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<(String, Value)> for Map<String, Value>

impl FromIterator<(Key, Value)> for Fields

impl FromIterator<char> for Box<str>

impl FromIterator<char> for ByteString

impl FromIterator<char> for String

impl FromIterator<u8> for ByteString

impl FromIterator<u8> for Bytes

impl FromIterator<u8> for BytesMut

impl FromIterator<u8> for IVec

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<ByteString> for ByteString

impl FromIterator<String> for Box<str>

impl FromIterator<String> for String

impl FromIterator<OsString> for OsString

impl FromIterator<HeaderValue> for HeaderValues

impl FromIterator<CreateFlags> for rustix::backend::event::epoll::CreateFlags

impl FromIterator<EventFlags> for rustix::backend::event::epoll::EventFlags

impl FromIterator<PollFlags> for rustix::backend::event::poll_fd::PollFlags

impl FromIterator<EventfdFlags> for rustix::backend::event::types::EventfdFlags

impl FromIterator<CreateFlags> for rustix::backend::fs::inotify::CreateFlags

impl FromIterator<CreateFlags> for rustix::backend::fs::inotify::CreateFlags

impl FromIterator<ReadFlags> for ReadFlags

impl FromIterator<WatchFlags> for rustix::backend::fs::inotify::WatchFlags

impl FromIterator<WatchFlags> for rustix::backend::fs::inotify::WatchFlags

impl FromIterator<Access> for rustix::backend::fs::types::Access

impl FromIterator<Access> for rustix::backend::fs::types::Access

impl FromIterator<AtFlags> for rustix::backend::fs::types::AtFlags

impl FromIterator<AtFlags> for rustix::backend::fs::types::AtFlags

impl FromIterator<FallocateFlags> for rustix::backend::fs::types::FallocateFlags

impl FromIterator<FallocateFlags> for rustix::backend::fs::types::FallocateFlags

impl FromIterator<MemfdFlags> for rustix::backend::fs::types::MemfdFlags

impl FromIterator<MemfdFlags> for rustix::backend::fs::types::MemfdFlags

impl FromIterator<Mode> for rustix::backend::fs::types::Mode

impl FromIterator<Mode> for rustix::backend::fs::types::Mode

impl FromIterator<MountFlags> for MountFlags

impl FromIterator<MountPropagationFlags> for MountPropagationFlags

impl FromIterator<OFlags> for rustix::backend::fs::types::OFlags

impl FromIterator<OFlags> for rustix::backend::fs::types::OFlags

impl FromIterator<RenameFlags> for rustix::backend::fs::types::RenameFlags

impl FromIterator<RenameFlags> for rustix::backend::fs::types::RenameFlags

impl FromIterator<ResolveFlags> for rustix::backend::fs::types::ResolveFlags

impl FromIterator<ResolveFlags> for rustix::backend::fs::types::ResolveFlags

impl FromIterator<SealFlags> for rustix::backend::fs::types::SealFlags

impl FromIterator<SealFlags> for rustix::backend::fs::types::SealFlags

impl FromIterator<StatVfsMountFlags> for rustix::backend::fs::types::StatVfsMountFlags

impl FromIterator<StatVfsMountFlags> for rustix::backend::fs::types::StatVfsMountFlags

impl FromIterator<StatxFlags> for rustix::backend::fs::types::StatxFlags

impl FromIterator<UnmountFlags> for UnmountFlags

impl FromIterator<CreateFlags> for rustix::backend::io::epoll::CreateFlags

impl FromIterator<EventFlags> for rustix::backend::io::epoll::EventFlags

impl FromIterator<PollFlags> for rustix::backend::io::poll_fd::PollFlags

impl FromIterator<DupFlags> for rustix::backend::io::types::DupFlags

impl FromIterator<DupFlags> for rustix::backend::io::types::DupFlags

impl FromIterator<EventfdFlags> for rustix::backend::io::types::EventfdFlags

impl FromIterator<FdFlags> for rustix::backend::io::types::FdFlags

impl FromIterator<FdFlags> for rustix::backend::io::types::FdFlags

impl FromIterator<PipeFlags> for rustix::backend::io::types::PipeFlags

impl FromIterator<ReadWriteFlags> for rustix::backend::io::types::ReadWriteFlags

impl FromIterator<ReadWriteFlags> for rustix::backend::io::types::ReadWriteFlags

impl FromIterator<SpliceFlags> for rustix::backend::io::types::SpliceFlags

impl FromIterator<RecvFlags> for RecvFlags

impl FromIterator<ReturnFlags> for ReturnFlags

impl FromIterator<SendFlags> for SendFlags

impl FromIterator<PipeFlags> for rustix::backend::pipe::types::PipeFlags

impl FromIterator<SpliceFlags> for rustix::backend::pipe::types::SpliceFlags

impl FromIterator<TimerfdFlags> for TimerfdFlags

impl FromIterator<TimerfdTimerFlags> for TimerfdTimerFlags

impl FromIterator<IFlags> for IFlags

impl FromIterator<StatxAttributes> for StatxAttributes

impl FromIterator<StatxFlags> for rustix::fs::statx::StatxFlags

impl FromIterator<XattrFlags> for rustix::fs::xattr::XattrFlags

impl FromIterator<XattrFlags> for rustix::fs::xattr::XattrFlags

impl FromIterator<SocketFlags> for SocketFlags

impl FromIterator<TxTimeFlags> for TxTimeFlags

impl FromIterator<SocketAddrXdpFlags> for SocketAddrXdpFlags

impl FromIterator<XdpDescOptions> for XdpDescOptions

impl FromIterator<XdpOptionsFlags> for XdpOptionsFlags

impl FromIterator<XdpRingFlags> for XdpRingFlags

impl FromIterator<XdpUmemRegFlags> for XdpUmemRegFlags

impl FromIterator<PidfdFlags> for PidfdFlags

impl FromIterator<PidfdGetfdFlags> for PidfdGetfdFlags

impl FromIterator<FloatingPointEmulationControl> for FloatingPointEmulationControl

impl FromIterator<FloatingPointExceptionMode> for FloatingPointExceptionMode

impl FromIterator<SpeculationFeatureControl> for SpeculationFeatureControl

impl FromIterator<SpeculationFeatureState> for SpeculationFeatureState

impl FromIterator<UnalignedAccessControl> for UnalignedAccessControl

impl FromIterator<WaitIdOptions> for WaitIdOptions

impl FromIterator<WaitOptions> for WaitOptions

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 ByteString

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

impl<'a> FromIterator<&'a u8> for BytesMut

impl<'a> FromIterator<&'a ByteStr> for ByteString

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

impl<'a> FromIterator<&'a [u8]> for ByteString

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, A, B, C, K0, K1, V> FromIterator<(A, B, C)> for ZeroMap2d<'a, K0, K1, V>

where A: Borrow<K0>, B: Borrow<K1>, C: Borrow<V>, K0: ZeroMapKV<'a> + Ord + ?Sized, K1: ZeroMapKV<'a> + Ord + ?Sized, V: ZeroMapKV<'a> + ?Sized,

impl<'a, A, B, K, V> FromIterator<(A, B)> for ZeroMap<'a, K, V>

where A: Borrow<K>, B: Borrow<V>, K: ZeroMapKV<'a> + Ord + ?Sized, V: ZeroMapKV<'a> + ?Sized,

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

where T: Clone,

impl<A> FromIterator<<A as Array>::Item> for arrayvec::ArrayVec<A>

where A: Array,

Create an ArrayVec from an iterator.

Does not extract more items than there is space for. No error occurs if there are more iterator elements.

impl<A> FromIterator<<A as Array>::Item> for SmallVec<A>

where A: Array,

impl<A> FromIterator<<A as Array>::Item> for TinyVec<A>

where A: Array,

impl<A> FromIterator<<A as Array>::Item> for tinyvec::arrayvec::ArrayVec<A>

where A: Array,

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

where V: FromIterator<A>,

impl<A, EA> FromIterator<(EA₁, EA₂, …, EAₙ)> for (A₁, A₂, …, Aₙ)

where A: Default + Extend<EA>,

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";

// Example given for a 2-tuple, but 1- through 12-tuples are supported
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]);

This trait is implemented for tuples up to twelve items long. The impls for 1- and 3- through 12-ary tuples were stabilized after 2-tuples, in 1.85.0.

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

where V: FromIterator<A>,

impl<F> FromIterator<F> for JoinAll<F>

where F: Future,

impl<F> FromIterator<F> for TryJoinAll<F>

where F: TryFuture,

impl<Fut> FromIterator<Fut> for futures_util::future::select_all::SelectAll<Fut>

where Fut: Future + Unpin,

impl<Fut> FromIterator<Fut> for SelectOk<Fut>

where Fut: TryFuture + Unpin,

impl<Fut> FromIterator<Fut> for FuturesOrdered<Fut>

where Fut: Future,

impl<Fut> FromIterator<Fut> for FuturesUnordered<Fut>

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

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

where K: Into<String>, V: Into<Value>,

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<K, V, S> FromIterator<(K, V)> for LiteMap<K, V, S>

where K: Ord, S: StoreFromIterable<K, V>,

impl<P> FromIterator<P> for std::path::PathBuf

where P: AsRef<Path>,

impl<P> FromIterator<P> for async_std::path::pathbuf::PathBuf

where P: AsRef<Path>,

impl<St> FromIterator<St> for futures_util::stream::select_all::SelectAll<St>

where St: Stream + Unpin,

impl<T> FromIterator<(usize, T)> for Slab<T>

Create a slab from an iterator of key-value pairs.

If the iterator produces duplicate keys, the previous value is replaced with the later one. The keys does not need to be sorted beforehand, and this function always takes O(n) time. Note that the returned slab will use space proportional to the largest key, so don’t use Slab with untrusted keys.

Examples

let vec = vec![(2,'a'), (6,'b'), (7,'c')];
let slab = vec.into_iter().collect::<Slab<char>>();
assert_eq!(slab.len(), 3);
assert!(slab.capacity() >= 8);
assert_eq!(slab[2], 'a');

With duplicate and unsorted keys:

let vec = vec![(20,'a'), (10,'b'), (11,'c'), (10,'d')];
let slab = vec.into_iter().collect::<Slab<char>>();
assert_eq!(slab.len(), 3);
assert_eq!(slab[10], 'd');

impl<T> FromIterator<T> for Value

where T: Into<Value>,

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> FromIterator<T> for ZeroVec<'_, T>

where T: AsULE,

impl<T, F> FromIterator<F> for JoinSet<T>

where F: Future<Output = T> + Send + 'static, T: Send + 'static,

Collect an iterator of futures into a JoinSet.

This is equivalent to calling JoinSet::spawn on each element of the iterator.

Examples

The main example from JoinSet’s documentation can also be written using collect:

use tokio::task::JoinSet;

#[tokio::main]
async fn main() {
    let mut set: JoinSet<_> = (0..10).map(|i| async move { i }).collect();

    let mut seen = [false; 10];
    while let Some(res) = set.join_next().await {
        let idx = res.unwrap();
        seen[idx] = true;
    }

    for i in 0..10 {
        assert!(seen[i]);
    }
}

impl<T, N> FromIterator<T> for GenericArray<T, N>

where N: ArrayLength<T>,

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

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

impl<T, const CAP: usize> FromIterator<T> for arrayvec::arrayvec::ArrayVec<T, CAP>

Create an ArrayVec from an iterator.

Panics if the number of elements in the iterator exceeds the arrayvec’s capacity.