Flag – Abnormal Template Library

Flag

C++ enum class lacks option composition, so it can’t be used directly as flag. abl::flag is an elegant tool to bring flag functionality.

Caution

Imports <concepts> and <type_traits>.

Tip

To import use #include "atl/types/flag.hpp".

Code

Godbolt example

`abl::flag<E> : abl::enum_wrapper<E>`

Functionality extension for enum class E, where std::is_enum_v<E> is satisfied. The base class provides bitwise & (&=) and | (|=) operators, .has() method for inclusion check, and .get_value() to obtain underlying integer value. Flag class provides functions for redefining basic options and composite flags.

Important

abl::flag needs an enum for basic flags. For correct work make sure that underlying values occupy different bits. The simplest way to do so is to define one option equal to 0, and the following as 1 << n, starting with n = 0.

Usage

Whether the solution is elegant, as a developer you will need to write one weird struct which you will actually use. The struct will have redefined basic flags together with complex combined flags.

Tip

An enum is a mandatory requirement, and you might not want to have it in the global scope. The recommended way is to define a nested enum class within the private section of your struct or class.

struct Colors {
   private:
    enum class BasicColors {
        NONE = 0,
        RED = 1 << 0,
        GREEN = 1 << 1,
        BLUE = 1 << 2,
    };

   public:
    using value_type = abl::flag<BasicColors>;

    // Reexport
    static constexpr auto RED =
        value_type::template value<BasicColors::RED>();
    static constexpr auto GREEN =
        value_type::template value<BasicColors::GREEN>();
    static constexpr auto BLUE =
        value_type::template value<BasicColors::BLUE>();

    // Reexport and rename
    static constexpr auto BLACK =
        value_type::template value<BasicColors::NONE>();

    // Mix
    static constexpr auto WHITE = value_type::template combine<
        BasicColors::RED, BasicColors::GREEN, BasicColors::BLUE>();
};

Code

Godbolt example