Trait core::iter::ExactSizeIterator

pub trait ExactSizeIterator: Iterator {
    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.

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 counter = Counter::new();

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

Provided methods

fn len(&self) -> usize

Returns the exact 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 five = 0..5;

assert_eq!(5, five.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);

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Implementors

impl ExactSizeIterator for core::ascii::EscapeDefault

impl ExactSizeIterator for EscapeDebug

impl ExactSizeIterator for core::char::EscapeDefault

fn len(&self) -> usize

impl ExactSizeIterator for EscapeUnicode

fn len(&self) -> usize

impl ExactSizeIterator for ToLowercase

impl ExactSizeIterator for ToUppercase

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<'_>

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I> where
    I: ExactSizeIterator<Item = &'a T>,
    T: Clone, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<'a, I, T: 'a> ExactSizeIterator for Copied<I> where
    I: ExactSizeIterator<Item = &'a T>,
    T: Copy, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

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

fn is_empty(&self) -> bool

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

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, B> ExactSizeIterator for Zip<A, B> where
    A: ExactSizeIterator,
    B: ExactSizeIterator, 

impl<A, F: FnOnce() -> A> ExactSizeIterator for OnceWith<F>

fn len(&self) -> usize

impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F> where
    F: FnMut(I::Item) -> B, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<I> ExactSizeIterator for Enumerate<I> where
    I: ExactSizeIterator, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<I> ExactSizeIterator for Fuse<I> where
    I: ExactSizeIterator, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<I> ExactSizeIterator for Rev<I> where
    I: ExactSizeIterator + DoubleEndedIterator, 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

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: ExactSizeIterator + ?Sized> ExactSizeIterator for &mut I

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>

impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F> where
    F: FnMut(&I::Item), 

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

impl<T> ExactSizeIterator for Empty<T>

fn len(&self) -> usize

impl<T> ExactSizeIterator for Once<T>

fn len(&self) -> usize

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

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

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

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

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

fn is_empty(&self) -> bool

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

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

fn is_empty(&self) -> bool

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

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

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

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

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

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

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

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

fn is_empty(&self) -> bool

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

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

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

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

fn len(&self) -> usize

fn is_empty(&self) -> bool

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

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

fn is_empty(&self) -> bool

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

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

fn is_empty(&self) -> bool

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

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

fn is_empty(&self) -> bool

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

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