Automatically obtaining the address of another member of the same object

[Lindley]

Let's say I create an object:

template <typename T>
struct RelativeLoc
{
    ptrdiff_t offset;
    RelativeLoc(T* other)
    {
         offset = reinterpret_cast<const char*>(other) - reinterpret_cast<char*>(this);
    }
    T* getOtherAddr() const
    {
         return reinterpret_cast<T*>(reinterpret_cast<const char*>(this) + offset);
    }
}

And then put this in an object:

struct MyClass
{
    std::vector<short> stuff;
    RelativeLoc<std::vector<short>> relative;
    MyClass(): relative(&stuff);
};

Is this safe?

The goal here is to have some other object in MyClass which always needs a reference to the "stuff" in the same object. I'd like to develop a way of automatically updating this on copies/moves which doesn't require writing an entire copy constructor for MyClass. If I can fill RelativeLoc with callbacks or something, it would fit the bill.