Compilare: user.cpp: In function ‘int valid(int)’: user.cpp:21: warning: unused variable ‘i’ user.cpp: In function ‘void back()’: user.cpp:37: warning: comparison between signed and unsigned integer expressions user.cpp:41: warning: comparison between signed and unsigned integer expressions user.cpp:53: warning: comparison between signed and unsigned integer expressions user.cpp: In function ‘int main()’: user.cpp:60: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result user.cpp:61: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result user.cpp:62: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result user.cpp:64: warning: ignoring return value of ‘char* gets(char*)’, declared with attribute warn_unused_result In file included from /usr/include/stdio.h:910, from /usr/include/c++/4.4/cstdio:45, from user.cpp:1: In function ‘char* gets(char*)’, inlined from ‘int main()’ at user.cpp:64: /usr/include/bits/stdio2.h:230: warning: call to ‘__gets_warn’ declared with attribute warning: please use fgets or getline instead, gets can't specify buffer size /tmp/ccetuACa.o: In function `main': user.cpp:(.text+0x4bc): warning: the `gets' function is dangerous and should not be used.