pub trait FromIterator<A>: Sized {
// Required method
fn from_iter<T>(iter: T) -> Self
where T: IntoIterator<Item = A>;
}Expand description
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§
1.6.0 · sourcefn from_iter<T>(iter: T) -> Selfwhere
T: IntoIterator<Item = A>,
fn from_iter<T>(iter: T) -> Selfwhere
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 CompactString
impl FromIterator<char> for Box<str>
no_global_oom_handling only.impl FromIterator<char> for String
no_global_oom_handling only.impl FromIterator<u8> for Bytes
impl FromIterator<u8> for BytesMut
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<usize> for FixedBitSet
Return a FixedBitSet containing bits set to true for every bit index in the iterator, other bits are set to false.
impl FromIterator<CompactString> for Cow<'_, str>
impl FromIterator<CompactString> for CompactString
impl FromIterator<CompactString> for String
impl FromIterator<Box<str>> for CompactString
impl FromIterator<String> for CompactString
impl FromIterator<String> for Box<str>
no_global_oom_handling only.impl FromIterator<String> for String
no_global_oom_handling only.impl FromIterator<OsString> for OsString
impl FromIterator<EventMask> for EventMask
impl FromIterator<Flags> for Flags
impl FromIterator<FsyncFlags> for FsyncFlags
impl FromIterator<TimeoutFlags> for TimeoutFlags
impl FromIterator<TrustAnchor<'static>> for RootCertStore
impl FromIterator<WatchMask> for WatchMask
impl<'a> FromIterator<&'a char> for CompactString
impl<'a> FromIterator<&'a char> for Box<str>
no_global_oom_handling only.impl<'a> FromIterator<&'a char> for String
no_global_oom_handling only.impl<'a> FromIterator<&'a str> for CompactString
impl<'a> FromIterator<&'a str> for Box<str>
no_global_oom_handling only.impl<'a> FromIterator<&'a str> for String
no_global_oom_handling only.impl<'a> FromIterator<&'a u8> for BytesMut
impl<'a> FromIterator<&'a OsStr> for OsString
impl<'a> FromIterator<Cow<'a, str>> for CompactString
impl<'a> FromIterator<Cow<'a, str>> for Box<str>
no_global_oom_handling only.impl<'a> FromIterator<Cow<'a, str>> for String
no_global_oom_handling only.impl<'a> FromIterator<Cow<'a, OsStr>> for OsString
impl<'a> FromIterator<char> for Cow<'a, str>
no_global_oom_handling only.impl<'a> FromIterator<String> for Cow<'a, str>
no_global_oom_handling only.impl<'a, 'b> FromIterator<&'b str> for Cow<'a, str>
no_global_oom_handling only.impl<'a, T> FromIterator<T> for Cow<'a, [T]>where
T: Clone,
impl<A> FromIterator<<A as Array>::Item> for ArrayVec<A>where
A: Array,
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<Box<str, A>> for Box<str>where
A: Allocator,
no_global_oom_handling only.impl<A> FromIterator<Box<str, A>> for Stringwhere
A: Allocator,
no_global_oom_handling only.impl<A> FromIterator<A> for Arc<[A]>
impl<A> FromIterator<A> for UniqueArc<[A]>
impl<A, B, AE, BE> FromIterator<(AE, BE)> for (A, B)
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<F> FromIterator<F> for JoinAll<F>where
F: Future,
impl<F> FromIterator<F> for TryJoinAll<F>where
F: TryFuture,
impl<Fut> FromIterator<Fut> for FuturesOrdered<Fut>where
Fut: Future,
impl<Fut> FromIterator<Fut> for FuturesUnordered<Fut>
impl<Fut> FromIterator<Fut> for SelectAll<Fut>
impl<Fut> FromIterator<Fut> for SelectOk<Fut>where
Fut: TryFuture + Unpin,
impl<I> FromIterator<I> for Box<[I]>
no_global_oom_handling only.impl<K, S> FromIterator<K> for DashSet<K, S>
impl<K, V> FromIterator<(K, V)> for BTreeMap<K, V>where
K: Ord,
impl<K, V, S> FromIterator<(K, V)> for kvarn::prelude::HashMap<K, V, S>
impl<K, V, S> FromIterator<(K, V)> for DashMap<K, V, S>
impl<K, V, S> FromIterator<(K, V)> for IndexMap<K, V, S>
impl<K, V, S, A> FromIterator<(K, V)> for HashMap<K, V, S, A>
impl<K, V, S, A> FromIterator<(K, V)> for HashMap<K, V, S, A>
impl<N, E, Ty, Item, S> FromIterator<Item> for GraphMap<N, E, Ty, S>
Create a new GraphMap from an iterable of edges.
impl<P> FromIterator<P> for PathBuf
impl<St> FromIterator<St> for SelectAll<St>
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<(HeaderName, T)> for HeaderMap<T>
impl<T> FromIterator<T> for kvarn::prelude::Arc<[T]>
no_global_oom_handling only.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]>
no_global_oom_handling only.impl<T> FromIterator<T> for Vec<T>
no_global_oom_handling only.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
pushingone 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, F> FromIterator<F> for JoinSet<T>
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]);
}
}