Hiding members in a C struct

Question

I\'ve been reading about OOP in C but I never liked how you can\'t have private data members like you can in C++. But then it came to my mind that you could create 2 structu

Answer 1

I found that bit-field might be a good solution if you really want to hide something.

struct person {
    unsigned long :64;
    char          *name;
    int           age;
};

struct wallet {
    char *currency;
    double balance;
};

The first member of struct person is an unnamed bit-field. used for a 64-bit pointer in this case. It's completely hidden and cannot be accessed by struct variable name.

Because of the first 64-bit in this struct is unused, so we can use it as a private pointer. We can access this member by its memory address instead of variable name.

void init_person(struct person* p, struct wallet* w) {
    *(unsigned long *)p = (unsigned long)w;
    // now the first 64-bit of person is a pointer of wallet
}

struct wallet* get_wallet(struct person* p) {
    return (struct wallet*)*(unsigned long *)p;
}

A small working example, tested on my intel mac:

//
// Created by Rieon Ke on 2020/7/6.
//

#include 
#include 
#include 


#if __x86_64__ || __LP64__
#define PRIVATE_SET(obj, val) *(unsigned long *) obj = (unsigned long) val;
#define PRIVATE_GET(obj, type) (type)*(unsigned long *) obj;
#define PRIVATE_POINTER unsigned long:64
#else
#define PRIVATE_SET(obj, val) *(unsigned int *) obj = (unsigned int) val;
#define PRIVATE_GET(obj, type) (type)*(unsigned int *) obj;
#define PRIVATE_POINTER unsigned int:32
#endif

struct person {
    PRIVATE_POINTER;
    char *name;
    int age;
};

struct wallet {
    char *currency;
    double balance;
};

int main() {

    struct wallet w;
    w.currency = strdup("$$");
    w.balance = 99.9;

    struct person p;
    PRIVATE_SET(&p, &w) //set private member

    p.name = strdup("JOHN");
    p.age = 18;

    struct wallet *pw = PRIVATE_GET(&p, struct wallet*) //get private member

    assert(strcmp(pw->currency, "$$") == 0);
    assert(pw->balance == 99.9);

    free(w.currency);
    free(p.name);

    return 0;
}