Polymorphism in C++ and plain C

polymorph.c

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

struct A {
    struct Atable *vptr;
};
struct Atable {
    int (*answerp)(struct A *this);
};

struct B {
    struct A base;
    double question;
};

int A_answer(struct A *this) {
    return 42;
}
struct Atable A_Atable = { &A_answer };

int B_answer(struct B *this) {
    return this->question;
}
struct Atable B_Atable = { (int (*)(struct A *))&B_answer };
  /* correct for different type of 'this' */

int main(void) {
    struct A *a; struct B *b;
    a = malloc(sizeof(struct A)); a->vptr = &A_Atable;
    b = malloc(sizeof(struct B)); b->base.vptr = &B_Atable;
    b->question = 57;

    struct A *aa = (struct A *)b;
    
    printf("%d %d\n", (*a->vptr->answerp)(a), (*aa->vptr->answerp)(aa));
    
    free(a); free(b);
    return 0;
}

polymorph.cpp

#include <cstdio>

// Virtual destructors in A and B omitted for brevity, although technically it
// means undefined behaviour

class A {
public:
    virtual int answer(void);
};

class B : public A {
public:
    virtual int answer(void);
    int question;
};

int A::answer(void) {
    return 42;
}

int B::answer(void) {
    return this->question;
}

int main(void) {
    A *a = new A();
    B *b = new B();
    b->question = 57;
    
    A *aa = b;
    
    std::printf("%d %d\n", a->answer(), aa->answer());
    
    delete a; delete b;
    return 0;
}