C++ dummy Observer

Listener.h

#ifndef LISTENABLE_H
#define LISTENABLE_H

class Listener {
};

#endif

main.cpp

#include <iostream>
#include "Observable.h"
#include "Listener.h"

using namespace std;

class A :  public Listener {
	public:
		void print(double val) {
			cout << "A::print "<< val << endl;
		}

		void dbl(double val) {
			cout << "A::dbl " << val * 2 << endl;
		}

};

int main() {
	Observable obs;
	A a;

	obs.registerListener(&a, (void (Listener::*) (double)) &A::print);
	obs.registerListener(&a, (void (Listener::*) (double)) &A::dbl);
	
	obs.setValue(42.0);

	return 0;
}

Observable.cpp

#include "Observable.h"

using namespace std;

void Observable::registerListener(Listener* listener, void (Listener::*callback) (double)) {
	m_listeners.insert(pair<Listener*, void (Listener::*) (double)>(listener, callback));
}


void Observable::setValue(double value) {
/* do whatever you want with value*/

    update(value);
}

void Observable::update(double value) {
	multimap< Listener*, void (Listener::*) (double) >::iterator it = m_listeners.begin();

	while (it != m_listeners.end()) {
        Listener* l = it->first;
        void (Listener::*meth) (double) = it->second;

        (l->*meth)(value);
        it++;
	}
}

Observable.h

#ifndef OBSERVABLE_H
#define OBSERVABLE_H

#include <map>
#include "Listener.h"

class Observable {
	public:
		void registerListener(Listener* listener, void (Listener::*callback) (double));

        void setValue(double value);

	protected:
		std::multimap< Listener*, void (Listener::*) (double)>	m_listeners;

        void update(double value);
};

#endif