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