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