Compilare: user.cpp: In instantiation of ‘T lista<T>::operator[](int) [with T = char]’: user.cpp:309:13: required from ‘T stiva<T>::top() [with T = char]’ user.cpp:332:19: required from here user.cpp:202:16: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] return NULL; ^ user.cpp:204:12: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] return NULL; ^