enum with struct Variants

Goal of This Episode

Learn to give enum variants named fields, struct-style.

Concept

Last episode’s tuple variants had unnamed fields, distinguished by position. But if a variant carries a lot of data, having no names makes things easy to mix up.

Rust lets you write variants in a style similar to named-field structs, giving every field a name:

enum Shape {
    Circle { radius: f64 },
    Rectangle { width: i32, height: i32 },
}

fn main() {}

Creating a value works just like creating a struct:

enum Shape {
    Circle { radius: f64 },
    Rectangle { width: i32, height: i32 },
}

fn main() {
    let s = Shape::Circle { radius: 5.0 };
    let r = Shape::Rectangle { width: 10, height: 20 };
}

Within a single enum, some variants can use the tuple form, some the struct form, and some can carry nothing at all — mixing and matching is completely fine.

Example Code

enum Shape {
    Circle { radius: f64 },
    Rectangle { width: i32, height: i32 },
    Dot, // A data-free variant can be mixed in too
}

fn main() {
    let s1 = Shape::Circle { radius: 5.0 };
    let s2 = Shape::Rectangle { width: 10, height: 20 };
    let s3 = Shape::Dot;

    // We can't extract the fields directly yet
    // Episode 10 covers extracting struct-variant data with match
    println!("All three shapes created!");

    // A more everyday example
    let event = Event::Click { x: 100, y: 200 };
    println!("Event created!");
}

enum Event {
    Click { x: i32, y: i32 },
    KeyPress(char), // The tuple form mixes in fine
    Quit,           // Carrying nothing works too
}

Recap

• Variants can carry named fields in struct form: Circle { radius: f64 }.
• Create a value: Shape::Circle { radius: 5.0 }.
• One enum can mix and match: tuple-form variants, struct-form variants, and data-free ones.
• The struct form’s advantage: named fields sometimes make the code easier to read.
• Extracting field data requires match (coming in Episode 10).