Structs

  • Rust doesn’t allow us to mark only certain fields as mutable. You can only mark whole struct as mutable or immutable.

  • For struct field you can use spread operator. But remember that data will be moved to new struct.

    fn main() {
  // --snip--

  let user1 = User {
      email: String::from("someone@example.com"),
      username: String::from("someusername123"),
      active: true,
      sign_in_count: 1,
  };

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

  print!("{}", user1.email); // Works
  print!("{}", user1.username); // Do not work, moved to user2 already
}

Tuple Structs

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

fn main() {
    let black = Color(0, 0, 0);
    let origin = Point(0, 0, 0);
}

Unit Structs

Structs with no fields

struct AlwaysEqual;

fn main() {
    let subject = AlwaysEqual;
}

Struct as Function Parameter

struct Rectangle {
    width: u32,
    height: u32,
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        area(&rect1)
    );
}

fn area(rectangle: &Rectangle) -> u32 {
    rectangle.width * rectangle.height
}

prints to stdout

#[derive(Debug)]   // Required for print of struct

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!("rect1 is {:?}", rect1);
     dbg!(&rect1);
}

prints to stderr

    let rect1 = Rectangle {
        width: dbg!(30 * scale),
        height: 50,
    };
    dbg!(&rect1);  // use ref here to prevent ownership changes

Method

  • Use impl to add method for a struct
#[derive(Debug)]
struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    // define method for Rectangle
    // the `&self` is shorthand for `self: &Self` 
    // the type Self is an alias for the type that the impl block is for.
    fn area(&self) -> u32 {  
        self.width * self.height
    }
}

fn main() {
    let rect1 = Rectangle {
        width: 30,
        height: 50,
    };

    println!(
        "The area of the rectangle is {} square pixels.",
        rect1.area()
    );
}

Calling method with Automatic Referencing and Dereferencing

In C and C++, two different operators are used for calling methods: you use . if you’re calling a method on the object directly and -> if you’re calling the method on a pointer to the object and need to dereference the pointer first. In other words, if object is a pointer, object->something() is similar to (*object).something().

Rust automatically adds in &, &mut, or * so object matches the signature of the method. In other words, the following are the same:

p1.distance(&p2);
(&p1).distance(&p2);

Constructor in methods

impl Rectangle {
    fn square(size: u32) -> Self {
        Self {
            width: size,
            height: size,
        }
    }
}


Rectangle::square(3);

Multiple impl

impl Rectangle {
    fn area(&self) -> u32 {
        self.width * self.height
    }
}

impl Rectangle {
    fn can_hold(&self, other: &Rectangle) -> bool {
        self.width > other.width && self.height > other.height
    }
}


// Caution: for method without ref on self, use `Rectangle::test()` instead 
impl Rectangle {
    fn test() {
        print!("Test")
    }
}