Compilare: main.cpp: In function 'bool is_substring(std::__cxx11::string, std::__cxx11::string)': main.cpp:29:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < a.length(); ++ i) { ~~^~~~~~~~~~~~ main.cpp:30:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] while (idx < b.length() && b[idx] != a[i]) { ~~~~^~~~~~~~~~~~ main.cpp:35:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (idx > b.length()) { ~~~~^~~~~~~~~~~~