Offset of pointer to member

Question

template<class T, typename U> ptrdiff_t foo(T U::* m)
{
    // return offset
}

How I can get the offset of the field 'm' in this context? I would prefer to use am compile-time expression.

Thanks in advance for any help. Best regards

Answer 1

Sounds like you're looking for the offsetof() macro.

Answer 2

@Michael J

Thanks for your answer. This wasn't exactly what I was looking for, but it gives me the inspiration to doing that:

template<class T, typename U>
std::ptrdiff_t member_offset(U T::* member)
{
    return reinterpret_cast<std::ptrdiff_t>(
        &(reinterpret_cast<T const volatile*>(NULL)->*member)
    );
}

Answer 3

The simple answer is that you can't. If the type U is a POD, you can use the macro offsetof, but formally, it's undefined behavior if the type isn't a POD: depending on the compiler, you'll get a compile time error, or simply wrong results some of the time. And you can't use it on a pointer to member. You have to invoke it with the name of the class, and the name of the member.