Intro to Rust - Structs

A struct is a data type that allows you to group multiple related values under a single type. To define a struct in Rust, we use the struct keyword:

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

And we create an instance of the struct like so:

let p = Point{ x: 3, y: 4 };

And to access a field on the struct we use dot notation:

println!("x: {}, y: {}", p.x, p.y);

If you wanted to change the value of a field, you could write:

p.x = 18;

However, this would result in an error as we did not create the struct with the mut keyword. Unlike languages like C#, the entire struct must be either mutable or immutable, you cannot delcare specific fields to be mutable.

let mut p = Point{ x: 3, y: 4 };

If you’re familiar with ES6 JavaScript then you’re probably aware of enhanced object literal syntax which allows you to declare just the property name if a variable exists in scope with the same name as the property. Rust has that too:

let x = 3;
let y = 4;
let p = Point{ x, y };

Similary, in JavaScript, you can spread an object into another one which you can also do in Rust:

let p = Point{ x, y };
let p2 = Point{ x: 10, ..p };

Struct Methods

Methods are like functions except they are defined on structs. To define a method on a struct:

struct Square {
    length: u32,
}

impl Square {
    fn area(&self) -> u32 {
        self.length * 4
    }
}

...

let s = Square{length: 3};
s.area();

The impl keyword stands for implementation which creates an implementation block to define our method in. Rust implicitly passes a reference to the struct as the first argument to the method. Here we are borrowing self, but the method can also take ownership of it or borrow a mutable copy. Also, Rust has automatic referencing and deferencing so given the receiver of our method (&self in this case), Rust will automatically call the method with the proper receiver.

There are also associated methods which are similar to static methods in OOP languages. An associated method is defined the same way:

impl Square {
    fn from(length: u32) -> Rectangle {
        Square{ length }
    }
}

The key difference is that the method has no receiver and it is also invoked differently:

let s = Square::from(3);