Downcasting non-template base class to templated derived class: is it possible?

Question

I'm implementing an event system for a game. It uses an event queue, and a data structure to hold all registered event handlers for a given event type. It works fine so far registering handlers, but when it comes to unregistering them (something that will take place when a game object is destroyed, for instance) I'm having a bit of trouble regarding templates and casting.

I've defined an EventHandler as some sort of functor, partially based on Szymon Gatner's article on http://www.gamedev.net/reference/programming/features/effeventcpp/ . To be precise, I took the HandlerFunctionBase and MemberFunctionHandler class definitions and came up with:

class BaseEventHandler
{
public:
    virtual ~BaseEventHandler(){}
    void handleEvent(const EventPtr evt)
    {
        invoke(evt);
    }
private:
    virtual void invoke(const EventPtr evt)=0;
};

template <class T, class TEvent>
class EventHandler: public BaseEventHandler
{
    public:
    typedef void (T::*TMemberFunction)(boost::shared_ptr<TEvent>);
    typedef boost::shared_ptr<T> TPtr;
    typedef boost::shared_ptr<TEvent> TEventPtr;

    EventHandler(TPtr instance, TMemberFunction memFn) : mInstance(instance), mCallback(memFn) {}

    void invoke(const EventPtr evt)
    {
        (mInstance.get()->*mCallback)(boost::dynamic_pointer_cast<TEvent>(evt));
    }
    TPtr getInstance() const{return mInstance;}
    TMemberFunction getCallback() const{return mCallback;}

    private:
        TPtr mInstance;
    TMemberFunction mCallback;
};

Then the initial implementation for the unregisterHandler() method on the EventManager class I've thought of would go like this:

// EventHandlerPtr is a boost::shared_ptr<BaseEventHandler>.
// mEventHandlers is an STL map indexed by TEventType, where the values are a std::list<EventHandlerPtr>
void EventManager::unregisterHandler(EventHandlerPtr hdl,TEventType evtType)
{
    if (!mEventHandlers.empty() && mEventHandlers.count(evtType))
    {
        mEventHandlers[evtType].remove(hdl);
        //remove entry if there are no more handlers subscribed for the event type
    if (mEventHandlers[evtType].size()==0)
        mEventHandlers.erase(evtType);
    }
}

To make "remove" work here I thought of overloading the == operator for BaseEventHandler, and then using a virtual method to perform the actual comparison...

bool BaseEventHandler::operator== (const BaseEventHandler& other) const
{
    if (typeid(*this)!=typeid(other)) return false;
    return equal(other);
}

and, on the template class EventHandler, implement the abstract method 'equal' like this:

bool  equal(const BaseEventHandler& other) const
{
    EventHandler<T,TEvent> derivedOther = static_cast<EventHandler<T,TEvent>>(other);
    return derivedOther.getInstance() == this->getInstance() && derivedOther.getCallback()==this->getCallback();
}

Of course, I'm getting a compile error on the static_cast line. I'm not even sure that it is possible at all to do that cast (not necessarily using static_cast). Is there a way to perform it, or at least some workaround that does the trick?

Thanks in advance =)

Answer 1

Thank you both, Mark and Steve: that led me on the right direction. There was another problem since I was trying to cast a const to a non-const but this one was much easier to spot after that.

This is what I've got in the end, after a few tweaks:

void EventManager::unregisterHandler(EventHandlerPtr hdl,TEventType evtType)
{
    if (!mEventHandlers.empty() && mEventHandlers.count(evtType))
    {
        TEventHandlerList::iterator newEnd=remove_if(mEventHandlers[evtType].begin(),
            mEventHandlers[evtType].end(),EventHandlerComparisonFunctor(hdl));
        mEventHandlers[evtType].erase(newEnd,mEventHandlers[evtType].end());
        if (mEventHandlers[evtType].size()==0)
            mEventHandlers.erase(evtType);
    }
}

I changed remove with remove_if because boost::shared_ptr implements the == operator by directly comparing the pointers, instead of their contents. The terribly-named EventHandlerComparisonFunctor is now the responsible to perform the equality check.

It is implemented like this:

class EventHandlerComparisonFunctor
{
private:
    EventHandlerPtr mInstance;
public:
    EventHandlerComparisonFunctor(EventHandlerPtr instance):mInstance(instance){}
    bool operator()(EventHandlerPtr& other) const
    {
        return *(mInstance.get())==*(other.get());
    }
};

And finally, the equal method in EventHandler (@gf, the method was indeed declared in the EventHandler template, but for some reason I cut it out to paste the class code in here, my mistake)

bool equal(const BaseEventHandler& other) const
{
    EventHandler<T,TEvent>& derivedOther = static_cast<EventHandler<T,TEvent>&>(const_cast<BaseEventHandler&>(other));
    return derivedOther.getInstance() == this->getInstance() && derivedOther.getCallback()==this->getCallback();
}

Everything is working OK now.

Answer 2

In general when closing templates, you need to make sure that > are separated by spaces so the compiler doesn't parse them as a right-shift operator.

Here you're trying to static cast a reference to a non-reference, which even if it worked could invoke object slicing. You need to static cast to a derived reference.

bool  equal(const BaseEventHandler& other) const
{
    EventHandler<T,TEvent>& derivedOther = static_cast<EventHandler<T,TEvent>&>(other);
    return derivedOther.getInstance() == this->getInstance() && derivedOther.getCallback()==this->getCallback();
}