Summary - Enums

Combine data-carrying enums and Option to model an IP address type.

Enums summary

An enum states that a value is one of a fixed set of options. Variants may carry no data, or carry data in tuple or struct form. match forces every variant to be handled. Option<T> (Some/None) replaces null and is used with helpers like unwrap_or, map and is_some. Methods can be added to enums with impl.

Methods on an enum

In enums carrying data, a method does a separate calculation for each variant with match self:

enum Sekil {
    Daire(f64),
    Kare(f64),
}

impl Sekil {
    fn alan(&self) -> f64 {
        match self {
            Sekil::Daire(r) => std::f64::consts::PI * r * r,
            Sekil::Kare(a) => a * a,
        }
    }
}

When a new shape is added, the compiler tells you to update the alan method.

Exercise Shape areas

Write the Sekil enum above and the alan method. Print the areas of Sekil::Daire(1.0) and Sekil::Kare(2.0) with the {:.2} format, separated by a space.

Show example solution

enum Sekil {
    Daire(f64),
    Kare(f64),
}

impl Sekil {
    fn alan(&self) -> f64 {
        match self {
            Sekil::Daire(r) => std::f64::consts::PI * r * r,
            Sekil::Kare(a) => a * a,
        }
    }
}

fn main() {
    println!("{:.2} {:.2}", Sekil::Daire(1.0).alan(), Sekil::Kare(2.0).alan());
}

Test yourself

What determines the memory size of an <code>enum Sekil { Daire(f64), Kare(f64) }</code> value?
- The largest variant + a discriminant tag
- The sum of all variants
- Always 1 byte
- It changes at runtime
Show answer

The largest variant + a discriminant tag — Since an enum value is one variant at a time, room is made for the largest one, plus a tag saying which variant it is.

Option variant

let x: Option<i32> = ____(4);

Show answer

let x: Option<i32> = Some(4);

Group summary

enum A { X, Y }

Set of options
B::X(5)

Variant with data
Option<T>

Instead of null
match self

Method by variant

Next lesson: match patterns

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