Is std::cout buffered?

Question

Just reading an old but interesting article by \"Scott Meyers\"

http://aristeia.com/Papers/C++ReportColumns/novdec95.pdf

Basically it is about preferring to

Answer 1

First, there is no requirement that std::cout (or even std::cerr) be unbuffered. The only requirement is that std::cerr have std::basic_ios::unitbuf set (so that it will flush at the end of each output function: << or a non-formatted output function). On the other hand, unless you've called std::basic_ios::sync_with_stdio(false), outputting to the C++ streams and outputting to the corresponding C streams (i.e. std::cout and stdout) must have the same effects. In theory, this can be done in several ways: stdout functions can forward to those of std::cout, std::cout output can forward to stdout, or they can share some common buffer implementation under the hood. In practice, practically all implementations have std::cout forwarding to stdout.

C specifies that stderr not be fully buffered, and that stdout may be fully buffered only if it can be determined not to refer to an interactive device (for some implementation defined meaning of "interactive device"). Typically, stdout will be line buffered (a concept which doesn't exist in iostream), and stderr will be unbuffered, but that's not guaranteed by the C standard (and may not be true today—the last time I really looked was over twenty years ago). Anyway, an implementation which just forwards to stdout will follow the rules of the C implementation it forwards to, and one that doesnt' still needs to take some steps to ensure that output to std::cout and stdout comes out in the right order, and that stdout behaves "as if" it obeyed the C rules.

If you're worried about performance, then you might want to run some trials. Try measuring the time it takes to output to an std::ofstream you've opened yourself, versus the time it takes to output to std::cout (both with and without having called sync_with_stdio), with the output redirected. The differences should be interesting.