C++ template Observer using function pointer callbacks

Observer.hpp

#ifndef OBSERVER_H
#define OBSERVER_H

#include <vector>
#include <iostream>


using namespace std;

template<class T>
class Observer
{
public:
    Observer();
    ~Observer();
    struct ObserverEventListener {
        void (*callback)(T);
    };
    vector<ObserverEventListener> listeners;
    void attachListener(ObserverEventListener el);
    void detachListener(ObserverEventListener el);
    void event(T arg);
};

template<class T>
Observer<T>::Observer()
{
}

template<class T>
Observer<T>::~Observer()
{
}


template<class T>
void Observer<T>::attachListener(Observer::ObserverEventListener el)
{
    listeners.push_back(el);
}


template<class T>
void Observer<T>::detachListener(Observer::ObserverEventListener el)
{
    //todo
    cout << "WARNING: Observer::detachListener called, WIP nonfunctional function!" << endl;
}


template<class T>
void Observer<T>::event(T argument) {
    for(uint16_t i = 0; i < listeners.size(); ++i) {
        listeners.at(i).callback(argument);
    }
}


#endif // OBSERVER_H