library/core/src/iter/traits/exact_size.rs

   1 /// An iterator that knows its exact length.
   2 ///
   3 /// Many [`Iterator`]s don't know how many times they will iterate, but some do.
   4 /// If an iterator knows how many times it can iterate, providing access to
   5 /// that information can be useful. For example, if you want to iterate
   6 /// backwards, a good start is to know where the end is.
   7 ///
   8 /// When implementing an `ExactSizeIterator`, you must also implement
   9 /// [`Iterator`]. When doing so, the implementation of [`Iterator::size_hint`]
  10 /// *must* return the exact size of the iterator.
  11 ///
  12 /// The [`len`] method has a default implementation, so you usually shouldn't
  13 /// implement it. However, you may be able to provide a more performant
  14 /// implementation than the default, so overriding it in this case makes sense.
  15 ///
  16 /// Note that this trait is a safe trait and as such does *not* and *cannot*
  17 /// guarantee that the returned length is correct. This means that `unsafe`
  18 /// code **must not** rely on the correctness of [`Iterator::size_hint`]. The
  19 /// unstable and unsafe [`TrustedLen`](super::marker::TrustedLen) trait gives
  20 /// this additional guarantee.
  21 ///
  22 /// [`len`]: ExactSizeIterator::len
  23 ///
  24 /// # Examples
  25 ///
  26 /// Basic usage:
  27 ///
  28 /// ```
  29 /// // a finite range knows exactly how many times it will iterate
  30 /// let five = 0..5;
  31 ///
  32 /// assert_eq!(5, five.len());
  33 /// ```
  34 ///
  35 /// In the [module-level docs], we implemented an [`Iterator`], `Counter`.
  36 /// Let's implement `ExactSizeIterator` for it as well:
  37 ///
  38 /// [module-level docs]: crate::iter
  39 ///
  40 /// ```
  41 /// # struct Counter {
  42 /// #     count: usize,
  43 /// # }
  44 /// # impl Counter {
  45 /// #     fn new() -> Counter {
  46 /// #         Counter { count: 0 }
  47 /// #     }
  48 /// # }
  49 /// # impl Iterator for Counter {
  50 /// #     type Item = usize;
  51 /// #     fn next(&mut self) -> Option<Self::Item> {
  52 /// #         self.count += 1;
  53 /// #         if self.count < 6 {
  54 /// #             Some(self.count)
  55 /// #         } else {
  56 /// #             None
  57 /// #         }
  58 /// #     }
  59 /// # }
  60 /// impl ExactSizeIterator for Counter {
  61 ///     // We can easily calculate the remaining number of iterations.
  62 ///     fn len(&self) -> usize {
  63 ///         5 - self.count
  64 ///     }
  65 /// }
  66 ///
  67 /// // And now we can use it!
  68 ///
  69 /// let counter = Counter::new();
  70 ///
  71 /// assert_eq!(5, counter.len());
  72 /// ```
  73 #[stable(feature = "rust1", since = "1.0.0")]
  74 pub trait ExactSizeIterator: Iterator {
  75     /// Returns the exact length of the iterator.
  76     ///
  77     /// The implementation ensures that the iterator will return exactly `len()`
  78     /// more times a [`Some(T)`] value, before returning [`None`].
  79     /// This method has a default implementation, so you usually should not
  80     /// implement it directly. However, if you can provide a more efficient
  81     /// implementation, you can do so. See the [trait-level] docs for an
  82     /// example.
  83     ///
  84     /// This function has the same safety guarantees as the
  85     /// [`Iterator::size_hint`] function.
  86     ///
  87     /// [trait-level]: ExactSizeIterator
  88     /// [`Some(T)`]: Some
  89     ///
  90     /// # Examples
  91     ///
  92     /// Basic usage:
  93     ///
  94     /// ```
  95     /// // a finite range knows exactly how many times it will iterate
  96     /// let five = 0..5;
  97     ///
  98     /// assert_eq!(5, five.len());
  99     /// ```
 100     #[inline]
 101     #[stable(feature = "rust1", since = "1.0.0")]
 102     fn len(&self) -> usize {
 103         let (lower, upper) = self.size_hint();
 104         // Note: This assertion is overly defensive, but it checks the invariant
 105         // guaranteed by the trait. If this trait were rust-internal,
 106         // we could use debug_assert!; assert_eq! will check all Rust user
 107         // implementations too.
 108         assert_eq!(upper, Some(lower));
 109         lower
 110     }
 111
 112     /// Returns `true` if the iterator is empty.
 113     ///
 114     /// This method has a default implementation using
 115     /// [`ExactSizeIterator::len()`], so you don't need to implement it yourself.
 116     ///
 117     /// # Examples
 118     ///
 119     /// Basic usage:
 120     ///
 121     /// ```
 122     /// #![feature(exact_size_is_empty)]
 123     ///
 124     /// let mut one_element = std::iter::once(0);
 125     /// assert!(!one_element.is_empty());
 126     ///
 127     /// assert_eq!(one_element.next(), Some(0));
 128     /// assert!(one_element.is_empty());
 129     ///
 130     /// assert_eq!(one_element.next(), None);
 131     /// ```
 132     #[inline]
 133     #[unstable(feature = "exact_size_is_empty", issue = "35428")]
 134     fn is_empty(&self) -> bool {
 135         self.len() == 0
 136     }
 137 }
 138
 139 #[stable(feature = "rust1", since = "1.0.0")]
 140 impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for &mut I {
 141     fn len(&self) -> usize {
 142         (**self).len()
 143     }
 144     fn is_empty(&self) -> bool {
 145         (**self).is_empty()
 146     }
 147 }