Structs

Structs are custom data types that group related values under named fields. They’re fundamental for organizing data in Fe programs.

Defining Structs

Define a struct with the struct keyword:

struct Point {
    x: i32,
    y: i32,
}

Each field has a name and a type. Fields are separated by commas.

Field Visibility

By default, struct fields are private. Use pub to make them publicly accessible:

pub struct User {
    pub name: String<20>,
    pub balance: u256,
    internal_id: u256,  // Private field
}

The struct itself can also be public or private:

pub struct PublicStruct {
    pub field: u256,
}

struct PrivateStruct {
    field: u256,
}

Creating Instances

Create a struct instance by specifying values for all fields:

struct Point {
    x: i32,
    y: i32,
}

let origin = Point { x: 0, y: 0 }
let point = Point { x: 10, y: 20 }

All fields must be initialized. There are no default values.

Field Init Shorthand

When a variable has the same name as a field, use shorthand syntax:

let x: i32 = 10
let y: i32 = 20

let point = Point { x, y }  // Same as Point { x: x, y: y }

Accessing Fields

Access struct fields with dot notation:

let point = Point { x: 10, y: 20 }

let x_value = point.x  // 10
let y_value = point.y  // 20

Updating Fields

For mutable struct instances, update fields directly:

let mut point = Point { x: 0, y: 0 }

point.x = 10
point.y = 20

Generic Structs

Structs can have generic type parameters:

struct Pair<T> {
    first: T,
    second: T,
}

let int_pair: Pair<u256> = Pair { first: 1, second: 2 }
let bool_pair = Pair { first: true, second: false }

Multiple Type Parameters

Use multiple generic parameters for different field types:

struct KeyValue<K, V> {
    key: K,
    value: V,
}

let entry: KeyValue<String<4>, u256> = KeyValue { key: "name", value: 42 }

Trait Bounds

Constrain generic types with trait bounds:

struct Container<T: Default> {
    item: T,
}

Nested Structs

Structs can contain other structs:

struct Point {
    x: i32,
    y: i32,
}

struct Rectangle {
    top_left: Point,
    bottom_right: Point,
}

let rect = Rectangle {
    top_left: Point { x: 0, y: 10 },
    bottom_right: Point { x: 10, y: 0 },
}

let x = rect.top_left.x  // 0

Struct Patterns

Destructure structs in patterns:

let point = Point { x: 10, y: 20 }

let Point { x, y } = point
// x is 10, y is 20

Use .. to ignore remaining fields:

struct User {
    name: u256,
    balance: u256,
    created_at: u256,
}

let User { name, .. } = user  // Only extract name

Patterns in Match

Match on struct patterns:

let location = match point {
    Point { x: 0, y: 0 } => "origin"
    Point { x: 0, y: _ } => "on y-axis"
    Point { x: _, y: 0 } => "on x-axis"
    Point { x: _, y: _ } => "somewhere else"
}

Structs with Tuples

Combine structs and tuples:

struct Line {
    start: (i32, i32),
    end: (i32, i32),
}

let line = Line {
    start: (0, 0),
    end: (100, 100),
}

let start_x = line.start.0

Summary

Syntax	Description
struct Name { }	Define a struct
pub field: Type	Public field
Struct { field: value }	Create instance
Struct { field }	Shorthand when variable matches field name
instance.field	Access field
struct Name<T>	Generic struct
let Struct { field } = s	Destructure struct
let Struct { field, .. } = s	Destructure, ignoring other fields