Structs in Rust

In this lesson, we will talk about Structs in Rust

What are Structs?

Structs are a way to create more complex data types.

They are similar to tuples, but with a few differences.

For example, you can name the fields of a struct (and you can also define methods for structs).

If you prefer a video version

All the code is available on GitHub (link available in the video description)

Define a Struct

Let's say that we want to crete a User, with name, email, is_active and age. We can define a struct like this:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

And then you can create an instance of the struct like this:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

fn main() {
    let user = User {
        name: String::from("Jhon Doe"),
        email: String::from("doe@mail.com"),
        is_active: true,
        age: 25,
    };
}

Accessing Struct Fields

You can access the fields of a struct using dot notation:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

fn main() {
    let user = User {
        name: String::from("Jhon Doe"),
        email: String::from("doe@mail.com"),
        is_active: true,
        age: 25,
    };

    println!("User name: {}", user.name);
}

Explanaition:

• We define a struct called User with four fields: name, email, is_active and age.
• We create an instance of the struct and assign it to the user variable.
• We access the name field of the user struct using dot notation (user.name).

Mutable Structs

You can make a struct mutable by using the mut keyword:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

fn main() {
    let mut user = User {
        name: String::from("Jhon Doe"),
        email: String::from("doe@mail.com"),
        is_active: true,
        age: 25,
    };

    user.name = String::from("Francesco");
    println!("User name: {}", user.name);

}

Note the you can't make individual fields of a struct mutable, you can only make the entire struct mutable.

Create a Struct instance using a function

You can create a function that returns a struct instance:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

fn main() {
    let user = create_user(String::from("Jhon Doe"), String::from("doe@mail"),);

    println!("User name: {}", user.name);
}

fn create_user(name: String, email: String,) -> User {
    User {
        name,
        email,
        is_active: true,
        age : 25,
    }
}

You can see the init shorthand syntax in the create_user function. This is a shorthand for initializing fields with variables that have the same name as the fields.

Creating Instances from Other Instances with Struct Update Syntax

You can create a new instance of a struct using another instance with the struct update syntax:

struct User {
    name: String,
    email: String,
    is_active: bool,
    age: u8,
}

fn main() {
    let user1 = User {
        name: String::from("Jhon Doe"),
        email: String::from("doe@mail.com"),
        is_active: false,
        age: 40
    };

    let new_user = User {
        name: String::from("Francesco"),
        email: user1.email,
        is_active: user1.is_active,
        age: user1.age,
    };

    println!("User name: {}", new_user.name);
}

In this example, we create a new instance of the User struct using the user1 instance with the struct update syntax.

Tuple Structs

Tuple structs are similar to regular structs, but their fields are not named:

struct Color(i32, i32, i32);
struct Point(i32, i32, i32);

fn main() {
    let black = Color(0, 0, 0);
    let origin = Point(0, 0, 0);
    println!("Black color: {}, {}, {}", black.0, black.1, black.2);
    println!("Origin: {}, {}, {}", origin.0, origin.1, origin.2);
}

In this example, we define two tuple structs: Color and Point. We create instances of these tuple structs and access their fields using dot notation.

Unit-Like Structs

You can define a struct with no fields, called a unit-like struct:

#[derive(Debug)]
struct User;

fn main() {
    let user = User;
    println!("User: {:?}", user);
}

Unit-like structs are useful when you need to implement a trait on a type but don't have any data to store in the type.

Conclusion

Structs are a way to create more complex data types in Rust. They are similar to tuples, but with a few differences. You can name the fields of a struct (and you can also define methods for structs). You can also make a struct mutable by using the mut keyword.

You can create a function that returns a struct instance. You can create a new instance of a struct using another instance with the struct update syntax. You can define tuple structs and unit-like structs.

I hope this lesson was useful. In the next lesson, we will see a practical example of using structs in Rust.

If you prefer a video version

All the code is available on GitHub (link available in the video description)

You can find me on https://francescociulla.com and on Twitter/X