Removing dependencies on subclass-specific behavior

Question

I have a Message class which can contain multiple types of payloads (or sometimes no payload), each derived from a common Payload class. However, this becomes problematic because the Message class wants to know about the Payload subclasses for various reasons such as:

• Checking for Message equality

  if (message parts besides the payload are equal) {
      switch(type) {
          case Payload::Type::RESPONSE:
              return *static_cast<ResponsePayload*>(payload.get())
                  == *static_cast<ResponsePayload*>(o.payload.get());
              break;
  
          case Payload::Type::SETUP:
              return *static_cast<SetupPayload*>(payload.get())
                  == *static_cast<SetupPayload*>(o.payload.get());
              break;
  
          ...
      }
  }
  

• Deserializing (since the deserialization methods are static because the payloads are immutable)

  switch(type) {
      case Payload::Type::RESPONSE:
          load = ResponsePayload::fromJSON(payloadValue);
          break;
  
      case Payload::Type::SETUP:
          load = SetupPayload::fromJSON(payloadValue);
          break;
  
      ...
  
      case Payload::Type::START:
      case Payload::Type::STOP:
      case ...:
          break; // Load stays null
  
      default:
          THROW(Exception, "Error in program logic: we forgot to parse some payload");
  }
  

• Ensuring a Payload is attached to a Message while constructing it:

  switch(type) {
      case Payload::Type::RESPONSE:
      case Payload::Type::SETUP:
      case ...:
          ENFORCE(IOException, payload != nullptr, "For this message type, a payload is required.");
          break;
  
      case Payload::Type::START:
      case Payload::Type::STOP:
      case ...:
          ENFORCE(IOException, payload == nullptr, "For this message type, the payload should be null");
          break;
  
      default:
          THROW(Exception, "Error in program logic: we forgot to handle some payload");
  }
  

Alarm bells are going are going off in my head - this violates SOLID like there's no tomorrow and obviously doesn't scale well since I have to add case statements every time I add a new payload. Is there a cleaner approach I could take?

Answer 1

Alarm bells should be going off. But some top of my head initial suggestions (meaning I didn't take the time to come up with the best solution, just a much better one than you've got).

1 - Why is there a separate Message and Payload class, aren't they the same thing?

2 - Include the equality check in the subclasses where they take a payload instance as the parameter. Subclasses only know about their own type. If you pass a payload that isn't the right type then the type conversion will fail and you know they aren't equal.

3 - Create a Payload factory instead of creating all the various types in the base message class.

4 - Subclass the Message class if it requires parameters to be built properly.