46 lines
1.4 KiB
Rust
46 lines
1.4 KiB
Rust
// cow1.rs
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// This exercise explores the Cow, or Clone-On-Write type.
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// Cow is a clone-on-write smart pointer.
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// It can enclose and provide immutable access to borrowed data, and clone the data lazily when mutation or ownership is required.
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// The type is designed to work with general borrowed data via the Borrow trait.
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use std::borrow::Cow;
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fn abs_all<'a, 'b>(input: &'a mut Cow<'b, [i32]>) -> &'a mut Cow<'b, [i32]> {
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for i in 0..input.len() {
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let v = input[i];
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if v < 0 {
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// Clones into a vector if not already owned.
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input.to_mut()[i] = -v;
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}
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}
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input
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}
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fn main() {
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// No clone occurs because `input` doesn't need to be mutated.
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let slice = [0, 1, 2];
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let mut input = Cow::from(&slice[..]);
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match abs_all(&mut input) {
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Cow::Borrowed(_) => println!("I borrowed the slice!"),
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_ => panic!("expected borrowed value"),
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}
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// Clone occurs because `input` needs to be mutated.
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let slice = [-1, 0, 1];
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let mut input = Cow::from(&slice[..]);
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match abs_all(&mut input) {
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Cow::Owned(_) => println!("I modified the slice and now own it!"),
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_ => panic!("expected owned value"),
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}
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// No clone occurs because `input` is already owned.
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let slice = vec![-1, 0, 1];
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let mut input = Cow::from(slice);
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match abs_all(&mut input) {
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Cow::Owned(_) => println!("I own this slice!"),
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_ => panic!("expected borrowed value"),
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}
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}
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