Compilare: user.cpp:4:0: warning: "NULL" redefined [enabled by default] #define NULL dead ^ In file included from /usr/include/time.h:38:0, from /usr/include/pthread.h:26, from /usr/include/i386-linux-gnu/c++/4.8/bits/gthr-default.h:35, from /usr/include/i386-linux-gnu/c++/4.8/bits/gthr.h:148, from /usr/include/c++/4.8/ext/atomicity.h:33, from /usr/include/c++/4.8/bits/basic_string.h:39, from /usr/include/c++/4.8/string:52, from /usr/include/c++/4.8/random:41, from /usr/include/c++/4.8/bits/stl_algo.h:65, from /usr/include/c++/4.8/algorithm:62, from user.cpp:1: /usr/lib/gcc/i686-linux-gnu/4.8/include/stddef.h:399:0: note: this is the location of the previous definition #define NULL __null ^