Compilare: user.cpp: In function `void read_data()': user.cpp:21: warning: int format, different type arg (arg 2) user.cpp:24: warning: int format, different type arg (arg 2) user.cpp: In function `void use(int)': user.cpp:34: warning: comparison between signed and unsigned integer expressions user.cpp: In function `void unuse(int)': user.cpp:40: warning: comparison between signed and unsigned integer expressions