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<main>
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<h2><a class="header" href="#implementing-structs-and-enums" id="implementing-structs-and-enums">Implementing structs and enums</a></h2>
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<p>This is where you can start to give your structs and enums some real power. To call functions on a <code>struct</code> or an <code>enum</code>, use an <code>impl</code> block. These functions are called <strong>methods</strong>. There are two kinds of methods in an <code>impl</code> block.</p>
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<ul>
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<li>Regular methods: these take <strong>self</strong> (or <strong>&self</strong> or <strong>&mut self</strong>). Regular methods use a <code>.</code> (a period). <code>.clone()</code> is an example of a regular method.</li>
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<li>Associated methods (or "static" methods): these do not take self. Associated means "related to". They are written differently, using <code>::</code>. <code>String::from()</code> is an associated method, and so is <code>Vec::new()</code>. You usually see associated methods used to create new variables.</li>
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</ul>
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<p>In our example we are going to create animals and print them.</p>
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<p>For a new <code>struct</code> or <code>enum</code>, you need to give it <strong>Debug</strong> if you want to use <code>{:?}</code> to print, so we will do that. If you write <code>#[derive(Debug)]</code> above the struct or enum then you can print it with <code>{:?}</code>. These messages with <code>#[]</code> are called <strong>attributes</strong>. You can sometimes use them to tell the compiler to give your struct an ability like <code>Debug</code>. There are many attributes and we will learn about them later. But <code>derive</code> is probably the most common and you see it a lot above structs and enums.</p>
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<pre><pre class="playground"><code class="language-rust">#[derive(Debug)]
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struct Animal {
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age: u8,
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animal_type: AnimalType,
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}
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#[derive(Debug)]
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enum AnimalType {
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Cat,
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Dog,
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}
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impl Animal {
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fn new() -> Self {
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// Self means Animal.
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//You can also write Animal instead of Self
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Self {
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// When we write Animal::new(), we always get a cat that is 10 years old
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age: 10,
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animal_type: AnimalType::Cat,
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}
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}
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fn change_to_dog(&mut self) { // because we are inside Animal, &mut self means &mut Animal
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// use .change_to_dog() to change the cat to a dog
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// with &mut self we can change it
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println!("Changing animal to dog!");
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self.animal_type = AnimalType::Dog;
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}
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fn change_to_cat(&mut self) {
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// use .change_to_cat() to change the dog to a cat
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// with &mut self we can change it
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println!("Changing animal to cat!");
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self.animal_type = AnimalType::Cat;
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}
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fn check_type(&self) {
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// we want to read self
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match self.animal_type {
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AnimalType::Dog => println!("The animal is a dog"),
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AnimalType::Cat => println!("The animal is a cat"),
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}
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}
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}
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fn main() {
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let mut new_animal = Animal::new(); // Associated method to create a new animal
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// It is a cat, 10 years old
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new_animal.check_type();
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new_animal.change_to_dog();
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new_animal.check_type();
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new_animal.change_to_cat();
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new_animal.check_type();
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}
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</code></pre></pre>
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<p>This prints:</p>
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<pre><code class="language-text">The animal is a cat
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Changing animal to dog!
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The animal is a dog
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Changing animal to cat!
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The animal is a cat
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</code></pre>
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<p>Remember that Self (the type Self) and self (the variable self) are abbreviations. (abbreviation = short way to write)</p>
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<p>So in our code, Self = Animal. Also, <code>fn change_to_dog(&mut self)</code> means <code>fn change_to_dog(&mut Animal)</code>.</p>
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<p>Here is one more small example. This time we will use <code>impl</code> on an <code>enum</code>:</p>
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<pre><pre class="playground"><code class="language-rust">enum Mood {
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Good,
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Bad,
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Sleepy,
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}
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impl Mood {
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fn check(&self) {
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match self {
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Mood::Good => println!("Feeling good!"),
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Mood::Bad => println!("Eh, not feeling so good"),
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Mood::Sleepy => println!("Need sleep NOW"),
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}
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}
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}
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fn main() {
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let my_mood = Mood::Sleepy;
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my_mood.check();
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}
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</code></pre></pre>
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<p>This prints <code>Need sleep NOW</code>.</p>
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