Специализация vector<bool>

void flip() noexcept;

std::vector<bool> arr = {true, false};
arr.flip();
std::cout << std::boolalpha << arr[0] << ' '
          << arr[1] << std::endl; // false true

static void swap(reference left, reference right) noexcept;

std::vector<bool> arr = {true, false};
arr.swap(arr[0], arr[1]);
std::cout << std::boolalpha << arr[0] << ' '
          << arr[1] << std::endl; // false true