Enums¶

Enum¶

Enum allows you to define a type by enumerating it's possible values.

For example, we work with IP addresses with 2 versions - IPv4 and IPv6.

enum IpAddrKind {
    V4,
    V6
}

let four = IpAddrKind::V4;
let six = IpAddrKind::V6;

The reason this is useful is that both of these variants are now of the same type IpAddrKind and we can define a function that takes any IpAddrKind.

fn route(ip_type: IpAddrKind) {}

Enum values¶

• Using enum as a type for fields of the structure.

let home = IPAddr { kind: IPAddrKind::V4, address: String::from("127.0.0.1) }

let loopback = IPAddr { kind: IPAddrKind::V6, address: String::from("::1) }

• Predefined values

enum Fruits {
    Apple = 0,
    Mango = 10,
    Watermelon = 20,
}

fn main() {
    let f = Fruits::Mango;
    println!("{:?}", f as i32);
}

• Using different types inside enum

enum IPAddr {
    V4(u8,u8,u8,u8),
    V6(String)
}

let home = IPAddr::V4(String::from(127,0,0,1));
let loopback = IPAddr::V6(String::from("::1"));

Option Enum¶

Option, which is another enum defined by standard library. The option type is used in many places because it encodes the very common scenario in which a value could be something or could be nothing.

This functionality can prevent bugs that are extremely common in other programming languages.

Rust does not have Null types, but it does have an enum that can encode the concept of a value being present or absent.

This enum is Option<T> and is defined by the standard library as follows:

enum Option<T> {
    Some(T),
    None
}