How do I properly add lifetimes to an iterator containing other iterators in Rust? - rust

Question

I currently have code that looks kind of like this:
struct People {
names: Vec<String>,
ages: Vec<i32>,
}
impl People {
fn iter_people<'a>(&'a self) -> PeopleIterator<'a> {
return PeopleIterator {
names_iterator: Box::new(self.names.iter()),
ages: Box::new(self.ages.iter()),
};
}
}
struct PeopleIterator<'a> {
names_iterator: Box<dyn Iterator<Item = &'a String>>,
ages: Box<dyn Iterator<Item = &'a i32>>,
}
impl<'a> Iterator for PeopleIterator<'a> {
...snip...
}
I am aware that I should model a person as a struct Person and then have a Vec<Person> to model people but this is just a simplification of my actual code.
Anyway, the Rust compiler tells me this:
lifetime may not live long enough
requirement occurs because of the type PeopleIterator<'_>, which makes the generic argument '_ invariant
I have looked at the suggested link for subtyping and variance but I need to read it a few more times to actually understand it.
What stumps me is that I would expect both my iterators self.names.iter() and self.ages.iter() to live as long as self and I have declared that self should live as long as PeopleIterator. However, when I look at the iter() function, it does not make this constraint but instead has an anonymous lifetime '_. I am guessing this is the problem but I am confused and don't know how to fix it :(

Answer 1

The problem is the lifetime of the iterator itself in Box<dyn Iterator<Item = &'a String>> is by default bound to be 'static, but that's not possible for an iterator containing non static references like anything from &'a self. The solution is to specify an explicit lifetime bound:
struct PeopleIterator<'a> {
names_iterator: Box<dyn Iterator<Item = &'a String> + 'a>,
ages: Box<dyn Iterator<Item = &'a i32> + 'a>,
}
Personally I'd just use generics instead of static dispatch avoiding some indirection and the whole problem from the beginning:
impl People {
fn iter_people(&self) -> PeopleIterator<impl Iterator<Item = &String>, impl Iterator<Item = &i32>> {
return PeopleIterator {
names_iterator: self.names.iter(),
ages: self.ages.iter(),
};
}
}
struct PeopleIterator<N, A> {
names_iterator: N,
ages: A,
}