Event manager

What you're looking for is a 'Design Pattern' that meets your criteria. Wikipedia is a good starting point. There are plenty of patterns that could be used to implement an event/messaging system. You need to decide exactly what you want from the system first.

Here are three possible solutions:
1) SystemA calls SystemB::AddMessage to pass a message to SystemB, SystemB adds the message to a pending queue. SystemB implements a function to process the queue.

2) SystemB registers a handler with an EventManager. SystemA calls EventManager::SendMessage. EventManager than calls all registered handlers.

3) SystemB registers a handler for each message it is interested in receiving with an EventManager. SystemA calls EventManager::SendMessage. EventManager calls all registered handlers for the given message.

Skizz

I'll check that out. Thanks for the hints :)

I don't quite see how this falls under 'DirectX' so I'm moving this over to 'Game Programming'.

Cheers,
Jack