Re: [boost] Improved typesafe bitfield

On 6 Feb 2016 at 12:00, Vicente J. Botet Escriba wrote:

Do you see something wrong having two classes one for the bits enum and one for the set of those bits enums? Mixing both, the element and the set in a single class, has some drawbacks. The flag type is not an enum, that is, is_enum<flag>::value is false.

Tongari's solution split them as you describe. I'm not against that solution, it's just a distinction I need to care about during using them and I don't want to care. I just want C bitfields which don't accidentally substitute for integers and therefore lead to bugs.

I don't see the added value of the macros and the inheritance

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

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

template<class Enum> struct bitfield : public Enum

Why do you need to inherit from Enum? to introduce the enumerators inside the defined type?

Exactly. I'm effectively wrapping an enum into a typesafe struct with member functions.

What about

template<class Enum> struct bitfield_set { using enum_type = Enum; using underlying_type = std::underlying_type_t; private: underlying_type _value;

...

enum class flag = unsigned { none=0, delete_on_close=1, disable_safety_fsyncs=2 };

using flags = bitfield_set<flag>;

Example usage:

flags f(flag::disable_safety_fsyncs); flags f2(f&flag::none); f2|=flag::delete_on_close; constexpr flags f3(flag::disable_safety_fsyncs); constexpr flags f4(f3&flag::none); if(f) ...

That's absolutely fine if you don't mind distinct types for the enumerators and the flags. For me personally, I find myself typing the wrong one in my code, and that's a source of compile errors and therefore wasted productivity. What is very annoying is that global operator overloads on a class enum gets you 98% of the way there. If only one could overload a global explicit bool operator ... we'd have our typesafe bit flags type. Niall -- ned Productions Limited Consulting http://www.nedproductions.biz/ http://ie.linkedin.com/in/nialldouglas/