std::flush (3) - Linux Manuals

std::flush: std::flush

NAME

std::flush - std::flush

Synopsis

Defined in header <ostream>
template< class CharT, class Traits >
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.

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.

Parameters

os - reference to output stream

Return value

os (reference to the stream after manipulation)

Example

Without std::flush, the output would be the same, but may not appear in real time.
// Run this code

  #include <iostream>
  #include <chrono>
  template<typename Diff>
  void log_progress(Diff d)
  {
      std::cout << "..("
                << std::chrono::duration_cast<std::chrono::milliseconds>(d).count()
                << " ms).." << 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:

  ..(450 ms)....(901 ms)....(1350 ms)....(1800 ms)....(2250 ms)..