Trait ExactSizeIterator 

pub trait ExactSizeIterator: Iterator {
    // Provided methods
    fn len(&self) -> usize { ... }
    fn is_empty(&self) -> bool { ... }
}

An iterator that knows its exact length.

Many Iterators don’t know how many times they will iterate, but some do. If an iterator knows how many times it can iterate, providing access to that information can be useful. For example, if you want to iterate backwards, a good start is to know where the end is.

When implementing an ExactSizeIterator, you must also implement Iterator. When doing so, the implementation of Iterator::size_hint must return the exact size of the iterator.

The len method has a default implementation, so you usually shouldn’t implement it. However, you may be able to provide a more performant implementation than the default, so overriding it in this case makes sense.

Note that this trait is a safe trait and as such does not and cannot guarantee that the returned length is correct. This means that unsafe code must not rely on the correctness of Iterator::size_hint. The unstable and unsafe TrustedLen trait gives this additional guarantee.

When shouldn’t an adapter be ExactSizeIterator?

If an adapter makes an iterator longer, then it’s usually incorrect for that adapter to implement ExactSizeIterator. The inner exact-sized iterator might already be usize::MAX-long, and thus the length of the longer adapted iterator would no longer be exactly representable in usize.

This is why Chain<A, B> isn’t ExactSizeIterator, even when A and B are both ExactSizeIterator.

Examples

Basic usage:

// a finite range knows exactly how many times it will iterate
let five = 0..5;

assert_eq!(5, five.len());

In the module-level docs, we implemented an Iterator, Counter. Let’s implement ExactSizeIterator for it as well:

impl ExactSizeIterator for Counter {
    // We can easily calculate the remaining number of iterations.
    fn len(&self) -> usize {
        5 - self.count
    }
}

// And now we can use it!

let mut counter = Counter::new();

assert_eq!(5, counter.len());
let _ = counter.next();
assert_eq!(4, counter.len());

Provided Methods

fn len(&self) -> usize

Available on non-crate feature ferrocene_certified only.

Returns the exact remaining length of the iterator.

The implementation ensures that the iterator will return exactly len() more times a Some(T) value, before returning None. This method has a default implementation, so you usually should not implement it directly. However, if you can provide a more efficient implementation, you can do so. See the trait-level docs for an example.

This function has the same safety guarantees as the Iterator::size_hint function.

Examples

Basic usage:

// a finite range knows exactly how many times it will iterate
let mut range = 0..5;

assert_eq!(5, range.len());
let _ = range.next();
assert_eq!(4, range.len());

fn is_empty(&self) -> bool

🔬This is a nightly-only experimental API. (exact_size_is_empty #35428)

Returns true if the iterator is empty.

This method has a default implementation using ExactSizeIterator::len(), so you don’t need to implement it yourself.

Examples

Basic usage:

#![feature(exact_size_is_empty)]

let mut one_element = std::iter::once(0);
assert!(!one_element.is_empty());

assert_eq!(one_element.next(), Some(0));
assert!(one_element.is_empty());

assert_eq!(one_element.next(), None);

Implementors

impl ExactSizeIterator for std::ascii::EscapeDefault

impl ExactSizeIterator for EscapeDebug

Available on non-crate feature ferrocene_certified only.

impl ExactSizeIterator for std::char::EscapeDefault

Available on non-crate feature ferrocene_certified only.

impl ExactSizeIterator for EscapeUnicode

Available on non-crate feature ferrocene_certified only.

impl ExactSizeIterator for ToLowercase

impl ExactSizeIterator for ToUppercase

impl ExactSizeIterator for Args

impl ExactSizeIterator for ArgsOs

impl ExactSizeIterator for Range<i8>

impl ExactSizeIterator for Range<i16>

impl ExactSizeIterator for Range<i32>

impl ExactSizeIterator for Range<isize>

impl ExactSizeIterator for Range<u8>

impl ExactSizeIterator for Range<u16>

impl ExactSizeIterator for Range<u32>

impl ExactSizeIterator for Range<usize>

impl ExactSizeIterator for RangeInclusive<i8>

impl ExactSizeIterator for RangeInclusive<i16>

impl ExactSizeIterator for RangeInclusive<u8>

impl ExactSizeIterator for RangeInclusive<u16>

impl ExactSizeIterator for IterRange<i8>

impl ExactSizeIterator for IterRange<i16>

impl ExactSizeIterator for IterRange<isize>

impl ExactSizeIterator for IterRange<u8>

impl ExactSizeIterator for IterRange<u16>

impl ExactSizeIterator for IterRange<usize>

impl ExactSizeIterator for IterRangeInclusive<i8>

impl ExactSizeIterator for IterRangeInclusive<u8>

impl ExactSizeIterator for Bytes<'_>

Available on non-crate feature ferrocene_certified only.

impl<'a> ExactSizeIterator for CommandArgs<'a>

impl<'a> ExactSizeIterator for CommandEnvs<'a>

impl<'a, I, T> ExactSizeIterator for Cloned<I>

where T: 'a + Clone, I: ExactSizeIterator<Item = &'a T>,

Available on non-crate feature ferrocene_certified only.

impl<'a, I, T> ExactSizeIterator for Copied<I>

where T: 'a + Copy, I: ExactSizeIterator<Item = &'a T>,

impl<'a, T> ExactSizeIterator for RChunksExact<'a, T>

Available on non-crate feature ferrocene_certified only.

impl<A> ExactSizeIterator for std::option::IntoIter<A>

Available on non-crate feature ferrocene_certified only.

impl<A> ExactSizeIterator for std::option::Iter<'_, A>

Available on non-crate feature ferrocene_certified only.

impl<A> ExactSizeIterator for std::option::IterMut<'_, A>

Available on non-crate feature ferrocene_certified only.

impl<A> ExactSizeIterator for RepeatN<A>

where A: Clone,

impl<A, B> ExactSizeIterator for Zip<A, B>

where A: ExactSizeIterator, B: ExactSizeIterator,

Available on non-crate feature ferrocene_certified only.

impl<A, F> ExactSizeIterator for OnceWith<F>

where F: FnOnce() -> A,

impl<B, I, F> ExactSizeIterator for Map<I, F>

where I: ExactSizeIterator, F: FnMut(<I as Iterator>::Item) -> B,

Available on non-crate feature ferrocene_certified only.

impl<F, A> ExactSizeIterator for Take<RepeatWith<F>>

where F: FnMut() -> A,

Available on non-crate feature ferrocene_certified only.

impl<I> ExactSizeIterator for &mut I

where I: ExactSizeIterator + ?Sized,

impl<I> ExactSizeIterator for Enumerate<I>

where I: ExactSizeIterator,

Available on non-crate feature ferrocene_certified only.

impl<I> ExactSizeIterator for Fuse<I>

where I: ExactSizeIterator,

impl<I> ExactSizeIterator for Peekable<I>

where I: ExactSizeIterator,

impl<I> ExactSizeIterator for Rev<I>

where I: ExactSizeIterator + DoubleEndedIterator,

Available on non-crate feature ferrocene_certified only.

impl<I> ExactSizeIterator for Skip<I>

where I: ExactSizeIterator,

Available on non-crate feature ferrocene_certified only.

impl<I> ExactSizeIterator for StepBy<I>

where I: ExactSizeIterator,

Available on non-crate feature ferrocene_certified only.

impl<I> ExactSizeIterator for Take<I>

where I: ExactSizeIterator,

Available on non-crate feature ferrocene_certified only.

impl<I, A> ExactSizeIterator for Box<I, A>

where I: ExactSizeIterator + ?Sized, A: Allocator,

impl<I, A> ExactSizeIterator for Splice<'_, I, A>

where I: Iterator, A: Allocator,

impl<I, F> ExactSizeIterator for Inspect<I, F>

where I: ExactSizeIterator, F: FnMut(&<I as Iterator>::Item),

impl<I, F, R, const N: usize> ExactSizeIterator for MapWindows<I, F, N>

where I: ExactSizeIterator, F: FnMut(&[<I as Iterator>::Item; N]) -> R,

impl<I, const N: usize> ExactSizeIterator for ArrayChunks<I, N>

where I: ExactSizeIterator,

impl<K> ExactSizeIterator for std::collections::hash_set::Drain<'_, K>

impl<K> ExactSizeIterator for std::collections::hash_set::IntoIter<K>

impl<K> ExactSizeIterator for std::collections::hash_set::Iter<'_, K>

impl<K, V> ExactSizeIterator for std::collections::btree_map::Iter<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::btree_map::IterMut<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::btree_map::Keys<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::btree_map::Values<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::btree_map::ValuesMut<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::Drain<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoIter<K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoKeys<K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoValues<K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::Iter<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::IterMut<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::Keys<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::Values<'_, K, V>

impl<K, V> ExactSizeIterator for std::collections::hash_map::ValuesMut<'_, K, V>

impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoIter<K, V, A>

where A: Allocator + Clone,

impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoKeys<K, V, A>

where A: Allocator + Clone,

impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoValues<K, V, A>

where A: Allocator + Clone,

impl<T> ExactSizeIterator for std::collections::binary_heap::Iter<'_, T>

impl<T> ExactSizeIterator for std::collections::btree_set::Iter<'_, T>

impl<T> ExactSizeIterator for std::collections::linked_list::Iter<'_, T>

impl<T> ExactSizeIterator for std::collections::linked_list::IterMut<'_, T>

impl<T> ExactSizeIterator for std::collections::vec_deque::Iter<'_, T>

impl<T> ExactSizeIterator for std::collections::vec_deque::IterMut<'_, T>

impl<T> ExactSizeIterator for std::result::IntoIter<T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for std::result::Iter<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for std::result::IterMut<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for Chunks<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for ChunksExact<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for ChunksExactMut<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for ChunksMut<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for std::slice::Iter<'_, T>

impl<T> ExactSizeIterator for std::slice::IterMut<'_, T>

impl<T> ExactSizeIterator for RChunks<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for RChunksExactMut<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for RChunksMut<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for Windows<'_, T>

Available on non-crate feature ferrocene_certified only.

impl<T> ExactSizeIterator for Empty<T>

impl<T> ExactSizeIterator for Once<T>

impl<T> ExactSizeIterator for Take<Repeat<T>>

where T: Clone,

Available on non-crate feature ferrocene_certified only.

impl<T, A> ExactSizeIterator for std::collections::binary_heap::Drain<'_, T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for DrainSorted<'_, T, A>

where T: Ord, A: Allocator,

impl<T, A> ExactSizeIterator for std::collections::binary_heap::IntoIter<T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for IntoIterSorted<T, A>

where T: Ord, A: Allocator,

impl<T, A> ExactSizeIterator for std::collections::btree_set::IntoIter<T, A>

where A: Allocator + Clone,

impl<T, A> ExactSizeIterator for std::collections::linked_list::IntoIter<T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for std::collections::vec_deque::Drain<'_, T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for std::collections::vec_deque::IntoIter<T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for std::vec::Drain<'_, T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for std::vec::IntoIter<T, A>

where A: Allocator,

impl<T, const N: usize> ExactSizeIterator for std::array::IntoIter<T, N>

Available on non-crate feature ferrocene_certified only.

impl<T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N>

Available on non-crate feature ferrocene_certified only.