std::flush (3) - Linux Man Pages
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.
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.
os - reference to output stream
os (reference to the stream after manipulation)
Without std::flush, the output would be the same, but may not appear in real time.
// Run this code
endl (function template)
flush (public member function of std::basic_ostream<CharT,Traits>)