# 21 Methods

Methods are functions attached to objects. These methods have access to the data of the object and its other methods via the self keyword. Methods are defined under a impl block.

#![feature(box_syntax)] use std::num::Float; // To be able to use .abs(), you must include this struct Point { x: f64, y: f64, } // Implementation block, all Point methods go in here impl Point { // This is a static method // Static methods don't need to be called by an instance // These methods are generally used as constructors fn origin() -> Point { Point { x: 0.0, y: 0.0 } } // Another static method, that takes two arguments fn new(x: f64, y: f64) -> Point { Point { x: x, y: y } } } struct Rectangle { p1: Point, p2: Point, } impl Rectangle { // This is an instance method // &self is sugar for self: &Self, where Self is the type of the // caller object. In this case Self = Rectangle fn area(&self) -> f64 { // self gives access to the struct fields via the dot operator let Point { x: x1, y: y1 } = self.p1; let Point { x: x2, y: y2 } = self.p2; // abs is a f64 method that returns the absolute value of the // caller ((x1 - x2) * (y1 - y2)).abs() } fn perimeter(&self) -> f64 { let Point { x: x1, y: y1 } = self.p1; let Point { x: x2, y: y2 } = self.p2; 2.0 * (x1 - x2).abs() + 2.0 * (y1 - y2).abs() } // This method requires the caller object to be mutable // &mut self desugars to self: &mut Self fn translate(&mut self, x: f64, y: f64) { self.p1.x += x; self.p2.x += x; self.p1.y += y; self.p2.y += y; } } // Pair owns resources: two heap allocated integers struct Pair(Box<i32>, Box<i32>); impl Pair { // This method "consumes" the resources of the caller object // self desugars to self: Self fn destroy(self) { // Destructure self let Pair(first, second) = self; println!("Destroying Pair({}, {})", first, second); // first and second go out of scope and get freed } } fn main() { let rectangle = Rectangle { // Static methods are called using double colons p1: Point::origin(), p2: Point::new(3.0, 4.0), }; // Instance method are called using the dot operator // Note that the first argument &self is implicitly passed, i.e. // rectangle.perimeter() === perimeter(&rectangle) println!("Rectangle perimeter: {}", rectangle.perimeter()); println!("Rectangle area: {}", rectangle.area()); let mut square = Rectangle { p1: Point::origin(), p2: Point::new(1.0, 1.0), }; // Error! rectangle is immutable, but this method requires a mutable // object //rectangle.translate(1.0, 0.0); // TODO ^ Try uncommenting this line // Ok, mutable object can call mutable methods square.translate(1.0, 1.0); let pair = Pair(box 1, box 2); pair.destroy(); // Error! Previous destroy call "consumed" pair //pair.destroy(); // TODO ^ Try uncommenting this line }