std::iter::ExactSizeIterator

Trait std::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 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());

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

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

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

#![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);

Implementors

• impl<'a, T> ExactSizeIterator for std::collections::binary_heap::Iter<'a, T>
• impl<T> ExactSizeIterator for std::collections::binary_heap::IntoIter<T>
• impl<'a, T> ExactSizeIterator for std::collections::binary_heap::Drain<'a, T> where
    T: 'a, 
• impl<'a, K, V> ExactSizeIterator for std::collections::btree_map::Iter<'a, K, V> where
    K: 'a,
    V: 'a, 
• impl<'a, K, V> ExactSizeIterator for std::collections::btree_map::IterMut<'a, K, V> where
    K: 'a,
    V: 'a, 
• impl<K, V> ExactSizeIterator for std::collections::btree_map::IntoIter<K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::btree_map::Keys<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::btree_map::Values<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::btree_map::ValuesMut<'a, K, V>
• impl<'a, T> ExactSizeIterator for std::collections::btree_set::Iter<'a, T>
• impl<T> ExactSizeIterator for std::collections::btree_set::IntoIter<T>
• impl<'a, T> ExactSizeIterator for std::collections::linked_list::Iter<'a, T>
• impl<'a, T> ExactSizeIterator for std::collections::linked_list::IterMut<'a, T>
• impl<T> ExactSizeIterator for std::collections::linked_list::IntoIter<T>
• impl<T> ExactSizeIterator for std::vec::IntoIter<T>
• impl<'a, T> ExactSizeIterator for std::vec::Drain<'a, T>
• impl<'a, T> ExactSizeIterator for std::collections::vec_deque::Iter<'a, T>
• impl<'a, T> ExactSizeIterator for std::collections::vec_deque::IterMut<'a, T>
• impl<T> ExactSizeIterator for std::collections::vec_deque::IntoIter<T>
• impl<'a, T> ExactSizeIterator for std::collections::vec_deque::Drain<'a, T> where
    T: 'a, 
• impl<I> ExactSizeIterator for Box<I> where
    I: ExactSizeIterator + ?Sized, 
• impl ExactSizeIterator for EscapeUnicode
• impl ExactSizeIterator for std::char::EscapeDefault
• impl ExactSizeIterator for EscapeDebug
• impl<T> ExactSizeIterator for Empty<T>
• impl<T> ExactSizeIterator for Once<T>
• impl<'a, I> ExactSizeIterator for &'a mut I where
    I: ExactSizeIterator + ?Sized, 
• impl<I> ExactSizeIterator for Rev<I> where
    I: ExactSizeIterator + DoubleEndedIterator, 
• impl<'a, I, T> ExactSizeIterator for Cloned<I> where
    I: ExactSizeIterator<Item = &'a T>,
    T: 'a + Clone, 
• 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 as Iterator>::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
    F: FnMut(&<I as Iterator>::Item) -> (),
    I: ExactSizeIterator, 
• impl<'a, A> ExactSizeIterator for std::option::Iter<'a, A>
• impl<'a, A> ExactSizeIterator for std::option::IterMut<'a, A>
• impl<A> ExactSizeIterator for std::option::IntoIter<A>
• impl<'a, T> ExactSizeIterator for std::result::Iter<'a, T>
• impl<'a, T> ExactSizeIterator for std::result::IterMut<'a, T>
• impl<T> ExactSizeIterator for std::result::IntoIter<T>
• impl<'a, T> ExactSizeIterator for std::slice::Iter<'a, T>
• impl<'a, T> ExactSizeIterator for std::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>
• 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 ExactSizeIterator for std::ascii::EscapeDefault
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::Iter<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::IterMut<'a, K, V>
• impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoIter<K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::Keys<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::Values<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::ValuesMut<'a, K, V>
• impl<'a, K, V> ExactSizeIterator for std::collections::hash_map::Drain<'a, K, V>
• impl<'a, K> ExactSizeIterator for std::collections::hash_set::Iter<'a, K>
• impl<K> ExactSizeIterator for std::collections::hash_set::IntoIter<K>
• impl<'a, K> ExactSizeIterator for std::collections::hash_set::Drain<'a, K>
• impl ExactSizeIterator for Args
• impl ExactSizeIterator for ArgsOs