// the interface of the wrapper to back-end,
// synchronized collection of pointers

template
<
    class Listener,
    class Mutex,
    class BagType
>
class ListenersCollection
{
    // this is a helper class that locks a given mutex
    // and unlocks when exiting the scope
    class MutexHolder
    {
        // ...
    };

public:

    void add(Listener *p);
    void remove(Listener *p);

    BagType getFrozen() const;

private:

    BagType bag_;
    Mutex mutex_;
};