gistfile1.cpp

class BaseSubscriberInterface
{
  publlic:
    /* stuff */
};

class DerivedSubscriber : public BaseSubscriberInterface
{
  public:
    void funcThatReceivesMessage(const Message& message);
};

DerivedSubscriber* subscriber = new DerivedSubscriber();
server.subscribe(messageType, subscriber, &DerivedSubscriber::funcThatReceivesMessage); // <- server does not know about DerivedSubscriber
server.sendMessages() // <- does subscriber->(*funcPtr);