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:
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: