std::endl (3) - Linux Man Pages
Defined in header <ostream>
template< class CharT, class Traits >
std::basic_ostream<CharT, Traits>& endl( std::basic_ostream<CharT, Traits>& os );
Inserts a newline character into the output sequence os and flushes it as if by calling os.put(os.widen('\n')) followed by os.flush().
This is an output-only I/O manipulator, it may be called with an expression such as out << std::endl for any out of type std::basic_ostream.
This manipulator may be used to produce a 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. 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(). Use of std::endl in place of '\n', encouraged by some sources, may significantly degrade output performance.
In many implementations, standard output is line-buffered, and writing '\n' causes a flush anyway, unless std::ios::sync_with_stdio(false) was executed. In those situations, unnecessary endl only degrades the performance of file output, not standard output.
The code samples on this wiki follow_Bjarne_Stroustrup and The_C++_Core_Guidelines in flushing the standard output only where necessary.
When an incomplete line of output needs to be flushed, the std::flush manipulator may be used.
When every character of output needs to be flushed, the std::unitbuf manipulator may be used.
os - reference to output stream
os (reference to the stream after manipulation)
With \n instead of endl, the output would be the same, but may not appear in real time.
// Run this code
flush (function template)
flush (public member function of std::basic_ostream<CharT,Traits>)