Compilare: user.cpp: In function 'void back(long unsigned int, long unsigned int)': user.cpp:52: warning: comparison between signed and unsigned integer expressions user.cpp: In function 'int main()': user.cpp:69: warning: statement has no effect user.cpp:75: warning: format '%s' expects type 'char*', but argument 3 has type 'char (*)[31]' user.cpp:77: warning: comparison between signed and unsigned integer expressions