alloc/vec/
extract_if.rs

1use core::ops::{Range, RangeBounds};
2use core::{fmt, ptr, slice};
3
4use super::Vec;
5use crate::alloc::{Allocator, Global};
6
7/// An iterator which uses a closure to determine if an element should be removed.
8///
9/// This struct is created by [`Vec::extract_if`].
10/// See its documentation for more.
11///
12/// # Example
13///
14/// ```
15/// let mut v = vec![0, 1, 2];
16/// let iter: std::vec::ExtractIf<'_, _, _> = v.extract_if(.., |x| *x % 2 == 0);
17/// ```
18#[stable(feature = "extract_if", since = "1.87.0")]
19#[must_use = "iterators are lazy and do nothing unless consumed; \
20    use `retain_mut` or `extract_if().for_each(drop)` to remove and discard elements"]
21pub struct ExtractIf<
22    'a,
23    T,
24    F,
25    #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
26> {
27    vec: &'a mut Vec<T, A>,
28    /// The index of the item that will be inspected by the next call to `next`.
29    idx: usize,
30    /// Elements at and beyond this point will be retained. Must be equal or smaller than `old_len`.
31    end: usize,
32    /// The number of items that have been drained (removed) thus far.
33    del: usize,
34    /// The original length of `vec` prior to draining.
35    old_len: usize,
36    /// The filter test predicate.
37    pred: F,
38}
39
40impl<'a, T, F, A: Allocator> ExtractIf<'a, T, F, A> {
41    pub(super) fn new<R: RangeBounds<usize>>(vec: &'a mut Vec<T, A>, pred: F, range: R) -> Self {
42        let old_len = vec.len();
43        let Range { start, end } = slice::range(range, ..old_len);
44
45        // Guard against the vec getting leaked (leak amplification)
46        unsafe {
47            vec.set_len(0);
48        }
49        ExtractIf { vec, idx: start, del: 0, end, old_len, pred }
50    }
51
52    /// Returns a reference to the underlying allocator.
53    #[unstable(feature = "allocator_api", issue = "32838")]
54    #[inline]
55    pub fn allocator(&self) -> &A {
56        self.vec.allocator()
57    }
58}
59
60#[stable(feature = "extract_if", since = "1.87.0")]
61impl<T, F, A: Allocator> Iterator for ExtractIf<'_, T, F, A>
62where
63    F: FnMut(&mut T) -> bool,
64{
65    type Item = T;
66
67    fn next(&mut self) -> Option<T> {
68        while self.idx < self.end {
69            let i = self.idx;
70            // SAFETY:
71            //  We know that `i < self.end` from the if guard and that `self.end <= self.old_len` from
72            //  the validity of `Self`. Therefore `i` points to an element within `vec`.
73            //
74            //  Additionally, the i-th element is valid because each element is visited at most once
75            //  and it is the first time we access vec[i].
76            //
77            //  Note: we can't use `vec.get_unchecked_mut(i)` here since the precondition for that
78            //  function is that i < vec.len(), but we've set vec's length to zero.
79            let cur = unsafe { &mut *self.vec.as_mut_ptr().add(i) };
80            let drained = (self.pred)(cur);
81            // Update the index *after* the predicate is called. If the index
82            // is updated prior and the predicate panics, the element at this
83            // index would be leaked.
84            self.idx += 1;
85            if drained {
86                self.del += 1;
87                // SAFETY: We never touch this element again after returning it.
88                return Some(unsafe { ptr::read(cur) });
89            } else if self.del > 0 {
90                // SAFETY: `self.del` > 0, so the hole slot must not overlap with current element.
91                // We use copy for move, and never touch this element again.
92                unsafe {
93                    let hole_slot = self.vec.as_mut_ptr().add(i - self.del);
94                    ptr::copy_nonoverlapping(cur, hole_slot, 1);
95                }
96            }
97        }
98        None
99    }
100
101    fn size_hint(&self) -> (usize, Option<usize>) {
102        (0, Some(self.end - self.idx))
103    }
104}
105
106#[stable(feature = "extract_if", since = "1.87.0")]
107impl<T, F, A: Allocator> Drop for ExtractIf<'_, T, F, A> {
108    fn drop(&mut self) {
109        if self.del > 0 {
110            // SAFETY: Trailing unchecked items must be valid since we never touch them.
111            unsafe {
112                ptr::copy(
113                    self.vec.as_ptr().add(self.idx),
114                    self.vec.as_mut_ptr().add(self.idx - self.del),
115                    self.old_len - self.idx,
116                );
117            }
118        }
119        // SAFETY: After filling holes, all items are in contiguous memory.
120        unsafe {
121            self.vec.set_len(self.old_len - self.del);
122        }
123    }
124}
125
126#[stable(feature = "extract_if", since = "1.87.0")]
127impl<T, F, A> fmt::Debug for ExtractIf<'_, T, F, A>
128where
129    T: fmt::Debug,
130    A: Allocator,
131{
132    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
133        let peek = if self.idx < self.end { self.vec.get(self.idx) } else { None };
134        f.debug_struct("ExtractIf").field("peek", &peek).finish_non_exhaustive()
135    }
136}
alloc/vec/extract_if.rs

alloc/vec/
extract_if.rs
