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Template and static abuse?

Started by lexs Jul 31, 2008 at 12:30 PM 5 replies 1k views

Original Post

Jul 31, 2008 at 12:30 PM

I'm exploring abuse of templates and statics. I don't think I ever would use code like this, but what are the gotchas?


template <typename Event>
struct EventQueue
{
	static std::queue<Event *> event_queue;
};

class System
{
public:
	template <typename Event>
	void postEvent(Event * event)
	{
		EventQueue<Event>::event_queue.push(event);
	}

	template <typename Event>
	std::queue<Event *> & getEventQueue()
	{
		return EventQueue<Event>::event_queue;
	}
private:
};

Note: Untested code.

godecho

Jul 31, 2008 at 12:47 PM

Thread safety.

Out of curiosity, why does your event queue need to be static?

lexs

Jul 31, 2008 at 12:50 PM

Quote:
Original post by godecho
Out of curiosity, why does your event queue need to be static?

It's static so you can get only one type of events without using something like a std::string for comparison. That's the abuse :)

Antheus

Jul 31, 2008 at 12:57 PM

That's not really abuse, it's common with templates. It's common patter to select instances by type.

Same gotchas apply as with usual template members, this is why such approach works more reliably with stateless objects.

Quote:
It's static so you can get only one type of events without using something like a std::string for comparison.

EventQueue<MessageA> aq;EventQueue<MessageB> bq;EventQueue<MessageC> cq;

This is the way events are usually handled, so why would it be abuse?

lexs

Jul 31, 2008 at 1:05 PM

Quote:
Original post by Antheus
This is the way events are usually handled, so why would it be abuse?

I never meant for EventQueue to be instantiated outside System, I'll code up a more complete example soon.

lexs

Jul 31, 2008 at 1:10 PM

Something more among the lines of this:

class EventManager{public:	template <typename Event>	void subscribeToEvent(Delegate delegate)	{		EventSubscribers<Event>::subscribers.push_back(delegate);	}	template <typename Event>	void postEvent(Event * event)	{		for each subscriber in EventSubscribers<Event>::subscriber			Delegate(event);	}private:	template <typename Event>	struct EventSubscribers	{		static std::list<Delegate> subscribers;	};};

Would this cause problems with the statics if used across DLL's for example?

donny dont

Jul 31, 2008 at 1:23 PM

A quick gotcha with using static member variables that are or contain pointers is what happens at the end of execution. In this example if the queue still has pointers in then they will not be deleted.

Using a smart pointer of some sort will insure this doesn't happen.

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