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 should the output stream be flushed and when is it an issue?

What exactly is flushing?

Answer 1

Flushing forces an output stream to write any buffered characters. Read streamed input/output.

It depends on your application, in real-time or interactive applications you need to flush them immediately but in many cases you can wait until closing the file and leave the program to flush it automatically.

Answer 2

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

When you want to be sure that data written to it is visible to other programs or (in the case of file streams) to other streams reading the same file which aren't tied to this one; and when you want to be certain that the output is written even if the program terminates abnormally.

So you would want to do this when printing a message before a lengthy computation, or for printing a message to indicate that something's wrong (although you'd usually use cerr for that, which is automatically flushed after each output).

There's usually no need to flush cerr (which, by default, has its unitbuf flag set to flush after each output), or to flush cout before reading from cin (these streams are tied so that cout is flushed automatically before reading cin).

If the purpose of your program is to produce large amounts of output, either to cout or to a file, then don't flush after each line - that could slow it down significantly.

What exactly is flushing?

Output streams contain memory buffers, which are typically much faster to write to than the underlying output. Output operations put data into the buffer; flushing sends it to the final output.