Trait 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 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.

§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());

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());

Provided Methods§

1.0.0 · Source

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());

Source

fn is_empty(&self) -> bool

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

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);

Dyn Compatibility§

This trait is dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Implementors§

1.0.0 · Source§

impl ExactSizeIterator for Args

1.0.0 · Source§

impl ExactSizeIterator for ArgsOs

1.0.0 · Source§

ExactSizeIterator

Trait ExactSizeIterator Copy item path

§When shouldn’t an adapter be ExactSizeIterator?

§Examples

Provided Methods§

fn len(&self) -> usize

§Examples

fn is_empty(&self) -> bool

§Examples

Dyn Compatibility§

Implementors§

impl ExactSizeIterator for Args

impl ExactSizeIterator for ArgsOs

impl ExactSizeIterator for core::str::iter::Bytes<'_>

impl ExactSizeIterator for DirEventIter

impl ExactSizeIterator for EscapeDebug

impl ExactSizeIterator for core::ascii::EscapeDefault

impl ExactSizeIterator for core::char::EscapeDefault

impl ExactSizeIterator for EscapeUnicode

impl ExactSizeIterator for rand::seq::index_::IndexVecIntoIter

impl ExactSizeIterator for rand::seq::index_::IndexVecIntoIter

impl ExactSizeIterator for rand::seq::index_::IndexVecIter<'_>

impl ExactSizeIterator for rand::seq::index_::IndexVecIter<'_>

impl ExactSizeIterator for serde_json::map::IntoIter

impl ExactSizeIterator for toml::map::IntoIter

impl ExactSizeIterator for serde_json::map::IntoValues

impl ExactSizeIterator for Range<NonZero<u8>>

impl ExactSizeIterator for Range<NonZero<u16>>

impl ExactSizeIterator for Range<NonZero<usize>>

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<NonZero<u8>>

impl ExactSizeIterator for RangeInclusive<NonZero<u16>>

impl ExactSizeIterator for RangeInclusive<NonZero<usize>>

impl ExactSizeIterator for RangeInclusive<i8>

impl ExactSizeIterator for RangeInclusive<i16>

impl ExactSizeIterator for RangeInclusive<u8>

impl ExactSizeIterator for RangeInclusive<u16>

impl ExactSizeIterator for RangeInclusiveIter<i8>

impl ExactSizeIterator for RangeInclusiveIter<u8>

impl ExactSizeIterator for RangeIter<i8>

impl ExactSizeIterator for RangeIter<i16>

impl ExactSizeIterator for RangeIter<isize>

impl ExactSizeIterator for RangeIter<u8>

impl ExactSizeIterator for RangeIter<u16>

impl ExactSizeIterator for RangeIter<usize>

impl ExactSizeIterator for RelayIdTypeIter

impl ExactSizeIterator for SchedulingPolicyIter

impl ExactSizeIterator for ToCasefold

impl ExactSizeIterator for ToLowercase

impl ExactSizeIterator for ToTitlecase

impl ExactSizeIterator for ToUppercase

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

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

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Drain<'a, K, V>where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Drain<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Drain<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Drain<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Drain<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for phf::map::Entries<'a, K, V>

impl<'a, K, V> ExactSizeIterator for phf::ordered_map::Entries<'a, K, V>

impl<'a, K, V> ExactSizeIterator for toml::map::Iter<'a, K, V>

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Iter<'a, K, V>where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for enum_map::iter::Iter<'a, K, V>where K: EnumArray<V>,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Iter<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Iter<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Iter<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Iter<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for toml::map::IterMut<'a, K, V>

impl<'a, K, V> ExactSizeIterator for slotmap::dense::IterMut<'a, K, V>where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for enum_map::iter::IterMut<'a, K, V>where K: EnumArray<V>,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::IterMut<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::IterMut<'a, K, V>where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::IterMut<'a, K, V>where K: Key,

Trait ExactSizeIterator

§When shouldn’t an adapter be `ExactSizeIterator`?

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

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

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Drain<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Drain<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Drain<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Drain<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Drain<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Iter<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for enum_map::iter::Iter<'a, K, V>
where K: EnumArray<V>,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Iter<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Iter<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Iter<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Iter<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::dense::IterMut<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for enum_map::iter::IterMut<'a, K, V>
where K: EnumArray<V>,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::IterMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::IterMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::IterMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::IterMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Keys<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Keys<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Keys<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Keys<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Keys<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::dense::Values<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::Values<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::Values<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::Values<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::Values<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::dense::ValuesMut<'a, K, V>
where K: 'a + Key,

impl<'a, K, V> ExactSizeIterator for slotmap::basic::ValuesMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::hop::ValuesMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::secondary::ValuesMut<'a, K, V>
where K: Key,

impl<'a, K, V> ExactSizeIterator for slotmap::sparse_secondary::ValuesMut<'a, K, V>
where K: Key,

impl<'a, S, T> ExactSizeIterator for IndexedSamples<'a, S, T>
where S: Index<usize, Output = T> + 'a + ?Sized, T: 'a,

impl<'a, S, T> ExactSizeIterator for SliceChooseIter<'a, S, T>
where S: Index<usize, Output = T> + 'a + ?Sized, T: 'a,

impl<'a, T, F> ExactSizeIterator for VarZeroSliceIter<'a, T, F>
where T: VarULE + ?Sized, F: VarZeroVecFormat,

impl<'a, T, O, P> ExactSizeIterator for RSplitMutNoAlias<'a, T, O, P>
where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, RSplitMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for RSplitNMutNoAlias<'a, T, O, P>
where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, RSplitNMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitInclusiveMutNoAlias<'a, T, O, P>
where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitInclusiveMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitMutNoAlias<'a, T, O, P>
where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O, P> ExactSizeIterator for SplitNMutNoAlias<'a, T, O, P>
where T: 'a + BitStore, O: BitOrder, P: FnMut(usize, &bool) -> bool, SplitNMut<'a, T, O, P>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for ChunksExactMutNoAlias<'a, T, O>
where T: 'a + BitStore, O: BitOrder, ChunksExactMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for ChunksMutNoAlias<'a, T, O>
where T: 'a + BitStore, O: BitOrder, ChunksMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for IterMutNoAlias<'a, T, O>
where T: 'a + BitStore, O: BitOrder, IterMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for RChunksExactMutNoAlias<'a, T, O>
where T: 'a + BitStore, O: BitOrder, RChunksExactMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, O> ExactSizeIterator for RChunksMutNoAlias<'a, T, O>
where T: 'a + BitStore, O: BitOrder, RChunksMut<'a, T, O>: ExactSizeIterator,

impl<'a, T, const CAP: usize> ExactSizeIterator for arrayvec::arrayvec::Drain<'a, T, CAP>
where T: 'a,

impl<'a, T> ExactSizeIterator for smallvec::Drain<'a, T>
where T: Array,

impl<'a, T> ExactSizeIterator for ListByRelayIdsIter<'a, T>
where Iter<'a, usize>: ExactSizeIterator,

impl<'a, T> ExactSizeIterator for ZeroSliceIter<'a, T>
where T: AsULE,

impl<'a, V> ExactSizeIterator for enum_map::iter::Values<'a, V>
where V: 'a,

impl<'a, V> ExactSizeIterator for enum_map::iter::ValuesMut<'a, V>
where V: 'a,