use const ref

Author: cmnrd

include/reactor-cpp/multiport.hh

@@ -35,7 +35,7 @@ private:
   [[nodiscard]] auto get_set_callback(std::size_t index) noexcept -> PortCallback;
   const PortCallback clean_callback_{[this]([[maybe_unused]] const BasePort& port) { this->reset(); }};
 
-  [[nodiscard]] auto get_clean_callback() const noexcept -> PortCallback { return clean_callback_; }
+  [[nodiscard]] auto get_clean_callback() const noexcept -> const PortCallback& { return clean_callback_; }
 
 protected:
   [[nodiscard]] inline auto present_ports() const -> const auto& { return present_ports_; }