Compilare: user.cpp: In member function ‘void matrix::operator*=(const matrix&)’: user.cpp:24:30: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < a.size(); ++i) ^ user.cpp:25:34: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int k = 0; k < a[0].size(); ++k) ^ user.cpp:26:35: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < b[0].size(); ++j) ^