Day 10: Methods & impl

Methods are functions defined within the context of a struct (or enum/trait).

Defining Methods

struct Circle {
    radius: f64,
}

impl Circle {
    // Associated function (no self) - like a constructor
    fn new(radius: f64) -> Self {
        Circle { radius }
    }
    
    // Method (takes &self)
    fn area(&self) -> f64 {
        std::f64::consts::PI * self.radius * self.radius
    }
    
    // Method that modifies self
    fn grow(&mut self, amount: f64) {
        self.radius += amount;
    }
}

The self Parameter

Syntax	Meaning
`self`	Takes ownership
`&self`	Immutable borrow
`&mut self`	Mutable borrow

Self (capital S) is an alias for the type being implemented. self (lowercase) is the instance.

Calling Methods

let mut c = Circle::new(5.0);  // associated function
let a = c.area();              // method with &self
c.grow(2.0);                   // method with &mut self

The Task

Create a Rectangle with:

new(width, height) - associated function
area(&self) - returns width × height
is_square(&self) - returns true if width == height

Requirements

Rectangle::new(5, 5).area() → 25
Rectangle::new(5, 5).is_square() → true
Rectangle::new(5, 3).is_square() → false

Hints

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