Re: [boost] interest in a flag library

Subject: Re: [boost] interest in a flag library
From: Masse Nicolas (masse.nicolas_at_[hidden])
Date: 2018-11-05 20:36:11

Le lun. 29 oct. 2018 Ã  00:01, Arvid Norberg <arvid.norberg_at_[hidden]> a
écrit :

> 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
>

Hi,

I had a quick look at your presentation. While I do think that using a
class is an option, I still prefer to use enums for that, for several
reasons:
- The use of an enum for the values makes a clear differences betwen them
and the flags (which are a combinaison of those values).
- The use of enums allows to scope the values inside their name, while
using your solution makes them visible directly inside your namespace.
- The need of a tag struct is be a bit odd, espescially for now comers
Anyway, I should say I like the idea of having an helper class to avoid the
specification of to power of two. As a consequence, I will probably add
something similar in my code.

Nicolas.