Structs - The Rust Programming Language

# Structs

`struct`s are a way of creating more complex data types. For example, if we were doing calculations involving coordinates in 2D space, we would need both an `x` and a `y` value:

    # #![allow(unused_variables)]
    #fn main() {
    let origin_x = 0;
    let origin_y = 0;

    #}

A `struct` lets us combine these two into a single, unified datatype with `x` and `y` as field labels:

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

    fn main() {
        let origin = Point { x: 0, y: 0 }; // origin: Point

        println!("The origin is at ({}, {})", origin.x, origin.y);
    }

There’s a lot going on here, so let’s break it down. We declare a `struct` with the `struct` keyword, and then with a name. By convention, `struct`s begin with a capital letter and are camel cased: `PointInSpace`, not `Point_In_Space`.

We can create an instance of our `struct` via `let`, as usual, but we use a `key: value` style syntax to set each field. The order doesn’t need to be the same as in the original declaration.

Finally, because fields have names, we can access them through dot notation: `origin.x`.

The values in `struct`s are immutable by default, like other bindings in Rust. Use `mut` to make them mutable:

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

    fn main() {
        let mut point = Point { x: 0, y: 0 };

        point.x = 5;

        println!("The point is at ({}, {})", point.x, point.y);
    }

This will print `The point is at (5, 0)`.

Rust does not support field mutability at the language level, so you cannot write something like this:

    struct Point {
        mut x: i32, // This causes an error.
        y: i32,
    }

Mutability is a property of the binding, not of the structure itself. If you’re used to field-level mutability, this may seem strange at first, but it significantly simplifies things. It even lets you make things mutable on a temporary basis:

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

    fn main() {
        let mut point = Point { x: 0, y: 0 };

        point.x = 5;

        let point = point; // `point` is now immutable.

        point.y = 6; // This causes an error.
    }

Your structure can still contain `&mut` references, which will let you do some kinds of mutation:

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

    struct PointRef<'a> {
        x: &'a mut i32,
        y: &'a mut i32,
    }

    fn main() {
        let mut point = Point { x: 0, y: 0 };

        {
            let r = PointRef { x: &mut point.x, y: &mut point.y };

            *r.x = 5;
            *r.y = 6;
        }

        assert_eq!(5, point.x);
        assert_eq!(6, point.y);
    }

Initialization of a data structure (struct, enum, union) can be simplified when fields of the data structure are initialized with variables of the same names as the fields.

    #[derive(Debug)]
    struct Person<'a> {
        name: &'a str,
        age: u8
    }

    fn main() {
        // Create struct with field init shorthand
        let name = "Peter";
        let age = 27;
        let peter = Person { name, age };

        // Debug-print struct
        println!("{:?}", peter);
    }

# Update syntax

A `struct` can include `..` to indicate that you want to use a copy of some other `struct` for some of the values. For example:

    # #![allow(unused_variables)]
    #fn main() {
    struct Point3d {
        x: i32,
        y: i32,
        z: i32,
    }

    let mut point = Point3d { x: 0, y: 0, z: 0 };
    point = Point3d { y: 1, .. point };

    #}

This gives `point` a new `y`, but keeps the old `x` and `z` values. It doesn’t have to be the same `struct` either, you can use this syntax when making new ones, and it will copy the values you don’t specify:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Point3d {
    #     x: i32,
    #     y: i32,
    #     z: i32,
    # }
    let origin = Point3d { x: 0, y: 0, z: 0 };
    let point = Point3d { z: 1, x: 2, .. origin };

    #}

# Tuple structs

Rust has another data type that’s like a hybrid between a tuple and a `struct`, called a ‘tuple struct’. Tuple structs have a name, but their fields don't. They are declared with the `struct` keyword, and then with a name followed by a tuple:

    # #![allow(unused_variables)]
    #fn main() {
    struct Color(i32, i32, i32);
    struct Point(i32, i32, i32);

    let black = Color(0, 0, 0);
    let origin = Point(0, 0, 0);

    #}

Here, `black` and `origin` are not the same type, even though they contain the same values.

The members of a tuple struct may be accessed by dot notation or destructuring `let`, just like regular tuples:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Color(i32, i32, i32);
    # struct Point(i32, i32, i32);
    # let black = Color(0, 0, 0);
    # let origin = Point(0, 0, 0);
    let black_r = black.0;
    let Point(_, origin_y, origin_z) = origin;

    #}

Patterns like `Point(_, origin_y, origin_z)` are also used in match expressions.

One case when a tuple struct is very useful is when it has only one element. We call this the ‘newtype’ pattern, because it allows you to create a new type that is distinct from its contained value and also expresses its own semantic meaning:

    # #![allow(unused_variables)]
    #fn main() {
    struct Inches(i32);

    let length = Inches(10);

    let Inches(integer_length) = length;
    println!("length is {} inches", integer_length);

    #}

As above, you can extract the inner integer type through a destructuring `let`. In this case, the `let Inches(integer_length)` assigns `10` to `integer_length`. We could have used dot notation to do the same thing:

    # #![allow(unused_variables)]
    #fn main() {
    # struct Inches(i32);
    # let length = Inches(10);
    let integer_length = length.0;

    #}

It's always possible to use a `struct` instead of a tuple struct, and can be clearer. We could write `Color` and `Point` like this instead:

    # #![allow(unused_variables)]
    #fn main() {
    struct Color {
        red: i32,
        blue: i32,
        green: i32,
    }

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

    #}

Good names are important, and while values in a tuple struct can be referenced with dot notation as well, a `struct` gives us actual names, rather than positions.

# Unit-like structs

You can define a `struct` with no members at all:

    # #![allow(unused_variables)]
    #fn main() {
    struct Electron {} // Use empty braces...
    struct Proton;     // ...or just a semicolon.

    // Use the same notation when creating an instance.
    let x = Electron {};
    let y = Proton;
    let z = Electron; // Error

    #}

Such a `struct` is called ‘unit-like’ because it resembles the empty tuple, `()`, sometimes called ‘unit’. Like a tuple struct, it defines a new type.

This is rarely useful on its own (although sometimes it can serve as a marker type), but in combination with other features, it can become useful. For instance, a library may ask you to create a structure that implements a certain trait to handle events. If you don’t have any data you need to store in the structure, you can create a unit-like `struct`.