Trait FromIterator

pub trait FromIterator<A>: Sized {
    // Required method
    fn from_iter<T>(iter: T) -> Self
       where T: IntoIterator<Item = A>;
}

Conversion from an Iterator.

By implementing FromIterator for a type, you define how it will be created from an iterator. This is common for types which describe a collection of some kind.

If you want to create a collection from the contents of an iterator, the Iterator::collect() method is preferred. However, when you need to specify the container type, FromIterator::from_iter() can be more readable than using a turbofish (e.g. ::<Vec<_>>()). See the Iterator::collect() documentation for more examples of its use.

See also: IntoIterator.

Examples

Basic usage:

let five_fives = std::iter::repeat(5).take(5);

let v = Vec::from_iter(five_fives);

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Using Iterator::collect() to implicitly use FromIterator:

let five_fives = std::iter::repeat(5).take(5);

let v: Vec<i32> = five_fives.collect();

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Using FromIterator::from_iter() as a more readable alternative to Iterator::collect():

use std::collections::VecDeque;
let first = (0..10).collect::<VecDeque<i32>>();
let second = VecDeque::from_iter(0..10);

assert_eq!(first, second);

Implementing FromIterator for your type:

// A sample collection, that's just a wrapper over Vec<T>
#[derive(Debug)]
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 {
        MyCollection(Vec::new())
    }

    fn add(&mut self, elem: i32) {
        self.0.push(elem);
    }
}

// and we'll implement FromIterator
impl FromIterator<i32> for MyCollection {
    fn from_iter<I: IntoIterator<Item=i32>>(iter: I) -> Self {
        let mut c = MyCollection::new();

        for i in iter {
            c.add(i);
        }

        c
    }
}

// Now we can make a new iterator...
let iter = (0..5).into_iter();

// ... and make a MyCollection out of it
let c = MyCollection::from_iter(iter);

assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

// collect works too!

let iter = (0..5).into_iter();
let c: MyCollection = iter.collect();

assert_eq!(c.0, vec![0, 1, 2, 3, 4]);

Required Methods

fn from_iter<T>(iter: T) -> Self

where T: IntoIterator<Item = A>,

Creates a value from an iterator.

See the module-level documentation for more.

Examples

let five_fives = std::iter::repeat(5).take(5);

let v = Vec::from_iter(five_fives);

assert_eq!(v, vec![5, 5, 5, 5, 5]);

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl FromIterator<char> for Box<str>

impl FromIterator<char> for String

impl FromIterator<()> for ()

Collapses all unit items from an iterator into one.

This is more useful when combined with higher-level abstractions, like collecting to a Result<(), E> where you only care about errors:

use std::io::*;
let data = vec![1, 2, 3, 4, 5];
let res: Result<()> = data.iter()
    .map(|x| writeln!(stdout(), "{x}"))
    .collect();
assert!(res.is_ok());

impl FromIterator<OsString> for OsString

impl FromIterator<String> for Box<str>

impl FromIterator<String> for String

impl<'a> FromIterator<&'a char> for Box<str>

impl<'a> FromIterator<&'a char> for String

impl<'a> FromIterator<&'a str> for Box<str>

impl<'a> FromIterator<&'a str> for String

impl<'a> FromIterator<&'a OsStr> for OsString

impl<'a> FromIterator<Cow<'a, str>> for Box<str>

impl<'a> FromIterator<Cow<'a, str>> for String

impl<'a> FromIterator<Cow<'a, OsStr>> for OsString

impl<'a> FromIterator<char> for Cow<'a, str>

impl<'a> FromIterator<String> for Cow<'a, str>

impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>

impl<'a, T> FromIterator<T> for Cow<'a, [T]>

where T: Clone,

impl<A> FromIterator<Box<str, A>> for Box<str>

where A: Allocator,

impl<A> FromIterator<Box<str, A>> for String

where A: Allocator,

impl<A, B, AE, BE> FromIterator<(AE, BE)> for (A, B)

where A: Default + Extend<AE>, B: Default + Extend<BE>,

This implementation turns an iterator of tuples into a tuple of types which implement Default and Extend.

This is similar to Iterator::unzip, but is also composable with other FromIterator implementations:

let string = "1,2,123,4";

let (numbers, lengths): (Vec<_>, Vec<_>) = string
    .split(',')
    .map(|s| s.parse().map(|n: u32| (n, s.len())))
    .collect::<Result<_, _>>()?;

assert_eq!(numbers, [1, 2, 123, 4]);
assert_eq!(lengths, [1, 1, 3, 1]);

impl<A, E, V> FromIterator<Result<A, E>> for Result<V, E>

where V: FromIterator<A>,

impl<A, V> FromIterator<Option<A>> for Option<V>

where V: FromIterator<A>,

impl<I> FromIterator<I> for Box<[I]>

impl<K, V> FromIterator<(K, V)> for BTreeMap<K, V>

where K: Ord,

impl<K, V, S> FromIterator<(K, V)> for HashMap<K, V, S>

where K: Eq + Hash, S: BuildHasher + Default,

impl<P: AsRef<Path>> FromIterator<P> for PathBuf

impl<T> FromIterator<T> for BinaryHeap<T>

where T: Ord,

impl<T> FromIterator<T> for BTreeSet<T>

where T: Ord,

impl<T> FromIterator<T> for LinkedList<T>

impl<T> FromIterator<T> for VecDeque<T>

impl<T> FromIterator<T> for Rc<[T]>

impl<T> FromIterator<T> for Arc<[T]>

impl<T> FromIterator<T> for Vec<T>

Collects an iterator into a Vec, commonly called via Iterator::collect()

Allocation behavior

In general Vec does not guarantee any particular growth or allocation strategy. That also applies to this trait impl.

Note: This section covers implementation details and is therefore exempt from stability guarantees.

Vec may use any or none of the following strategies, depending on the supplied iterator:

• preallocate based on Iterator::size_hint()
  • and panic if the number of items is outside the provided lower/upper bounds
• use an amortized growth strategy similar to pushing one item at a time
• perform the iteration in-place on the original allocation backing the iterator

The last case warrants some attention. It is an optimization that in many cases reduces peak memory consumption and improves cache locality. But when big, short-lived allocations are created, only a small fraction of their items get collected, no further use is made of the spare capacity and the resulting Vec is moved into a longer-lived structure, then this can lead to the large allocations having their lifetimes unnecessarily extended which can result in increased memory footprint.

In cases where this is an issue, the excess capacity can be discarded with Vec::shrink_to(), Vec::shrink_to_fit() or by collecting into Box<[T]> instead, which additionally reduces the size of the long-lived struct.

static LONG_LIVED: Mutex<Vec<Vec<u16>>> = Mutex::new(Vec::new());

for i in 0..10 {
    let big_temporary: Vec<u16> = (0..1024).collect();
    // discard most items
    let mut result: Vec<_> = big_temporary.into_iter().filter(|i| i % 100 == 0).collect();
    // without this a lot of unused capacity might be moved into the global
    result.shrink_to_fit();
    LONG_LIVED.lock().unwrap().push(result);
}

impl<T, S> FromIterator<T> for HashSet<T, S>

where T: Eq + Hash, S: BuildHasher + Default,