CONTAINER_OF macro to downcast types more easily in C

container_of.c

//===--------------------~ Source ~--------------------===//

#include <stdint.h>

#ifndef offsetof
#define offsetof(type, member) ((uintptr_t)&((type*)0)->member)
#endif

#define CONTAINER_OF_CONST(member_ptr, container_ty, member_name) \
    ((container_ty const*) ((uintptr_t) member_ptr - offsetof(container_ty, member_name)))

#define CONTAINER_OF(member_ptr, container_ty, member_name) \
    ((container_ty *) ((uintptr_t) member_ptr - offsetof(container_ty, member_name)))



//===--------------------~ Usage ~--------------------===//

// Parent type
typedef struct {
    float x, y;
    float width, height;
} Widget;

// Child type aka. container, because it contains the parent (widget).
typedef struct {
    char const *label;
    Widget widget;
    int times_clicked;
} ButtonWidget;

// Cast a Widget* to a ButtonWidget*
// It doesn't matter where widget is declared in the struct,
// it doesn't have to be the first member.
#define WIDGET_DOWNCAST_BUTTON(ptr) CONTAINER_OF(ptr, ButtonWidget, widget)

// Cast a ButtonWidget* to a Widget*
#define BUTTON_UPCAST_WIDGET(ptr) (&(ptr)->widget)

//===--------------------~ Example ~--------------------===//

void button_on_click(Widget *w) {
    ButtonWidget *button = WIDGET_DOWNCAST_BUTTON(w);
    ++button->times_clicked;
}

void example() {
    ButtonWidget bw = {
        .label = "my button",
        .widget = {
            .x = .0f,
            .y = .0f,
            .width = 32.0f,
            .height = 32.0f,
        },
        .times_clicked = 0,
    };

    button_on_click(BUTTON_UPCAST_WIDGET(&bw));
    assert(bw.times_clicked == 1);
}