Compilare: user.cpp: In function ‘int main()’: user.cpp:44:22: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] char o = NULL; ^ user.cpp:62:22: warning: NULL used in arithmetic [-Wpointer-arith] if (o != NULL) { ^ user.cpp:63:43: warning: array subscript has type ‘char’ [-Wchar-subscripts] while (!op.empty() && pr[o] <= pr[op.top()]) move_to_polo(); ^ user.cpp:63:59: warning: array subscript has type ‘char’ [-Wchar-subscripts] while (!op.empty() && pr[o] <= pr[op.top()]) move_to_polo(); ^ user.cpp:79:22: warning: array subscript has type ‘char’ [-Wchar-subscripts] val[change[i]] = !val[change[i]]; ^ user.cpp:79:40: warning: array subscript has type ‘char’ [-Wchar-subscripts] val[change[i]] = !val[change[i]]; ^ user.cpp:82:30: warning: array subscript has type ‘char’ [-Wchar-subscripts] ev.push(val[c]); ^