Improved enum bitfield thanks to feedback from Boost dev list

v2_gen2.cpp

#include <stdio.h>
#include <type_traits>

#define BOOST_CXX14_CONSTEXPR constexpr

template<class Enum> struct bitfield : public Enum
{
  using enum_type = typename Enum::enum_type;
  using underlying_type = std::underlying_type_t<enum_type>;
private:
  underlying_type _value;
public:
  //! Default construct to all bits zero
  constexpr bitfield() : _value(0) { }
  //! Implicit construction from the C style enum
  constexpr bitfield(enum_type v) : _value(v) { }
  //! Explicit construction from the underlying type of the C enum
  explicit constexpr bitfield(underlying_type v) : _value(v) { }

  //! Permit explicit casting to the underlying type
  explicit constexpr operator underlying_type() const { return _value; }
  //! Test for non-zeroness
  explicit constexpr operator bool() const { return !!_value; }
  //! Test for zeroness
  constexpr bool operator !() const { return !_value; }

  //! Performs a bitwise NOT
  constexpr bitfield operator ~() const noexcept { return bitfield(~_value); }
  //! Performs a bitwise AND
  constexpr bitfield operator &(bitfield o) const noexcept { return bitfield(_value&o._value); }
  //! Performs a bitwise AND
  BOOST_CXX14_CONSTEXPR bitfield &operator &=(bitfield o) noexcept { _value &= o._value; return *this; }
  //! Performs a bitwise OR
  constexpr bitfield operator |(bitfield o) const noexcept { return bitfield(_value | o._value); }
  //! Performs a bitwise OR
  BOOST_CXX14_CONSTEXPR bitfield &operator |=(bitfield o) noexcept { _value |= o._value; return *this; }
  //! Performs a bitwise XOR
  constexpr bitfield operator ^(bitfield o) const noexcept { return bitfield(_value ^ o._value); }
  //! Performs a bitwise XOR
  BOOST_CXX14_CONSTEXPR bitfield &operator ^=(bitfield o) noexcept { _value ^= o._value; return *this; }
};

#define BOOST_AFIO_BITFIELD_BEGIN(type) \
struct type##_base \
{ \
  enum enum_type

#define BOOST_AFIO_BITFIELD_END(type) \
;}; \
using type = bitfield<type##_base>; 

BOOST_AFIO_BITFIELD_BEGIN(flag)
{
  none=0,
  delete_on_close=1,
  disable_safety_fsyncs=2
}
BOOST_AFIO_BITFIELD_END(flag)

int main(void)
{
  flag f(flag::disable_safety_fsyncs);
  flag f2(f&flag::none);
  f2|=flag::delete_on_close;
  constexpr flag f3(flag::disable_safety_fsyncs);
  constexpr flag f4(f3&flag::none);
  printf("f=%d, f2=%d, f3=%d, f4=%d\n", (unsigned) f, (unsigned) f2, (unsigned) f3, (unsigned) f4);
  if(f)
    printf("f is null\n");
  return 0;
}