How to hash and compare a pointer-to-member-function?

Question

How can i hash (std::tr1::hash or boost::hash) a c++ pointer-to-member-function?

Example:

I have several bool (Class::*functionPointer)() (not static) that

Answer 1

If your member function pointer is unique, which is true in most of cases for callback-based subscriptions, then you can use the tick with type_index, which uniqueness is guaranteed by uniqueness of type (i.e. Class::Method) in your program, and it is suitable to be stored in unordered_map, i.e.

struct MyEvent {

    using fn_t = std::function;
    using map_t = std::unordered_map;


    template 
    void subscribe(Object& obj, Handler&& handler) {
        fn_t fn = [&, handler = std::move(handler)](MyEvent& event) {
            (obj.*handler)(event);
        }
        std::type_index index = typeid(Handler);
        subscribers.emplace(std::move(index), std::move(fn));
    }

    void fire() {
        for(auto& pair: subscribers) {
            auto& fn = pair.second;
            fn(*this);
        }
    }

    map_t subscribers;
}

And the subscription and fire event example:

MyEvent event;
MyObject obj = ...;
event.subscribe(obj, &MyObject::on_event );
...
event.fire();

So, example above gives you class/method uniqueness, and if you need object/method uniqueness, then you should have an struct, which provides combined hash, assuming that there is std::hash and there is already std::hash for a member function pointer.