< cpp‎ | io‎ | manip
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.


[edit] 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::cout.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.

[edit] Parameters

os - reference to output stream

[edit] Return value

os (reference to the stream after manipulation)

[edit] Example

With \n instead of endl, the output would be the same, but may not appear in real time.

#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);


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

[edit] See also

controls whether output is flushed after each operation
(function) [edit]
flushes the output stream
(function template) [edit]
synchronizes with the underlying storage device
(public member function of std::basic_ostream) [edit]