enum (C-style)

Goal of This Episode

Learn to use an enum to define a fixed set of options, so a variable can only be one of those values.

Concept

Last episode we learned struct — for defining new types that “group several values together.” This episode covers another way to define a new type: enum.

Sometimes we want to express “this thing can only be one of a few options.” For instance, a traffic light can only be red, yellow, or green.

An enum (enumeration) is for defining exactly this kind of “pick one of several” type. Like a struct, defining an enum tells Rust: “I want a new type whose value can only be one of these options.” The simplest enum looks like this:

enum Color {
    Red,
    Green,
    Blue,
}

fn main() {}

Each option is called a variant. To create an enum value, use the TypeName::VariantName syntax:

enum Color {
    Red,
    Green,
    Blue,
}

fn main() {
    let c = Color::Red;
}

Note the two colons :: in the middle — in Rust this is the “path operator,” meaning “the Red under Color.”

This most basic kind of enum — where no variant carries any extra data — is sometimes called a C-style enum, because that’s what enums are like in the C language.

Example Code

enum Direction {
    Up,
    Down,
    Left,
    Right,
}

fn main() {
    let _dir = Direction::Up;

    // We can't do much with enums yet
    // Next episode we'll learn match, which lets us act on an enum's value
    // For now, here's how to create different enum values

    let _d1 = Direction::Down;
    let _d2 = Direction::Left;
    let _d3 = Direction::Right;

    println!("Direction is set!");
    println!("(Next episode, match lets us act based on the direction)");
}

Recap

• Like struct, an enum is a way to define a new type.
• struct: groups several values together; enum: picks one from several options.
• Use :: to name a specific variant, e.g. Direction::Up.
• Every variant of a C-style enum carries no extra data.
• Like structs, enum definitions generally go outside fn main(), above or below.
• We can’t directly print an enum’s value yet (we can once we learn match next episode).