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Structs, Enums and Traits

Rust Enums

A type that is exactly one of several variants, where each variant can carry its own data.

  • Intermediate
  • 3 min read
  • 1 runnable example
What you'll learn
  • Variants with data
  • match on an enum
  • Methods on enums
Run code →
enum Shape
  • Shape
    • Circle(f64)
    • Rectangle { width, height }
    • Point

One of several shapes

An enum is a type whose value is exactly one of a fixed list of variants. In Rust each variant can carry its own data, so one enum can describe several different shapes of information.

Circle carries one number, Rectangle carries two named fields and Point carries nothing. match must handle every variant, so the compiler reports an error if you add a variant and forget a case. {:.2} prints two digits after the decimal point.

Key points

  1. Each variant can hold different data.
  2. match must cover every variant.
  3. Forgetting one is a compile error.

Runnable Rust example

Rust
Run code →
main.rs
enum Shape {
    Circle(f64),
    Rectangle { width: f64, height: f64 },
    Point,
}

impl Shape {
    fn area(&self) -> f64 {
        match self {
            Shape::Circle(radius) => 3.14159 * radius * radius,
            Shape::Rectangle { width, height } => width * height,
            Shape::Point => 0.0,
        }
    }
}

fn main() {
    let shapes = [
        Shape::Circle(1.0),
        Shape::Rectangle { width: 2.0, height: 3.5 },
        Shape::Point,
    ];
    for shape in &shapes {
        println!("{:.2}", shape.area());
    }
}
Get a Run button for this example Result of running the code
3.14
7.00
0.00

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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