static_assert(_offsetof_(MyClass, my_field) == 0, "my_field must be first field");

constexpr_offsetof.hpp

#pragma once


#ifdef _MSC_VER
#define _offsetof_(s, m) offsetof(s, m)
#else
namespace detail {

template<typename T> struct declval_helper { static T value; };

template<typename T, typename Z, Z T::*MPtr>
struct offset_helper {
	using TV = declval_helper<T>;
	char for_sizeof[
		(char *)&(TV::value.*MPtr) -
		(char *)&TV::value
	];
};

template<typename T, typename Z, Z T::*MPtr>
constexpr size_t offset_of() {
	return sizeof(offset_helper<T, Z, MPtr>::for_sizeof);
}

}

#define _offsetof_(s, m) vm::detail::offset_of<s, decltype(s::m), &s::m>()

#endif // _MSC_VER