Offset from member pointer without temporary instance

Question

I would like to get the offset of a standard layout member variable when provided with a poiner to that variable. I cannot use offsetof since I have a pointer and n

Answer 1

How about:

template
struct {
  ptrdiff_t get_offset( int (T::*mem) )
  {
    union {
      int used;
      T unused;
    } dummy;
    return reinterpret_cast(&(dummy.unused.*mem)) 
      - reinterpret_cast(&dummy.unused);
  }
};

The address of a union member doesn't depend on the union member being constructed. Works already in C++03, but then only for PODs.