Can I stop std::cout flushing on "\n"?

Question

According to to this post std::cout will automatically flush on \n when it is attached to an interactive device (e.g. a terminal window). Otherwise (e.g. when being piped to a file) it will act fully buffered and will only flush on .flush() or std::endl.

Is there a way to override this behaviour in Microsoft Visual C++ so that I can select whether I want fully buffered or line buffered mode?

Answer 1

Contrary to anon's (Apr 28 '09) answer, this behavior has nothing to do with the operating system or "console software."

C++'s <iostream> streams are designed to be interoperable with C's <stdio.h> streams. The goal is to allow uses of std::cout to be intermixed with uses of printf/puts. To achieve this, std::cout's streambuf is implemented atop C's stdout stream. It is actually C's stdout that is line-buffered when the standard output is attached to a terminal device.

You can call std::ios_base::sync_with_stdio(false) (before your program uses any of C++'s standard I/O streams) to tell the C++ streams library to communicate directly with the underlying file descriptors rather than layering atop C's streams library. This avoids C's stdout stream entirely and speeds up C++'s I/O streams at the cost of the two libraries no longer mixing well.

An alternative is to unconditionally set stdout to fully buffered by calling std::setvbuf(stdout, nullptr, _IOFBF, BUFSIZ). Then, even though std::cout is still writing through stdout, you will not have stdout flushing after every newline.