better-enums/doc/demo/102-bitset.md

Bit sets

If you want to use std::bitset or a similar library to use enums as keys into a bit set, you need to know the number of bits at compile time. You can easily automate this with Better Enums, even when constants are not declared in increasing order.

---

We simply need to find the maximum value of any given enum type.

#include <bitset>
#include <enum.h>

template <typename Enum>
constexpr Enum max_loop(Enum accumulator, size_t index)
{
    return
        index >= Enum::_size ? accumulator :
        Enum::_values()[index] > accumulator ?
            max_loop<Enum>(Enum::_values()[index], index + 1) :
            max_loop<Enum>(accumulator, index + 1);
}

template <typename Enum>
constexpr Enum max()
{
    return max_loop<Enum>(Enum::_values()[0], 1);
}

And use that to declare a bit set template:

template <typename Enum>
using EnumSet = std::bitset<max<Enum>()._to_integral() + 1>;

Then rest is straightforward. The only issue is that, in $cxx11, it is necessary to keep calling to_integral on the enums when passing them to bitset functions. You may want to implement a more enum-friendly bit set type, or overload unary operator -.

ENUM(Channel, int, Red, Green, Blue)
ENUM(Depth, int, TrueColor = 1, HighColor = 0)

int main()
{
    EnumSet<Channel>    channels;
    EnumSet<Depth>      depths;

    return 0;
}