Print offsets of object field in Nim

printoffsets.c

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

#define PRINTOFFSET(typ, field) printf("%s(%ld): %ld\n", #field, sizeof(((typ*)0)->field), offsetof(typ, field))

typedef struct {
  char a;
  int b;
  short c;
  double d;
} someStrct;

int main() {
  PRINTOFFSET(someStrct, a);
  PRINTOFFSET(someStrct, b);
  PRINTOFFSET(someStrct, c);
  PRINTOFFSET(someStrct, d);

  return 0;
}

printoffsets.nim

proc printSizeAndOffset(x: typedesc[object | tuple]) =
  echo x, ':'
  for name, field in cast[ptr x](0)[].fieldPairs:
    echo "  ", name, "(", sizeof(field), "): ", cast[uint](addr field)

(a:10, b:"aa").typeof.printSizeAndOffset

type
  someObj = object
    a: char
    b: int
    c: int16
    d: float

someObj.printSizeAndOffset