Structs & Enums

TL;DR

• Struct = product type (all fields). Enum = sum type (one variant).
• Use impl blocks for methods. self = instance, Self = type.
• Enums can hold data per variant. Pattern match with match.

Structs

struct User {
    name: String,
    age: u32,
    active: bool,
}

impl User {
    // Constructor (associated function)
    fn new(name: &str, age: u32) -> Self {
        Self { name: name.to_string(), age, active: true }
    }

    // Method
    fn greet(&self) -> String {
        format!("Hi, I'm {} ({})", self.name, self.age)
    }
}

Enums

enum Shape {
    Circle(f64),           // radius
    Rectangle(f64, f64),   // width, height
    Triangle { base: f64, height: f64 },
}

impl Shape {
    fn area(&self) -> f64 {
        match self {
            Shape::Circle(r) => std::f64::consts::PI * r * r,
            Shape::Rectangle(w, h) => w * h,
            Shape::Triangle { base, height } => 0.5 * base * height,
        }
    }
}

Key Patterns