Alterantive for callbacks using std::function

Currently I am trying out a code that does essentially the following:

void f(int x) { cout << "f("<<x<<")" << endl; }

class C
{
public:
   void m(int x) { cout << "C::m("<<x<<")" << endl; }
};

class C2
{
public:
   void registerCallback(function<void(int)> f)
   {
      v.push_back(f);
   }

private:
   vector<function<void(int)>> v;

   void callThem()
   {
      for (int i=0; i<v.size(); i++)
      {
         v[i](i);
      }
   }
};

int main()
{
   C2 registrar;

   C c;
   registrar.registerCallback(&f); // Static function
   registrar.registerCallback(bind(&C::m, &c, placeholders::_1)); // Method

   return 0;
}

This works pretty well. However I got stuck with this pattern. I would like to check if a callback already has been registered and I would like to be able to unregister a callback by removing it from the vector. I just learned that std::function objects can not be compared which means it is impossible to search for their existence in the container.

So I need an alternative. Of course I would like to keep compile-time type checks and the ability to register methods of arbitrary classes.

How can I achieve a similar solution that allows unregistering the callback and checking for double registration? Are there any trade-offs I need to accept?