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 · Sourcefn len(&self) -> usize
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());Sourcefn is_empty(&self) -> bool
🔬This is a nightly-only experimental API. (exact_size_is_empty)
fn is_empty(&self) -> bool
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);Implementors§
impl ExactSizeIterator for IndexVecIntoIter
impl ExactSizeIterator for InstructionCodeIter
impl ExactSizeIterator for DecimalTypeVariantIter
impl ExactSizeIterator for IntegerTypeVariantIter
impl ExactSizeIterator for datex_core::stdlib::ascii::EscapeDefault
impl ExactSizeIterator for EscapeDebug
impl ExactSizeIterator for datex_core::stdlib::char::EscapeDefault
impl ExactSizeIterator for EscapeUnicode
impl ExactSizeIterator for ToLowercase
impl ExactSizeIterator for ToUppercase
impl ExactSizeIterator for Args
impl ExactSizeIterator for ArgsOs
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<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 Bytes<'_>
impl ExactSizeIterator for NaiveDateDaysIterator
impl ExactSizeIterator for NaiveDateWeeksIterator
impl ExactSizeIterator for ParamsIter<'_, '_, '_>
impl ExactSizeIterator for U32Digits<'_>
impl ExactSizeIterator for U64Digits<'_>
impl ExactSizeIterator for plist::dictionary::IntoIter
impl ExactSizeIterator for serde_json::map::IntoIter
impl ExactSizeIterator for serde_json::map::IntoValues
impl<'a> ExactSizeIterator for IndexVecIter<'a>
impl<'a> ExactSizeIterator for CommandArgs<'a>
impl<'a> ExactSizeIterator for CommandEnvs<'a>
impl<'a> ExactSizeIterator for BitStringIter<'a>
impl<'a> ExactSizeIterator for plist::dictionary::Iter<'a>
impl<'a> ExactSizeIterator for plist::dictionary::IterMut<'a>
impl<'a> ExactSizeIterator for plist::dictionary::Keys<'a>
impl<'a> ExactSizeIterator for plist::dictionary::Values<'a>
impl<'a> ExactSizeIterator for plist::dictionary::ValuesMut<'a>
impl<'a> ExactSizeIterator for CapturesPatternIter<'a>
impl<'a> ExactSizeIterator for GroupInfoPatternNames<'a>
impl<'a> ExactSizeIterator for serde_json::map::Iter<'a>
impl<'a> ExactSizeIterator for serde_json::map::IterMut<'a>
impl<'a> ExactSizeIterator for serde_json::map::Keys<'a>
impl<'a> ExactSizeIterator for serde_json::map::Values<'a>
impl<'a> ExactSizeIterator for serde_json::map::ValuesMut<'a>
impl<'a> ExactSizeIterator for Segments<'a>
impl<'a, A> ExactSizeIterator for arrayvec::Drain<'a, A>
impl<'a, B> ExactSizeIterator for Blocks<'a, B>where
B: BitBlock,
impl<'a, B> ExactSizeIterator for bit_vec::Iter<'a, B>where
B: BitBlock,
impl<'a, B> ExactSizeIterator for bit_vec::IterMut<'a, B>where
B: BitBlock,
impl<'a, I, T> ExactSizeIterator for Cloned<I>
impl<'a, I, T> ExactSizeIterator for Copied<I>
impl<'a, K> ExactSizeIterator for hashbrown::set::Iter<'a, K>
impl<'a, K, V> ExactSizeIterator for linked_hash_map::Drain<'a, K, V>
impl<'a, K, V> ExactSizeIterator for linked_hash_map::Iter<'a, K, V>
impl<'a, K, V> ExactSizeIterator for linked_hash_map::IterMut<'a, K, V>
impl<'a, K, V> ExactSizeIterator for linked_hash_map::Keys<'a, K, V>
impl<'a, K, V> ExactSizeIterator for linked_hash_map::Values<'a, K, V>
impl<'a, S, T> ExactSizeIterator for SliceChooseIter<'a, S, T>
Available on crate feature
alloc only.