Polymorphism by GCC transparent union extension (works in clang as well)

union.c

typedef enum GenericType GenericType;
typedef struct A A;
typedef struct B B;

enum GenericType
{
  TYPE_A = 0,
  TYPE_B = 1,
};

struct A
{
  GenericType type;
  ...
};

struct B
{
  GenericType type;
  ...
};

union GenericPtr
{
  GenericType* type;
  A* A;
  B* B;
} __attribute__((__transparent_union__));

void foo (GenericPtr ptr) {
  switch(*ptr.type) {
    case TYPE_A:
	  ptr.A->a_elements;
	  break;
	case TYPE_B:
	  ptr.B->b_elements;
	  break;
	default:
	  assert(false);
  }
}

A* a;
B* b;
foo(a); // works
foo(b); // works