#include <stdio.h>

#define ShapeText(TYPE) void (*new)(struct TYPE*);float (*area)(struct TYPE*)

typedef struct _Shape { // Shape 物件，沒有欄位
  ShapeText(_Shape);
 } Shape;


float ShapeArea(Shape *obj) { return 0; }

void ShapeNew(Shape *obj) {
  obj->new = ShapeNew;
  obj->area = ShapeArea;
}

typedef struct _Circle {
  ShapeText(_Circle);
  float r;
} Circle;

float CircleArea(Circle *obj) { return 3.14 * obj->r * obj->r; }

void CircleNew(Circle *obj) {
  obj->new = CircleNew;
  obj->area = CircleArea;
}

int main() {
  Shape s;
  ShapeNew(&s);  
  Circle c;
  CircleNew(&c);
  c.r = 3.0;
  Shape *list[] = { &s, (Shape*) &c };
  int i;
  for (i=0; i<2; i++) {
    Shape *o = list[i];
    printf("s.area()=%G\n", o->area(o));
  }
}