2405C++,编译期根据成员变量地址获取其名称

原文

最近根据iguana的一个需求,给iguana增加了一个特征,支持在编译期根据成员变量地址取其名字.
用法如下:

struct point_t {
  int x;
  double y;
};
REFLECTION(point_t, x, y);
void test_name_of() {
  constexpr auto s1 = iguana::name_of<&point_t::y>();
  static_assert(s1 == "y");
  constexpr auto s2 = iguana::name_of<&person::name>();
  static_assert(s2 == "name");
}

iguana可在编译期取反射类型的串数组,如果能根据成员变量地址取对应索引,则就可用索引从数组中得到对应名字.

name_of的实现:

template <class T>
struct member_tratis {};
template <class T, class Owner>
struct member_tratis<T Owner::*> {
  using owner_type = Owner;
  using value_type = T;
};
template <auto member>
constexpr auto name_of() {
  using T = typename member_tratis<decltype(member)>::owner_type; //#1
  using M = Reflect_members<T>;
  constexpr auto s = M::arr()[index_of<member>()]; //#2
  return std::string_view(s.data(), s.size());
}

#1代码根据成员变量地址的类型萃取出其父类型.
#2代码,index_of<member>()根据成员变量地址获得对应索引.index_of的实现:

template <auto member>
constexpr size_t index_of() {
  using namespace detail;
  using T = typename member_tratis<decltype(member)>::owner_type;
  using M = Reflect_members<T>;
  constexpr auto tp = M::apply_impl(); //#1
  constexpr size_t Size = std::tuple_size_v<decltype(tp)>;
  constexpr size_t index = member_index(member, tp); //#2
  static_assert(index < Size, "out of range");
  return index;
}

index_of的实现思路是,遍历反射类型所有成员变量地址生成的元组,然后判断,如果给定成员变量地址和元组中的某个元素相等,则返回该索引.

#1代码,在编译期获得反射类型所有成员变量地址的元组,#2代码调用member_index得到对应索引.member_index的实现:

template <typename T, typename U>
constexpr bool get_index_imple(T ptr, U ele) {
  if constexpr (std::is_same_v< typename iguana::member_tratis<decltype(ptr)>::value_type, typename iguana::member_tratis< decltype(ele)>::value_type>) {
    if (ele == ptr) {
      return true;
    }
    else {
      return false;
    }
  }
  else {
    return false;
  }
}
template <typename T, typename Tuple, size_t... I>
constexpr size_t member_index_impl(T ptr, Tuple &tp, std::index_sequence<I...>) {
  bool r = false;
  size_t index = 0;
  ((void)(!r && (r = get_index_imple(ptr, std::get<I>(tp)), !r ? index++ : index, true)),
   ...);
  return index;
}
template <typename T, typename Tuple>
constexpr size_t member_index(T ptr, Tuple &tp) {
  return member_index_impl(
      ptr, tp,
      std::make_index_sequence<
          std::tuple_size_v<std::decay_t<decltype(tp)>>>{});
}

member_index_impl函数中遍历元组,然后折叠式调用get_index_imple来检查给定成员变量地址的ptr和元组中的元素的地址是否相等,如果相等则index++,否则索引不变,如果检查地址相等,则后面不会再进入到get_index_imple函数.