When must the output stream in C++ be flushed?

Question

I understand cout << \'\\n\' is preferred over cout << endl; but cout << \'\\n\' doesn\'t flush the output stream. When

Answer 1

Flushing can be disastrous if you are writing a large file with frequent spaces.

For example

for(int i = 0 ;i < LARGENUMBER;i++)
{//Slow?
  auto point = xyz[i];
  cout<< point.x <<",",point.y<

vs

for(int i = 0 ;i < LARGENUMBER;i++)
{//faster
  auto point = xyz[i];
  cout<< point.x <<",",point.y<<"\n";
}

vs

for(int i = 0 ;i < LARGENUMBER;i++)
{//fastest?
  auto point = xyz[i];
  printf("%i,%i\n",point.x,point.y);
}

endl() was often know for doing other things, for example synchronize threads when in a so-called debug mode on MSVC, resulting in multithreaded programs that, contrary to expectation, printed uninterrupted phrases from different threads.