polymorphic.c

#include <stdio.h>
#include <stdlib.h>

#define ALLOC(t) (t *) malloc(sizeof(t))

typedef unsigned int OBJ;

typedef enum {
  tVector,
  tPoint
} Type;
  
// --- Vector  

typedef struct _Vector {
  int x, y;
  Type t;
} *Vector;

inline Vector
Vector_alloc() {
  return ALLOC(struct _Vector);
}

OBJ
Vector_new(int x, int y) {
  Vector v = Vector_alloc();
  v->x = x; v->y = y;
  v->t = tVector;
  return (OBJ) v;
}

// --- Point

typedef struct _Point {
  int x, y;
  Type t;
} *Point;

inline Point
Point_alloc() {
  return ALLOC(struct _Point);
}

OBJ
Point_new(int x, int y) {
  Point p = Point_alloc();
  p->x = x; p->y = y;
  p->t = tPoint;
  return (OBJ) p;
}

static void
inspect(Vector self) {
  char *type;
  switch (self->t) {
    case tVector : type = "Vector" ;break;
    case tPoint  : type = "Point"  ;break;
  }
  printf("<%s x:%d y:%d>\n", type, self->x, self->y);
}

int 
main(int argc, char **argv)
{
  OBJ a = Vector_new(10, 20);
  inspect(a);
  
  OBJ b = Point_new(30, 40);
  inspect(b);
  
  return 0;
}