exact_size.rs.html -- source
Description: Source to the Rust file `library/core/src/iter/traits/exact_size.rs`.
Keywords: rust, rustlang, rust-lang

      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141

    /// An iterator that knows its exact length.
    ///
    /// Many [`Iterator`]s 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.
    ///
    /// [`len`]: ExactSizeIterator::len
    ///
    /// # 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][moddocs], we implemented an [`Iterator`],
    /// `Counter`. Let's implement `ExactSizeIterator` for it as well:
    ///
    /// [moddocs]: index.html
    ///
    /// ```
    /// # struct Counter {
    /// #     count: usize,
    /// # }
    /// # impl Counter {
    /// #     fn new() -> Counter {
    /// #         Counter { count: 0 }
    /// #     }
    /// # }
    /// # impl Iterator for Counter {
    /// #     type Item = usize;
    /// #     fn next(&mut self) -> Option<Self::Item> {
    /// #         self.count += 1;
    /// #         if self.count < 6 {
    /// #             Some(self.count)
    /// #         } else {
    /// #             None
    /// #         }
    /// #     }
    /// # }
    /// 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 counter = Counter::new();
    ///
    /// assert_eq!(5, counter.len());
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait ExactSizeIterator: Iterator {
        /// Returns the exact 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.
        ///
        /// [trait-level]: ExactSizeIterator
        /// [`Some(T)`]: Some
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // a finite range knows exactly how many times it will iterate
        /// let five = 0..5;
        ///
        /// assert_eq!(5, five.len());
        /// ```
        #[inline]
        #[stable(feature = "rust1", since = "1.0.0")]
        fn len(&self) -> usize {
            let (lower, upper) = self.size_hint();
            // Note: This assertion is overly defensive, but it checks the invariant
            // guaranteed by the trait. If this trait were rust-internal,
            // we could use debug_assert!; assert_eq! will check all Rust user
            // implementations too.
            assert_eq!(upper, Some(lower));
            lower
        }

        /// 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);
        /// ```
        #[inline]
        #[unstable(feature = "exact_size_is_empty", issue = "35428")]
        fn is_empty(&self) -> bool {
            self.len() == 0
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<I: ExactSizeIterator + ?Sized> ExactSizeIterator for &mut I {
        fn len(&self) -> usize {
            (**self).len()
        }
        fn is_empty(&self) -> bool {
            (**self).is_empty()
        }
    }