I have some C++ code that uses cout statements for debug purposes and for some reason I can't get all the data to print unless I do a std::cout.flush(); at the end.
I don't quite understand why this flush operation is needed.
Anyone have any insight?
To add to the other answers: your debugging statements should instead go to cerr, because:
cerr by default is unbuffered, which means that, after each output operation, it will automatically flush itself, and in general this is desirable for errors and debug output.(source: C++ standard, §27.3.1 ¶4-5, §27.4.2.1.2 table 83)
Are you using std::endl to terminate your lines. This should be the
usual practice, until performance issues require otherwise, but for some
reason, I see a lot of code which uses '\n' instead.
Otherwise, you can always do:
std::cout.setf( std::ios_base::unitbuf );
as one of the first things in main. This will cause a flush at the
end of every <<, which is more than you need, but for diagnostic
output to the console, is probably quite acceptable.
unitbuf.
std::endl be usual practice?\n. But that should be thought of as an optimization, not as default behavior when performance doesn't matter.)Is the data that isn't automatically getting flushed lacking a \n at the end? By default, standard out doesn't get delivered until a carriage return is seen.
iostream works considerably differently than FILE. std::cout will be flushed by an explicit call to flush (including that triggered by std::endl or by a read on std::cin), or at other unspecified times decided by the implementation. There is no line buffering in iostream.
\r sign. I just have no comments. @Mah, by the way, as I know, the stdout should be flushed not on a «carriage return», but on a «newline»."When you send output to a stream, it does not necessarily get printed immediately. Rather, it may wait in a buffer until some unspecified event, e.g. buffer full enough, reading from input, or exit from program. The details may vary."
http://www.cs.hmc.edu/~geoff/classes/hmc.cs070.200109/notes/io.html
In C++ you can use endl formatter with cout operator rather then flush.
It is the right behavior. You probably use std::endl that add \n and flush the buffer. http://www.cplusplus.com/reference/ostream/endl/
You need to flush the stream, if you want to see the output.
The answer of std::endl is only valid if you want a return.
Not sure how you would do this if you wanted to flush a command prompt out.
