Structs and Enums

Structs and enums are Rust's primary tools for creating custom data types. Combined with pattern matching, they form a powerful and expressive type system.

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Defining Structs

A struct groups related data together:

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

Creating Instances

let user1 = User {
    username: String::from("alice"),
    email: String::from("alice@example.com"),
    sign_in_count: 1,
    active: true,
};

Field Init Shorthand

When variable names match field names:

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

Struct Update Syntax

Create a new struct based on an existing one:

let user2 = User {
    email: String::from("bob@example.com"),
    ..user1 // remaining fields from user1
};

Tip: The ..user1 syntax moves any non-Copy fields from user1. After this, user1.username is no longer valid (it was moved), but user1.active and user1.sign_in_count are still valid (they are Copy types).

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Tuple Structs and Unit Structs

Tuple Structs

Named tuples with struct semantics:

struct Colour(i32, i32, i32);
struct Point(f64, f64, f64);