use std::optional to control access to Packet data

protocol/artistic/artnet/packet.h

@@ -27,6 +27,7 @@
 #include "config.h"
 #include "bufferstream.h"
 #include "../../esta/rdm/uid.h"
+#include <optional>
 #include <vector>
 
 namespace ARTNET {
@@ -553,27 +554,12 @@ struct Packet
    * @return
    */
   template<class T>
-  std::shared_ptr<T> data()
+  auto data() const
   {
-#ifdef RTTI_ENABLED
-    return std::dynamic_pointer_cast<T>(_data);
-#else
-    return std::static_pointer_cast<T>(_data);
-#endif
-  }
-
-  /**
-   * @brief createData
-   * @param stream
-   */
-  template<class T>
-  void createData(std::shared_ptr<bufferstream> stream = nullptr)
-  {
-    if (_data)
-      return; // already has a data segment
-    _data = std::make_shared<T>();
-    if (stream && stream->good())
-      _data->iStream(stream);
+    static_assert(std::is_base_of<packet_data, T>::value,
+                  "type parameter of data must derive from ARTNET::packet_data");
+    return _data ? std::optional<std::shared_ptr<T>>{std::static_pointer_cast<T>(_data)}
+                 : std::nullopt;
   }
 
   size_t streamSize() const override;