Struct functions

Struct functions are another new feature in C2.

Struct functions are syntactic-sugar that makes code more readable.

The example below shows how it works:

type Point struct {
    i32 x;
    i32 y;

fn void Point.add(Point* p, i32 x) {
    p.x = x;

fn void example() {
    Point p = { 1, 2 }

    // with struct-functions

    // without struct-functions (if defined as non struct-function)
    /// point_add(&p, 10);


  • Struct functions only work on struct/union types
  • Struct functions are defined in the same module as the struct (not necessarily the same file!)
  • For a type type named Foo, struct functions must start with Foo. prefix.
  • There cannot be a struct member x and a struct function Foo.x.
  • A (non-static) struct function is required to have 'Type*' or 'const Type*' as its first argument.
  • A static struct function has no argument requirements and can only be called on the type itself, not an instance of the type. Foo.add() and f.add() cannot both exist.
  • A struct function may also be assigned to variables of type Function with the correct prototype: callback = Foo.add;
  • Sub-structs cannot have struct functions

Extra notes:

  • It's possible to have public or non-public struct functions for a public struct.
  • A struct-function itself is a regular function and can also be used as such
  • It's also possible to use a struct function call if the variable of struct type is a member of another struct
type Outer struct {
    Inner inner;

type Inner struct {
    // ...

fn void Inner.modify(Inner* inner, /* ... */) {
    // ...

fn void example() {
    Outer outer;
    outer.inner.modify(/* ... */);
    // translates to Inner.modify(&outer.inner);
  • The following is OK, as struct functions implicitly dereference the object on which they are called:
    Type* t = nil;

for more examples, see the tests in c2compiler/test/Functions/struct_functions/

Complex example

See below a more complex example, which uses opaque pointers. The module inner offers an API:

module inner;

import stdlib;
import stdio;

public type Shape struct @(opaque) {
    u8 sides;
    // ..

// a non-public struct-function
fn void Shape.init(Shape* shape, u8 sides) {
   shape.sides = sides;

// a static struct-function, called as Shape.create(..)
public fn Shape* Shape.create(u8 sides) {
    Shape* shape = stdlib.malloc(sizeof(Shape));
    return shape;

// a public, const struct-function, first argument: const Shape*
public fn void Shape.print(const Shape* shape) {
    stdio.printf("shape with %d sides\n", shape.sides);

// a public, struct-function, first argument: Shape*
public fn void* shape) {;

which is used by the module outer:

module outer;

import inner local;

fn void example() {
    Shape* s = Shape.create(3);

NOTE: outer is not allowed to access Shape's regular members directly.