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std::flush(3)		       C++ Standard Libary		   std::flush(3)

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 expres-
     sion 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 im-
     mediately,
	e.g. when displaying output from a long-running process, logging  activ-
     ity of
	multiple  threads  or logging activity of a program that may crash unex-
     pectedly. An
	explicit flush of std::cout is also necessary before a call to std::sys-
     tem, if the
	spawned process performs any screen I/O (a common example  is  std::sys-
     tem("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 manip-
     ulator may be
	used.

	When every output operation needs to be flushed, the std::unitbuf manip-
     ulator 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 <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 ...

See also
	unitbuf   controls whether output is flushed after each operation
	nounitbuf (function)
	endl	  outputs '\n' and flushes the output stream
		  (function template)
	flush	  synchronizes with the underlying storage device
		  (public member function of std::basic_ostream<CharT,Traits>)

http://cppreference.com 	   2024.06.10			   std::flush(3)
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