Compilare: main.cpp: In function 'std::istream& operator>>(std::istream&, rational&)': main.cpp:31:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < str.length(); i++) ~~^~~~~~~~~~~~~~ main.cpp:40:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < str.find('('); i++) ~~^~~~~~~~~~~~~~~