Question

Are there more output buffers? If so, does endl only flush the output buffer which is associated with cout? If there is only one buffer, then what makes the two objects cerr and clog distinct?

Answer 1

endl flushes the output buffer of whichever stream you stream it to. For example:

cout << ... << endl;   // flushes the output buffer of 'cout'
cerr << ... << endl;   // flushes the output buffer of 'cerr'
ofstream file("filename");
file << ... << endl;   // flushes the output buffer of 'file'

The only difference between cerr and clog is that clog is buffered by default, while cerr is not. You can turn buffering on or off for any stream using the std::unitbuf and std::nounitbuf manipulators. For example:

cerr << nounitbuf;   // cerr is now buffered
clog << unitbuf;     // clog is now unbuffered

Note that “unitbuf” means “make the buffer size 1”, i.e. turn off buffering.