Re: [boost] How best to implement a bitfield in C++ 14?

Subject: Re: [boost] How best to implement a bitfield in C++ 14?
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2016-02-03 18:47:12

Le 03/02/2016 10:01, Niall Douglas a écrit :
> Dear Boost,
>
> As part of the AFIO v2 rewrite I've been trying to improve on how
> AFIO v1 did bitfields by making better use of constexpr and the type
> system to make usage more natural. I'd appreciate feedback on my
> efforts.
>
>
> AFIO v1 has an example of usage at https://goo.gl/oVLIjU or
> https://gist.github.com/ned14/9ef2a9d00da0ffc877b8 and it took the
> following form:
>
> enum class flag : size_t
> {
>    none=0,
>    delete_on_close=1,
>    disable_safety_fsyncs=2
> };
> BOOST_AFIO_DECLARE_CLASS_ENUM_AS_BITFIELD(flag)
>
> The BOOST_AFIO_DECLARE_CLASS_ENUM_AS_BITFIELD macro declares all the
> sensible global overloads you'd expect, is type safe and you can use
> it without surprise except in one single instance: testing for
> emptiness. The problem is that you cannot declare as a global
> operator an explicit operator bool nor member functions on enums, so
> one cannot do if(f) though if(!f) is fine. In AFIO v1, I used if(!!f)
> and if(!!(f & flag::foo)) everywhere, which is fine if a little
> abtuse.
TBoost.Enums [1] has an alternative design, given an ordinal enum E we 
can create a enum_set<E> [2] that behaves like a bitset<N>, but we can 
use the enumerators instead of the position. I'm not saying it is better 
or worst, I'm just giving the pointer here in case this can help someone.
>
> AFIO v2's current bitfield has an example of usage at
> https://goo.gl/LsjSGD or
> https://gist.github.com/ned14/89ee39c6b8eb5254116a and it takes the
> following form:
>
> struct flag : bitwise_flags<flag>
> {
>    flag() = default;
>    constexpr flag(bitwise_flags<flag> v) noexcept :
> bitwise_flags<flag>(v) { }
>    static constexpr auto none(){ return bit(0);}
>    static constexpr auto delete_on_close(){ return bit(1); }
>    static constexpr auto disable_safety_fsyncs(){ return bit(2); }
> };

What is the bit above? what is the decltype of bit(0)?
Could flag store bit(5)?
>
>
> So, do Boosters think we can actually make a C++ 14 bitfield which:
>
> 1. Is typesafe, so not a C bitfield.
>
> 2. Is convenient for programmers to declare (i.e. little boilerplate
> in a specific bitfield declaration).
>
> 3. Works as you'd expect a bitfield to work, including
> bitfield::flag.
>
> 4. if(bf) works.
>
> 5. Is 100% constexpr and generates zero runtime overhead.
>
>
>
Yes, I believe we need something like that.

Vicente

[1] 
https://htmlpreview.github.io/?https://github.com/viboes/enums/blob/master/libs/enums/doc/html/index.html

[2] 
https://htmlpreview.github.io/?https://github.com/viboes/enums/blob/master/libs/enums/doc/html/index.html#header.boost.enums.containers.enum_set_hpp

P.S. This library is an old one that needs to be refactored to take 
advantage of C++14 and is not ready for review :(.