# Associated functions & Methods

## Examples

``````struct Point {
x: f64,
y: f64,
}

// Implementation block, all `Point` associated functions & methods go in here.
impl Point {
// This is an "associated function" because this function is associated with
// a particular type, that is, Point.
//
// Associated functions don't need to be called with an instance.
// These functions are generally used like constructors.
fn origin() -> Point {
Point { x: 0.0, y: 0.0 }
}

// Another associated function, taking two arguments:
fn new(x: f64, y: f64) -> Point {
Point { x: x, y: y }
}
}

struct Rectangle {
p1: Point,
p2: Point,
}

impl Rectangle {
// This is a 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() + (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 {
// Associated functions are called using double colons
p1: Point::origin(),
p2: Point::new(3.0, 4.0),
};

// Methods are called using the dot operator.
// Note that the first argument `&self` is implicitly passed, i.e.
// `rectangle.perimeter()` === `Rectangle::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

// Okay! Mutable objects can call mutable methods
square.translate(1.0, 1.0);

let pair = Pair(Box::new(1), Box::new(2));

pair.destroy();

// Error! Previous `destroy` call "consumed" `pair`
//pair.destroy();
// TODO ^ Try uncommenting this line
}
``````

## Exercises

### Method

1. 🌟🌟 Methods are similar to functions: Declare with `fn`, have parameters and a return value. Unlike functions, methods are defined within the context of a struct (or an enum or a trait object), and their first parameter is always `self`, which represents the instance of the struct the method is being called on.
``````struct Rectangle {
width: u32,
height: u32,
}

impl Rectangle {
// Complete the area method which return the area of a Rectangle.
fn area
}

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

assert_eq!(rect1.area(), 1500);

println!("Success!");
}
``````
1. 🌟🌟 `self` will take the ownership of current struct instance, however, `&self` will only borrow a reference from the instance.
``````// Only fill in the blanks, DON'T remove any line!
#[derive(Debug)]
struct TrafficLight {
color: String,
}

impl TrafficLight {
pub fn show_state(__)  {
println!("the current state is {}", __.color);
}
}
fn main() {
let light = TrafficLight{
color: "red".to_owned(),
};
// Don't take the ownership of `light` here.
light.show_state();
// ... Otherwise, there will be an error below
println!("{:?}", light);
}
``````
1. 🌟🌟 The `&self` is actually short for `self: &Self`. Within an `impl` block, the type `Self` is an alias for the type that the `impl` block is for. Methods must have a parameter named `self` of type `Self` for their first parameter, so Rust lets you abbreviate this with only the name `self` in the first parameter spot.
``````struct TrafficLight {
color: String,
}

impl TrafficLight {
// Using `Self` to fill in the blank.
pub fn show_state(__)  {
println!("the current state is {}", self.color);
}

// Fill in the blank, DON'T use any variants of `Self`.
pub fn change_state(__) {
self.color = "green".to_string()
}
}
fn main() {
println!("Success!");
}
``````

### Associated functions

1. 🌟🌟 All functions defined within an `impl` block are called associated functions because they’re associated with the type named after the `impl`. We can define associated functions that don’t have `self` as their first parameter (and thus are not methods) because they don’t need an instance of the type to work with.
``````#[derive(Debug)]
struct TrafficLight {
color: String,
}

impl TrafficLight {
// 1. Implement an assotiated function `new`,
// 2. It will return a TrafficLight contains color "red"
// 3. Must use `Self`, DONT use `TrafficLight` in fn signatures or body
pub fn new()

pub fn get_state(&self) -> &str {
&self.color
}
}

fn main() {
let light = TrafficLight::new();
assert_eq!(light.get_state(), "red");

println!("Success!");
}
``````

### Multiple `impl` blocks

1. 🌟 Each struct is allowed to have multiple impl blocks.
``````
struct Rectangle {
width: u32,
height: u32,
}

// Using multiple `impl` blocks to rewrite the code below.
impl Rectangle {
fn area(&self) -> u32 {
self.width * self.height
}

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

fn main() {
println!("Success!");
}
``````

### Enums

1. 🌟🌟🌟 We can also implement methods for enums.
``````
#[derive(Debug)]
enum TrafficLightColor {
Red,
Yellow,
Green,
}

// Implement TrafficLightColor with a method.
impl TrafficLightColor {

}

fn main() {
let c = TrafficLightColor::Yellow;

assert_eq!(c.color(), "yellow");

println!("{:?}",c);
}
``````

## Practice

@todo

You can find the solutions here(under the solutions path), but only use it when you need it