Retrieving parent object from structure's member

gistfile1.txt

// Example program
#include <iostream>
#include <string>

#include <cstddef>
#include <iostream>

using namespace std;

template <typename Owner, typename Tag>
struct offset_aware
{
    Owner *owner()
    {
        return reinterpret_cast<Owner *>(
            reinterpret_cast<char *>(this) - Tag::offset());
    }
};

#define OFFSET_AWARE(Owner, name) \
    struct name ## _tag { \
        static ptrdiff_t offset() { \
            return offsetof(Owner, name); \
        } \
    }; \
    offset_aware<Owner, name ## _tag> name

struct foo
{
    int x;
    OFFSET_AWARE(foo, a);
    OFFSET_AWARE(foo, b);
    OFFSET_AWARE(foo, c);
    int y;
};

int main()
{
    foo f;
    
    cout << "foo f = " << &f << endl
        << "f.a: owner = " << f.a.owner() << endl
        << "f.b: owner = " << f.b.owner() << endl
        << "f.c: owner = " << f.c.owner() << endl
        << "Size of foo = " << sizeof(foo) << endl;
}