possible solution to registering/deregistering function callbacks using this to disambiguate.

foo.cpp

class dispatch
{
public:

    void register(void* instance, std::function& callback)
    {
        auto id = hash(instance, callback.target());
        map[id] = callback;
    }
    
    void unregister(void* instance, std::function& callback)
    {
        auto id = hash(instance, callback.target());
        auto it = map.find(id);
        if (it != map.end())
        {
            map.erase(it);
        }
    }
};