Defining and Instantiating Structs

Structs are similar to tuples, discussed in the Data Types section, in that both hold multiple related values. Like tuples, the pieces of a struct can be different types. Unlike with tuples, in a struct you’ll name each piece of data so it’s clear what the values mean. Adding these names means that structs are more flexible than tuples: you don’t have to rely on the order of the data to specify or access the values of an instance.

To define a struct, we enter the keyword struct and name the entire struct. A struct’s name should describe the significance of the pieces of data being grouped together. Then, inside curly brackets, we define the names and types of the pieces of data, which we call fields. For example, Listing 5-1 shows a struct that stores information about a user account.

Filename: src/lib.cairo

#[derive(Copy, Drop)]
struct User {
    active: bool,
    username: felt252,
    email: felt252,
    sign_in_count: u64,
}

Listing 5-1: A User struct definition

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. We create an instance by stating the name of the struct and then add curly brackets containing key: value pairs, where the keys are the names of the fields and the values are the data we want to store in those fields. We don’t have to specify the fields in the same order in which we declared them in the struct. In other words, the struct definition is like a general template for the type, and instances fill in that template with particular data to create values of the type.

For example, we can declare a particular user as shown in Listing 5-2.

Filename: src/lib.cairo

#[derive(Copy, Drop)]
struct User {
    active: bool,
    username: felt252,
    email: felt252,
    sign_in_count: u64,
}
fn main() {
    let user1 = User {
        active: true, username: 'someusername123', email: 'someone@example.com', sign_in_count: 1
    };
}

Listing 5-2: Creating an instance of the User struct

To get a specific value from a struct, we use dot notation. For example, to access this user’s email address, we use user1.email. If the instance is mutable, we can change a value by using the dot notation and assigning into a particular field. Listing 5-3 shows how to change the value in the email field of a mutable User instance.

Filename: src/lib.cairo

#[derive(Copy, Drop)]
struct User {
    active: bool,
    username: felt252,
    email: felt252,
    sign_in_count: u64,
}
fn main() {
    let mut user1 = User {
        active: true, username: 'someusername123', email: 'someone@example.com', sign_in_count: 1
    };
    user1.email = 'anotheremail@example.com';
}

fn build_user(email: felt252, username: felt252) -> User {
    User { active: true, username: username, email: email, sign_in_count: 1, }
}

fn build_user_short(email: felt252, username: felt252) -> User {
    User { active: true, username, email, sign_in_count: 1, }
}

Listing 5-3: Changing the value in the email field of a User instance

Tenga en cuenta que toda la instancia debe ser mutable; Cairo no nos permite marcar solo ciertos campos como mutables.

Como con cualquier expresión, podemos construir una nueva instancia de la estructura como la última expresión en el cuerpo de la función para devolver implícitamente esa nueva instancia.

Listing 5-4 shows a build_user function that returns a User instance with the given email and username. The active field gets the value of true, and the sign_in_count gets a value of 1.

Filename: src/lib.cairo

#[derive(Copy, Drop)]
struct User {
    active: bool,
    username: felt252,
    email: felt252,
    sign_in_count: u64,
}
fn main() {
    let mut user1 = User {
        active: true, username: 'someusername123', email: 'someone@example.com', sign_in_count: 1
    };
    user1.email = 'anotheremail@example.com';
}

fn build_user(email: felt252, username: felt252) -> User {
    User { active: true, username: username, email: email, sign_in_count: 1, }
}

fn build_user_short(email: felt252, username: felt252) -> User {
    User { active: true, username, email, sign_in_count: 1, }
}

Listing 5-4: A build_user function that takes an email and username and returns a User instance

Tiene sentido nombrar los parámetros de la función con el mismo nombre que los campos de la estructura, porque tener que repetir los nombres y variables de los campos emaily username es un poco tedioso. Si la estructura tuviera más campos, repetir cada nombre sería aún más molesto. ¡Afortunadamente, hay una forma abreviada!

Using the Field Init Shorthand

Because the parameter names and the struct field names are exactly the same in Listing 5-4, we can use the field init shorthand syntax to rewrite build_user so it behaves exactly the same but doesn’t have the repetition of username and email, as shown in Listing 5-5.

Filename: src/lib.cairo

#[derive(Copy, Drop)]
struct User {
    active: bool,
    username: felt252,
    email: felt252,
    sign_in_count: u64,
}
fn main() {
    let mut user1 = User {
        active: true, username: 'someusername123', email: 'someone@example.com', sign_in_count: 1
    };
    user1.email = 'anotheremail@example.com';
}

fn build_user(email: felt252, username: felt252) -> User {
    User { active: true, username: username, email: email, sign_in_count: 1, }
}

fn build_user_short(email: felt252, username: felt252) -> User {
    User { active: true, username, email, sign_in_count: 1, }
}

Listing 5-5: A build_user function that uses field init shorthand because the username and email parameters have the same name as struct fields

Aquí, estamos creando una nueva instancia de la estructura User, que tiene un campo llamado email. Queremos establecer el valor del campo email con el valor del parámetro email de la función build_user. Debido a que el campo email y el parámetro email tienen el mismo nombre, solo necesitamos escribir email en lugar de email: email.