Compound Types

Structs

A struct groups and names data of different types.

Definition

#![allow(unused)]
fn main() {
struct Point {
    x: i32,
    y: i32,
}
}

Note:

The fields may not be laid out in memory in the order they are written (unless you ask the compiler to ensure that they are).

Construction

• there is no partial initialization

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 1, y: 2 };
}

Construction

• but you can copy from an existing variable of the same type

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 1, y: 2 };
    let q = Point { x: 4, ..p };
}

Field Access

struct Point {
    x: i32,
    y: i32,
}

fn main() {
    let p = Point { x: 1, y: 2 };
    println!("{}", p.x);
    println!("{}", p.y);
}

Tuples

• Holds values of different types together.
• Like an anonymous struct, with fields numbered 0, 1, etc.

fn main() {
    let p = (1, 2);
    println!("{}", p.0);
    println!("{}", p.1);
}

()

• the empty tuple
• represents the absence of data
• we often use this similarly to how you’d use void in C

#![allow(unused)]
fn main() {
fn prints_but_returns_nothing(data: &str) -> () {
    println!("passed string: {}", data);
}
}

Tuple Structs

• Like a struct, with fields numbered 0, 1, etc.

struct Point(i32,i32);

fn main() {
    let p = Point(1, 2);
    println!("{}", p.0);
    println!("{}", p.1);
}

Enums

• An enum represents different variations of the same subject.
• The different choices in an enum are called variants

enum: Definition and Construction

enum Shape {
    Square,
    Circle,
    Rectangle,
    Triangle,
}

fn main() {
    let shape = Shape::Rectangle;
}

Enums with Values

pub enum Shapes {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn main() {
    let dot = Shapes::Dot;
    let square = Shapes::Square(10);
    let rectangle = Shapes::Rectangle { width: 10, length: 20 };
}

Enums with Values

• An enum value is the same size, no matter which variant is picked
• It will be the size of the largest variant (plus a tag)

Note:

• From a computer science perspective, enums are tagged unions.
• The tag in an enum specifies which variant is currently valid, and is stored as the smallest integer the compiler can get away with - it depends how many variants you have. Of course, if none of the variants have any data, the enum is just the tag.
• If you have a C background, you can think of this as being a struct containing an int and a union.

Doing a match on an enum

• When an enum has variants, you use match to extract the data
• New variables are created from the pattern (e.g. radius)

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn check_shape(shape: Shape) {
    match shape {
        Shape::Square(width) => {
            println!("It's a square, with the width {}", width);
        }
        _ => {
            println!("Try a square instead");
        }
    }
}
}

Doing a match on an enum

• There are two variables called width
• The binding of width in the pattern on line 10 hides the width variable on line 8

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn check_shape(shape: Shape) {
    let width = 10;
    match shape {
        Shape::Square(width) => {
            println!("It's a square, with width {}", width);
        }
        _ => {
            println!("Try a square instead");
        }
    }
}
}

Note:

• Rust allows the variable shadowing shown above in general

Match guards

Match guards allow further refining of a match

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn check_shape(shape: Shape) {
    match shape {
        Shape::Square(width) if width > 10 => {
            println!("It's a BIG square, with width {}", width);
        }
        _ => {
            println!("Try a big square instead");
        }
    }
}
}

Combining patterns

• You can use the | operator to join patterns together

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn test_shape(shape: Shape) {
    match shape {
        Shape::Rectangle { width, .. } | Shape::Square(width) => {
            println!("Shape has a width of {}", width);
        }
        _ => {
            println!("Not a rectangle, nor a square");
        }
    }
}
}

Shorthand: if let conditionals

• You can use if let if only one case is of interest.
• Still pattern matching

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn test_shape(shape: Shape) {
    if let Shape::Square(width) = shape {
        println!("Shape is a Square with width {}", width);
    }
}
}

if let chains in newer Rust versions

Newer Rust versions (edition 2024) allow if let chaining, for example:

enum Shape {
    Circle(i32),
    Rectangle(i32, i32),
}

fn test_shape(shape: Shape) {
    // Hardcoded here, but could be determined by other logic.
    let ignore_rectangle = true;
    if !ignore_rectangle && let Shape::Rectangle(length, height) = shape {
        println!("Shape is a Rectangle with {length} x {height}");
    }
}

Shorthand: let else conditionals

• If you expect it to match, but want to handle the error...
• The else block must diverge

#![allow(unused)]
fn main() {
pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn test_shape(shape: Shape) {
    let Shape::Square(width) = shape else {
        println!("I only like squares");
        return;
    };
    println!("Shape is a square with width {}", width);
}
}

Shorthand: while let conditionals

• Keep looping whilst the pattern still matches

pub enum Shape {
    Dot,
    Square(u32),
    Rectangle { width: u32, length: u32 }
}

fn main() {
    while let Shape::Square(width) = make_shape() {
        println!("got square, width {}", width);
    }
}

fn make_shape() -> Shape {
    todo!()
}

Foreshadowing! 👻

Two very important enums

#![allow(unused)]
fn main() {
enum Option<T> {
    Some(T),
    None,
}

enum Result<T, E> {
    Ok(T),
    Err(E)
}
}

We'll come back to them after we learn about error handling.