cow1.rs (2310B)
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 6 use std::borrow::Cow; 7 8 fn 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 18 fn main() { 19 // You can optionally experiment here. 20 } 21 22 #[cfg(test)] 23 mod 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 // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`. 42 assert!(matches!(input, todo!())); 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 // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`. 55 assert!(matches!(input, todo!())); 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 // TODO: Replace `todo!()` with `Cow::Owned(_)` or `Cow::Borrowed(_)`. 67 assert!(matches!(input, todo!())); 68 } 69 }