Trait std::iter::ExactSizeIterator

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

知道其确切长度的迭代器。

许多 Iterator 不知道它们将迭代多少次，但是有些迭代器知道。 如果迭代器知道可以迭代多少次，则提供对该信息的访问将很有用。 例如，如果要向后迭代，一个好的开始就是知道终点在哪里。

实现 ExactSizeIterator 时，还必须实现 Iterator。 这样做时，Iterator::size_hint 的实现 必须 返回迭代器的确切大小。

len 方法具有默认实现，因此通常不应该实现它。 但是，您可能能够提供比默认设置更有效的实现，因此在这种情况下将其覆盖是有道理的。

请注意，此 trait 是安全的 trait，因此 not 和 cannot 不能保证返回的长度正确。 这意味着 unsafe 代码 ** 一定不要依赖 Iterator::size_hint 的正确性。 不稳定且不安全的 TrustedLen trait 提供了此额外的保证。

Examples

基本用法:

// 一个有限的范围确切地知道它将迭代多少次
let five = 0..5;

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

在 module-level docs 中，我们实现了 Iterator， Counter. 让我们也为其实现 ExactSizeIterator:

impl ExactSizeIterator for Counter {
    // 我们可以轻松计算剩余的迭代次数。
    fn len(&self) -> usize {
        5 - self.count
    }
}

// 现在我们可以使用它了!

let counter = Counter::new();

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

Provided methods

fn len(&self) -> usize

返回迭代器的确切长度。

该实现可确保迭代器在返回 None 之前，将返回 Some(T) 值的次数正好多于 len()。

此方法具有默认实现，因此通常不应直接实现它。 但是，如果您可以提供更有效的实现，则可以这样做。 有关示例，请参见 trait-level 文档。

该函数与 Iterator::size_hint 函数具有相同的安全保证。

Examples

基本用法:

// 一个有限的范围确切地知道它将迭代多少次
let five = 0..5;

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

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

如果迭代器为空，则返回 true。

此方法具有使用 ExactSizeIterator::len() 的默认实现，因此您无需自己实现。

Examples

基本用法:

#![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);

Implementations on Foreign Types

impl<'a> ExactSizeIterator for EscapeAscii<'a>

Implementors

impl ExactSizeIterator for std::ascii::EscapeDefault

impl ExactSizeIterator for EscapeDebug

impl ExactSizeIterator for std::char::EscapeDefault

pub fn len(&self) -> usize

impl ExactSizeIterator for EscapeUnicode

pub fn len(&self) -> usize

impl ExactSizeIterator for ToLowercase

impl ExactSizeIterator for ToUppercase

impl ExactSizeIterator for Args

fn len(&self) -> usize

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl ExactSizeIterator for ArgsOs

fn len(&self) -> usize

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

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 Bytes<'_>

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, A> ExactSizeIterator for std::option::Iter<'_, A>

impl<'_, A> ExactSizeIterator for std::option::IterMut<'_, A>

impl<'_, I> ExactSizeIterator for &'_ mut I where
    I: ExactSizeIterator + ?Sized, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, I, A> ExactSizeIterator for Splice<'_, I, A> where
    I: Iterator,
    A: Allocator, 

impl<'_, K, V> ExactSizeIterator for std::collections::btree_map::Iter<'_, K, V>

pub fn len(&self) -> usize

impl<'_, K, V> ExactSizeIterator for std::collections::btree_map::IterMut<'_, K, V>

pub fn len(&self) -> usize

impl<'_, K, V> ExactSizeIterator for std::collections::btree_map::Keys<'_, K, V>

pub fn len(&self) -> usize

impl<'_, K, V> ExactSizeIterator for std::collections::btree_map::Values<'_, K, V>

pub fn len(&self) -> usize

impl<'_, K, V> ExactSizeIterator for std::collections::btree_map::ValuesMut<'_, K, V>

pub fn len(&self) -> usize

impl<'_, T> ExactSizeIterator for std::collections::binary_heap::Drain<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for DrainSorted<'_, T> where
    T: Ord, 

impl<'_, T> ExactSizeIterator for std::collections::binary_heap::Iter<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for std::collections::btree_set::Iter<'_, T>

pub fn len(&self) -> usize

impl<'_, T> ExactSizeIterator for std::collections::linked_list::Iter<'_, T>

impl<'_, T> ExactSizeIterator for std::collections::linked_list::IterMut<'_, T>

impl<'_, T> ExactSizeIterator for std::collections::vec_deque::Drain<'_, T>

impl<'_, T> ExactSizeIterator for std::collections::vec_deque::Iter<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for std::collections::vec_deque::IterMut<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for std::result::Iter<'_, T>

impl<'_, T> ExactSizeIterator for std::result::IterMut<'_, T>

impl<'_, T> ExactSizeIterator for Chunks<'_, T>

impl<'_, T> ExactSizeIterator for ChunksExact<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for ChunksExactMut<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for ChunksMut<'_, T>

impl<'_, T> ExactSizeIterator for std::slice::Iter<'_, T>

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for std::slice::IterMut<'_, T>

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for RChunks<'_, T>

impl<'_, T> ExactSizeIterator for RChunksExactMut<'_, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T> ExactSizeIterator for RChunksMut<'_, T>

impl<'_, T> ExactSizeIterator for Windows<'_, T>

impl<'_, T, A> ExactSizeIterator for std::vec::Drain<'_, T, A> where
    A: Allocator, 

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T, const N: usize> ExactSizeIterator for ArrayChunks<'_, T, N>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T, const N: usize> ExactSizeIterator for ArrayChunksMut<'_, T, N>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'_, T, const N: usize> ExactSizeIterator for ArrayWindows<'_, T, N>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'a> ExactSizeIterator for CommandArgs<'a>

fn len(&self) -> usize

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'a> ExactSizeIterator for CommandEnvs<'a>

fn len(&self) -> usize

fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

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

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

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

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<'a, T> ExactSizeIterator for RChunksExact<'a, T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<A> ExactSizeIterator for std::option::IntoIter<A>

impl<A, B> ExactSizeIterator for Zip<A, B> where
    B: ExactSizeIterator,
    A: ExactSizeIterator, 

impl<A, F> ExactSizeIterator for OnceWith<F> where
    F: FnOnce() -> A, 

pub fn len(&self) -> usize

impl<B, I, F> ExactSizeIterator for Map<I, F> where
    F: FnMut(<I as Iterator>::Item) -> B,
    I: ExactSizeIterator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<I> ExactSizeIterator for Enumerate<I> where
    I: ExactSizeIterator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<I> ExactSizeIterator for Fuse<I> where
    I: ExactSizeIterator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<I> ExactSizeIterator for Peekable<I> where
    I: ExactSizeIterator, 

impl<I> ExactSizeIterator for Rev<I> where
    I: ExactSizeIterator + DoubleEndedIterator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<I> ExactSizeIterator for Skip<I> where
    I: ExactSizeIterator, 

impl<I> ExactSizeIterator for StepBy<I> where
    I: ExactSizeIterator, 

impl<I> ExactSizeIterator for Take<I> where
    I: ExactSizeIterator, 

impl<I, A> ExactSizeIterator for Box<I, A> where
    I: ExactSizeIterator + ?Sized,
    A: Allocator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<I, F> ExactSizeIterator for Inspect<I, F> where
    F: FnMut(&<I as Iterator>::Item),
    I: ExactSizeIterator, 

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<K> ExactSizeIterator for std::collections::hash_set::Drain<'_, K>

fn len(&self) -> usize

impl<K> ExactSizeIterator for std::collections::hash_set::IntoIter<K>

fn len(&self) -> usize

impl<K> ExactSizeIterator for std::collections::hash_set::Iter<'_, K>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::btree_map::IntoIter<K, V>

pub fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::btree_map::IntoKeys<K, V>

pub fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::btree_map::IntoValues<K, V>

pub fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::Drain<'_, K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoIter<K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoKeys<K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::IntoValues<K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::Iter<'_, K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::IterMut<'_, K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::Keys<'_, K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::Values<'_, K, V>

fn len(&self) -> usize

impl<K, V> ExactSizeIterator for std::collections::hash_map::ValuesMut<'_, K, V>

fn len(&self) -> usize

impl<T> ExactSizeIterator for std::collections::binary_heap::IntoIter<T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<T> ExactSizeIterator for IntoIterSorted<T> where
    T: Ord, 

impl<T> ExactSizeIterator for std::collections::btree_set::IntoIter<T>

pub fn len(&self) -> usize

impl<T> ExactSizeIterator for std::collections::linked_list::IntoIter<T>

impl<T> ExactSizeIterator for std::collections::vec_deque::IntoIter<T>

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<T> ExactSizeIterator for std::result::IntoIter<T>

impl<T> ExactSizeIterator for Empty<T>

pub fn len(&self) -> usize

impl<T> ExactSizeIterator for Once<T>

pub fn len(&self) -> usize

impl<T, A> ExactSizeIterator for std::vec::IntoIter<T, A> where
    A: Allocator, 

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)

impl<T, const N: usize> ExactSizeIterator for std::array::IntoIter<T, N>

pub fn len(&self) -> usize

pub fn is_empty(&self) -> bool

🔬 This is a nightly-only experimental API. (exact_size_is_empty #35428)