Trait std::iter::Extend

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


Extend implementation that copies elements out of references before pushing them onto the Vec.

This implementation is specialized for slice iterators, where it uses copy_from_slice to append the entire slice at once.

Inserts all new key-values from the iterator and replaces values with existing keys with new values returned from the iterator.