/in/foo.cc: In function 'int main()':
/in/foo.cc:107:18: error: invalid initialization of non-const reference of type 'Matrix&' from an rvalue of type 'Matrix'
  Matrix ans = (m * m);
               ~~~^~~~
/in/foo.cc:13:2: note:   initializing argument 1 of 'Matrix::Matrix(Matrix&)'
  Matrix(Matrix& obj)
  ^~~~~~
/in/foo.cc:83:6: warning: unused variable 'k' [-Wunused-variable]
  int k, a;
      ^