Messaging system: Callbacks can be anything

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

You'll find that most generic function wrappers do not support function equality.

Why is this? Well, just look at your functor there:

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

This Functor has no operator==, and therefore cannot be compared for equality. So when you pass it to boost::signal by value, a new instance is created; this will compare false for pointer-equality, and has no operator to test for value-equality.

Most functors don't, in fact, have value-equality predicates. It's not useful very much. The usual way to deal with this is to have a handle to the callback instead; boost::signals does this with its connection object. For example, take a look at this example from the documentation:

boost::signals::connection c = sig.connect(HelloWorld());
if (c.connected()) {
// c is still connected to the signal
  sig(); // Prints "Hello, World!"
}

c.disconnect(); // Disconnect the HelloWorld object
assert(!c.connected()); c isn't connected any more

sig(); // Does nothing: there are no connected slots

With this, HelloWorld doesn't need to have an operator==, as you're referring directly to the signal registration.

Answer 2

Have you ever tried libsigc and libsigc++? I started using them in linux and fell in love with them. I now use them in my Windows applications as well. I believe it is more extensible and flexible than boost. It is also a breeze to implement.

Answer 3

I highly recommend you consider Don Clugston's "Member Function Pointers and the Fastest Possible C++ Delegates". You can find the article and download the code from here:

http://www.codeproject.com/KB/cpp/FastDelegate.aspx

Among many other benefits, his delegates provide comparison operators (==, !=, <) out of the box. I'm currently using them for a realtime system and find them excellent in every way. I do seem to recall we had to make a minor modification to fix a compiler portability issue; but, that experience will vary based on platform etc.

Also, the article is several years old so you may want to google around for updated code/discussion regarding this delegate implementation if you run into any problems.

Answer 4

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.