std::cerr for debug output

  #include  <iostream>
  int  main()  {
     std::cout  <<  "A";
     function_which_might_crash();
     std::cout  <<  "B";
  }

  std::cout  <<  "A"  <<  std::flush;
  std::cout  <<  "A"  <<  std::endl;
  std::cerr  <<  "A";