CTRP and C++ template magic for compile-time control

example.cpp

#include <iostream>

enum class msg_count_status_t
   {
      undefined,
      defined
   };

struct basic_data_t
   {
      int to_extract_{};
      int to_handle_{};

      int common_payload_{};
   };

template<typename Basic_Data>
class basic_data_holder_t
   {
   private :
      Basic_Data data_;

   protected :
      void set_to_extract(int v) { data_.to_extract_ = v; }
      void set_to_handle(int v) { data_.to_handle_ = v; }

      void set_common_payload(int v) { data_.common_payload_ = v; }

      const auto & data() const { return data_; }

   public :
      basic_data_holder_t() = default;

      basic_data_holder_t(Basic_Data data) : data_{std::move(data)} {}

      int to_extract() const { return data_.to_extract_; }
      int to_handle() const { return data_.to_handle_; }

      int common_payload() const { return data_.common_payload_; }
   };

template<typename Data, typename Derived>
class basic_params_t : public basic_data_holder_t<Data>
   {
      using base_type = basic_data_holder_t<Data>;

   public :
      using actual_type = Derived;
      using data_type = Data;

   protected :
      actual_type & self_reference()
         { return static_cast<actual_type &>(*this); }

      decltype(auto) clone_as_defined()
         {
            return self_reference().template clone_if_necessary<
                  msg_count_status_t::defined >();
         }

   public :
      basic_params_t() = default;

      basic_params_t(data_type data) : base_type{std::move(data)} {}

      decltype(auto) handle_all()
         {
            this->set_to_handle(0);
            return clone_as_defined();
         }

      decltype(auto) handle_n(int v)
         {
            this->set_to_handle(v);
            return clone_as_defined();
         }

      decltype(auto) extract_n(int v)
         {
            this->set_to_extract(v);
            return clone_as_defined();
         }

      actual_type & common_payload(int v)
         {
            this->set_common_payload(v);
            return self_reference();
         }
      using base_type::common_payload;
   };

struct receive_specific_data_t final : public basic_data_t
   {
      int receive_payload_{};

      receive_specific_data_t() = default;

      receive_specific_data_t(int v) : receive_payload_{v} {}
   };

template< msg_count_status_t Msg_Count_Status >
class receive_specific_params_t final
   : public basic_params_t<
         receive_specific_data_t,
         receive_specific_params_t<Msg_Count_Status> >
   {
      using base_type = basic_params_t<
            receive_specific_data_t,
            receive_specific_params_t<Msg_Count_Status> >;

   public :
      template<msg_count_status_t New_Msg_Count_Status>
      std::enable_if_t<
            New_Msg_Count_Status != Msg_Count_Status,
            receive_specific_params_t<New_Msg_Count_Status> >
      clone_if_necessary() const
         {
            return { this->data() };
         }

      template<msg_count_status_t New_Msg_Count_Status>
      std::enable_if_t<
            New_Msg_Count_Status == Msg_Count_Status,
            receive_specific_params_t& >
      clone_if_necessary()
         {
            return *this;
         }

      receive_specific_params_t(int receive_payload)
         :  base_type{ typename base_type::data_type{receive_payload} }
         {}

      receive_specific_params_t(typename base_type::data_type data)
         :  base_type{ std::move(data) }
         {}

      int receive_payload() const { return this->data().receive_payload_; }
   };

receive_specific_params_t<msg_count_status_t::undefined>
make_receive_params(int receive_payload)
   {
      return {receive_payload};
   }

template<msg_count_status_t Msg_Count_Status>
void do_something(const receive_specific_params_t<Msg_Count_Status> & params)
   {
      static_assert(
            Msg_Count_Status == msg_count_status_t::defined,
            "message count to be processed/extracted should be defined "
            "by using handle_all()/handle_n()/extract_n() methods" );

      std::cout << "to_handle: " << params.to_handle() << std::endl;
      std::cout << "to_extract: " << params.to_extract() << std::endl;
      std::cout << "common_payload: " << params.common_payload() << std::endl;
      std::cout << "receive_payload: " << params.receive_payload() << std::endl;
   }

int main()
   {
      do_something( make_receive_params(42).handle_all().common_payload(13) );
   }

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