Structures are the record type in Onyx. A structure is declared using the struct keyword and is normally bound to a symbol. Members of a structure are declared like declarations in a procedure.

Point :: struct {
	x: i32;
	y: i32;

Accessing Members

Member access is done through the . operator. Note that accessing a member on a pointer to a structure uses the same . syntax.

p: Point;
p.x = 10;
p.y = 20;

ptr := &p;
ptr.x = 30;

Structure Literals

Structure literals are a quicker way of creating a value of a struct type. They have the form, Type.{ members }. The members can be partially, or completely named. The same rules apply for when giving members as do for arguments when calling a procedure. If a value is not provided for a member, and no default value is given in the structure, a zeroed-value is used.

// Naming members
p1 := Point.{x=10, y=20};

// Leaving out names. Follows order of members declared in the structure.
p2 := Point.{10, 20};

Defaulted Members

Members can be given default values. These values are used in structure literals if no other value is provided for a member. They are also used by __initialize to initialize a structure.

Person :: struct {
	name: str = "Joe";

	// If the type can be inferred, the type can be omitted.
	age := 30;

sally := Person.{ name="Sally", age=42 };

// Because name is omitted, it defaults to "Joe".
joe := Person.{ age=31 };

// Leaving out all members simply sets the members with initializers to
// their default values, and all other members to zero.
joe2 := Person.{};


Structures have a variety of directives that can be applied to them to change their properties. Directives go before the { of the structure definition.

#size nSet a minimum size
#align nSet a minimum alignment
#packDisable automatic padding
#unionA members are at offset 0 (C Union)

Polymorphic Structures

Structures can be polymorphic, meaning they accept a number of compile time arguments, and generate a new version of the structure for each set of arguments.

// A 2d-point in any field.
Point :: struct (T: type_expr) {
	x, y: T;

Complex :: struct {
	real, imag: f32;

int_point: Point(i32);
complex_point: Point(Complex);

Polymorphic structures are immensely useful when creating data structure. Consider this binary tree of any type.

Tree :: struct (T: type_expr) {
	data: T;
	left, right: &Tree(T);	

root: Tree([] u8);

When declaring a procedure that accepts a polymorphic structure, the polymorphic variables can be explicitly listed.

HashMap :: struct (Key: type_expr, Value: type_expr, hash: (Key) -> u32) {
	// ...

put :: (map: ^HashMap($Key, $Value, $hash), key: Key, value: Value) {
	h := hash(key);
	// ...

Or they can be omitted and a polymorphic procedure will be created automatically. The parameters to the polymorphic structure can be accessed as though they were members of the structure.

HashMap :: struct (Key: type_expr, Value: type_expr, hash: (Key) -> u32) {
	// ...

put :: (map: ^HashMap, key: map.Key, value: map.Value) {
	h := map.hash(key);
	// ...

Structure Composition

Onyx does not support inheritance. Instead, a composition model is preferred. The use keyword specifies that all members of a member should be directly accessible.

Name_Component :: struct {
	name: str;

Age_Component :: struct {
	age: u32;

Person :: struct {
	use name_comp: Name_Component;
	use age_comp:  Age_Component;

// 'name' and 'age' are directly accessible.
p: Person; = "Joe";
p.age = 42;

Sub-Type Polymorphism

Onyx supports sub-type polymorphism, which enable a safe and automatic conversion between pointer types &B to &A if the following conditions are met:

  1. The first member of B is of type A.
  2. The first member of B is used.
Person :: struct {
	name: str;
	age:  u32;

Joe :: struct {
	use base: Person;
	pet_name: str;

say_name :: (person: ^Person) {
	printf("Hi, I am {}.\n",;

joe: Joe; = "Joe";

// This is safe, because Joe "extends" Person.

In this example, you can pass a pointer to Joe when a pointer to Person is expected, because the first member of Joe is a Person, and that member is used.