#include <bit>  // P0237R9, targeting C++2a

namespace nonstd {

template<class T>
struct compressed_vector_traits;

template<>
struct compressed_vector_traits<bool> {
    static constexpr int width_in_bits = 1;
    static bool from_bits(std::bit_pointer<const std::byte> first) {
        return *first;
    }
    static void to_bits(bool value, std::bit_pointer<std::byte> first) {
        *first = value;
    }
};

template<class T, class Alloc = std::allocator<std::byte>,
         class Traits = compressed_vector_traits<T>>
class compressed_vector {
    // ...
};

} // namespace nonstd

struct Cardinal {
    enum Which : int { NORTH, SOUTH, EAST, WEST };
    Which direction : 2;
};

template<>
struct nonstd::compressed_vector_traits<Cardinal> {
    static constexpr int width_in_bits = 2;
    static Cardinal from_bits(std::bit_pointer<const std::byte> first) {
        int result = *first++;
        result = (2 * result) + *first++;
        return Cardinal{ Which(result) };
    }
    static void to_bits(const Cardinal& value, std::bit_pointer<std::byte> first) {
        *first++ = (int(value.direction) / 2);
        *first++ = (int(value.direction) % 2);
    }
};

// uses only 1MB, not 8MB
nonstd::compressed_vector<bool> bools(8'000'000, true);

// uses only 2MB, not 8MB
nonstd::compressed_vector<Cardinal> cards(8'000'000, Cardinal{NORTH});