Enum definition

Define enumerations with enum.

Defining an enum

An enum (enumeration) is a type expressing that a value can be exactly one of a fixed set of options. Each option is called a variant:

#[derive(Debug)]
enum Yon {
    Kuzey,
    Guney,
    Dogu,
    Bati,
}

let y = Yon::Dogu;
println!("{:?}", y);   // Dogu

Variants are qualified with the enum name: Yon::Dogu. A struct models an "and" relation (name and score), an enum models an "or" relation (north or south).

Enums and integers

Variants that carry no data are stored with a number (the discriminant) behind the scenes; it can be set explicitly and converted to an integer with as:

enum Durum {
    Bekliyor = 1,
    Hazir = 2,
    Hata = 9,
}

println!("{}", Durum::Hata as i32);   // 9

The reverse (integer to enum) isn't automatic; since invalid numbers are possible, it's written by hand with match.

Exercise Direction enum

Define enum Yon with the variants Kuzey, Guney, Dogu and Bati, with #[derive(Debug)]. Assign Yon::Dogu to a variable and print it with {:?}.

Show example solution

#[derive(Debug)]
enum Yon {
    Kuzey,
    Guney,
    Dogu,
    Bati,
}

fn main() {
    let y = Yon::Dogu;
    println!("{:?}", y);
}

Test yourself

Which is the best fit for an enum?
- A piece of text
- A traffic light colour: red, yellow, green
- A person's name and age
- A list of numbers
Show answer

A traffic light colour: red, yellow, green — If the value can only be one of fixed options, an enum is the natural choice.
Pick a variant
```
let y = Yon____Bati;
```
Show answer
```
let y = Yon::Bati;
```
Concept and example

enum Yon { .. }

Enum definition
Yon::Dogu

Variant value
Durum::Hata as i32

Discriminant
#[derive(Debug)]

Prints the variant name

Next lesson: Variants

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies