struct A {
   inline constexpr A(int a) : _a{a} {}

   int _a;
};

#define DEFINE_A(name, value) \
   inline constexpr static A name() noexcept {\
       return A(value);\
   }

#define Register_A(name) std::pair{#name, name}

struct Registry {
   DEFINE_A(one, 1)
   DEFINE_A(two, 2)
   DEFINE_A(three, 3)

   static constexpr const std::array m_values {
       Register_A(one),
       Register_A(two),
       Register_A(three)
   };
};

int main() {
  std::cout << Registry::one()._a;
}