Trait core::iter::Extend

1.0.0 · source · []
pub trait Extend<A> {
    fn extend<T: IntoIterator<Item = A>>(&mut self, iter: T);

    fn extend_one(&mut self, item: A) { ... }
    fn extend_reserve(&mut self, additional: usize) { ... }
Expand description

Extend a collection with the contents of an iterator.

Iterators produce a series of values, and collections can also be thought of as a series of values. The Extend trait bridges this gap, allowing you to extend a collection by including the contents of that iterator. When extending a collection with an already existing key, that entry is updated or, in the case of collections that permit multiple entries with equal keys, that entry is inserted.


Basic usage:

// You can extend a String with some chars:
let mut message = String::from("The first three letters are: ");

message.extend(&['a', 'b', 'c']);

assert_eq!("abc", &message[29..32]);

Implementing Extend:

// A sample collection, that's just a wrapper over Vec<T>
struct MyCollection(Vec<i32>);

// Let's give it some methods so we can create one and add things
// to it.
impl MyCollection {
    fn new() -> MyCollection {

    fn add(&mut self, elem: i32) {

// since MyCollection has a list of i32s, we implement Extend for i32
impl Extend<i32> for MyCollection {

    // This is a bit simpler with the concrete type signature: we can call
    // extend on anything which can be turned into an Iterator which gives
    // us i32s. Because we need i32s to put into MyCollection.
    fn extend<T: IntoIterator<Item=i32>>(&mut self, iter: T) {

        // The implementation is very straightforward: loop through the
        // iterator, and add() each element to ourselves.
        for elem in iter {

let mut c = MyCollection::new();


// let's extend our collection with three more numbers
c.extend(vec![1, 2, 3]);

// we've added these elements onto the end
assert_eq!("MyCollection([5, 6, 7, 1, 2, 3])", format!("{c:?}"));

Required Methods

Extends a collection with the contents of an iterator.

As this is the only required method for this trait, the trait-level docs contain more details.


Basic usage:

// You can extend a String with some chars:
let mut message = String::from("abc");

message.extend(['d', 'e', 'f'].iter());

assert_eq!("abcdef", &message);

Provided Methods

🔬This is a nightly-only experimental API. (extend_one #72631)

Extends a collection with exactly one element.

🔬This is a nightly-only experimental API. (extend_one #72631)

Reserves capacity in a collection for the given number of additional elements.

The default implementation does nothing.