Question

I’m trying to write an event system for my game. The callbacks that my event manager will store can be both plain functions as well as functors. I also need to be able to compare functions/functors so I know which one I need to disconnect from the event manager.

• Initially I tried using boost::function; it handles functions and functors perfectly well, except it has no operator==, so I can’t remove callbacks if I want to.

class EventManager
{
    typedef boost::function<void (boost::weak_ptr<Event>)> Callback;
    std::map<Event::Type, std::vector<Callback>> eventHandlerMap_;
};

• I also tried using boost::signal, but that also gives me a compilation problem related to operator==:

binary ‘==’ : no operator found which takes a left-hand operand of type ‘const Functor’ (or there is no acceptable conversion)

void test(int c) {
    std::cout << "test(" << c << ")";
}

struct Functor
{
    void operator()(int g) {
        std::cout << "Functor::operator(" << g << ")";
    }
};

int main()
{
    boost::signal<void (int)> sig;

    Functor f;

    sig.connect(test);
    sig.connect(f);

    sig(7);

    sig.disconnect(f); // Error
}

Any other suggestions about how I might implement this? Or maybe how I can make either boost:: function or boost::signal work? (I’d rather use boost:: function though, since I’ve heard signal is rather slow for small collections of items.)

---

Edit: This is the interface of that I’d like EventManager to have.

class EventManager
{
  public:
    void addEventHandler(Event::Type evType, Callback func);
    void removeEventHandler(Event::Type evType, Callback func);

    void queueEvent(boost::shared_ptr<Event> ev);
    void dispatchNextEvent();
};

Answer 1

No matter, I found the solution. A little template magic and things become simple(r):

template<typename F>
void EventManager::removeEventHandler(Event::Type evType, F func)
{
    auto compare = [func](const Callback& other) -> bool {
        F const* f = other.target<F>();
        if (f == nullptr) return false;
        return *f == func;
    };

    std::vector<Callback>& callbacks = ...;
    auto pend = std::remove_if(callbacks.begin(), callbacks.end(), compare);
    callbacks.erase(pend, callbacks.end());
}


template<typename R, typename F, typename L>
void EventManager::removeEventHandler(
    Event::Type evType, const boost::_bi::bind_t<R, F, L>& func)
{
    auto compare = [&func](const Callback& other) -> bool {
        auto const* f = other.target<boost::_bi::bind_t<R, F, L>>();
        if (f == nullptr) return false;
        return func.compare(*f);
    };

    std::vector<Callback>& callbacks = ...;
    auto pend = std::remove_if(callbacks.begin(), callbacks.end(), compare);
    callbacks.erase(pend, callbacks.end());
}

I need to handle Boost.Bind objects separately because operator== doesn’t actually do comparison for Bind objects, but produce a new functor that compares the result of the other two (read more). To compare Boost.Bind you have to use the member function compare().

The type boost::_bi::bind_t seems to be an internal type of Boost (I guess that’s what the underscore in namespace ‘_bi’ means), however it should be safe to use it as all overloads of boost::function_equal also use this type (reference).

This code will work for all types of functors as long as there is an operator== defined that does comparison, or if you’re using Boost.Bind. I had a superficial look into std::bind (C++0x), but that doesn’t seem to be comparable, so it won’t work with the code I posted above.