Boost logo

Boost :

Subject: [boost] [Library idea] Type-safe Flagsets
From: Julien Vernay (julienvernay99_at_[hidden])
Date: 2018-06-24 18:43:21

Hello everybody,

I want to know if some people are interested in a type-safe flagsets. I
define flagsets as a container similar to a bitset, but where each bit has
a meaning.

For example, we could have in some game :

    enum Direction : unsigned {
        UP = 0b0001,
        DOWN = 0b0010,
        LEFT = 0b0100,
        RIGHT = 0b1000
    unsigned player_dir = UP | RIGHT;
    if ((player_dir & (UP|LEFT)) == (UP|LEFT))
    player_dir &= ~UP; //removing UP from player_dir

I think there are some issues with this kind of flagset :
- you need to think about power-of-twos when attributing values to your
- you need to use bitwise operations which can be non-trivial, even for
removing only one flag.
- it is not safe:
    for example with another flagset using the flag INIT_VIDEO (in a window
library like SDL),
    you can do " UP | INIT_VIDEO " even if the two flagsets have nothing in

With a type-safe flagset library, this code would become :

    namespace Direction {
        struct UP;
        struct DOWN;
        struct LEFT;
        struct RIGHT;
        using Type = flagset<unsigned, UP, DOWN, LEFT, RIGHT>;
//unsigned is the underlying type
    auto player_dir = Direction::Type::Value<Direction::UP, Direction::RIGHT>();
    if (player_dir.allOf<Direction::UP,Direction::LEFT>())

It is more verbose but also more safe and easy to use.

I actually have a partial implementation on GitHub, supporting read, write
and conversions to strings :

I would like to know if some people are interested in this kind of library,
and if yes, what they think about the current syntax/implementation.

Thanks for reading !

Boost list run by bdawes at, gregod at, cpdaniel at, john at