# Trait std::iter::ExactSizeIterator

1.0.0 · source ·

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

Expand description

An iterator that knows its exact length.

Many `Iterator`s 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());

Run

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());

Run

## Provided Methods§

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#### 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());

Run
source

#### 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);

Run

## Implementors§

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### impl ExactSizeIterator for std::ascii::EscapeDefault

1.20.0 · source§

### impl ExactSizeIterator for EscapeDebug

1.11.0 · source§

### impl ExactSizeIterator for std::char::EscapeDefault

1.11.0 · source§

### impl ExactSizeIterator for EscapeUnicode

1.35.0 · source§

### impl ExactSizeIterator for ToLowercase

1.35.0 · source§

### impl ExactSizeIterator for ToUppercase

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### impl ExactSizeIterator for Args

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### impl ExactSizeIterator for ArgsOs

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### impl ExactSizeIterator for Range<i8>

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### impl ExactSizeIterator for Range<i16>

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### impl ExactSizeIterator for Range<i32>

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### impl ExactSizeIterator for Range<isize>

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### impl ExactSizeIterator for Range<u8>

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### impl ExactSizeIterator for Range<u16>

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### impl ExactSizeIterator for Range<u32>

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### impl ExactSizeIterator for Range<usize>

1.26.0 · source§

### impl ExactSizeIterator for RangeInclusive<i8>

1.26.0 · source§

### impl ExactSizeIterator for RangeInclusive<i16>

1.26.0 · source§

### impl ExactSizeIterator for RangeInclusive<u8>

1.26.0 · source§

### impl ExactSizeIterator for RangeInclusive<u16>

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### impl ExactSizeIterator for Bytes<'_>

1.57.0 · source§

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

1.57.0 · source§

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

1.1.0 · source§

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

1.36.0 · source§

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

1.31.0 · source§

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

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

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

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

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

1.43.0 · source§

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

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

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### impl<I> ExactSizeIterator for &mut I where I: ExactSizeIterator + ?Sized,

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

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

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### impl<I> ExactSizeIterator for Peekable<I>where I: ExactSizeIterator,

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

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### impl<I> ExactSizeIterator for Skip<I>where I: ExactSizeIterator,

1.28.0 · source§

### impl<I> ExactSizeIterator for StepBy<I>where I: ExactSizeIterator,

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### impl<I> ExactSizeIterator for Take<I>where I: ExactSizeIterator,

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### impl<I, A> ExactSizeIterator for Box<I, A>where I: ExactSizeIterator + ?Sized, A: Allocator,

1.21.0 · source§

### impl<I, A> ExactSizeIterator for Splice<'_, I, A>where I: Iterator, A: Allocator,

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

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### impl<I, F, R, const N: usize> ExactSizeIterator for MapWindows<I, F, N>where I: ExactSizeIterator, F: FnMut(&[<I as Iterator>::Item; N]) -> R,

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### impl<I, const N: usize> ExactSizeIterator for std::iter::ArrayChunks<I, N>where I: ExactSizeIterator,

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

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

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

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

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

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

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

1.10.0 · source§

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

1.6.0 · source§

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

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

1.54.0 · source§

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

1.54.0 · source§

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

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

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

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

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

1.10.0 · source§

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

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### impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoIter<K, V, A>where A: Allocator + Clone,

1.54.0 · source§

### impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoKeys<K, V, A>where A: Allocator + Clone,

1.54.0 · source§

### impl<K, V, A> ExactSizeIterator for std::collections::btree_map::IntoValues<K, V, A>where A: Allocator + Clone,

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

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

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

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

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

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

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

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

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

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

1.31.0 · source§

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

1.31.0 · source§

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

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

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

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

1.31.0 · source§

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

1.31.0 · source§

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

1.31.0 · source§

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

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

1.2.0 · source§

### impl<T> ExactSizeIterator for Empty<T>

1.2.0 · source§

### impl<T> ExactSizeIterator for Once<T>

1.6.0 · source§

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

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### impl<T, A> ExactSizeIterator for DrainSorted<'_, T, A>where T: Ord, A: Allocator,

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

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### impl<T, A> ExactSizeIterator for IntoIterSorted<T, A>where T: Ord, A: Allocator,

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### impl<T, A> ExactSizeIterator for std::collections::btree_set::IntoIter<T, A>where A: Allocator + Clone,

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

1.6.0 · source§

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

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

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

1.6.0 · source§

### impl<T, A, FH> ExactSizeIterator for std::vec::Drain<'_, T, A, FH>where A: Allocator, FH: FailureHandling,

1.40.0 · source§

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

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

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

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