std::flush
<ostream>
std::basic_ostream <CharT, Traits>& flush( std::basic_ostream <CharT, Traits>& os );
Flushes the output sequence os as if by calling os.flush().
This is an output-only I/O manipulator, it may be called with an expression such as out << std::flush for any out of type std::basic_ostream .
Contents
[edit] Notes
This manipulator may be used to produce an incomplete line of output immediately, e.g. when displaying output from a long-running process, logging activity of multiple threads or logging activity of a program that may crash unexpectedly. An explicit flush of std::cout is also necessary before a call to std::system , if the spawned process performs any screen I/O (a common example is std::system ("pause") on Windows). In most other usual interactive I/O scenarios, std::endl is redundant when used with std::cout because any input from std::cin , output to std::cerr , or program termination forces a call to std::cout.flush().
When a complete line of output needs to be flushed, the std::endl manipulator may be used.
When every output operation needs to be flushed, the std::unitbuf manipulator may be used.
[edit] Parameters
[edit] Return value
os (reference to the stream after manipulation).
[edit] Example
Without std::flush, the output would be the same, but may not appear in real time.
#include <chrono> #include <iostream> template<typename Diff> void log_progress(Diff d) { std::cout << std::chrono::duration_cast <std::chrono::milliseconds >(d) << " ... " << std::flush; } int main() { volatile int sink = 0; auto t1 = std::chrono::high_resolution_clock::now (); for (int j = 0; j < 5; ++j) { for (int n = 0; n < 10000; ++n) for (int m = 0; m < 20000; ++m) sink += m * n; // do some work auto now = std::chrono::high_resolution_clock::now (); log_progress(now - t1); } std::cout << '\n'; }
Possible output:
567ms ... 1137ms ... 1707ms ... 2269ms ... 2842ms ...