How to implement iterator using trait

Question

I have a struct called Library, which has a vector of strings (titles). I have implemented an iterator for this. Here is my code.

#[derive(Debug, Clone)]
struct Library {
    books: Vec<String>
}

impl Iterator for Library {
    fn next(&mut self) -> Option<Self::Item> {
        ...
    }
}

Now, I am trying to implement an Iterator using a trait, like this:

fn foo(x: Vec<u32>) -> impl Iterator<Item=u32> {
    //Unsure if correct method
    fn next() -> Option<...> {
       x.into_iter()....

    }
}

But I'm unsure of how to proceed in this case. Do I simply have to define a next() method again? That doesn't seem to be the case, according to other resources. Why is that? Shouldn't an iterator (which is being returned) have a next() method?

What is the general method for implementing an iterator this way?

Does this answer your question? Implement Iterator trait for a struct containing an iterable field — Locke
– Locke, Commented Mar 2, 2022 at 6:29

cdhowie · Accepted Answer · 2022-03-02 05:18:48Z

8

You wouldn't implement the trait directly on Library. The library is not an iterator, but it is something that could be iterated by an iterator.

Instead, just declare a method that returns an iterator, and you can simply return an iterator directly from your vector. There is no need for a custom iterator implementation. For example:

impl Library {
    fn iter(&self) -> impl Iterator<Item=&String> {
        self.books.iter()
    }
}

For your second case, you can turn the vector into an iterator using into_iter(), provided by the IntoIterator trait:

fn foo(x: Vec<u32>) -> impl Iterator<Item=u32> {
    x.into_iter()
}

answered Mar 2, 2022 at 5:18

cdhowie

172k25 gold badges303 silver badges324 bronze badges

Sign up to request clarification or add additional context in comments.

6 Comments

user545642 Over a year ago

Right, so the .into_iter() I understand. What if I had a more complicated type? Such as fn foo<A>(x: Vec<A>) -> impl Iterator<Item = (u32,A)>? I want the iterator to go through each element in x and return (0,A) or (1, A) based on some condition?

user545642 Over a year ago

Similarly, what if I had fn foo(x: Vec<u32>) -> impl Iterator<Item = (u32, u32)>?

cdhowie Over a year ago

@DarrelGulseth That's where the standard iterator utilities like .map() come in. So you could do x.into_iter().map(|v| /* some expression here */)

cdhowie Over a year ago

@DarrelGulseth It all depends how you want to iterate that. If you want to flatten the nested vector into one iterator of u32 then you just do x.into_iter().flatten().

cdhowie Over a year ago

@DarrelGulseth No problem. The Iterator trait provides dozens of such utilities. Using them you can do just about any sequence manipulation you can think of. It's very rare you need to implement your own iterator, unless you are implementing something like a paged API client or are implementing your own data structure from scratch.

|

Collectives™ on Stack Overflow

How to implement iterator using trait

1 Answer 1

6 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

6 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related