layout.cc

#include <iostream>
#include <utility>

template <typename T> constexpr size_t align_to(size_t n) {
  return (n + alignof(T) - 1) & -alignof(T);
}

template <typename T> constexpr size_t skip(size_t n) {
  return align_to<T>(n) + sizeof(T);
}

template <typename... T> struct layout_helper;

template <> struct layout_helper<> {

  static constexpr size_t size(size_t n = 0) { return n; }

  template <size_t N> static constexpr size_t offset(size_t n = 0) { return n; }
};

template <typename T, typename... Ts> struct layout_helper<T, Ts...> {

  static constexpr size_t size(size_t n = 0) {
    return layout_helper<Ts...>::size(skip<T>(n));
  }

  template <size_t N> static constexpr size_t offset(size_t n = 0) {
    return N == 0 ? align_to<T>(n)
                  : layout_helper<Ts...>::template offset<N - 1>(skip<T>(n));
  }
};

int main() {
  using layout = layout_helper<int, char, char, float>;

  std::cout << "size: " << layout::size() << std::endl;
  std::cout << "0: " << layout::offset<0>() << std::endl;
  std::cout << "1: " << layout::offset<1>() << std::endl;
  std::cout << "2: " << layout::offset<2>() << std::endl;
  std::cout << "3: " << layout::offset<3>() << std::endl;
  std::cout << "4: " << layout::offset<4>() << std::endl;
  std::cout << "5: " << layout::offset<5>() << std::endl;
}