Need to pass data with notification in the observer design pattern

I am implementing a system using the observer pattern to decouple my data generating/modifying code from the data handling code. As there are different notifications my notify method takes a 'message' parameter; if I make this a struct

I need to pass some data with the notification - there are several different types of change that can occur, so my
There is additional data that I feel should be provided with the call to notify(), in particular the updated data.

Looking at the code below, does anyone have any feeling on the alternatives I propose, or can suggest something different? I'm not sure I've explained it supremely well, let me know if I can clarify.

TIA,
--rob

struct Message
{
   virtual ~Message(void) {}
};
 
 
// methods not referred to are omitted
struct IMessageObserver
{
   virtual void notify(const Message& msg) = 0;
   virtual void notify2(const Message& msg, void* data) = 0;
};
 
 
struct MessageWithoutData
   : public IMessage
{
} MSG_NO_DATA; // no members and this instance mean I can compare (msg == MSG_NO_DATA) to determine what notification I've been given
 
 
struct MessageWithData
   : public IMessage
{
   MessageWithData(int newValue)
      : value(newValue)
   {}
 
   int value;
} // can't do a straightforward comparison in notify() now - RTTI instead?
 
 
// the example notifications
void doANotification(IMessageObserver& observer)
{
   int newValue(24);
 
   // should I construct with the data?
   observer.notify(MessageWithData(newValue));
 
   // or pass the data separately
   observer.notify2(MSG_NO_DATA, &newValue);
}

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