# Trait std::iter::ExactSizeIterator [−] [src]

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

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:

fn main() { // a finite range knows exactly how many times it will iterate let five = 0..5; assert_eq!(5, five.len()); }

    // 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:

fn main() { struct Counter { count: usize, } impl Counter { fn new() -> Counter { Counter { count: 0 } } } impl Iterator for Counter { type Item = usize; fn next(&mut self) -> Option<usize> { self.count += 1; if self.count < 6 { Some(self.count) } else { None } } } impl ExactSizeIterator for Counter { // We already have the number of iterations, so we can use it directly. fn len(&self) -> usize { self.count } } // And now we can use it! let counter = Counter::new(); assert_eq!(0, counter.len()); }

    impl ExactSizeIterator for Counter {
        // We already have the number of iterations, so we can use it directly.
        fn len(&self) -> usize {
            self.count
        }
    }

    // And now we can use it!

    let counter = Counter::new();

    assert_eq!(0, counter.len());

## 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:

fn main() { // a finite range knows exactly how many times it will iterate let five = 0..5; assert_eq!(5, five.len()); }

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

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

## Implementors

- `impl<I> ExactSizeIterator for Rev<I> where I: ExactSizeIterator + DoubleEndedIterator`
- `impl<'a, I, T> ExactSizeIterator for Cloned<I> where T: 'a + Clone, I: ExactSizeIterator<Item=&'a T>`
- `impl<A, B> ExactSizeIterator for Zip<A, B> where A: ExactSizeIterator, B: ExactSizeIterator`
- `impl<B, I, F> ExactSizeIterator for Map<I, F> where F: FnMut(I::Item) -> B, I: ExactSizeIterator`
- `impl<I> ExactSizeIterator for Enumerate<I> where I: ExactSizeIterator`
- `impl<I> ExactSizeIterator for Peekable<I> where I: ExactSizeIterator`
- `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, F> ExactSizeIterator for Inspect<I, F> where I: ExactSizeIterator, F: FnMut(&I::Item) -> ()`
- `impl<T> ExactSizeIterator for Empty<T>`
- `impl<T> ExactSizeIterator for Once<T>`
- `impl ExactSizeIterator for Range<i32>`
- `impl ExactSizeIterator for Range<i16>`
- `impl ExactSizeIterator for Range<i8>`
- `impl ExactSizeIterator for Range<isize>`
- `impl ExactSizeIterator for Range<u32>`
- `impl ExactSizeIterator for Range<u16>`
- `impl ExactSizeIterator for Range<u8>`
- `impl ExactSizeIterator for Range<usize>`
- `impl ExactSizeIterator for RangeInclusive<i32>`
- `impl ExactSizeIterator for RangeInclusive<i16>`
- `impl ExactSizeIterator for RangeInclusive<i8>`
- `impl ExactSizeIterator for RangeInclusive<isize>`
- `impl ExactSizeIterator for RangeInclusive<u32>`
- `impl ExactSizeIterator for RangeInclusive<u16>`
- `impl ExactSizeIterator for RangeInclusive<u8>`
- `impl ExactSizeIterator for RangeInclusive<usize>`
- `impl<'a, T> ExactSizeIterator for Iter<'a, T>`
- `impl<'a, T> ExactSizeIterator for IterMut<'a, T>`
- `impl<T> ExactSizeIterator for IntoIter<T>`
- `impl<'a, A> ExactSizeIterator for Iter<'a, A>`
- `impl<'a, A> ExactSizeIterator for IterMut<'a, A>`
- `impl<A> ExactSizeIterator for IntoIter<A>`
- `impl<I> ExactSizeIterator for Box<I> where I: ExactSizeIterator + ?Sized`
- `impl<'a, T> ExactSizeIterator for Chunks<'a, T>`
- `impl<'a, T> ExactSizeIterator for Iter<'a, T>`
- `impl<'a, T> ExactSizeIterator for ChunksMut<'a, T>`
- `impl<'a, T> ExactSizeIterator for IterMut<'a, T>`
- `impl<'a, T> ExactSizeIterator for Windows<'a, T>`
- `impl<'a> ExactSizeIterator for Bytes<'a>`
- `impl<T> ExactSizeIterator for IntoIter<T>`
- `impl<'a, T> ExactSizeIterator for Drain<'a, T>`
- `impl ExactSizeIterator for EscapeDefault`
- `impl<'a, T> ExactSizeIterator for Iter<'a, T>`
- `impl<T> ExactSizeIterator for IntoIter<T>`
- `impl<'a, T> ExactSizeIterator for Drain<'a, T> where T: 'a`
- `impl<'a, K, V> ExactSizeIterator for Keys<'a, K, V>`
- `impl<K, V> ExactSizeIterator for IntoIter<K, V>`
- `impl<'a, K, V> ExactSizeIterator for Values<'a, K, V>`
- `impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V> where V: 'a, K: 'a`
- `impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V> where V: 'a, K: 'a`
- `impl<'a, T> ExactSizeIterator for Iter<'a, T>`
- `impl<T> ExactSizeIterator for IntoIter<T>`
- `impl<'a, A> ExactSizeIterator for Iter<'a, A>`
- `impl<'a, A> ExactSizeIterator for IterMut<'a, A>`
- `impl<A> ExactSizeIterator for IntoIter<A>`
- `impl<'a, T> ExactSizeIterator for Iter<'a, T>`
- `impl<'a, T> ExactSizeIterator for IterMut<'a, T>`
- `impl<T> ExactSizeIterator for IntoIter<T>`
- `impl<'a, T> ExactSizeIterator for Drain<'a, T> where T: 'a`
- `impl<'a, K, V> ExactSizeIterator for Iter<'a, K, V>`
- `impl<'a, K, V> ExactSizeIterator for IterMut<'a, K, V>`
- `impl<K, V> ExactSizeIterator for IntoIter<K, V>`
- `impl<'a, K, V> ExactSizeIterator for Keys<'a, K, V>`
- `impl<'a, K, V> ExactSizeIterator for Values<'a, K, V>`
- `impl<'a, K, V> ExactSizeIterator for Drain<'a, K, V>`
- `impl<'a, K> ExactSizeIterator for Iter<'a, K>`
- `impl<K> ExactSizeIterator for IntoIter<K>`
- `impl<'a, K> ExactSizeIterator for Drain<'a, K>`
- `impl ExactSizeIterator for Args`
- `impl ExactSizeIterator for ArgsOs`

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