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

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)

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 IndexVecIntoIter

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 Bytes<'_>

impl ExactSizeIterator for core::ascii::EscapeDefault

impl ExactSizeIterator for EscapeDebug

impl ExactSizeIterator for core::char::EscapeDefault

impl ExactSizeIterator for EscapeUnicode

impl ExactSizeIterator for ToLowercase

impl ExactSizeIterator for ToUppercase

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 Args

impl ExactSizeIterator for ArgsOs

impl ExactSizeIterator for Chain<'_>

impl ExactSizeIterator for U32Digits<'_>

impl ExactSizeIterator for U64Digits<'_>

impl ExactSizeIterator for serde_json::map::IntoIter

impl ExactSizeIterator for serde_json::map::IntoValues

impl<'a> ExactSizeIterator for IndexVecIter<'a>

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

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

impl<'a> ExactSizeIterator for serde_json::map::Iter<'a>

impl<'a> ExactSizeIterator for serde_json::map::IterMut<'a>

impl<'a> ExactSizeIterator for serde_json::map::Keys<'a>

impl<'a> ExactSizeIterator for serde_json::map::Values<'a>

impl<'a> ExactSizeIterator for serde_json::map::ValuesMut<'a>

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

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

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

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

impl<'a, K, V> ExactSizeIterator for lru::Iter<'a, K, V>

impl<'a, K, V> ExactSizeIterator for lru::IterMut<'a, K, V>

impl<'a, S, T> ExactSizeIterator for SliceChooseIter<'a, S, T>

where S: Index<usize, Output = T> + 'a + ?Sized, T: 'a,

Available on crate feature alloc only.

impl<'a, T> ExactSizeIterator for cairo_vm::with_std::slice::RChunksExact<'a, T>

impl<'a, T, O> ExactSizeIterator for ChunksExactMutNoAlias<'a, T, O>

where T: 'a + BitStore, O: BitOrder, ChunksExactMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for ChunksMutNoAlias<'a, T, O>

where T: 'a + BitStore, O: BitOrder, ChunksMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for IterMutNoAlias<'a, T, O>

where T: 'a + BitStore, O: BitOrder, IterMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for RChunksExactMutNoAlias<'a, T, O>

where T: 'a + BitStore, O: BitOrder, RChunksExactMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for RChunksMutNoAlias<'a, T, O>

where T: 'a + BitStore, O: BitOrder, RChunksMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for RSplitMutNoAlias<'a, T, O, P>

where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, RSplitMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for RSplitNMutNoAlias<'a, T, O, P>

where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, RSplitNMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitInclusiveMutNoAlias<'a, T, O, P>

where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitInclusiveMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitMutNoAlias<'a, T, O, P>

where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitNMutNoAlias<'a, T, O, P>

where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitNMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, const CAP: usize> ExactSizeIterator for arrayvec::arrayvec::Drain<'a, T, CAP>

where T: 'a,

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

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

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

impl<A> ExactSizeIterator for RepeatN<A>

where A: Clone,

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

where A: ExactSizeIterator, B: ExactSizeIterator,

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

where F: FnOnce() -> A,

impl<A, O> ExactSizeIterator for bitvec::array::iter::IntoIter<A, O>

where A: BitViewSized, O: BitOrder,

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

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

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

where F: FnMut() -> A,

impl<I> ExactSizeIterator for &mut I

where I: ExactSizeIterator + ?Sized,

impl<I> ExactSizeIterator for Bidi<I>

where I: DoubleEndedIterator + ExactSizeIterator,

impl<I> ExactSizeIterator for Enumerate<I>

where I: ExactSizeIterator,

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,

impl<I> ExactSizeIterator for Skip<I>

where I: ExactSizeIterator,

impl<I> ExactSizeIterator for StepBy<I>

where I: ExactSizeIterator,

impl<I> ExactSizeIterator for Take<I>

where I: ExactSizeIterator,

impl<I, A> ExactSizeIterator for cairo_vm::with_std::boxed::Box<I, A>

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

impl<I, A> ExactSizeIterator for cairo_vm::with_std::vec::Splice<'_, I, A>

where I: Iterator, A: Allocator,

impl<I, A> ExactSizeIterator for allocator_api2::stable::boxed::Box<I, A>

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

impl<I, A> ExactSizeIterator for allocator_api2::stable::vec::splice::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> ExactSizeIterator for hashbrown::set::Iter<'_, K>

impl<K, A> ExactSizeIterator for hashbrown::set::Drain<'_, K, A>

where A: Allocator,

impl<K, A> ExactSizeIterator for hashbrown::set::IntoIter<K, A>

where A: Allocator,

impl<K, V> ExactSizeIterator for alloc::collections::btree::map::Iter<'_, K, V>

impl<K, V> ExactSizeIterator for alloc::collections::btree::map::IterMut<'_, K, V>

impl<K, V> ExactSizeIterator for alloc::collections::btree::map::Keys<'_, K, V>

impl<K, V> ExactSizeIterator for alloc::collections::btree::map::Values<'_, K, V>

impl<K, V> ExactSizeIterator for alloc::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> ExactSizeIterator for hashbrown::map::Iter<'_, K, V>

impl<K, V> ExactSizeIterator for hashbrown::map::IterMut<'_, K, V>

impl<K, V> ExactSizeIterator for hashbrown::map::Keys<'_, K, V>

impl<K, V> ExactSizeIterator for hashbrown::map::Values<'_, K, V>

impl<K, V> ExactSizeIterator for hashbrown::map::ValuesMut<'_, K, V>

impl<K, V> ExactSizeIterator for lru::IntoIter<K, V>

where K: Hash + Eq,

impl<K, V, A> ExactSizeIterator for alloc::collections::btree::map::IntoIter<K, V, A>

where A: Allocator + Clone,

impl<K, V, A> ExactSizeIterator for alloc::collections::btree::map::IntoKeys<K, V, A>

where A: Allocator + Clone,

impl<K, V, A> ExactSizeIterator for alloc::collections::btree::map::IntoValues<K, V, A>

where A: Allocator + Clone,

impl<K, V, A> ExactSizeIterator for hashbrown::map::Drain<'_, K, V, A>

where A: Allocator,

impl<K, V, A> ExactSizeIterator for hashbrown::map::IntoIter<K, V, A>

where A: Allocator,

impl<K, V, A> ExactSizeIterator for hashbrown::map::IntoKeys<K, V, A>

where A: Allocator,

impl<K, V, A> ExactSizeIterator for hashbrown::map::IntoValues<K, V, A>

where A: Allocator,

impl<L, R> ExactSizeIterator for Either<L, R>

where L: ExactSizeIterator, R: ExactSizeIterator<Item = <L as Iterator>::Item>,

impl<L, R> ExactSizeIterator for IterEither<L, R>

where L: ExactSizeIterator, R: ExactSizeIterator,

impl<M, T, O> ExactSizeIterator for BitPtrRange<M, T, O>

where M: Mutability, T: BitStore, O: BitOrder,

impl<T> ExactSizeIterator for cairo_vm::with_std::result::IntoIter<T>

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

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

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::Chunks<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::ChunksExact<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::ChunksExactMut<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::ChunksMut<'_, T>

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

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

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::RChunks<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::RChunksExactMut<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::RChunksMut<'_, T>

impl<T> ExactSizeIterator for cairo_vm::with_std::slice::Windows<'_, T>

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

impl<T> ExactSizeIterator for alloc::collections::btree::set::Iter<'_, T>

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

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

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

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

impl<T> ExactSizeIterator for hashbrown::table::Iter<'_, T>

impl<T> ExactSizeIterator for hashbrown::table::IterMut<'_, T>

impl<T> ExactSizeIterator for Empty<T>

impl<T> ExactSizeIterator for Once<T>

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

where T: Clone,

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

where A: Allocator,

impl<T, A> ExactSizeIterator for cairo_vm::with_std::vec::IntoIter<T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for alloc::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 alloc::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 alloc::collections::btree::set::IntoIter<T, A>

where A: Allocator + Clone,

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

where A: Allocator,

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

where A: Allocator,

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

where A: Allocator,

impl<T, A> ExactSizeIterator for allocator_api2::stable::vec::drain::Drain<'_, T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for allocator_api2::stable::vec::into_iter::IntoIter<T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for hashbrown::table::Drain<'_, T, A>

where A: Allocator,

impl<T, A> ExactSizeIterator for hashbrown::table::IntoIter<T, A>

where A: Allocator,

impl<T, N> ExactSizeIterator for GenericArrayIter<T, N>

where N: ArrayLength<T>,

impl<T, O> ExactSizeIterator for Domain<'_, Const, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::boxed::iter::IntoIter<T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for BitValIter<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::Chunks<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::ChunksExact<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::ChunksExactMut<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::ChunksMut<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::Iter<'_, T, O>

where T: BitStore, O: BitOrder,

Original and Original

impl<T, O> ExactSizeIterator for bitvec::slice::iter::IterMut<'_, T, O>

where T: BitStore, O: BitOrder,

Original and Original

impl<T, O> ExactSizeIterator for IterOnes<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for IterZeros<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::RChunks<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::RChunksExact<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::RChunksExactMut<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::RChunksMut<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::slice::iter::Windows<'_, T, O>

where T: BitStore, O: BitOrder,

impl<T, O> ExactSizeIterator for bitvec::vec::iter::Drain<'_, T, O>

where T: BitStore, O: BitOrder,

Available on non-tarpaulin_include only.

Original

impl<T, O, I> ExactSizeIterator for bitvec::vec::iter::Splice<'_, T, O, I>

where T: BitStore, O: BitOrder, I: Iterator<Item = bool>,

Available on non-tarpaulin_include only.

impl<T, const CAP: usize> ExactSizeIterator for arrayvec::arrayvec::IntoIter<T, CAP>

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

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