迭代器

Rust 迭代器模式讲解和代码示例

迭代器是一种行为设计模式，让你能在不暴露复杂数据结构内部细节的情况下遍历其中所有的元素。

在迭代器的帮助下，客户端可以用一个迭代器接口以相似的方式遍历不同集合中的元素。

进一步了解迭代器模式

Standard Iterator

Iterators are heavily used in idiomatic Rust code. This is how to use iterators over a standard array collection.

let array = &[1, 2, 3];
let iterator = array.iter();

// Traversal over each element of the vector.
iterator.for_each(|e| print!("{}, ", e));

Custom Iterator

In Rust, the recommended way to define your custom iterator is to use a standard Iterator trait. The example doesn't contain a synthetic iterator interface, because it is really recommended to use the idiomatic Rust way.

let users = UserCollection::new();
let mut iterator = users.iter();

iterator.next();

A next method is the only Iterator trait method which is mandatory to be implemented. It makes accessible a huge range of standard methods, e.g. fold, map, for_each.

impl Iterator for UserIterator<'_> {
    fn next(&mut self) -> Option<Self::Item>;
}

users.rs: Collection & Iterator

pub struct UserCollection {
    users: [&'static str; 3],
}

/// A custom collection contains an arbitrary user array under the hood.
impl UserCollection {
    /// Returns a custom user collection.
    pub fn new() -> Self {
        Self {
            users: ["Alice", "Bob", "Carl"],
        }
    }

    /// Returns an iterator over a user collection.
    ///
    /// The method name may be different, however, `iter` is used as a de facto
    /// standard in a Rust naming convention.
    pub fn iter(&self) -> UserIterator {
        UserIterator {
            index: 0,
            user_collection: self,
        }
    }
}

/// UserIterator allows sequential traversal through a complex user collection
/// without exposing its internal details.
pub struct UserIterator<'a> {
    index: usize,
    user_collection: &'a UserCollection,
}

/// `Iterator` is a standard interface for dealing with iterators
/// from the Rust standard library.
impl Iterator for UserIterator<'_> {
    type Item = &'static str;

    /// A `next` method is the only `Iterator` trait method which is mandatory to be
    /// implemented. It makes accessible a huge range of standard methods,
    /// e.g. `fold`, `map`, `for_each`.
    fn next(&mut self) -> Option<Self::Item> {
        if self.index < self.user_collection.users.len() {
            let user = Some(self.user_collection.users[self.index]);
            self.index += 1;
            return user;
        }

        None
    }
}

main.rs: Client code

use crate::users::UserCollection;

mod users;

fn main() {
    print!("Iterators are widely used in the standard library: ");

    let array = &[1, 2, 3];
    let iterator = array.iter();

    // Traversal over each element of the array.
    iterator.for_each(|e| print!("{} ", e));

    println!("\n\nLet's test our own iterator.\n");

    let users = UserCollection::new();
    let mut iterator = users.iter();

    println!("1nd element: {:?}", iterator.next());
    println!("2nd element: {:?}", iterator.next());
    println!("3rd element: {:?}", iterator.next());
    println!("4th element: {:?}", iterator.next());

    print!("\nAll elements in user collection: ");
    users.iter().for_each(|e| print!("{} ", e));

    println!();
}

Output

Iterators are widely used in the standard library: 1 2 3

Let's test our own iterator.

1nd element: Some("Alice")
2nd element: Some("Bob")
3rd element: Some("Carl")
4th element: None


All elements in user collection: Alice Bob Carl

迭代器在其他编程语言中的实现