Enum Basics
Defining a Simple Enum
An enum defines a type whose value can be exactly one of a fixed set of named variants, useful for representing a closed set of possibilities.
Example: Defining a Simple Enum
#[derive(Debug)]
enum Direction {
North,
South,
East,
West,
}
fn main() {
let heading = Direction::North;
println!("{:?}", heading);
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Enums With Data
Unlike simple labels, Rust enum variants can carry their own associated data, and different variants can carry different types and amounts of data.
Example: Enums With Data
#[derive(Debug)]
enum Shape {
Circle(f64),
Rectangle(f64, f64),
}
fn main() {
let shapes = [Shape::Circle(2.0), Shape::Rectangle(3.0, 4.0)];
for s in shapes.iter() {
println!("{:?}", s);
}
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Matching on an Enum
match is the primary way to inspect which variant an enum value holds and to extract any data it carries, with a separate arm for each possible variant.
Example: Matching on an Enum
enum TrafficLight {
Red,
Yellow,
Green,
}
fn main() {
let light = TrafficLight::Yellow;
match light {
TrafficLight::Red => println!("Stop"),
TrafficLight::Yellow => println!("Slow down"),
TrafficLight::Green => println!("Go"),
}
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Comparing Enum Values
Deriving PartialEq on an enum allows comparing its values directly with ==, which is useful for simple checks without writing a full match.
Example: Comparing Enum Values
#[derive(PartialEq, Debug)]
enum Status {
Active,
Inactive,
}
fn main() {
let current = Status::Active;
println!("Is active: {}", current == Status::Active);
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- Forgetting an enum variant can carry its own data, not just be a plain label like in some other languages.
- Trying to compare enum variants with
==before derivingPartialEqon the enum. - Assuming enum variant names are globally unique -- they must be qualified with
EnumName::Variantunless imported.
- An enum defines a type that can be exactly one of several named variants.
- Variants can optionally carry associated data, including different data per variant.
- Enum values are typically handled with
matchto branch on which variant is present. - Deriving traits like
DebugandPartialEqon an enum enables printing and comparison.
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