
    type IntoIter = GridIter;
    fn into_iter(self) -> GridIter {
        GridIter { grid: self, i: 0, j: 0 }
    }
}

struct GridIter {
    grid: Grid,
    i: usize,
    j: usize,
}

impl Iterator for GridIter {
    type Item = (u32, u32);

    fn next(&mut self) -> Option<(u32, u32)> {
        if self.i >= self.grid.x_coords.len() {
            self.i = 0;
            self.j += 1;
            if self.j >= self.grid.y_coords.len() {
                return None;
            }
        }
        let res = Some((self.grid.x_coords[self.i], self.grid.y_coords[self.j]));
        self.i += 1;
        res
    }
}

fn main() {
    let grid = Grid { x_coords: vec![3, 5, 7, 9], y_coords: vec![10, 20, 30, 40] };
    for (x, y) in grid {
        println!("point = {x}, {y}");
    }
}
This slide should take about 5 minutes.
  • IntoIterator is the trait that makes for loops work. It is implemented by collection types such as Vec<T> and references to them such as &Vec<T> and &[T]. Ranges also implement it. This is why you can iterate over a vector with for i in some_vec { .. } but some_vec.next() doesn’t exist.

Click through to the docs for IntoIterator. Every implementation of IntoIterator must declare two types:

  • Item: the type to iterate over, such as i8,
  • IntoIter: the Iterator type returned by the into_iter method.

Note that IntoIter and Item are linked: the iterator must have the same Item type, which means that it returns Option<Item>

The example iterates over all combinations of x and y coordinates.

Try iterating over the grid twice in main. Why does this fail? Note that IntoIterator::into_iter takes ownership of self.

Fix this issue by implementing IntoIterator for &Grid and creating a GridRefIter that iterates by reference. A version with both GridIter and GridRefIter is available in this playground.

The same problem can occur for standard library types: for e in some_vector will take ownership of some_vector and iterate over owned elements from that vector. Use for e in &some_vector instead, to iterate over references to elements of some_vector.