Compilare: user.cpp: In function ‘std::stack<node_data> to_polish(std::string)’: user.cpp:89:41: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < expression.size(); i++) { ^ user.cpp: In function ‘int main()’: user.cpp:140:39: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("evaluare.in", "r", stdin); ^ user.cpp:141:41: warning: ignoring return value of ‘FILE* freopen(const char*, const char*, FILE*)’, declared with attribute warn_unused_result [-Wunused-result] freopen("evaluare.out", "w", stdout); ^