/in/foo.cc:1:9: warning: #pragma once in main file
 #pragma once
         ^~~~
/in/foo.cc: In instantiation of 'List<T>::~List() [with T = int]':
/in/foo.cc:266:12:   required from here
/in/foo.cc:250:18: warning: NULL used in arithmetic [-Wpointer-arith]
   if (head->data != NULL)
                  ^
/in/foo.cc: In instantiation of 'void List<T>::add(T) [with T = int]':
/in/foo.cc:272:12:   required from here
/in/foo.cc:82:13: warning: converting to non-pointer type 'int' from NULL [-Wconversion-null]
  tail->data = NULL;
             ^