Compilare: user.cpp: In function ‘std::ifstream& operator>>(std::ifstream&, cost_matrix&)’: user.cpp:16:16: warning: no return statement in function returning non-void [-Wreturn-type] f >> x; } } } ^ user.cpp: In function ‘long long int calc_cost(const string&, const cost_matrix&)’: user.cpp:31:32: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0; i+1 < str.size(); ++i){ ^ user.cpp:34:37: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int dist = 4; dist <= str.size(); dist += 2){ ^ user.cpp:35:43: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for(int i = 0, j = dist-1; j < str.size(); ++i, ++j){ ^