Object-oriented programming in C (without casts)

oopc.c

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

void *malloc_safe(size_t n)
{
    void *p = malloc(n);
    if (!p) {
        fprintf(stderr, "malloc(%zd) failed\n", n);
        abort();
    }
    return p;
}

struct type_a {
    int a;
};

struct type_b {
    int b;
};

enum typeid {
    TYPE_A,
    TYPE_B,
};

struct virt {
    enum typeid typeid;

    union {
        char _mark[0];
        struct type_a a;
        struct type_b b;
    };
    /* Don't add anything after this union */
};

#define virt_sizeof(type)   \
    (offsetof(struct virt, _mark) + sizeof(type))

struct virt *new_a(int a)
{
    struct virt *o = malloc_safe( virt_sizeof(struct type_a) );
    o->typeid = TYPE_A;
    o->a.a = a;
    return o;
}

struct virt *new_b(int b)
{
    struct virt *o = malloc_safe( virt_sizeof(struct type_b) );
    o->typeid = TYPE_B;
    o->b.b = b;
    return o;
}

static void handle_a(struct type_a *o)
{
    printf("%s a=%d\n", __FUNCTION__, o->a);
}

static void handle_b(struct type_b *o)
{
    printf("%s b=%d\n", __FUNCTION__, o->b);
}

void handle_virt(struct virt *o)
{
    switch (o->typeid) {
        case TYPE_A:    return handle_a(&o->a);
        case TYPE_B:    return handle_b(&o->b);
        default:
            fprintf(stderr, "Invalid type %d\n", o->typeid);
            abort();
    }
}

int main(void)
{
    struct virt *o;

    o = new_a(33);
    handle_virt(o);
    free(o);

    o = new_b(66);
    handle_virt(o);
    free(o);

    return 0;
}