Question

I’d like to have a dynamic messaging system in my C++ project, one where there is a fixed list of existing events, events can be triggered anywhere during runtime, and where you can subscribe callback functions to certain events.

There should be an option for arguments passed around in those events. For example, one event might not need any arguments (EVENT_EXIT), and some may need multiple ones (EVENT_PLAYER_CHAT: Player object pointer, String with message)

The first option for making this possible is allowing to pass a void pointer as argument to the event manager when triggering an event, and receiving it in the callback function.

Although: I was told that void pointers are unsafe and I shouldn’t use them.

• How can I keep (semi) dynamic argument types and counts for my events whilst not using void pointers?

Answer 1

You could use a base class, optionally abstract, and use dynamic_cast. The argument will be checked at run-time. A compile-time would probably be better, though.

class EventArgs
{
public:
   virtual ~EventArgs();
};

class PlayerChatEventArgs : public EventArgs
{
public:
   PlayerChatEventArgs(Player* player, const std::string& message);
   virtual ~PlayerChatEventArgs();
   Player* GetPlayer() const;
   const std::string& GetMessage() const;
private:
   Player* player;
   std::string message;
};

class Event
{
public:
   virtual ~Event() = 0;
   virtual void Handle(const EventArgs& args) = 0;
};

class ExitEvent : public Event
{
public:
   virtual ~ExitEvent();
   virtual void Handle(const EventArgs& /*args*/)
   {
      // Perform exit stuff.
   }
};

class PlayerChatEvent : public Event
{
public:
   virtual ~PlayerChatEvent();
   virtual void Handle(const EventArgs& args)
   {
      // this will throw a bad_cast exception if cast fails.
      const PlayerChatEventArgs& playerchatargs =
         dynamic_cast<const PlayerChatEventArgs&>(args);
      // Perform player chat stuff.
   }
};