Compilare: user.cpp: In function ‘std::ostream& operator<<(std::ostream&, const std::vector<int>&)’: user.cpp:11:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < x.size(); i++) ^ user.cpp: In function ‘std::ostream& operator<<(std::ostream&, const std::vector<std::vector<int> >&)’: user.cpp:18:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < x.size(); i++) ^