Like a tuple, a struct can have different types of data. Unlike with a tuple, in a struct you’ll name each piece of data so it’s clear what the values mean.

struct User {
    active: bool,
    username: String,
    email: String,
    sign_in_count: u64,
}

This shows to change the value in the email field of a mutable User instance.

fn main() {
    let mut user1 = User {
        active: true,
        username: String::from("someusername123"),
        email: String::from("[email protected]"),
        sign_in_count: 1,
    };

    user1.email = String::from("[email protected]");
}

Note that the entire instance must be mutable; Rust doesn’t allow us to mark only certain fields as mutable. As with any expression, we can construct a new instance of the struct as the last expression in the function body to implicitly return that new instance.

fn build_user(email: String, username: String) -> User {
    User {
        active: true,
        username: username,
        email: email,
        sign_in_count: 1,
    }
}

It makes sense to name the function parameters with the same name as the struct fields, but having to repeat the email and username field names and variables is a bit tedious. If the struct had more fields, repeating each name would get even more annoying. Luckily, there’s a convenient shorthand!

Derive Traits

In addition to the Debug trait, Rust has provided a number of traits for us to use with the derive attribute that can add useful behavior to our custom types. Those traits and their behaviors are listed in Appendix C. We’ll cover how to implement these traits with custom behavior as well as how to create your own traits in Chapter 10. There are also many attributes other than derive; for more information, see the “Attributes” section of the Rust Reference.

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!("rect1 is {rect1:?}");
}

Methods

Methods are similar to functions: we declare them with the fn keyword and a name, they can have parameters and a return value, and they contain some code that’s run when the method is called from somewhere else. Unlike functions, methods are defined within the context of a struct (or an enum or a trait object, which we cover in Chapter 6 and Chapter 17, respectively), and their first parameter is always self, which represents the instance of the struct the method is being called on.

#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        rect1.area()
    );
}

Rust's struct blocks with method and implementation blocks are similar to classes in languages like JavaScript and Java, but with some key differences:

• Rust separates data (structs) from behavior (impl blocks), while traditional classes combine both. 1
• Rust structs can be extended with traits in other modules, offering more flexibility than traditional class inheritance. 2
• Rust's approach allows for explicit self arguments on methods, which is not typical in class-based languages. 1

While similar in purpose, Rust's struct and impl combination provides a different paradigm for organizing code compared to traditional OOP classes.

Associated Functions

Associated functions that aren’t methods are often used for constructors that will return a new instance of the struct. These are often called new, but new isn’t a special name and isn’t built into the language. For example, we could choose to provide an associated function named square that would have one dimension parameter and use that as both width and height, thus making it easier to create a square Rectangle rather than having to specify the same value twice.