chay.dev
1// This exercise explores the `Cow` (Clone-On-Write) smart pointer. It can
2// enclose and provide immutable access to borrowed data and clone the data
3// lazily when mutation or ownership is required. The type is designed to work
4// with general borrowed data via the `Borrow` trait.
5
6use std::borrow::Cow;
7
8fn abs_all(input: &mut Cow<[i32]>) {
9 for ind in 0..input.len() {
10 let value = input[ind];
11 if value < 0 {
12 // Clones into a vector if not already owned.
13 input.to_mut()[ind] = -value;
14 }
15 }
16}
17
18fn main() {
19 // You can optionally experiment here.
20}
21
22#[cfg(test)]
23mod tests {
24 use super::*;
25
26 #[test]
27 fn reference_mutation() {
28 // Clone occurs because `input` needs to be mutated.
29 let vec = vec![-1, 0, 1];
30 let mut input = Cow::from(&vec);
31 abs_all(&mut input);
32 assert!(matches!(input, Cow::Owned(_)));
33 }
34
35 #[test]
36 fn reference_no_mutation() {
37 // No clone occurs because `input` doesn't need to be mutated.
38 let vec = vec![0, 1, 2];
39 let mut input = Cow::from(&vec);
40 abs_all(&mut input);
41 assert!(matches!(input, Cow::Borrowed(_)));
42 // ^^^^^^^^^^^^^^^^
43 }
44
45 #[test]
46 fn owned_no_mutation() {
47 // We can also pass `vec` without `&` so `Cow` owns it directly. In this
48 // case, no mutation occurs (all numbers are already absolute) and thus
49 // also no clone. But the result is still owned because it was never
50 // borrowed or mutated.
51 let vec = vec![0, 1, 2];
52 let mut input = Cow::from(vec);
53 abs_all(&mut input);
54 assert!(matches!(input, Cow::Owned(_)));
55 // ^^^^^^^^^^^^^
56 }
57
58 #[test]
59 fn owned_mutation() {
60 // Of course this is also the case if a mutation does occur (not all
61 // numbers are absolute). In this case, the call to `to_mut()` in the
62 // `abs_all` function returns a reference to the same data as before.
63 let vec = vec![-1, 0, 1];
64 let mut input = Cow::from(vec);
65 abs_all(&mut input);
66 assert!(matches!(input, Cow::Owned(_)));
67 // ^^^^^^^^^^^^^
68 }
69}