Member function with pure C

impl.c

#include "impl.h"
#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

struct obj_impl
{
  int a;
};

static void add_and_print(struct obj_impl *o, int val)
{
        assert(o != 0 && "Invalid instance");
        o->a += val;
        printf("a = %d\n", o->a);
}

static void substract_and_print(struct obj_impl *o, int val)
{
        assert(o != 0 && "Invalid instance");
        o->a -= val;
        printf("a = %d\n", o->a);
}

static fn_ptr_table_t mfn_table = {
        .add_and_print = &add_and_print,
        .substract_and_print = &substract_and_print
};

obj_t create_obj(int a)
{
        obj_t wobj = {
                .inst_ = malloc(sizeof(struct obj_impl)),
                .fn_ptr_table_ = &mfn_table
        };
        wobj.inst_->a = a;
        return wobj;
}

void destroy_obj(obj_t *obj)
{
        free(obj->inst_);
        obj->inst_ = 0;
}

impl.h

#ifndef IMPL_H_
#define IMPL_H_

struct obj_impl;

typedef struct fn_ptr_table
{
  void (*add_and_print)(struct obj_impl *, int);
  void (*substract_and_print)(struct obj_impl*, int);
} fn_ptr_table_t;

typedef struct obj
{
  fn_ptr_table_t const *fn_ptr_table_;
  struct obj_impl *inst_;
} obj_t;

obj_t create_obj(int a);
void destroy_obj(obj_t *obj);
#endif

main.c

#include "impl.h"
#include <stdio.h>

#define call_mfn(Handle, Mfn, ...) \
        Handle.fn_ptr_table_->Mfn(Handle.inst_, __VA_ARGS__)

int main()
{
        obj_t wobj = create_obj(123);
        printf("sizeof obj_t: %zu\n", sizeof(obj_t));
        call_mfn(wobj, add_and_print, 456);
        call_mfn(wobj, substract_and_print, 456);
        destroy_obj(&wobj);

        call_mfn(wobj, add_and_print, 455);
        return 0;
}