Compilare: user.cpp: In function ‘std::string process_line(std::string&)’: user.cpp:51: warning: comparison between signed and unsigned integer expressions user.cpp:75: warning: comparison between signed and unsigned integer expressions user.cpp: In function ‘bool eval_expression(std::string&)’: user.cpp:90: warning: comparison between signed and unsigned integer expressions user.cpp:87: warning: unused variable ‘ret’ user.cpp: In function ‘int main()’: user.cpp:142: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result