A mutable reference, written &mut T, temporarily borrows a value and permits its modification without taking ownership. Rust allows either one mutable reference or any number of immutable references at a time, and references must remain valid. section 2.3.1 section 2.3.2
Use the reference type to make a function’s permitted behavior explicit:
- &Artwork lets a function inspect artwork repeatedly without moving or recreating the owned value. section 2.3.1
- &mut Artwork lets a function update artwork, but the owner must be declared mutable and passed with &mut. section 2.3.2
- Sequential calls remain legal because each mutable reference ends when the function returns; no two mutable references overlap. section 2.3.2
Incrementing a view counter
Define admire_art as fn admire_art(art: &mut Artwork), increment art.view_count inside it, declare the owner as let mut art1, and call admire_art(&mut art1) twice. section 2.3.2
Listing 2.11 Incrementing a view counter on an artwork using mutable references
struct Artwork {
view_count: i32,
name: String,
}
fn admire_art(art: &mut Artwork) { #1
println!("{} people have seen {} today!",
art.view_count, art.name);
art.view_count += 1; #2
}
fn main() {
let mut art1 = Artwork {
view_count: 0, name: "".to_string() }; #3
admire_art(&mut art1); #4
admire_art(&mut art1);
}
Inspect the complete mutable-reference example, including the Artwork type, function signature, mutation, mutable owner, and calls. section 2.3.2
