class Listener
{
public:
    virtual void onEvent() = 0;
}

class ListenerImplA : public Listener
{
private:
    virtual void onEvent() override {printf("hoolayA!");}
}

class ListenerImplB : public Listener
{
protected:
    virtual void onEvent() override {printf("hoolayB!");}
}

void emit(Listener* listener)
{
    listener->onEvent();
}

void main()
{
    emit(new ListenerImplA);
    emit(new ListenerImplB);
}