#[derive(Debug)]
struct MyCollection(Vec<i32>);

impl MyCollection {
    fn new() -> MyCollection {
        MyCollection(Vec::new())
    }

    fn add(&mut self, elem: i32) {
        self.0.push(elem);
    }

    fn my_filter(&self) -> impl Iterator<Item=&i32> {
        self.0.iter().filter(|&n| *n > 0)
    }

    fn my_map(&self) -> impl Iterator<Item=i32> {
        self.0.iter().map(|n| n * 2)
    }
}

fn main() {
    let mut c = MyCollection::new();

    c.add(0);
    c.add(1);
    c.add(2);
    c.add(3);
    c.add(4);
    c.add(5);

    for n in c.my_filter().map(|&n| n * 2) {
        println!("{}", n);
    }

    for n in c.my_map().filter(|&n| n > 0) {
        println!("{}", n);
    }
}