Structs, Enums and Traits

Rust Methods and impl Blocks

Attach behavior to a struct: methods with &self, changes with &mut self and constructors like new.

Intermediate
4 min read
1 runnable example

What you'll learn

impl blocks
&self and &mut self
Associated functions

Run code →

Receiver → what the method can do

fn area(&self)read

fn scale(&mut self, f)change

fn new(w, h) -> Selfcreate

Functions that belong to a type

An impl block holds the functions that belong to a type. A method takes self as its first parameter and is called with a dot. A function without self, such as new, is called with the type name and ::.

&self borrows the rectangle for reading and &mut self borrows it for changing, which is why rect must be mut to call scale. Self is a short name for the type of the impl block. new and square are associated functions used as constructors.

Key points

&self reads the value.
&mut self changes it.
Rectangle::new has no self: it is a constructor.

Runnable Rust example

Rust

Run code →

main.rs

struct Rectangle {
    width: f64,
    height: f64,
}

impl Rectangle {
    fn new(width: f64, height: f64) -> Self {
        Self { width, height }
    }

    fn square(size: f64) -> Self {
        Self::new(size, size)
    }

    fn area(&self) -> f64 {
        self.width * self.height
    }

    fn scale(&mut self, factor: f64) {
        self.width *= factor;
        self.height *= factor;
    }
}

fn main() {
    let mut rect = Rectangle::new(3.0, 4.0);
    println!("area: {}", rect.area());

    rect.scale(2.0);
    println!("scaled: {} x {} = {}", rect.width, rect.height, rect.area());
    println!("square: {}", Rectangle::square(5.0).area());
}

Get a Run button for this example Result of running the code

area: 12
scaled: 6 x 8 = 48
square: 25

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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