Structs and Enums¶

Structs and Enums are custom data types that let you create complex, logical data structures by giving names and types to varying pieces of data (Structs) or differing variants of a type (Enums).

Structs¶

A struct, or structure, is a custom data type that lets you package together and name multiple related values that make up a meaningful group.

Defining a Struct¶

struct User {
    active: bool,
    username: string,
    email: string,
    sign_in_count: int,
}

Instantiating a Struct¶

To use a struct after we've defined it, we create an instance of that struct by specifying concrete values for each of the fields.

let mut user1 = User {
    active: true,
    username: "someusername123",
    email: "someone@example.com",
    sign_in_count: 1,
};

// Accessing fields
user1.email = "anotheremail@example.com";

Remember, by default variables are immutable. You must use mut to allow modification of struct instances.

Methods¶

Methods are defined within an impl block and take self as the first parameter.

struct Rectangle {
    width: int,
    height: int,
}

impl Rectangle {
    fn area(self) -> int {
        return self.width * self.height;
    }

    async fn scale(mut self, factor: int) {
        self.width = self.width * factor;
        self.height = self.height * factor;
    }
}

fn main() {
    let rect1 = Rectangle { width: 30, height: 50 };
    fmt.println("The area of the rectangle is:", rect1.area());
}

Enums¶

Enums allow you to define a type by enumerating its possible variants. Where structs give you a way of grouping together related fields and data, like a Rectangle with its width and height, enums give you a way of saying a value is one of a possible set of values.

enum IpAddrKind {
    V4,
    V6,
}

We can then create instances of each of the two variants:

let four = IpAddrKind::V4;
let six = IpAddrKind::V6;

Enums with Data¶

An enum variant can optionally store arbitrary data of any type.

enum Message {
    Quit,
    Move { x: i32, y: i32 }, // Anonymous struct variant
    Write(String),           // Tuple variant
    ChangeColor(i32, i32, i32), // Tuple variant with multiple values
}

Like structs, enums can also have methods defined via impl blocks.