Заменить делегаты функциями обратного вызова - C++

In file included from NMClient.cpp:14:0,
                 from main.cpp:1:
EventHandler.h: In member function ‘bool EventHandler<EventArgs>::operator==(int)’:
EventHandler.h:38:47: warning: NULL used in arithmetic [-Wpointer-arith]
  inline bool operator==(int a) { return (a == NULL) && (callBackList.size() == 0); }
                                               ^
EventHandler.h: In member function ‘bool EventHandler<EventArgs>::operator!=(int)’:
EventHandler.h:40:47: warning: NULL used in arithmetic [-Wpointer-arith]
  inline bool operator!=(int a) { return (a == NULL) && (callBackList.size() != 0); }