Public/private struct members in C

main.c

#include <stdio.h>
/* #include "private.h" */
#include "public.h"
#include "s.h"

int main() {
  struct s foo;
  init_s(&foo);

  printf("foo.public.a %d\n", foo.public.a);
  printf("foo.public.b %d\n", foo.public.b);

  // won't compile w/o private.h
  /* printf("foo.private.c %d\n", foo.pointer_to_private->c); */
  
  return 0;
}

private.h

struct secretStuff {
   int c;
};

public.h

struct secretStuff; // prototype tells linker this is later defined

struct s {
    struct {
        int a;
        int b;
    } public;
    struct secretStuff *pointer_to_private;
};

s.c

#include "private.h"
#include "public.h"

// A secret-aware function to initialize the struct
void init_s(struct s* new) {
  new->public.a = 25;
  new->public.b = 50;
  new->pointer_to_private->c = 75;
}

s.h

void init_s(struct s* new);