loesung-2e.c

#include <stdio.h>

#define ANIMAL     0
#define WITH_WINGS 1
#define WITH_LEGS  2

typedef struct
{
  int type;
  char *name;
} base;

typedef struct
{
  int type;
  char *name;
  int wings;
} with_wings;

typedef struct
{
  int type;
  char *name;
  int legs;
} with_legs;

typedef union
{
  base b;
  with_wings w;
  with_legs l;
} animal;

int main (void)
{
  animal *a[2];

  animal duck;
  a[0] = &duck;
  duck.b.type = WITH_WINGS;
  duck.b.name = "duck";
  duck.w.wings = 2;

  animal cow;
  a[1] = &cow;
  cow.b.type = WITH_LEGS;
  cow.b.name = "cow";
  cow.l.legs = 4;

  for (int i = 0; i < 2; i++)
    if (a[i]->b.type == WITH_LEGS)
      printf ("A %s has %d legs.\n", a[i]->b.name,
              a[i]->l.legs);
    else if (a[i]->b.type == WITH_WINGS)
      printf ("A %s has %d wings.\n", a[i]->b.name,
              a[i]->w.wings);
    else
      printf ("Error in animal: %s\n", a[i]->b.name);

  return 0;
}