Re: [boost] interest in a flag library

Subject: Re: [boost] interest in a flag library
From: Arvid Norberg (arvid.norberg_at_[hidden])
Date: 2018-10-28 23:01:29

On Thu, Oct 25, 2018 at 11:28 PM Masse Nicolas via Boost <
boost_at_[hidden]> wrote:

> Hi all,
>
> [...]
>
> enum struct option : uint
> {
>     none = 0,
>     first = 1,
>     second = 2,
>     third = 4,
>     forth = 8
> };
>
> static constexpr flags<option> var1 = option::first;
> static constexpr flags<option> var2 = option::second | option::third;
> static constexpr auto var3 = var1 | var2; /* type will be resolved into
> flags<option> */
> static constexpr auto var4 = option::first | option::second |
> option::third; /* works too */
>

I think a better approach is to fully define the flag type as a class
(without the enum). It gives you slightly less typing for the user. Such
flags type would be defined similar to how the NamedType library is used.
There's also a neat trick to not have to manually specify powers of two
(and possibly get it wrong) using a separate type for bit numbers.

using my_flags_type = flag_type<std::uint8, struct my_flags_tag>;
constexpr my_flags_type red = 0_bit;
constexpr my_flags_type green = 1_bit;
constexpr my_flags_type blue = 2_bit;

I describe such a flag type in this lightning talk:
https://www.youtube.com/watch?v=NGrnKr9rSz4

-- 
Arvid Norberg