Why does using std::endl with ostringstream affect output speed?

Question

I\'m timing the difference between various ways to print text to standard output. I\'m testing cout, printf, and ostringstream using both

Answer 1

std::endl triggers a flush of the stream, which slows down printing a lot. See http://en.cppreference.com/w/cpp/io/manip/endl

It is often recommended to not use std::endl unless you really want the stream to be flushed. If this is really important to you, depends on your use case.

Regarding why flush has a performance impact even on a ostringstream (where no flushing should happen): It seems that an implementation is required to at least construct the sentry objects. Those need to check good and tie of the ostream. The call to pubsync should be able to be optimized out. This is based on my reading of libcpp and libstdc++.

After some more reading the interesting question seems to be this: Is an implementation of basic_ostringstream::flush really required to construct the sentry object? If not, this seems like a "quality of implementation" issues to me. But I actually think it needs to because even a basic_stringbug can change to have its badbit set.