# Trait core::iter::ExactSizeIterator 1.0.0 [−] [src]

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

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 `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.

# 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 counter = Counter::new();

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

## Provided Methods

### `fn len(&self) -> usize`

Returns the exact number of times the iterator will iterate.

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 `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());Run

### `fn is_empty(&self) -> bool`

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

Returns whether the iterator is empty.

This method has a default implementation using `self.len()`, so you don't need to implement it yourself.

# Examples

Basic usage:

    #![feature(exact_size_is_empty)]

    let mut one_element = 0..1;
    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

- `impl ExactSizeIterator for EscapeUnicode`
- `impl ExactSizeIterator for EscapeDefault`
- `impl ExactSizeIterator for EscapeDebug`
- `impl ExactSizeIterator for Range<usize>`
- `impl ExactSizeIterator for Range<u8>`
- `impl ExactSizeIterator for Range<u16>`
- `impl ExactSizeIterator for Range<u32>`
- `impl ExactSizeIterator for Range<isize>`
- `impl ExactSizeIterator for Range<i8>`
- `impl ExactSizeIterator for Range<i16>`
- `impl ExactSizeIterator for Range<i32>`
- `impl ExactSizeIterator for RangeInclusive<u8>`
- `impl ExactSizeIterator for RangeInclusive<u16>`
- `impl ExactSizeIterator for RangeInclusive<i8>`
- `impl ExactSizeIterator for RangeInclusive<i16>`
- `impl<T> ExactSizeIterator for Empty<T>`
- `impl<T> ExactSizeIterator for Once<T>`
- `impl<'a, I: ExactSizeIterator + ?Sized> ExactSizeIterator for &'a mut I`
- `impl<I> ExactSizeIterator for Rev<I> where
  I: ExactSizeIterator + DoubleEndedIterator,`
- `impl<'a, I, T: 'a> ExactSizeIterator for Cloned<I> where
  I: ExactSizeIterator<Item = &'a T>,
  T: Clone,`
- `impl<A, B> ExactSizeIterator for Zip<A, B> where
  A: ExactSizeIterator,
  B: ExactSizeIterator,`
- `impl<B, I: ExactSizeIterator, F> ExactSizeIterator for Map<I, F> where
  F: FnMut(I::Item) -> B,`
- `impl<I> ExactSizeIterator for Enumerate<I> where
  I: ExactSizeIterator,`
- `impl<I: ExactSizeIterator> ExactSizeIterator for Peekable<I>`
- `impl<I> ExactSizeIterator for Skip<I> where
  I: ExactSizeIterator,`
- `impl<I> ExactSizeIterator for Take<I> where
  I: ExactSizeIterator,`
- `impl<I> ExactSizeIterator for Fuse<I> where
  I: ExactSizeIterator,`
- `impl<I: ExactSizeIterator, F> ExactSizeIterator for Inspect<I, F> where
  F: FnMut(&I::Item),`
- `impl<'a, A> ExactSizeIterator for core::option::Iter<'a, A>`
- `impl<'a, A> ExactSizeIterator for core::option::IterMut<'a, A>`
- `impl<A> ExactSizeIterator for core::option::IntoIter<A>`
- `impl<'a, T> ExactSizeIterator for core::result::Iter<'a, T>`
- `impl<'a, T> ExactSizeIterator for core::result::IterMut<'a, T>`
- `impl<T> ExactSizeIterator for core::result::IntoIter<T>`
- `impl<'a, T> ExactSizeIterator for core::slice::Iter<'a, T>`
- `impl<'a, T> ExactSizeIterator for core::slice::IterMut<'a, T>`
- `impl<'a, T> ExactSizeIterator for Windows<'a, T>`
- `impl<'a, T> ExactSizeIterator for Chunks<'a, T>`
- `impl<'a, T> ExactSizeIterator for ChunksMut<'a, T>`
- `impl<'a> ExactSizeIterator for Bytes<'a>`

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