1.2 KiB
Mutable slices
Every time we've talked about slice types (like str and [T]), we've used their immutable borrow form (&str and &[T]).
But slices can also be mutable!
Here's how you create a mutable slice:
let mut numbers = vec![1, 2, 3];
let slice: &mut [i32] = &mut numbers;
You can then modify the elements in the slice:
slice[0] = 42;
This will change the first element of the Vec to 42.
Limitations
When working with immutable borrows, the recommendation was clear: prefer slice references over references to
the owned type (e.g. &[T] over &Vec<T>).
That's not the case with mutable borrows.
Consider this scenario:
let mut numbers = Vec::with_capacity(2);
let mut slice: &mut [i32] = &mut numbers;
slice.push(1);
It won't compile!
push is a method on Vec, not on slices. This is the manifestation of a more general principle: Rust won't
allow you to add or remove elements from a slice. You will only be able to modify/replace the elements that are
already there.
In this regard, a &mut Vec or a &mut String are strictly more powerful than a &mut [T] or a &mut str.
Choose the type that best fits based on the operations you need to perform.