4 ms·

Stateful metaprogramming (compile-time type list)

• kris-jusiak 3y ago
  https://wg21.link/P2996 https://wg21.link/P2996 allows form of stateful meta-programming without using friend injection. Meta-counter is pretty easy - https://godbolt.org/z/91M56a5dd https://godbolt.org/z/91M56a5dd as we just iterate over complete instantiations of the counter class (tricky part is to force the instantiation). The following is a bit more complex example of stateful meta-programming - compile-time type list. Firstly, C++20 version (based on friend injection) - works on gcc,clang,msvc template<class...> struct type_list {}; namespace detail { template<auto> struct nth { auto friend get(nth); auto friend get(nth); }; template<auto N, class T> struct set { auto friend get(nth<N>) { return T{}; } }; template<class T, template<class...> class TList, class... Ts> auto append(TList<Ts...>) -> TList<Ts..., T>; } // namespace detail template<class T, auto N = 0, auto unique = []{}> consteval auto append() { if constexpr (requires { get(detail::nth<N>{}); }) { append<T, N+1, unique>(); } else if constexpr (N == 0) { void(detail::set<N, type_list<T>>{}); } else { void(detail::set<N, decltype(detail::append<T>(get(detail::nth<N-1>{})))>{}); } } template<auto unique = []{}, auto N = 0> consteval auto get_list() { if constexpr (requires { get(detail::nth<N>{}); }) { return get_list<unique, N+1>(); } else if constexpr (N == 0) { return type_list{}; } else { return get(detail::nth<N-1>{}); } } int main() { static_assert(typeid(get_list()) == typeid(type_list<>)); append<int>(); static_assert(typeid(get_list()) == typeid(type_list<int>)); append<float>(); static_assert(typeid(get_list()) == typeid(type_list<int, float>)); } Full example -> https://godbolt.org/z/axPT88e3c https://godbolt.org/z/axPT88e3c Now, C++26 version with the reflection proposal (based on injecting classes with members) - works on EDG template<class...> struct type_list{}; namespace detail { template<auto> struct type_list; consteval auto append(auto new_member) { std::vector<std::meta::info> members{}; for (auto i = 0;; ++i) { if (auto mi = substitute(^type_list, { std::meta::reflect_value(i) }); std::meta::is_incomplete_type(mi)) { std::vector<std::meta::nsdm_description> new_members{}; for (auto member: members) { new_members.push_back({std::meta::type_of(member), {.name = std::meta::name_of(member)}}); } const char name[]{'_', char(i+'0'), 0}; // there are defo better ways to do that new_members.push_back({{new_member}, {.name = std::string_view(name, 2)}}); return define_class(mi, new_members); } else { members = std::meta::nonstatic_data_members_of(mi); } } } consteval auto get_list() { std::vector<std::meta::info> members{}; for (auto i = 0;; ++i) { if (auto mi = substitute(^type_list, { std::meta::reflect_value(i) }); std::meta::is_incomplete_type(mi)) { break; } else { members = std::meta::nonstatic_data_members_of(mi); } } std::vector<std::meta::info> new_members{}; for (auto member : members) { new_members.push_back(std::meta::type_of(member)); } return substitute(^::type_list, new_members); } } // namespace detail template<class T> using append = [:detail::append(^T):]; template<auto = []{}> using get_list = [:detail::get_list():]; int main() { static_assert(typeid(get_list<>) == typeid(type_list<>)); append<int>(); static_assert(typeid(get_list<>) == typeid(type_list<int>)); append<float>(); static_assert(typeid(get_list<>) == typeid(type_list<int, float>)); } Full example -> https://godbolt.org/z/5s8YvYqqr https://godbolt.org/z/5s8YvYqqr