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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

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<code>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;
...
}
}
</code>
<code>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; ... } } </code>
```
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)

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<code>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");
}
</code>
<code>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"); } </code>
```
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:

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<code>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");
}
</code>
<code>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"); } </code>
```
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?

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