std::endl (3) - Linux Manuals

std::endl: std::endl

NAME

std::endl - std::endl

Synopsis

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.

Notes

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.

Parameters

os - reference to output stream

Return value

os (reference to the stream after manipulation)

Example

With \n instead of endl, 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 passed" << std::endl;
  }

  int main()
  {
      std::cout.sync_with_stdio(false); // on some platforms, stdout flushes on \n
      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);
      }
  }

Possible output:

  487 ms passed
  974 ms passed
  1470 ms passed
  1965 ms passed
  2455 ms passed