typed_union.c

#include <assert.h>
#include <stddef.h>
#include <stdio.h>
#include <stdint.h>

#define tag(x) ((ptrdiff_t)&Tag##x)
#define is(x, T) ((x)->tag == tag(T))
#define as(x, T) ((struct T *)variant_cast((void *)(x), tag(T)))

static inline void*
variant_cast(void * variant_ptr, ptrdiff_t desired_tag) {
  ptrdiff_t * variant_tag = (ptrdiff_t *)variant_ptr;
  assert(*variant_tag == desired_tag);
  return (void *)((char *)variant_ptr + sizeof(ptrdiff_t));
}

#include <errno.h>

struct Error {
  int id;
} TagError;

struct Value {
  float z;
  float w;
} TagValue;

struct Result {
  ptrdiff_t tag;
  union {
    struct Error _;
    struct Value __;
  };
};

struct Result
function_with_possible_error(int n)
{
  struct Result res;
  if (n < 0) {
    res.tag = tag(Error);
    struct Error *e = as(&res, Error);
    e->id = ENOMEM;
    return res;  
  }
  res.tag = tag(Value);
  struct Value *v = as(&res, Value);
  v->w = 5.0f;
  v->z = 2.0f;
  return res;
}
struct Result
function_taking_some(struct Result r, float n)
{
  struct Result res = r;
  if (is(&res,Error))
    return r;
  if (n < 0) {
    res.tag = tag(Error);
    struct Error *e = as(&res, Error);
    e->id = EINVAL;
    return res;  
  }
  struct Value *v = as(&res, Value);
  v->w += n;
  return res;
}
int main() {
  struct Result res = {};
  res = function_with_possible_error(5);
  res = function_taking_some(res, 2.0f);
  if (is(&res, Error)) {
      struct Error *err = as(&res, Error);
      printf("%d\n", err->id);
  } else if (is(&res, Value)) {
      struct Value *v = as(&res, Value);
      printf("%.2f, %.2f\n", v->z, v->w);
  } return 0;
}