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<h1 id="associated-functions--methods"><a class="header" href="#associated-functions--methods">Associated functions &amp; Methods</a></h1>
<h2 id="examples"><a class="header" href="#examples">Examples</a></h2>
<pre><pre class="playground"><code class="language-rust editable edition2021">struct Point {
x: f64,
y: f64,
}
// Implementation block, all `Point` associated functions &amp; 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() -&gt; Point {
Point { x: 0.0, y: 0.0 }
}
// Another associated function, taking two arguments:
fn new(x: f64, y: f64) -&gt; Point {
Point { x: x, y: y }
}
}
struct Rectangle {
p1: Point,
p2: Point,
}
impl Rectangle {
// This is a method.
// `&amp;self` is sugar for `self: &amp;Self`, where `Self` is the type of the
// caller object. In this case `Self` = `Rectangle`
fn area(&amp;self) -&gt; 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(&amp;self) -&gt; 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
// `&amp;mut self` desugars to `self: &amp;mut Self`
fn translate(&amp;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&lt;i32&gt;, Box&lt;i32&gt;);
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 `&amp;self` is implicitly passed, i.e.
// `rectangle.perimeter()` === `Rectangle::perimeter(&amp;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
}</code></pre></pre>
<h2 id="exercises"><a class="header" href="#exercises">Exercises</a></h2>
<h3 id="method"><a class="header" href="#method">Method</a></h3>
<ol>
<li>🌟🌟 Methods are similar to functions: Declare with <code>fn</code>, 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 <code>self</code>, which represents the instance of the struct the method is being called on.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">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!");
}</code></pre></pre>
<ol start="2">
<li>🌟🌟 <code>self</code> will take the ownership of current struct instance, however, <code>&amp;self</code> will only borrow a reference from the instance.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">// 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);
}</code></pre></pre>
<ol start="3">
<li>🌟🌟 The <code>&amp;self</code> is actually short for <code>self: &amp;Self</code>. Within an <code>impl</code> block, the type <code>Self</code> is an alias for the type that the <code>impl</code> block is for. Methods must have a parameter named <code>self</code> of type <code>Self</code> for their first parameter, so Rust lets you abbreviate this with only the name <code>self</code> in the first parameter spot.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">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!");
}</code></pre></pre>
<h3 id="associated-functions"><a class="header" href="#associated-functions">Associated functions</a></h3>
<ol start="4">
<li>🌟🌟 All functions defined within an <code>impl</code> block are called associated functions because they’re associated with the type named after the <code>impl</code>. We can define associated functions that don’t have <code>self</code> as their first parameter (and thus are not methods) because they don’t need an instance of the type to work with.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">#[derive(Debug)]
struct TrafficLight {
color: String,
}
impl TrafficLight {
// 1. Implement an associated 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(&amp;self) -&gt; &amp;str {
&amp;self.color
}
}
fn main() {
let light = TrafficLight::new();
assert_eq!(light.get_state(), "red");
println!("Success!");
}</code></pre></pre>
<h3 id="multiple-impl-blocks"><a class="header" href="#multiple-impl-blocks">Multiple <code>impl</code> blocks</a></h3>
<ol start="5">
<li>🌟 Each struct is allowed to have multiple impl blocks.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
struct Rectangle {
width: u32,
height: u32,
}
// Using multiple `impl` blocks to rewrite the code below.
impl Rectangle {
fn area(&amp;self) -&gt; u32 {
self.width * self.height
}
fn can_hold(&amp;self, other: &amp;Rectangle) -&gt; bool {
self.width &gt; other.width &amp;&amp; self.height &gt; other.height
}
}
fn main() {
println!("Success!");
}</code></pre></pre>
<h3 id="enums"><a class="header" href="#enums">Enums</a></h3>
<ol start="6">
<li>🌟🌟🌟 We can also implement methods for enums.</li>
</ol>
<pre><pre class="playground"><code class="language-rust editable edition2021">
#[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);
}</code></pre></pre>
<h2 id="practice"><a class="header" href="#practice">Practice</a></h2>
<p>@todo</p>
<blockquote>
<p>You can find the solutions <a href="https://github.com/sunface/rust-by-practice">here</a>(under the solutions path), but only use it when you need it</p>
</blockquote>
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