vtable method dispatch about nimskull HOT 8 OPEN

// you can test this
// gcc vtable.c -o vtable
#include <stdio.h>
#include <stdlib.h>

// -------------------
// Function Pointer Types
// -----------------

typedef void (*widget_draw_proc)(void*);
typedef void (*widget_update_proc)(void*);
typedef void (*widget_layout_proc)(void*);
typedef void (*widget_event_proc)(void*);

// ----------------
// Default Widget
// ----------------

typedef struct {
  void** vtable;
  int x, y, w, h;
} widget_t;

typedef struct {
  widget_t root;
} default_t;

static void empty_draw(widget_t* self) { printf("drawed nothing\n"); }
static void empty_update(widget_t* self) { printf("update nothing\n"); }
static void empty_layout(widget_t* self) { printf("layout nothing\n"); }
static void empty_event(widget_t* self) { printf("event nothing\n"); }

// vtable for a default widget
const void* vtable_default[] = {
  (void*) empty_draw,
  (void*) empty_update,
  (void*) empty_layout,
  (void*) empty_event,
};

default_t* default_new() {
  default_t* w = malloc(sizeof(default_t));
  // Look, we define the vtable
  w->root.vtable = (void**) vtable_default;

  return w;
}

// ----------------
// Widget Button
// ----------------

typedef struct {
  widget_t root;
  char* label;
} button_t;

static void button_draw(button_t* self) { printf("%s\n", self->label); }
static void button_event(button_t* self) { printf("evented button\n"); }

// vtable for button widget, look overwrites
const void* vtable_button[] = {
  (void*) button_draw, // Overwrite
  (void*) empty_update,
  (void*) empty_layout,
  (void*) button_event // Overwrite
};

button_t* button_new(char* label) {
  button_t* w = malloc(sizeof(button_t));
  // Look, we define the vtable
  w->root.vtable = (void**) vtable_button;

  // Define Button Thing
  w->label = label;

  return w;
}

// -----------------------
// Usage of VTable
// -----------------------

int main() {
  button_t* btn0 = button_new("Button 0");
  button_t* btn1 = button_new("Button 1");
  default_t* default0 = default_new();

  // Usage of VTable, look at casting
  printf("-- Draw Method\n");
  ( (widget_draw_proc) btn0->root.vtable[0] ) (btn0);
  ( (widget_draw_proc) btn1->root.vtable[0] ) (btn1);
  ( (widget_draw_proc) default0->root.vtable[0] ) (default0);
  printf("-- Event Method\n");
  ( (widget_event_proc) btn0->root.vtable[3] ) (btn0);
  ( (widget_event_proc) btn1->root.vtable[3] ) (btn1);
  ( (widget_event_proc) default0->root.vtable[3] ) (default0);
  printf("-- Update Method\n");
  ( (widget_update_proc) btn0->root.vtable[1] ) (btn0);
  ( (widget_update_proc) btn1->root.vtable[1] ) (btn1);
  ( (widget_update_proc) default0->root.vtable[1] ) (default0);

  // Free Data
  free(btn0);
  free(btn1);
  free(default0);

  // Exit
  return 0;
}