Compilare: In file included from /usr/include/c++/4.2/backward/iostream.h:31, from user.cpp:1: /usr/include/c++/4.2/backward/backward_warning.h:32:2: warning: #warning This file includes at least one deprecated or antiquated header. Please consider using one of the 32 headers found in section 17.4.1.2 of the C++ standard. Examples include substituting the <X> header for the <X.h> header for C++ includes, or <iostream> instead of the deprecated header <iostream.h>. To disable this warning use -Wno-deprecated. user.cpp: In function 'int valid()': user.cpp:21: warning: comparison between signed and unsigned integer expressions user.cpp:22: warning: comparison between signed and unsigned integer expressions user.cpp: In function 'void back()': user.cpp:37: warning: comparison between signed and unsigned integer expressions user.cpp: In function 'int main()': user.cpp:61: warning: comparison between signed and unsigned integer expressions user.cpp:62: warning: comparison between signed and unsigned integer expressions user.cpp:67: warning: comparison between signed and unsigned integer expressions user.cpp:71: warning: comparison between signed and unsigned integer expressions