#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});