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< h2 id = "implementing-structs-and-enums" > < a class = "header" href = "#implementing-structs-and-enums" > Implementing structs and enums< / a > < / h2 >
< 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 >
< ul >
< 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 >
< 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 >
< / ul >
< p > In our example we are going to create animals and print them.< / p >
< 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 >
< pre > < pre class = "playground" > < code class = "language-rust" > #[derive(Debug)]
struct Animal {
age: u8,
animal_type: AnimalType,
}
#[derive(Debug)]
enum AnimalType {
Cat,
Dog,
}
impl Animal {
fn new() -> Self {
// Self means Animal.
//You can also write Animal instead of Self
Self {
// When we write Animal::new(), we always get a cat that is 10 years old
age: 10,
animal_type: AnimalType::Cat,
}
}
fn change_to_dog(& mut self) { // because we are inside Animal, & mut self means & mut Animal
// use .change_to_dog() to change the cat to a dog
// with & mut self we can change it
println!(" Changing animal to dog!" );
self.animal_type = AnimalType::Dog;
}
fn change_to_cat(& mut self) {
// use .change_to_cat() to change the dog to a cat
// with & mut self we can change it
println!(" Changing animal to cat!" );
self.animal_type = AnimalType::Cat;
}
fn check_type(& self) {
// we want to read self
match self.animal_type {
AnimalType::Dog => println!(" The animal is a dog" ),
AnimalType::Cat => println!(" The animal is a cat" ),
}
}
}
fn main() {
let mut new_animal = Animal::new(); // Associated method to create a new animal
// It is a cat, 10 years old
new_animal.check_type();
new_animal.change_to_dog();
new_animal.check_type();
new_animal.change_to_cat();
new_animal.check_type();
}
< / code > < / pre > < / pre >
< p > This prints:< / p >
< pre > < code class = "language-text" > The animal is a cat
Changing animal to dog!
The animal is a dog
Changing animal to cat!
The animal is a cat
< / code > < / pre >
< p > Remember that Self (the type Self) and self (the variable self) are abbreviations. (abbreviation = short way to write)< / p >
< 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 >
< p > Here is one more small example. This time we will use < code > impl< / code > on an < code > enum< / code > :< / p >
< pre > < pre class = "playground" > < code class = "language-rust" > enum Mood {
Good,
Bad,
Sleepy,
}
impl Mood {
fn check(& self) {
match self {
Mood::Good => println!(" Feeling good!" ),
Mood::Bad => println!(" Eh, not feeling so good" ),
Mood::Sleepy => println!(" Need sleep NOW" ),
}
}
}
fn main() {
let my_mood = Mood::Sleepy;
my_mood.check();
}
< / code > < / pre > < / pre >
< p > This prints < code > Need sleep NOW< / code > .< / p >
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