a simple virtual-table implemented polymorphism simulation in C

virtualTable.c

#include <stdio.h>
#include <stdlib.h>
struct Base;
struct Derived;

int f_Base(struct Base * b);
int f_Derived(struct Derived* d);

typedef struct {
    int (*f)(struct Base *);
} VTable_Base;

VTable_Base vtable_base;
VTable_Base vtable_derived;
int vptr_init() {
    vtable_base.f = f_Base;
    //warning: incompatible pointer types assigning to 'int (*)(struct Base *)' from 'int (struct Derived *)'
    vtable_derived.f = f_Derived;
    return 0;
}

struct Base {
    VTable_Base* vptr;
    int i;
};

struct Derived {
    VTable_Base* vptr;
    struct Base b;
    int j;
};


int f_Base(struct Base * b) {
    printf("base::f, i = %d\n", b->i);
    return b->i;
}

int f_Derived(struct Derived* d) {
    printf("derived::f, i = %d\n", d->b.i);
    return d->b.i;
}

int f(struct Base * b) {
    return b->vptr->f(b);
}

struct Base* make_Base() {
    struct Base * b = malloc(sizeof(struct Base));
    b->vptr = &vtable_base;
    return b;
}

struct Derived* make_Derived() {
    struct Derived * d = malloc(sizeof(struct Derived));
    d->vptr = &vtable_derived;
    return d;
}

int main() {
    vptr_init();
    struct Derived *d = make_Derived();
    d->b.i = 10;
    f((struct Base *)d); //derived::f, i = 10
    free(d);

    struct Base *b = make_Base();
    b->i = 15;
    f(b); //base::f, i = 15
    free(b);
}