offset to base

offset_of_base.cpp

#include <cstddef>
#include <cstdio>

struct A
{
    int a;
};

struct B
{
    int b;
};

struct C : A, B
{
    int c;
};

const C *my_c = reinterpret_cast<C *>(alignof(C));

const ptrdiff_t offset_from_c_to_a =
    reinterpret_cast<ptrdiff_t>(
        reinterpret_cast<char const *>(static_cast<A const *>(my_c))
            - reinterpret_cast<char const *>(my_c));

const ptrdiff_t offset_from_c_to_b =
    reinterpret_cast<ptrdiff_t>(
        reinterpret_cast<char const *>(static_cast<B const *>(my_c)) -
            reinterpret_cast<char const *>(my_c));

B *b_from_opaque_c_pointer(void *c_pointer)
{
    return reinterpret_cast<B *>((char *)c_pointer + offset_from_c_to_b);
}

int main()
{
    C data;
    data.a = 1;
    data.b = 2;
    data.c = 3;

    auto &data_as_b = *b_from_opaque_c_pointer(&data);
    printf("data_as_b.b = %d\n", data_as_b.b);
}