Compilare: user.cpp: In function ‘bool isOk(std::string)’: user.cpp:10:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < s.size(); ++i) ^ user.cpp: In function ‘int main()’: user.cpp:46:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < line.size() - 1; ++i) ^ user.cpp:50:37: warning: converting to non-pointer type ‘char’ from NULL [-Wconversion-null] newl[newl.size()-1] = NULL; ^