to_bitmask.cpp

#include <iostream>
#include <bitset>
#include <unordered_set>
#include <climits>

constexpr auto uint64_bits = CHAR_BIT * sizeof(uint64_t);
using show_bits = std::bitset<uint64_bits>;

uint64_t to_bitmask(const std::unordered_set<uint8_t>& from)
{
    uint64_t bits = 0;
    for (const auto i : from)
    {
        bits |= (uint64_t(1) << i);
    }
    return bits;
}

int main()
{
    std::cout << show_bits(to_bitmask({ 0, 2, 3, 32 })) << std::endl;
}