Compilare: user.cpp: In function ‘void is_ok(char*)’: user.cpp:19:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<strlen(sir); i++) ~^~~~~~~~~~~~ user.cpp:25:15: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wparentheses] if(!s.empty()) ^ user.cpp:38:15: warning: suggest explicit braces to avoid ambiguous ‘else’ [-Wparentheses] if(!s.empty() ) ^ user.cpp: In function ‘int main()’: user.cpp:70:24: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i=0; i<strlen(sir); i++) ~^~~~~~~~~~~~